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Acolyte
2008-Apr-25, 05:16 AM
OK, this may be a little difficult to ask properly, but here goes...

While reading about the expansion of space, the difficulty of observing local effects & the effects of gravity, I am left wondering about something.

If expansion operates only on space, if mass is not affected or the effects are covered up because of the local effects of gravity, am I correct in assuming that space itself is...
a. not affected by gravity (because if it was it wouldn't be able to be expanded)
b. not a nullness (because it has an effect on the universe)
c. permeates everywhere, up to & including the interstitial spaces between atoms & maybe even quarks & smaller entities

If this is so, is it possible that we could somehow measure expansion by viewing/finding space 'streaming' past mass as the former expands?

Or is there some reason why space, within the influence of mass & gravity, doesn't expand at all?

If space doesn't expand when it's near matter or gravity effects, maybe the increase in expansion rate (was it?) 5 billion years back ws caused by there eventually being enough distance between masses for spacial expansion to be unhampered in some areas?

Acolyte
2008-Apr-26, 03:04 AM
Hm... Too many Universe Expansion topics? Poorly explained questions? Stupid questions? Or nobody has any answers or approaches?

Neverfly
2008-Apr-26, 03:10 AM
am I correct in assuming that space itself is...
a. not affected by gravity (because if it was it wouldn't be able to be expanded)
Maybe this is bad wording. I'm not sure what prompted this thought...
Space is absolutely effected by gravity. That is what causes a gravity well and light to be 'bent' while travleing in a straight line.

b. not a nullness (because it has an effect on the universe)
I agree here- Space is not a "nothing".

c. permeates everywhere, up to & including the interstitial spaces between atoms & maybe even quarks & smaller entities
Good Question!
Not one I have an answer to...


If this is so, is it possible that we could somehow measure expansion by viewing/finding space 'streaming' past mass as the former expands?
At the distances involved?
You cannot "see" space. You can see the effects on objects in space. Which is what we are currently doing.


If space doesn't expand when it's near matter or gravity effects, maybe the increase in expansion rate (was it?) 5 billion years back was caused by there eventually being enough distance between masses for spacial expansion to be unhampered in some areas?

Others would probably answer this question better than I can- I would probably mess up explaining this one...:doh:

Neverfly
2008-Apr-26, 03:13 AM
Hm... Too many Universe Expansion topics?
Yep.

Poorly explained questions?
That too...

Stupid questions?
Nope.

Or nobody has any answers or approaches?
A little patience;)

Acolyte
2008-Apr-26, 04:16 AM
Thank you for the reply...

Maybe this is bad wording. I'm not sure what prompted this thought...
Space is absolutely effected by gravity. That is what causes a gravity well and light to be 'bent' while travelling in a straight line.
Well, it comes from thinking about expansion. I got told off in the 'ant & rope' thread (not upset OK?) for thinking that mass might be affected by expansion. As is my wont, I let it simmer for a bit then a little flag popped up with questions.

If only regions virtually empty of matter are affected by expansion it seems to me there's a bit of a puzzle. I'm not even sure I can word it properly but it's to do with just what space is. If space is 'not nothing' & can be distorted by mass as you say, how does it expand against the influence of mass? If we roll back in time, mass was much closer together than now, so if the distortions produced by matter affect space & prevent expansion, we'd have a situation where expansion would proceed along a ballistic model with mass gradually slowing expansion, until the point was reached where the distance between major clumps of matter became large enough for gravity effects to drop below a certain level.

At that point, it would seem the expansion effects would begin to appear & we'd have a surge in the expansion rate. As more space 'appeared' in between masses, the rate would increase because gravity is becoming less effective.

So this could be a mechanism to explain why we have Origin, Inflation, slowing expansion & then sudden increase in expansion rate as well as an ongoing increased rate.

What I don't know is whether this thought means anything or is at all useful in describing what is going on. Or if maybe it's been thought of & discarded as not useful.

It seems to me there'd be clues here as to what space might be. A beginning for finding things is to first describe what the properties might be.

If space can be affected by matter, does it follow that matter is affected by space?

If nearby mass prevents expansion of space, what must space be to be subject to that effect?


At the distances involved?
You cannot "see" space. You can see the effects on objects in space. Which is what we are currently doing.:doh:We can't see gravity either, but we can trace & model the effects of it. As we do we get closer to a description of what it might actually be. So I'm wondering what might be the effects of space on matter that we can look at.

speedfreek
2008-Apr-26, 09:54 AM
Now I come at it from the other perspective - that space is nothing, just an abstract concept. Space itself is a "nullness", it is simply the gaps between things. The expansion of the universe causes the size of any space to increase in between unbound objects.

Does "space" have properties of its own that might cause expansion? Or is there something that permeates space, a form of energy or pressure that is responsible? I tend to think it is the latter option.

Acolyte
2008-Apr-26, 10:08 AM
Now I come at it from the other perspective - that space is nothing, just an abstract concept. Space itself is a "nullness", it is simply the gaps between things. The expansion of the universe causes the size of any space to increase in between unbound objects.

Does "space" have properties of its own that might cause expansion? Or is there something that permeates space, a form of energy or pressure that is responsible? I tend to think it is the latter option.Hm... I think that means you're talking about the Aether. If this was ATM I'd talk about whether or not Michelson & Morley really did have a non-result or not, but as it isn't, I will just say that, I think that's what I'm asking - is there some property of Space that would interact with mass, the way standard theory says mass reacts with space?

Is it possible for mass to interact with space in any way if space is a nullness? That is the essence of Einstein's gravity - that mass deforms space to form the effects of gravity.

And if mass can interact with space, can it possibly be a one-way interaction or does what we know of this Universe mean that any interaction has to work both ways?

If we say that mass doesn't react with space, that becomes ATM in a big way because it invalidates the idea that gravity is deformation of space due to mass, plus the idea that, whatever dark energy is, it has to overcome the effects of mass (gravity) & we need to find a different paradigm for the Universe.

Among other things like mass deforming space to form gravity effects in the first place...

If space is nullness, how does the gap between masses increase at all - if I add two times nothing to nothing, I still have nothing. Surely space has to be a substrate or possess qualities of some kind for it to be able to separate masses in the first place? We know there is vacuum between masses within our galaxy & we're reasonably sure there is a vacuum between the galaxies, but light travels pretty much the same between the furthest galaxy & Earth as it does between Alpha Centauri & Earth. So we don't have any reason to suspect there is a substantial difference between intergalactic & interstellar vacuums.

speedfreek
2008-Apr-26, 11:59 AM
The aether is making a comeback in a different form - dark energy, but the question you ask is indeed one of the most profound in modern cosmology.

All we have so far are ways to describe what the expansion is doing. Einstein describes space using geometry, but we don't know how that geometry manages to manifest itself, we just see the result. There are many valid hypotheses for the mechanism behind expansion, some of which involve a form of negative pressure.

mugaliens
2008-Apr-26, 12:36 PM
Now I come at it from the other perspective - that space is nothing, just an abstract concept. Space itself is a "nullness", it is simply the gaps between things. The expansion of the universe causes the size of any space to increase in between unbound objects.

Does "space" have properties of its own that might cause expansion? Or is there something that permeates space, a form of energy or pressure that is responsible? I tend to think it is the latter option.

Space is all but 100% vacuum. There are approximately 4 molecules for every cubic meter, and that includes all matter in the universe! Thus, the space between the stars is even more of a vacuum.

But "nothing?" A "nullness?"

That's far from the truth. It does indeed have properties, such as it's ability to regulate the speed of light and change the mass and rate of time lapse of objects travelling through it depending upon their velocity with respect to the speed of light. Furthermore, fundamental particles are constantly winking in and out of existance. While only a very tiny fraction of matter itself, it nonetheless does happen.

speedfreek
2008-Apr-26, 01:33 PM
I am talking in terms of space as an abstract concept, the gaps in between things. What happens in that space is a different matter. The term "space" simply describes the dimensions within which events occur.

Jeff Root
2008-Apr-26, 02:18 PM
Does "space" have properties of its own that might cause expansion?
Or is there something that permeates space, a form of energy or
pressure that is responsible? I tend to think it is the latter option.
Setting aside the acceleration of the expansion, ignoring it as if
it were 11 years ago and we didn't know about it yet...

What reasons do we have to think that the expansion is anything
other than the simple ballistic motion of stuff given an initial impetus
approximately 14 billion years ago?

-- Jeff, in Minneapolis

mugaliens
2008-Apr-26, 02:24 PM
I am talking in terms of space as an abstract concept, the gaps in between things. What happens in that space is a different matter. The term "space" simply describes the dimensions within which events occur.

So you're talking Newtonian/Euclidian space.

However, the OP refers to the expansion of space, which involves GR issues, including energy and pressure as measured by the cosmological constant, hence my response to your post.

When discussions the expansion of space, space itself is most certainly not empty!

speedfreek
2008-Apr-26, 04:52 PM
We are both talking about the same thing from different angles, and my point was exactly the same as yours - it is the energy and pressure we should be concerned with, what those do to the measure of space.

Space is not empty, and I said I tended to think the energy and pressure are the cause of the expansion, not some property of empty space. I never implied space was empty, I implied the opposite, that empty space cannot expand as it has no inherent properties. The only things with inherent properties are the things in space (including the fundamental particles that appear and then disappear).

Something causes the spaces between unbound objects to increase in distance, but space itself doesn't "expand".

speedfreek
2008-Apr-26, 04:57 PM
Setting aside the acceleration of the expansion, ignoring it as if
it were 11 years ago and we didn't know about it yet...

What reasons do we have to think that the expansion is anything
other than the simple ballistic motion of stuff given an initial impetus
approximately 14 billion years ago?

-- Jeff, in Minneapolis

Can you define "ballistic motion" for us?

Tim Thompson
2008-Apr-26, 05:18 PM
Hm... Too many Universe Expansion topics? ...
Definitely.


... am I correct in assuming that space itself is ... a. not affected by gravity (because if it was it wouldn't be able to be expanded)
First and foremost, right out of the bag, "space itself" is an entirely unknown entity which, at least for the time being, is an entirely philosophical concept. The same is true for time, and the same is true for their combination of spacetime. We have no idea what "itself" is or means. What we do know is that the equations of general relativity describe how space, time & spacetime behave physically, in terms of their involvement in measurements & observations. And that is the real bottom line. We have nothing to say about what anything really is, we are limited to detailed examinations of how things behave dynamically, and what they look like when we observe & measure.

Now, with that behind us, the standard interpretation of general relativity tells us (metaphysically) that you are wrong to assume that space itself is not affected by gravity. In fact, the standard interpretation of general relativity is that space itself is gravity (or more correctly, spacetime itself). In pure general relativity, that which we normally refer to as the "force" of gravity is not a force at all, but rather a manifestation of the geometry & curvature of spacetime.


... am I correct in assuming that space itself is ... b. not a nullness (because it has an effect on the universe)
I am unsure what a "nullness" is supposed to be, but I tend to agree that space itself, whatever it may be, is likely not a "nullness". If nothing else, regardless of its physical properties, we do know that it can be mathematically described, and that is enough to show that something is there, even if the something is more metaphysical than physical. If nothing else, spacetime is geometry.


... am I correct in assuming that space itself is ... c. permeates everywhere, up to & including the interstitial spaces between atoms & maybe even quarks & smaller entities
Certainly that is mathematically true, so it seems physically valid as well.


If this is so, is it possible that we could somehow measure expansion by viewing/finding space 'streaming' past mass as the former expands?
I will call that impossible. The reason is that we have no idea how to observe or detect "space" or "spacetime" in any way. We are entirely limited to inferring the behavior of spacetime from the observed behavior of matter & energy entrained in spacetime.


Or is there some reason why space, within the influence of mass & gravity, doesn't expand at all?
Yes, there is a reason. Just consider what happens to a heavy steel bar when you try to bend it or stretch it Superman style in your own bare hands. It does not bend, not even a little. That steel bar won't bend at all until you reach the elastic strength of the bar. Likewise, the internal strength of the matter, or the gravitationally bound structure, overwhelms the expansion of spacetime over such short distances.


If space doesn't expand when it's near matter or gravity effects, maybe the increase in expansion rate (was it?) 5 billion years back was caused by there eventually being enough distance between masses for spacial expansion to be unhampered in some areas?
That is in fact the standard cosmology. The expansion history of the universe can be derived as a function of redshift (i.e., Shafieloo, 2007 (http://adsabs.harvard.edu/abs/2007MNRAS.380.1573S); Ishida, et al., 2008 (http://adsabs.harvard.edu/abs/2008APh....28..547I)). Early on the large structures are close enough for gravity to slow the expansion of the universe. But a constant, albeit small cosmological constant, will eventually get strong enough, as the large structures get farther apart, to turn the expansion around from slowing down to speeding up, and we are in the speeding up era now.

Cougar
2008-Apr-26, 05:28 PM
Just consider what happens to a heavy steel bar when you try to bend it or stretch it Superman style in your own bare hands. It does not bend, not even a little. That steel bar won't bend at all until you reach the elastic strength of the bar.

Great example. :exclaim:

Jeff Root
2008-Apr-26, 05:47 PM
Setting aside the acceleration of the expansion, ignoring it as if
it were 11 years ago and we didn't know about it yet...

What reasons do we have to think that the expansion is anything
other than the simple ballistic motion of stuff given an initial impetus
approximately 14 billion years ago?
Can you define "ballistic motion" for us?
The motion of any thrown object. Given an initial motion, the
motion simply continues. Your statement prompting my question
suggests that some force is constantly applied to matter, causing
the cosmic expansion. I am asking whether such a continuous
force is needed to explain what is observed, and if so, why?

-- Jeff, in Minneapolis

Jeff Root
2008-Apr-26, 06:11 PM
Tim,

As I said to speedfreek, to the best of my knowledge, the observed
cosmic expansion as described until the discovery of the acceleration
in 1998 could be the result of an initial impetus given to matter some
14 billion years ago, and continuing to now by simple inertia. Or it could
be continuously produced by some kind of force. If the latter, the force
could be present throughout all of space, but not seen inside of galaxies
or in lab experiments because it is so weak that it is overwhelmed by
nuclear, electromagnetic, and gravitational forces. Or the force might
only be present between galaxies, and not exist within galaxies at all.

-- Jeff, in Minneapolis

speedfreek
2008-Apr-26, 06:18 PM
The motion of any thrown object. Given an initial motion, the
motion simply continues. Your statement prompting my question
suggests that some force is constantly applied to matter, causing
the cosmic expansion. I am asking whether such a continuous
force is needed to explain what is observed, and if so, why?

-- Jeff, in Minneapolis

In that case nothing "weird" is required to explain the decelerating expansion of the universe. F=ma and gravity provide the force (as Ned Wright puts it). That is if we disregard the cosmological constant and/or the cause of the initial impetus.

I was really only trying to say what Tim said here:


First and foremost, right out of the bag, "space itself" is an entirely unknown entity which, at least for the time being, is an entirely philosophical concept.

But I think I should leave these concepts to the experts to explain.

Tim Thompson
2008-Apr-26, 10:34 PM
Tim, As I said to speedfreek, to the best of my knowledge, the observed cosmic expansion as described until the discovery of the acceleration in 1998 could be the result of an initial impetus given to matter some 14 billion years ago, and continuing to now by simple inertia.
That is in fact what cosmologists thought. That's the basis of the older cosmological models which described a universe which could stop expanding and collapse, or continue slowing asymptotically to a stop at infinite time, or continue slowing but still expand too fast to stop at infinity, depending on whether or not the universe was closed, flat or open, respectively.


... Or it could be continuously produced by some kind of force. If the latter, the force could be present throughout all of space, but not seen inside of galaxies or in lab experiments because it is so weak that it is overwhelmed by nuclear, electromagnetic, and gravitational forces.
And this is what cosmologists think now. The force is everywhere, and does not exist "only in the space between galaxies". Although the repulsive force that accelerates expansion, which is described best as a cosmological constant, exists everywhere, it actually has an effect only in the space between galaxy clusters. So we can expect that in the far future galaxy clusters, like our own local group, will remain intact, while other clusters move beyond detectability. See, for instance, Chiueh & He, 2002 (http://adsabs.harvard.edu/abs/2002PhRvD..65l3518C); Dünner, et al., 2006 (http://adsabs.harvard.edu/abs/2006MNRAS.366..803D); Krauss & Scherrer, 2007 (http://adsabs.harvard.edu/abs/2007GReGr..39.1545K).

Acolyte
2008-Apr-27, 01:42 AM
Sorry about the delay in getting back, but I've been trying to find a reference. A while back I read a site that looked at the various ways in which redshift, galaxy & supernova information agreed or otherwise with the various models of the universe.

IIRC, there was a possible bias built in to observations that had to be taken into account to do with samples being biased by distance - the further away we look, the more likely it is we're only looking at larger galaxies.

The reason I'm looking is because, again IIRC, there was a speculation in there that, if there are gravitons, there need not be a distance relationship with redshift at all. That light could be simply losing energy by passing through a graviton field & so appear redshifted.

And the reason for wanting to re-find that information is it seems to me that the puzzle most posters are having with Expansion has to do with just what is expanding. If we are misinterpreting the redshift as galactic recession when it isn't, the expansion problem(s) go(es) away.

I thought it might have relevance here but I can't recall enough of it to know that or if it would actually be an ATM argument. if anyone knows about it, I'd appreciate a link or suggestions.

Back to topic...

One of the suggestions I've seen for dark energy is that it is vacuum energy. (ie. energy stored in the fabric of spacetime) & that this could be the cosmological constant that drives expansion. Leaving aside the possibilities inherent in energy that permeates the cosmos, (could we use a vacuum pump to move our cars? *grins*) the expansion would seem to violate the idea that energy can't be created or destroyed.

If it's not an 'energy' as such, there is a strangeness about how it can interact with matter to push it apart isn't there? If it doesn't interact with matter & normal energy, then space would simply expand past all the mass, creating a larger void around the Universe but not affecting its development.

Or am I missing something?

tommac
2008-Apr-27, 02:02 AM
The reason I'm looking is because, again IIRC, there was a speculation in there that, if there are gravitons, there need not be a distance relationship with redshift at all. That light could be simply losing energy by passing through a graviton field & so appear redshifted.


I would love to see this ... I have been questioning the redshift a bit ... but was told on this board that is 100% from galaxies moving farther away from us due to the expansion of the universe.

Please find.

Acolyte
2008-Apr-27, 05:05 AM
Ah... found it. Unfortunately it isn't useful here although it may be for ATM - it's a page by Tom Van Flandern called "Did the Universe Have a Beginning?" at http://www.metaresearch.org/cosmology/DidTheUniverseHaveABeginning.asp#footnotes

A quick review ot TVF shows him, shall I say, not at the forefront of mainstream science.

Tim Thompson
2008-Apr-27, 06:17 AM
IIRC, there was a possible bias built in to observations that had to be taken into account to do with samples being biased by distance - the further away we look, the more likely it is we're only looking at larger galaxies.
Make the brightest rather than largest. This bias is so well know to astronomers that it even has a name: Malmquist Bias. Early determinations of the Hubble Constant (http://cfa-www.harvard.edu/~huchra/hubble/) were far higher than they are now because of Malmquist bias, as Trimble, 1996 (http://adsabs.harvard.edu/abs/1996PASP..108.1073T) shows. There have been many studies on the effect of Malmquist bias, and how to compensate for it (i.e., Teerikorpi, 1984 (http://adsabs.harvard.edu/abs/1984A%26A...141..407T); Feast, 1987 (http://adsabs.harvard.edu/abs/1987Obs...107..185F); Landy & Szalay, 1992 (http://adsabs.harvard.edu/abs/1992ApJ...391..494L); Triay, Lachieze-Rey & Rauzy, 1994 (http://adsabs.harvard.edu/abs/1994A%26A...289...19T); Teerikorpi, 2003 (http://adsabs.harvard.edu/abs/2003A%26A...399..829T); Butkevich, Berdyugin & Teerikorpi, 2005a (http://adsabs.harvard.edu/abs/2005A%26A...435..949B); Butkevich, Berdyugin & Teerikorpi, 2005b (http://adsabs.harvard.edu/abs/2005MNRAS.362..321B)). The point is the astronomers now routinely include Malmquist bias compensation. This is not an issue in the accelerated expansion cosmology (i.e, Perlmutter, et al., 1999 (http://adsabs.harvard.edu/abs/1999ApJ...517..565P); Li, Filippenko & Riess, 2001 (http://adsabs.harvard.edu/abs/2001ApJ...546..719L); Schaefer, 2007 (http://adsabs.harvard.edu/abs/2007ApJ...660...16S)).


The reason I'm looking is because, again IIRC, there was a speculation in there that, if there are gravitons, there need not be a distance relationship with redshift at all. That light could be simply losing energy by passing through a graviton field & so appear redshifted.
Tired light cosmologies are occasionally suggested (i.e., Crawford, 1999 (http://adsabs.harvard.edu/abs/1999AuJPh..52..753C)), but do not stand up to detailed examination. For instance time dilation is not expected by tired light cosmologies, but has been observed in supernova light curves (i.e., Foley, et al., 2005 (http://adsabs.harvard.edu/abs/2005ApJ...626L..11F)). Also see Ned Wright's Errors in Tired Light Cosmology (http://www.astro.ucla.edu/~wright/tiredlit.htm) page.

The other problem here is with the phrase "there need not be a distance relationship with redshift". The redshift distance relationship is a matter of observed fact, so it makes no sense to talk about whether or not it is "needed"; it is there, whether we like it or not. Edwin Hubble established this fact a long time ago (Hubble, 1929 (http://adsabs.harvard.edu/abs/1929PNAS...15..168H)), and observations since then have only increased the distance over which we know the relationship holds (i.e., Sandage, 1972a (http://adsabs.harvard.edu/abs/1972ApJ...173..485S); Sandage, 1972b (http://adsabs.harvard.edu/abs/1972ApJ...178....1S); Sandage, 1972c (http://adsabs.harvard.edu/abs/1972ApJ...178...25S); Sandage, 1973a (http://adsabs.harvard.edu/abs/1973ApJ...180..687S); Sandage, 1973b (http://adsabs.harvard.edu/abs/1973ApJ...183..711S); Sandage, 1973c (http://adsabs.harvard.edu/abs/1973ApJ...183..731S); Sandage, 1973d (http://adsabs.harvard.edu/abs/1973ApJ...183..743S); Sandage, 1975 (http://adsabs.harvard.edu/abs/1975ApJ...202..563S); Salpeter & Hoffman, 1986 (http://adsabs.harvard.edu/abs/1986PNAS...83.3056S); Sandage, 1986 (http://adsabs.harvard.edu/abs/1986ApJ...307....1S); Sofue, et al., 1996 (http://adsabs.harvard.edu/abs/1996PASJ...48..657S); da Costa, et al., 2000 (http://adsabs.harvard.edu/abs/2000AJ....120...95D); Bernardi, et al., 2002 (http://adsabs.harvard.edu/abs?bibcode=2002AJ....123.2159B&) and so on). The point of all the references is that even though the redshift distance relationship is considered as fact, it is still subject to constant observational verification.


Leaving aside the possibilities inherent in energy that permeates the cosmos, (could we use a vacuum pump to move our cars? *grins*) the expansion would seem to violate the idea that energy can't be created or destroyed.
That is correct. It is known that energy is not globally conserved by general relativity, because "energy" is impossible to define as a global variable (at least so far). Since energy can be defined only as a local observable, there is no guarantee that it can be globally conserved.


If it's not an 'energy' as such, there is a strangeness about how it can interact with matter to push it apart isn't there?
There is. But who says cosmology can't be "strange"?

Acolyte
2008-Apr-27, 09:38 AM
For me, Cosmology is already strange. With my questions here I'm trying to find out just how strange it might be... :lol:

So...

I’m certainly not competent to follow the math involved in some of the links. But I can read enough of it to get some understanding of what is being said.

From the link given, Hubble’s estimate for Virgo cluster uses luminosities to calculate how far away it is; it seems to be wrong by a factor of almost 10 (2,000,000Mpc as opposed to 18,000,000Mpc. I realize later measurements have resulted in the different figure, but it shows that luminosities are, at best, a poor way to gauge inter-galactic distances.

Also, the article linked to doesn’t appear, to my limited understanding, to demonstrate anything about redshift specifically, even though it provides distance estimates to a number of extra-galactic ‘nebulae.’ In fact, at the end of the piece, it mentions that these observations could be evidence of the de Sitter effect

The outstanding feature, however, is the possibility that the velocity/distance relation may represent the de Sitter effect, and hence that numerical data may be introduced into discussions of the general curvature of space. In the de Sitter cosmology, displacements of the spectra arise from two sources, an apparent slowing down of atomic vibrations and a general tendency of material particles to scatter. It does seem though, as if there’s a bit of circular calculation going on.

From: http://starchild.gsfc.nasa.gov/docs/StarChild/questions/redshift.html

In 1929, Edwin Hubble announced that almost all galaxies appeared to be moving away from us. In fact, he found that the Universe was expanding - with all of the galaxies moving away from each other. This phenomenon was observed as a redshift of a galaxy's spectrum. This redshift appeared to be larger for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth. You can see this trend in Hubble's data shown in the images above. The velocity of a galaxy could be expressed mathematically as


v = H x d

where v is the galaxy's radial outward velocity, d is the galaxy's distance from Earth, and H is the constant of proportionality called the Hubble constant.Hubble found redshift with fainter objects having a larger redshift so he deduced the fainter objects were further away & so were moving away from us faster.
But if there is another explanation for the reduced brightness, the initial assumption of increased apparent velocity due to distance isn’t really sound. And now we use redshift to give distances to explain brightness to back up the redshift distance.
Or reduced brightness of objects could mean they are further away but an alternate explanation, such as the effects of a graviton field or curvature pressure could explain the ‘redshift’ results. From what I read, if the putative gravitons are very small, there would be little or no scattering effect.

For that matter, could we be seeing the effect of strings or virtual particles? If they are a valid concept, space, empty or not, as well as matter & energy, would be formed by them. So a drag on light would be a result, with the effect greater over longer distances, & as Light can’t slow down in a given medium, it would alter the wavelength that reaches us.

Acolyte
2008-Apr-27, 10:05 AM
There are more problems with the redshift hypothesis than just trying to understand how it works. I'm unsure of his reputation on this forum but Halton Arp has shown a number of high redshift quasars directly associated with low redshift galaxies.

from - http://www.haltonarp.com/articles/rebuttals
In one case, NGC 4319 and Markarian 205, Arp showed a physical bridge linking the 2 objects. 31 years later, NASA published what seems to be a low res photo to try to disprove the bridge, but it still shows - I downloaded the NASA photo & simply ran an Autofix in Photoshop to show both the 'wings' of the galaxy & the link to the Quasar.

Arp has numerous other examples of high & low redshift objects in apparently close proximity - this should be impossible under the expanding Universe scenario, shouldn't it?

mugaliens
2008-Apr-27, 10:56 AM
OK, this may be a little difficult to ask properly, but here goes...

While reading about the expansion of space, the difficulty of observing local effects & the effects of gravity, I am left wondering about something.

If expansion operates only on space, if mass is not affected or the effects are covered up because of the local effects of gravity, am I correct in assuming that space itself is...

Please stop.

Expansion isn't "covered up" by the local effects of gravity. Rather, the local effects of gravity are so much greater than the effects of expansion that expansion has little effect compared to local gravitational effects. Expansion still has has an effect! It's just that it's miniscule compared to local (galaxy-class) effects of gravity.

Acolyte
2008-Apr-27, 10:59 AM
I don't understand. Why do I have to stop? And stop what? My term 'covered up' is not saying that expansion has no effect, just that any effect it might have is masked by the close up gravitational effects.

Did I offend you somehow?

mugaliens
2008-Apr-27, 12:25 PM
I don't understand. Why do I have to stop? And stop what? My term 'covered up' is not saying that expansion has no effect, just that any effect it might have is masked by the close up gravitational effects.

Did I offend you somehow?

No, acolyte - you didn't offend me in the least. Perhaps it's my fault. In learning this stuff I spent many hours in the books trying to discern what's what because others on this board and other boards kept telling me, "you still don't understand - get thee back to the books!"

Frustrating? Yes! But I do not wish to pass that on to you.

Nevertheless, your questions indicate a lack of basic understanding of the physical properties of the universe.

Instead of us spending countless hours trying to explain it to you, PLEASE take the time to tap into the countless hours that have ALREADY been spent on areas like Wikipedia, and not just by a handful of people, but by hundreds of subject matter experts.

It's not a matter of "don't waste my time." Rather, it's a matter of "tap into the vastly superior time that others have spent in terms of both quantity as well as quality (there are some exceptions, such as when Grant, Publius, or Astromark answer a question) than what you'll ever find here."

Garner an eduction first, then come back here and we'll be happy to help you fill in the gaps!

That didn't come out quite right...

While you are garnering an education elsewhere, whether it be a self-study or a formal course, we'll be happy to help you understand what you're learning, guiding you along the way (learning what we can in the process, as learning never stops - well over half of what I've learned here occurred while reasearching a topic so that I could explain it to someone else!).

We'd just like to see a bit more self-initiative to use the web, Wikipedia, Google, your local library, your teachers, your fellow students, etc., before you come to us asking us in a first (or even tenth) post to answer what's obviously a homework question.

We're not homework answer-people. We help others learn because we love to learn ourselves. However, it's up to you to make (and indicate to the rest of us) that you've taken your best stab at it, first, before coming to us.

And it's up to us to utilize considerable restrain to ensure we give you the best possible environment for you to learn how to conduct your own research, rather than co-dependantly coughing up an answer merely to assuage our own ego.

The former approach is healthy. The latter is not, neither for ourselves, or for you, for if we supply you with every answer, rather than helping guide you to learn how to find the answers on your own, we've merely given you a fish, and in the long run you'll starve, and we will have wasted our time.

If, on the other hand, we guide you towards being able to find the answers on your own, we will have taught you how to fish, and you'll never starve, and our time will have been well spent.

So, no offense at all!

However, in keeping with the spirit of this post, please stop asking questions and conduct a bit a research using the two main links in my signature block. If you get stuck on Wiki, follow the countless links. If you're still stuck, try varying the words.

That's what I do.

Become an originator of content, rather than merely a requestor thereof.

mugaliens
2008-Apr-27, 12:28 PM
I don't understand. Why do I have to stop? And stop what? My term 'covered up' is not saying that expansion has no effect, just that any effect it might have is masked by the close up gravitational effects.

You're correct in that local gravitation effects are vastly superior to the effects of expansion. Nevertheless, we observe expansion effects throughout the universe, even within the relatively small confines of our own galaxy.

Jeff Root
2008-Apr-27, 12:46 PM
There are certainly no effects of the cosmic expansion visible within the
Milky Way galaxy. Probably none visible within our local cluster.

If the expansion of space in fact permeates all of space, as you say, it
would not cause the same kind of expansion within the galaxy as is seen
on cosmic scales, where things get farther and farther apart. At most,
it would cause a very slight increase in the size of things, which would
not change with time. That is, for example, Earth might be a micrometre
larger in diameter than it would be if there were no expansion; the orbit
of Neptune might be a metre greater in diameter than if there were no
expansion. But those increases in size would not change over time.

-- Jeff, in Minneapolis

grant hutchison
2008-Apr-27, 01:12 PM
That is, for example, Earth might be a micrometre
larger in diameter than it would be if there were no expansion; the orbit
of Neptune might be a metre greater in diameter than if there were no
expansion. But those increases in size would not change over time.Accelerating expansion is required if there is to be an outward force. Lineweaver & Davis (http://www.sciam.com/article.cfm?id=misconceptions-about-the-2005-03) report that the current acceleration in the expansion of the Universe produces an outward force at the Earth's surface equal to about 10-30 of the inward force of gravity.

Grant Hutchison

Jeff Root
2008-Apr-27, 01:49 PM
Accelerating expansion is required if there is to be an outward force.
The cause and effect seems backwards: An outward force is required
to cause accelerating expansion between widely-separated galaxies.

However, even with no acceleration, expansion could either be a
ballistic flow of matter through space, or a flow of space whch drags
matter along with it. In the latter case, space is expanding, applying
a force to all the matter it passes through as it expands, causing the
unchanging size increases I described within galaxies, and constant
expansion between widely-separated galaxies.

I'll add the possibility that flowing matter drags space with it. That
actually sounds very reasonable, but I'm not sure it would have any
effects distinguishing it from a ballistic flow through space.

-- Jeff, in Minneapolis

grant hutchison
2008-Apr-27, 02:01 PM
However, even with no acceleration, expansion could either be a
ballistic flow of matter through space, or a flow of space whch drags
matter along with it. In the latter case, space is expanding, applying
a force to all the matter it passes through as it expands, causing the
unchanging size increases I described within galaxies, and constant
expansion between widely-separated galaxies.According to the Lineweaver & Davis (http://www.sciam.com/article.cfm?id=misconceptions-about-the-2005-03&page=5) article I linked to (See Is Brooklyn Expanding?), constant expansion produces no net force, and therefore no change in size of gravitationally or electromagnetically bound objects. Decelerating expansion produces an inward force, accelerating expansion an outward force.
I believe the expansion of space under the influence of gravity (and without a cosmological constant) can be viewed as effectively "ballastic": it doesn't have to "drag" matter along with it; everything moves together.

Grant Hutchison

Jeff Root
2008-Apr-27, 02:24 PM
According to the Lineweaver & Davis (http://www.sciam.com/article.cfm?id=misconceptions-about-the-2005-03&page=5) article I linked to
(See Is Brooklyn Expanding?), constant expansion produces no net force,
and therefore no change in size of gravitationally or electromagnetically bound objects. Decelerating expansion produces an inward force,
accelerating expansion an outward force.
I believe the expansion of space under the influence of gravity (and
without a cosmological constant) can be viewed as effectively "ballastic":
it doesn't have to "drag" matter along with it; everything moves together.
It appears that we are in complete agreement if you are saying that
expansion which can be considered ballistic would produce no force,
and thus would not affect the size of Brooklyn.

Only if the expansion could not be considered ballistic might there be
any outward force. But the expansion in that case would not necessarily
be accelerating. Since it now appears that it is accelerating, the
expansion cannot be considered entirely ballistic. But even then there
are possibilities that do not require an expansion force within galaxies.
The force might only be between galaxies.

-- Jeff, in Minneapolis

grant hutchison
2008-Apr-27, 02:32 PM
It appears that we are in complete agreement if you are saying that
expansion which can be considered ballistic would produce no force,
and thus would not affect the size of Brooklyn.Then you disagree with Lineweaver & Davis, who are quite explicit on this point, I think:
For example, if gravity got stronger, your spinal cord would compress until the electrons in your vertebrae reached a new equilibrium slightly closer together. You would be a shorter person, but you would not continue to shrink. In the same way, if we lived in a universe dominated by the attractive force of gravity, as most cosmologists thought until a few years ago, the expansion would decelerate, putting a gentle squeeze on bodies in the universe, making them reach a smaller equilibrium size. Having done so, they would not keep shrinking.
In fact, in our universe the expansion is accelerating, and that exerts a gentle outward force on bodies. Consequently, bound objects are slightly larger than they would be in a nonaccelerating universe, because the equilibrium among forces is reached at a slightly larger size. At Earth's surface, the outward acceleration away from the planet's center equals a tiny fraction (10-30) of the normal inward gravitational acceleration. If this acceleration is constant, it does not make Earth expand; rather the planet simply settles into a static equilibrium size slightly larger than the size it would have attained.(My bold)

Grant Hutchison

grant hutchison
2008-Apr-27, 03:39 PM
For more on how expanding space doesn't "drag" matter with it, except in misleading analogies, see Section 2.5 of Francis et al.'s Expanding Space: the Root of all Evil? (http://arxiv.org/abs/0707.0380):
We now turn to the issue of test particle motion, since
this is at the heart of many of the attacks on expanding
space. The classic thought experiment used is the
“tethered galaxy” problem (Harrison 1995). In this,
a test galaxy in an expanding universe is held at rest
with respect to the origin at a cosmological distance.
By Hubble’s law, we would expect this galaxy to be
receding, however we prevent this, artificially holding
the test galaxy in place. The question is, when the
galaxy is released, what does it do? Since critics of
the expanding space concept argue that the Newtonian
analogue of expanding space is the presence of
some kind of viscous force, dragging the galaxies apart
like objects carried along by a river: therefore, in this
thought experiment, the test particle should pick up
a velocity away from the origin due to the expanding
river of space. In fact, what the particle does once
being released depends on the acceleration of the universe.
If the scale factor is accelerating the particle
moves away but if it is decelerating the particle moves
towards the origin [see Barnes et al. (2006) for the full
details].Notice the connection, in the last sentence, to Lineweaver & Davis's remarks about the acceleration of universal expansion being required for a net force to be developed within extended objects.

For more detail, look at the Barnes et al. (2006) reference which concludes my quotation: Joining the Hubble Flow: Implications for Expanding Space (http://arxiv.org/abs/astro-ph/0609271).

Grant Hutchison

Tim Thompson
2008-Apr-27, 05:21 PM
From the link given, Hubble’s estimate for Virgo cluster uses luminosities to calculate how far away it is; it seems to be wrong by a factor of almost 10 (2,000,000Mpc as opposed to 18,000,000Mpc. I realize later measurements have resulted in the different figure, but it shows that luminosities are, at best, a poor way to gauge inter-galactic distances.
That's incorrect. Hubble made the classified galaxies by their morphology (Galaxy morphological classification (http://en.wikipedia.org/wiki/Galaxy_morphological_classification); Galaxy Classification (http://www.astr.ua.edu/keel/galaxies/classify.html)). He assumed that galaxies of the same type should have roughly the same intrinsic luminosity, and that therefore the farther galaxies should be apparently dimmer. This is one of those common sense things that in fact works, even in cosmology. So he decides in this way that there should be a redshift distance relationship. However, if you look at the published paper, Hubble 1929 (http://adsabs.harvard.edu/abs/1929PNAS...15..168H), you see that this is not how he actually established the relationship. He measured the distances independently of redshift by using Cepheid variable stars, and showed that the Cepheid distances and the redshifts (mutually independent) are correlated, and therefore there is a real, directly observed, redshift distance. I thought I made this point when I listed so many references showing that the redshift distance relationship is observationally determined independently from redshift. It is not based on luminosity, it is based on direct & indirect distance measurements, independently of redshift.

The reason Hubble severely underestimated the distances is that he was unaware of the fact that there are 2 classes of Cepheid variable stars, fundamental and overtone pulsators. Later corrections have stabilized the distances. But is is important to understand that the redshift distance relationship as it now is understood is based on direct & indirect distance measures, and is neither dependent nor based on simple luminosity.


Also, the article linked to doesn’t appear, to my limited understanding, to demonstrate anything about redshift specifically, even though it provides distance estimates to a number of extra-galactic ‘nebulae.’ In fact, at the end of the piece, it mentions that these observations could be evidence of the de Sitter effect.
That's because Hubble established the redshift distance relationship, but did not infer from it an expanding universe. Indeed, Hubble himself never accepted the expanding universe theory that became mainstream, and assigned the redshifts to another cause.


... It does seem though, as if there’s a bit of circular calculation going on. ... But if there is another explanation for the reduced brightness, the initial assumption of increased apparent velocity due to distance isn’t really sound. And now we use redshift to give distances to explain brightness to back up the redshift distance.
As I have already pointed out, there is no such "assumption" in play. Distances & redshifts are independently measured and the correlation is real.


... Or reduced brightness of objects could mean they are further away but an alternate explanation, such as the effects of a graviton field or curvature pressure could explain the ‘redshift’ results. From what I read, if the putative gravitons are very small, there would be little or no scattering effect.
Tired light cosmologies have long since been abandoned because they don't work. The observation of time dilation as a function of redshift (i.e., Foley, et al., 2005 (http://adsabs.harvard.edu/abs/2005ApJ...626L..11F)) falsifies this cosmology (also see ned Wright's Errors in Tired Light Cosmology (http://www.astro.ucla.edu/~wright/tiredlit.htm) page).


For that matter, could we be seeing the effect of strings or virtual particles? If they are a valid concept, space, empty or not, as well as matter & energy, would be formed by them. So a drag on light would be a result, with the effect greater over longer distances, & as Light can’t slow down in a given medium, it would alter the wavelength that reaches us.
Strings literally, no; that suffers from the same problem that all tired light cosmologies suffer from. But we could be seeing an effect of string theory, specifically higher dimensional structures called branes (http://en.wikipedia.org/wiki/Brane_cosmology). "Dark matter" or "dark energy" could be effects propagating along branes, but this is purely speculative.

eburacum45
2008-Apr-27, 06:33 PM
In fact, in our universe the expansion is accelerating, and that exerts a gentle outward force on bodies. Consequently, bound objects are slightly larger than they would be in a nonaccelerating universe, because the equilibrium among forces is reached at a slightly larger size.
So I'm slightly bigger than I would be in a non-accelerating universe. eh? Sounds good to me.

Bogie
2008-Apr-27, 07:44 PM
...Tired light cosmologies have long since been abandoned because they don't work. The observation of time dilation as a function of redshift (i.e., Foley, et al., 2005 (http://adsabs.harvard.edu/abs/2005ApJ...626L..11F)) falsifies this cosmology (also see ned Wright's Errors in Tired Light Cosmology (http://www.astro.ucla.edu/~wright/tiredlit.htm) page).Is the evidence for acceleration being exmined and confirmed at the same time as the evidence is improving that red shift is due to expansion?

Acolyte
2008-Apr-27, 08:32 PM
No, acolyte - you didn't offend me in the least. Perhaps it's my fault. In learning this stuff I spent many hours in the books trying to discern what's what because others on this board and other boards kept telling me, "you still don't understand - get thee back to the books!"...

...However, in keeping with the spirit of this post, please stop asking questions and conduct a bit a research using the two main links in my signature block. If you get stuck on Wiki, follow the countless links. If you're still stuck, try varying the words.

That's what I do.

Become an originator of content, rather than merely a requestor thereof.Thank you for taking the time. I had (to the annoyance of my wife) spent a number of hours away from the site reading up on references provided by members trying to give me information.

This raised questions so I returned to ask them. I did try to find answers by tracing links, but the knowledge represented here on BAUTforums can help reduce hours or days of link tracing to a non-divorce level.

I appreciate very much the efforts made to provide me with reasons & further information to follow & given the conversations that have come into the thread, I feel that perhaps in this instance, some of my questions impinge on areas that are not yet settled.

As in my question about Halton Arp, there are also times when it is difficult to know how reliable or respected a source is. Arp's point about high & low redshift objects being connected seems to be a valid objection to redshift-distance assumptions.

In another instance I thought I recalled other data that contradicted redshift-distance, & while I don't have the judgement to know yet if it is valid, further research showed me that it wasn't a source that would get much shrift on BAUT.

So I am doing work on my own, & trying not to just treat BAUT as an answer machine.

I realise it must be frustrating to be providing the same or similar content over & again, but I did take a look at other expansion questions & felt they weren't quite what I wanted to know. Asking them here gave me a point of leverage to crack open a problem that has been (slightly) bugging me for years.

The various fields of science have grown to the point where it can be difficult for someone not working in a specific area to sort out what is meaningful & what is transition ideas & even sometimes to discern valuable insight from crackpot theory.


Again, thanks for taking the time. (all of you who respond to (new) members' queries)

Neverfly
2008-Apr-27, 09:33 PM
Acolyte, going off that post, it sounds like you are doing a GREAT job.
Many folks don't bother to go look stuff up.

But please ensure you don't let your wife get too upset;)

Hopefully you will continue to use BAUT for learning- or arguing:p, What Mugaliens seems to have been talking about doesn't apply to you so much as those
that endlessly ask questions but never look anything up or try to do any research on their own. So it looks like you're on safeground.

dcl
2008-Apr-27, 10:20 PM
I'm amazed at how rapidly this thread is growing. In joining it, I feel as if I'm jumping into a raging torrent, perhaps to be swept away. That not withstanding, I'm about to present my thoughts on the subject under discusson and stand back to see what happens. If I'm not welcomed aboard, I'll quietly disappear.

I'll offer my view concerning the nature of what we call space and its relation to its material contents.

I see our Universe as having come into existence some 13.7 billion years ago at an infinitesimally small point in a supermedium of which we can presumably never learn anything. We have no basis for even speculating about the nature of this supermedium and the process through which our Universe came to existence in it. We can assume only that the infinitely small point contained only energy at an infinitely high temperature. This energy could only have been in the form of photons since the temperature was far too high for even the most basic forms of matter to exist.
The photons have a blackbody spectrum corresponding to an infinitely high temperature.

During the first instant, his energy was under infinite pressure, and the contents of the point began instantaely to expand at infinite speed but immediately slowing to an extremely high finite speed. As the point instantly expanded into a sphere, its temperature began to fall.

The expansion is four- rather than three-dimensional, and the entire contents of the point expands instantly into a rapidly expanding hollow hypersphere with empty interior and three-dimensional "surface" containing its entire contents. It is driven by both the temperature of the radiation that it contains and by an intrinsic property of space itself that is not understood beyond the fact that it is embodied in the cosmological constant in the field equations of the general theory of relativity.

As the expansion progressed and the temperature of the black-body radiation fell, some of the radiation was converted through the mechanism of Einstein's cosmological constant and his E = mc2 into matter to maintain an equilibrium between matter and energy. This matter took the form of quarks, electrons, and corresponding forms of antimatter. Nearly all of the matter and antimatter annihilated back into radiation, leaving only a much smaller amount of matter now popularly called "quark soup".

The universe continued to expand and cool, and the quarks combined into protons, neutrons, and mesons.

Now I'll answer the questions in the initial contribution that opened this thread:

Acolyte: If expansion operates only on space, if mass is not affected or the effects are covered up because of the local effects of gravity, am I correct in assuming that space itself is...
a. not affected by gravity (because if it was it wouldn't be able to be expanded).
b. not a nullness (because it has an effect on the universe)
c. permeates everywhere, up to & including the interstitial spaces between atoms & maybe even quarks & smaller entities

Answers: Expansion operates only on space, not on its contents. Mass is not affected by the expansion of space. The effects of expansion are covered up by the local effects of gravity. Space itself is not affected by the local effects of gravity. I can't answer your question about space not being a nullness because I don't understand your question. Space does have an effect on the universe, but it doesn't follow that space is a nullness, whatever that means. Space doers permeate everywhere, up to and including the interstitial spaces beetween atoms and quarks and smaller entities.

Acolyte: If this is so, is it possible that we could somehow measure expansion by viewing/finding space 'streaming' past mass as the former expands?

Bogie
2008-Apr-27, 10:51 PM
Welcome aboard, not that I have any standing on this thread either.
A couple of things dcl:


I see our Universe as having come into existence some 13.7 billion years ago at an infinitesimally small point in a supermedium of which we can presumably never learn anything. We have no basis for even speculating about the nature of this supermedium and the process through which our Universe came to existence in it. We can assume only that the infinitely small point contained only energy at an infinitely high temperature. This energy could only have been in the form of photons since the temperature was far too high for even the most basic forms of matter to exist.
Your say we have no basis to speculate about the nature of what you call the supermedium and the process through which our universe came to existence in it, right after you do just that.



During the first instant, his energy was under infinite pressure, and the contents of the point began instantaely to expand at infinite speed but immediately slowing to an extremely high finite speed. As the point instantly expanded into a sphere, its temperature began to fall.

The expansion is four- rather than three-dimensional, and the entire contents of the point expands instantly into a rapidly expanding hollow hypersphere with empty interior and three-dimensional "surface" containing its entire contents. It is driven by both the temperature of the radiation that it contains and by an intrinsic property of space itself that is not understood beyond the fact that it is embodied in the cosmological constant in the field equations of the general theory of relativity.

As the expansion progressed and the temperature of the black-body radiation fell, some of the radiation was converted through the mechanism of Einstein's cosmological constant and his E = mc2 into matter to maintain an equilibrium between matter and energy. This matter took the form of quarks, electrons, and corresponding forms of antimatter. Nearly all of the matter and antimatter annihilated back into radiation, leaving only a much smaller amount of matter now popularly called "quark soup".

The universe continued to expand and cool, and the quarks combined into protons, neutrons, and mesons.You sound like you look at that scenario as fact. Please note that it is theory and that your version of the theory is not strictly Big Bang Theory which makes no claim about a zero volume infinitely dense singularity. That concept is derived from GTR and implied but not incorporated.

My question to you is how you find believing in something that seems quite impossible like the infinitely dense zero volume point origin of the universe in place of engaging in speculation about possible causes that don’t require such a reach of faith.

Neverfly
2008-Apr-27, 11:10 PM
I'm amazed at how rapidly this thread is growing. In joining it, I feel as if I'm jumping into a raging torrent, perhaps to be swept away. That not withstanding, I'm about to present my thoughts on the subject under discusson and stand back to see what happens. If I'm not welcomed aboard, I'll quietly disappear.
65 posts since 2002?
I think you might need to do more jumping!


I see our Universe as having come into existence some 13.7 billion years ago at an infinitesimally small point in a supermedium of which we can presumably never learn anything. We have no basis for even speculating about the nature of this supermedium and the process through which our Universe came to existence in it.
Not that this stops our Bogie...;)


Answers: Expansion operates only on space, not on its contents. Mass is not affected by the expansion of space. The effects of expansion are covered up by the local effects of gravity.
Someone else mentioned the use of the words "Covered up" as a bit misleading.
Personally I have thought of it as Overwhelmed...

Acolyte
2008-Apr-28, 12:32 AM
Someone else mentioned the use of the words "Covered up" as a bit misleading.
Personally I have thought of it as Overwhelmed...Perhaps Gravity finds Expansion to be Underwhelming? Galaxies are on a Redshift diet? *grins*

Ok... I'll stop now...

dcl
2008-Apr-28, 02:12 AM
This really IS a fast-growing thread. Hardly had I got my two cents posted than I have three responses from two different people! My responses follow. Each starts with the name of the person to whom I'm responding followed by a specific statement to which I'm responding followed by ":--", then, starting on the next line, my response.

Bogie: You sound as if you look at that scenario as fact. (Big Bang theory) makes no claim about a zero volume infinite volume singularity:--
To be precise, I agree with your statement about the Big Bang theory not postulating an infinitely dense singularity. The known laws of physics do not allow us to track events all the way back to time zero. On the other hand, there is nothing we know of to stop the extrapolation from going all the way back to infinitesimal size at infinite density. If we try to start at two or more points separated by any distance at all, we run into a problem with causality: How can the same cause produce effects at two separated points, no matter how close together they are, without invoking a signal propagation time, thereby implying that there was an earlier time from which a signal traveled simultaneously from a single point to two other points? Admittedly, this is somewhat like arguing about how many angels can dance on the head of a pin.

Bogie: You look as if you look on that scenario as fact:--
I mean only to infer that we know of no plausible alternative to the proposed mechanism. We know of no plausible mechanism for allowing the Big Bang to begin over an extended source. I agree that it's only educated conjecture.

Neverfly: 65 posts since 2002? I think you need to do more jumping!:--
I looked into the Forum briefly back in 2002, then gave up on it on finding myself responding to Forum users who seemed too naive to benefit from what I might tell them. Just recently, Fraser Cain urged me to put my views on the Forum after I conveyed to both him and Dr. Gay my feeling regarding Dr. Gay's contention that the Universe has the shape of a doughnut. I'm now finding that contributors to the Forum are far better informed than I had gathered from my earlier experience. Incidentally, I'm a professional physicist, now retired, with a primarily but nonprofessional interest in cosmology. You'd be amazed at the list of extremely famous physicists that I've met during my active years.

Neverfly: Someone else mentioned the use of the words "Covered up" as a bit misleading. Personally I have thought of it as Overwhelmed...:--
I agree that "overwhelmed" is better in that context than "covered up".

Neverfly
2008-Apr-28, 02:17 AM
Neverfly: Someone else mentioned the use of the words "Covered up" as a bit misleading. Personally I have thought of it as Overwhelmed...:--
I agree that "overwhelmed" is better in that context than "covered up".

I can't remember who it was..
Speedfreak maybe? Or Grant Hutchison...

Either way- this poster gave a good reason but it was in one of Tommacs threads and I can't figure out which one- where -at what point...:doh:
Maybe whoever that was can come to the rescue...

ETA: dcl, glad to see that Fraser blackmailed you out of your silence then;) Will be looking forward to making you miserable with my ignorance later.

Neverfly
2008-Apr-28, 02:36 AM
Found it.
It was in Arcolyte's thread, not Tommacs.
Here:http://www.bautforum.com/questions-answers/73268-understanding-expansion-universe.html#post1227857
http://www.bautforum.com/1228322-post27.html
Mugaliens was the one who said it- Tim Thompson gave a great example of it earlier in the thread describing if you tried to bend a steel bar in your hands super-man style. It won't bend until it reaches it's elastic limit.

THERE! I have fulfilled my Destiny Luke...

Acolyte
2008-Apr-28, 04:07 AM
Bogie: You sound as if you look at that scenario as fact. (Big Bang theory) makes no claim about a zero volume infinite volume singularity...

...If we try to start at two or more points separated by any distance at all, we run into a problem with causality: How can the same cause produce effects at two separated points, no matter how close together they are, without invoking a signal propagation time, thereby implying that there was an earlier time from which a signal traveled simultaneously from a single point to two other points?There's been experimental information about instantaneous transmission of information between particles - twin particles know each other's data like spin for example. Non-locality I think - with 2 particles.

There's also some info about single particle non-locality which I think means Merlin the Magician was a real person... Check out http://www.physorg.com/news113824784.html

Acolyte
2008-Apr-28, 04:11 AM
Found it.
It was in Arcolyte's thread, not Tommacs.
Here:http://www.bautforum.com/questions-answers/73268-understanding-expansion-universe.html#post1227857
http://www.bautforum.com/1228322-post27.html
Mugaliens was the one who said it- Tim Thompson gave a great example of it earlier in the thread describing if you tried to bend a steel bar in your hands super-man style. It won't bend until it reaches it's elastic limit.

THERE! I have fulfilled my Destiny Luke...Ah, but grasshopper, in the real world, the steel bar does react to the pressure before it bends. It isn't a sudden breakpoint then a physical reaction, it's just that in our macro world nothing seems to happen. In reality the molecular interstices begin to react as soon as pressure is applied.

If I recall my high school physics, the bending happens only when catastrophic failure of the bonds is imminent & the bend is a permanent change in the structure. (which is why you can't bend it back to perfectly straight again without remelting it)

Neverfly
2008-Apr-28, 04:24 AM
Ah, but grasshopper, in the real world, the steel bar does react to the pressure before it bends. It isn't a sudden breakpoint then a physical reaction, it's just that in our macro world nothing seems to happen. In reality the molecular interstices begin to react as soon as pressure is applied.

If I recall my high school physics, the bending happens only when catastrophic failure of the bonds is imminent & the bend is a permanent change in the structure. (which is why you can't bend it back to perfectly straight again without remelting it)

Yes, in the case of the steel rod. But it's an example- an analogy- in this case referring to the expansion of space and why matter does not expand- why local systems don't expand.
Changing the analogy:
If you put two magnets on the table close enough together, you can watch them pull toward eachother until the click together. But if you stick an object between them they will not reach eachother. Sure, there may be interaction at the molecular level but it's rendered insignificant- the two magnets won't touch. They won't burrow through the obstacle even if you wait a million years.

Acolyte
2008-Apr-28, 06:28 AM
But the analogy holds up. The overwhelm of gravitational interactions doesn't mean there is no expansion effect at all.

The effect is there still - it can't be otherwise unless we start postulating that gravity is like some forcefield effect that has a definite cut-off point within which space-expansion cannot occur.

That was why I asked initially about space effects. And why i think it is Ok to use 'cover up' to describe how gravity effects conceal expansion effects when near mass.

For expansion to be affecting superclusters & larger but not clusters, gravity effects have to be much larger that expansion effects, vis-a-vis the huge spaces necessary before expansion becomes significant.

On another note...

Does anyone have info or comment about Halton Arp? The redshift inconsistency with objects apparently linked or close is a worry. As near as I can tell it takes only one of these pairs to invalidate the redshift - recession velocity due to universal expansion & he has quite a few candidates.

His data looks good & he has a long time in astronomy behind him.

Jeff Root
2008-Apr-28, 06:50 AM
Oops. Ignore everything I said in this post about Arp. My error.

-- Jeff, in Minneapolis

Ari Jokimaki
2008-Apr-28, 07:22 AM
Does anyone have info or comment about Halton Arp? The redshift inconsistency with objects apparently linked or close is a worry. As near as I can tell it takes only one of these pairs to invalidate the redshift - recession velocity due to universal expansion & he has quite a few candidates.

His data looks good & he has a long time in astronomy behind him.
There's an old thread about that in the ATM section, here's the link (http://www.bautforum.com/against-mainstream/26365-more-arp-et-al.html).

astromark
2008-Apr-28, 09:32 AM
Umm... Err... Hold the presses and here is the front page.
The Universe is Not expanding.
There is no red shift.
This Galaxy is just 5,000 years old. The light path was aged just to fool us....
The whole universe was created just for us...

The very fact that this Forum is here and we feel free to ask and answer questions and comment about the science of astronomy in a informed and intelligent manor.
The weight of modern popular opinion would have not printed that fools front page.
What are you saying Acoyte? would you clarify please.

Acolyte
2008-Apr-28, 09:43 AM
Oops. Ignore everything I said in this post about Arp. My error.
-- Jeff, in MinneapolisOkaaaay... Did you have the wrong Halton Arp? *grins*

OK, reading further... In the thread given, one of the objections is that Arp looks at 2D & ignores 3D. This seems a rather harsh criticism of someone who has been in Astronomy for most of his working life.

Aside from that, in the other things I've read, he is quoting redshifts as links. He looks at relative redshifts & finds patterns of low redshift old galaxies paired with almost identically higher redshift (usually) bright X-ray source galaxies or Quasars on opposite sides of the (suggested) source.

He gives clear descriptions (even for me) & references.

So... also from what I'm reading, the best weapon in the redshift arsenal seems to be the time dilation of Type 1a's. If that found a new explanation it would open up the field once more. The other evidence for redshift seems to have a range of possibilities for explanations.

Is that a correct thought or have I missed something?

Acolyte
2008-Apr-28, 10:00 AM
Umm... Err... Hold the presses and here is the front page.
The Universe is Not expanding.
There is no red shift.
This Galaxy is just 5,000 years old. The light path was aged just to fool us....
The whole universe was created just for us...

The very fact that this Forum is here and we feel free to ask and answer questions and comment about the science of astronomy in a informed and intelligent manor.
The weight of modern popular opinion would have not printed that fools front page.
What are you saying Acoyte? would you clarify please.Um... which fool are you speaking about?

Popular opinion is a notoriously fickle judge of what is worthwhile. Seen the Reality shows on TV?

As for what I'm saying, I don't yet have an opinion. I look, I learn & I ask questions.

I don't take someone's unsupported opinion of things as Truth & I try to find & eradicate beliefs in myself. If I am going to think of something as Truth I want to have valid reasons.

Since age 10 when the local pastor tried to sell me on the faith = belief = faith circular argument I've tried to avoid them. I'm probably a bit leery of them & so I'm checking out the 'Relativity says redshift shows increasing recession because redshift shows relativity effects' apparency in this area.

So I ask & explore. Some here have been good enough to give of their time & knowledge to help me on the path.

For example, one of the reason I read that it couldn't be a 'tired light' explanation (apart from the time dilation effect of Type 1a's) is that anything interacting with light enough to increase the wavelength would also scatter it & we don't see scattering.

What occurs to me is we have a candidate that I can't find to have been considered - it has apparently a small enough cross section that we know it doesn't interfere with the path of light, it has few other effects on the Universe around it & it exhibits gravity effects.

Dark matter could be a candidate for a mechanism to increase the wavelength of light - IF, & it's a big if, there is the possibility of another explanation for the Time dilation effects of the Type 1a's.

I'm presuming from the beginning of your post you are wondering if I'm
a Creationist out to provoke?

astromark
2008-Apr-28, 11:45 AM
Yes you had me worried... and no I avoid reality TV. I was simply demonstrating that in our community there are people who think like that... maybe they are the lucky ones. I have a faith in my fellow man. A empathy for humanity. I thought I saw a reference to creationism. I was wrong. Do not be discouraged, ask and explore. When I mentioned popular opinion I was to have added the scientific community but didn't. I should have. Else where in this forum I have seen drawn as a pie chart,the known universes make up. A very small fraction is mater as we are familiar with. about 7%. A larger fraction was depicted as 'Dark mater' and some thing like 90% was this 'Dark energy'... This is why we are building large Particle accelerators and making sub surface caverns bristling with sensors. We need to know this stuff is there., and what it is.

Bogie
2008-Apr-28, 11:47 AM
We may be getting out on thin ice discussing the imponderable issue of the “beginning” on Acolyte’s thread. The justification for continuing to pursue the issue with you (dcl) is that the OP was labeled “understanding the expansion of the universe”, and you attempted to do so with a scenario that employes speculations more complicated than some reasonable alternatives. If the moderator has a position on this I will understand. In the mean time let me address your scenario without committing rule violations regarding the introduction of Against the Mainstream ideas any more than anyone else has here:


The known laws of physics do not allow us to track events all the way back to time zero.(my bold)

I say, the known laws of physics do not allow for something to come from nothing any more than they allow such back tracking.
We either have to say we don’t know and cannot know and leave it at that, or we are speculating.
So we are speculating when we say there might be unknown physics that allows back tracking to a zero volume infinitely dense origin to our universe, and we are speculating when we suggest that there might be unknown physics that could cause our expanding universe from pre-conditions.


On the other hand, there is nothing we know of to stop the extrapolation from going all the way back to infinitesimal size at infinite density.
On the other hand, there is nothing we know of to suggest that the extrapolation of going back toward infinitesimal size at infinite density would actually be achievable. There is nothing we know of to suggest that we wouldn’t find that there were preconditions.

If we try to start at two or more points separated by any distance at all, we run into a problem with causality: How can the same cause produce effects at two separated points, no matter how close together they are, without invoking a signal propagation time, thereby implying that there was an earlier time from which a signal traveled simultaneously from a single point to two other points?
Bingo!
Expansion could have an origin and cause put into play from preconditions.

tommac
2008-Apr-28, 05:33 PM
Really? I thought there were ... it expands the orbits slightly and I think I even read something saying that the circumference of the earth is expanding due to its effects.



There are certainly no effects of the cosmic expansion visible within the
Milky Way galaxy. Probably none visible within our local cluster.

If the expansion of space in fact permeates all of space, as you say, it
would not cause the same kind of expansion within the galaxy as is seen
on cosmic scales, where things get farther and farther apart. At most,
it would cause a very slight increase in the size of things, which would
not change with time. That is, for example, Earth might be a micrometre
larger in diameter than it would be if there were no expansion; the orbit
of Neptune might be a metre greater in diameter than if there were no
expansion. But those increases in size would not change over time.

-- Jeff, in Minneapolis

Tim Thompson
2008-Apr-28, 05:35 PM
Is the evidence for acceleration being exmined and confirmed at the same time as the evidence is improving that red shift is due to expansion?
Yes. The observational evidence in favor of both conclusions is really quite strong. I already listed many papers showing the strength of the redshift distance relationship back in post #24 (http://www.bautforum.com/questions-answers/73268-understanding-expansion-universe.html#post1228264). The original reports of supernova data implying accelerated expansion come from Riess, et al., 1998 (http://adsabs.harvard.edu/abs/1998AJ....116.1009R) and confirmed independently by Perlmutter, et al., 1999 (http://adsabs.harvard.edu/abs/1999ApJ...517..565P). Both papers have already garnered over 3000 citations each in just 10 years. Followup supernova surveys include Riess, et al., 2001 (http://adsabs.harvard.edu/abs/2001ApJ...560...49R); Turner & Riess, 2002 (http://adsabs.harvard.edu/abs/2002ApJ...569...18T); Knop, et al., 2003 (http://adsabs.harvard.edu/abs/2003MNRAS.340.1057S); Sullivan, et al., 2003 (http://adsabs.harvard.edu/abs/2003MNRAS.340.1057S); Tonry, et al., 2003 (http://adsabs.harvard.edu/abs/2003ApJ...594....1T); Riess, et al., 2004 (http://adsabs.harvard.edu/abs/2004ApJ...607..665R); Clocchiatti, et al]., 2006 (http://adsabs.harvard.edu/abs/2006ApJ...642....1C); Astier, et al., 2006 (http://adsabs.harvard.edu/abs/2006A%26A...447...31A); Conley, et al., 2006 (http://adsabs.harvard.edu/abs/2006ApJ...644....1C); Wood-Vasey, et al., 2007 (http://adsabs.harvard.edu/abs/2007ApJ...666..694W); Bronder, et al., 2008 (http://adsabs.harvard.edu/abs/2008A%26A...477..717B); Kowalski, et al., 2008 (http://adsabs.harvard.edu/abs/2008arXiv0804.4142K). All that is continuation of the research on the acceleration of the expansion as implied by type Ia supernovae. This is the most effective evidence in favor of accelerating expansion. But other observations, such as CMB & gravitational microlensing, while less effective, are consistent with the supernova data.


Ah, but grasshopper, in the real world, the steel bar does react to the pressure before it bends. It isn't a sudden breakpoint then a physical reaction, it's just that in our macro world nothing seems to happen. In reality the molecular interstices begin to react as soon as pressure is applied.
"React" and "bend" are not the same thing. The "reaction" is that the bar reaches a new equilibrium configuration to match the miniscule force applied. So, in the presence of a miniscule expansion, a material body, or a gravitationally bound system, will have a slightly larger equilibrium size, where the interior strength of the body or bound system balances the force of expansion (a point made by somebody else in one of far too many threads on the same topic). The material body, or the gravitationally bound system, will not expand with the expanding universe for the same reason that the bar does not bend, because the applied force is too weak.

tommac
2008-Apr-28, 05:38 PM
So I'm slightly bigger than I would be in a non-accelerating universe. eh? Sounds good to me.

Yes I agree.

parejkoj
2008-Apr-28, 06:14 PM
OK, reading further... In the thread given, one of the objections is that Arp looks at 2D & ignores 3D. This seems a rather harsh criticism of someone who has been in Astronomy for most of his working life.


Sadly, it happens. Einstein never really thought Quantum Mechanics was valid, and he was quite wrong, though that doesn't invalidate GR. Even the best of us make mistakes; some just persist in hanging on to them...

If you want to discuss Arp in more detail, please start a thread in the ATM section. Arp is mostly ignored by astronomers these days, though his catalog of peculiar galaxies is still widely cited. But his ideas about quasars are now hopelessly out of date. Shields' Brief History of Active Galactic Nuclei (http://adsabs.harvard.edu/abs/1999PASP..111..661S) provides a thorough summary, though it too is now somewhat dated (e.g. the Gunn-Peterson trough has since been observed (http://adsabs.harvard.edu/abs/2001AJ....122.2850B), and the X-ray background resolved (http://www.nature.com/nature/journal/v404/n6777/full/404459a0.html) and confirmed to be AGN). I'll have to see if there is a more recent summary of our knowledge of AGN, but I don't know of any that are as broad as that one.



Aside from that, in the other things I've read, he is quoting redshifts as links. He looks at relative redshifts & finds patterns of low redshift old galaxies paired with almost identically higher redshift (usually) bright X-ray source galaxies or Quasars on opposite sides of the (suggested) source.


Again, if you want to talk more about Arp, start a thread in ATM. Suffice it to say, we know a lot more about quasars now, and Arp's "patterns" were a combination of wishful thinking, confirmation bias and selection effects (the early quasar catalogs were missing a lot of sources, because we didn't know how to find them all).

Besides the slew of papers that Tim has cited (I'm always impressed at his "survey completeness"), some strong evidence for the standard cosmology model comes from quasars: the Lyman alpha forest and Gunn-Peterson troughs.

speedfreek
2008-Apr-28, 08:17 PM
Really? I thought there were ... it expands the orbits slightly and I think I even read something saying that the circumference of the earth is expanding due to its effects.

Now I know what you mean here, but to make sure no misconceptions have been introduced, the theory has it that orbits settle at an ever so slightly larger radius due to accelerating expansion, but orbits (and the size of the Earth) are not "expanding" with the expansion of the universe, they settle at a slightly different size due a change in the rate of the expansion.

As the rate of expansion increases, so does the size that things "settle" at, and vice versa, be we are talking about a (currently) tiny and physically immeasurable amount of change.

This then begs the question.. (here we go again..!) Do our meter sticks settle at a slightly different length and if so how do we take that into account? Does this tiny difference have an effect on the frequency that atoms "vibrate" at? Will this have implications as the expansion continues to change rate in the future?

Acolyte
2008-Apr-28, 08:36 PM
Hm... flowers perhaps...? Maybe a nice dinner & wine...? There must be something to placate the missus for all this extra reading I've now got. *grins*

le sigh...

(which is my way of saying thanks for the links guys)

Note I'm not actually wanting to talk specifically about Arp - he was a blip in the reading I was doing.

The idea that Dark Matter (DM) could be affecting what we see is still hanging in there though. To affect galaxy formation & evolution it must have gravitational effects & if so, it should also have effects on light as it passes by. And there's more DM than Normal Matter (NM)

parejkoj
2008-Apr-28, 09:20 PM
Why don't you bring her in on it? Two people doing research can cover more ground and gain more understanding than just one.




The idea that Dark Matter (DM) could be affecting what we see is still hanging in there though. To affect galaxy formation & evolution it must have gravitational effects & if so, it should also have effects on light as it passes by.

It does. (http://en.wikipedia.org/wiki/Bullet_cluster) There is also the Baryon Acoustic Peak (http://cmb.as.arizona.edu/~eisenste/acousticpeak/), among other effects.

transreality
2008-Apr-29, 01:03 AM
Acolyte: If expansion operates only on space, if mass is not affected or the effects are covered up because of the local effects of gravity, am I correct in assuming that space itself is...
a. not affected by gravity (because if it was it wouldn't be able to be expanded).
b. not a nullness (because it has an effect on the universe)
c. permeates everywhere, up to & including the interstitial spaces between atoms & maybe even quarks & smaller entities

Answers: Expansion operates only on space, not on its contents. Mass is not affected by the expansion of space. The effects of expansion are covered up by the local effects of gravity. Space itself is not affected by the local effects of gravity. I can't answer your question about space not being a nullness because I don't understand your question. Space does have an effect on the universe, but it doesn't follow that space is a nullness, whatever that means. Space doers permeate everywhere, up to and including the interstitial spaces beetween atoms and quarks and smaller entities.



This may not be true beyond a certain scale.

Doesn't the casimir effect imply that at a certain interstitial size virtual particles can no longer manifest in the gap so expansion at that point cannot occur. The gaps between the atoms of a planet (for example) or a star may be sufficiently close to stop local expansion within the body of the object.

On a large scale even if a region of space has 0.01% density of mass, then expansion in that space would by 0.01% less than in a volume with no or much less mass?

mugaliens
2008-Apr-29, 05:37 PM
That is, for example, Earth might be a micrometre
larger in diameter than it would be if there were no expansion; the orbit
of Neptune might be a metre greater in diameter than if there were no
expansion. But those increases in size would not change over time.

-- Jeff, in Minneapolis

If there is expansion at all, it's highly unlikely that the rate of expension is a constant (critically damped by gravity). It's either slowing, or it's accelerating. Common sense says it ought to be slowing, but whether it's slowing fast enough (overdamped by gravity) to cause a Big Collapse or not fast enough (underdamped by gravity) had remained to be seen, until 1998, when my precise measurements showed that the rate of expansion was accelerating, which gave rise to postulations of various mechanisms which might be causing this.

(I know you alreay know this. I just wanted to clarify for other readers. Thanks).

mugaliens
2008-Apr-29, 05:43 PM
The cause and effect seems backwards: An outward force is required
to cause accelerating expansion between widely-separated galaxies.

Not necessarily, as I postulated in another thread.

Please allow me to summarize: It's the amount of matter within the observable universe that would tend to slow expansion by means of gravitational attraction. Gravity works at the speed of light.

However, due to expansion itself, matter at the edge is moving at the speed of light away from us, crossing over the comoving distance, thus, relative to us, it's now moving away from us faster than the speed of light (red-shifted beyond a wavelength of zero). Thus, that matter no longer exerts it's gravitational pull on us.

This is why we appear to be at the exact center of the universe. We're not, really. It's just that due to expansion the very edge has moved beyond the comoving distance.

The less total mass within the universe, the less that's holding us together, and the greater the rate of expansion.

Well, that's my pet theory for the week, anyway... :lol:

speedfreek
2008-Apr-29, 06:25 PM
The less total mass within the universe, the less that's holding us together, and the greater the rate of expansion.

Well, that's my pet theory for the week, anyway... :lol:

I know you mean the less total mass within the observable universe, as you said gravity works at the speed of light. But...

We used to think that, depending on the amount of matter within the universe, the expansion would either slow down to a constant speed or slow down but never get to a halt or slow to a halt and start to reverse. We thought we had a universe that would either end up expanding at a constant rate, would continuously slow or would end up contracting, and it all depended on the overall gravitational influence (at the speed of light) of everything relative to everything else, as you say.

But gravity could never have been responsible for an acceleration of the rate of expansion, which is why we were so surprised when we found that the rate of expansion was only decelerating until around 6 billion years ago. The rate levelled out and since then the rate of expansion has been increasing.

The whole point is that expansion is only measurable between systems that are already unbound by gravity. If the clusters of galaxies exert an attractive force on their neighbours, they will slow down the rate of expansion between them. The rate of expansion between might continue to decelerate towards a constant, where there is essentially no gravitational influence between them, or there might always be some small influence between them slowing the rate of expansion, or there might always be enough influence to slow the rate of expansion to zero and start a contracting of the universe.

An acceleration of the rate of expansion between those clusters would require another cause and this seems to have kicked in around 6 billion years ago, when something resembling repulsive gravity started to manifest itself as the rate of expansion (and overall gravity density) reached a certain value.

Acolyte
2008-Apr-29, 08:20 PM
I know you mean the less total mass within the observable universe, as you said gravity works at the speed of light. But...

...The whole point is that expansion is only measurable between systems that are already unbound by gravity. If the clusters of galaxies exert an attractive force on their neighbours, they will slow down the rate of expansion between them. The rate of expansion between might continue to decelerate towards a constant, where there is essentially no gravitational influence between them, or there might always be some small influence between them slowing the rate of expansion, or there might always be enough influence to slow the rate of expansion to zero and start a contracting of the universe.

An acceleration of the rate of expansion between those clusters would require another cause and this seems to have kicked in around 6 billion years ago, when something resembling repulsive gravity started to manifest itself as the rate of expansion (and overall gravity density) reached a certain value.First, could we know if there is expansion and the rate of it between cluster members or even between clusters in a supercluster? Do we have the sophistication in measurement yet?

Seems to me, if there is expansion in the smaller entities, it could be a repeat process of what occurred 5 - 6 billion years back. ir. the expansion finally reached a break point where the effects of gravity on a large scale were finally overcome & expansion could begin to be a dominant force.

Perhaps that will occur eventually to break up super clusters and between member galaxies of clusters? It wouldn't require a new force, just the continuation of the process we're talking about here.

mugaliens
2008-Apr-30, 08:42 PM
I know you mean the less total mass within the observable universe, as you said gravity works at the speed of light. But...

We used to think that, depending on the amount of matter within the universe, the expansion would either slow down to a constant speed or slow down but never get to a halt or slow to a halt and start to reverse. We thought we had a universe that would either end up expanding at a constant rate, would continuously slow or would end up contracting, and it all depended on the overall gravitational influence (at the speed of light) of everything relative to everything else, as you say.

But gravity could never have been responsible for an acceleration of the rate of expansion, which is why we were so surprised when we found that the rate of expansion was only decelerating until around 6 billion years ago. The rate levelled out and since then the rate of expansion has been increasing.

The whole point is that expansion is only measurable between systems that are already unbound by gravity. If the clusters of galaxies exert an attractive force on their neighbours, they will slow down the rate of expansion between them. The rate of expansion between might continue to decelerate towards a constant, where there is essentially no gravitational influence between them, or there might always be some small influence between them slowing the rate of expansion, or there might always be enough influence to slow the rate of expansion to zero and start a contracting of the universe.

An acceleration of the rate of expansion between those clusters would require another cause and this seems to have kicked in around 6 billion years ago, when something resembling repulsive gravity started to manifest itself as the rate of expansion (and overall gravity density) reached a certain value.

Are you absolutely certain that as mass moves beyond the comoving distance that this effect, the lessening of the total mass of the observable universe, isn't responsible for the accelerating expansion?

Also, I'm unschooled with respect to how we know that "the rate of expansion was only decelerating until around 6 billion years ago."

How do we know that?

I'm not entirely unschooled, but I'm still learning, here. Detailed links, if you can, please (some math, I understand and can reproduce GR/SR and generally 3rd yr physics/engineering, but not higher).

Thanks.

tommac
2008-Apr-30, 08:46 PM
Now I know what you mean here, but to make sure no misconceptions have been introduced, the theory has it that orbits settle at an ever so slightly larger radius due to accelerating expansion, but orbits (and the size of the Earth) are not "expanding" with the expansion of the universe, they settle at a slightly different size due a change in the rate of the expansion.

As the rate of expansion increases, so does the size that things "settle" at, and vice versa, be we are talking about a (currently) tiny and physically immeasurable amount of change.

This then begs the question.. (here we go again..!) Do our meter sticks settle at a slightly different length and if so how do we take that into account? Does this tiny difference have an effect on the frequency that atoms "vibrate" at? Will this have implications as the expansion continues to change rate in the future?

Yes I have those same questions. But I think some is counteracted by the speeding up of time ... so if space changed and time changes ... atoms can vibrate at the same rate.

tommac
2008-Apr-30, 08:49 PM
If there is expansion at all, it's highly unlikely that the rate of expension is a constant (critically damped by gravity). It's either slowing, or it's accelerating. Common sense says it ought to be slowing, but whether it's slowing fast enough (overdamped by gravity) to cause a Big Collapse or not fast enough (underdamped by gravity) had remained to be seen, until 1998, when my precise measurements showed that the rate of expansion was accelerating, which gave rise to postulations of various mechanisms which might be causing this.

(I know you alreay know this. I just wanted to clarify for other readers. Thanks).

Is it even constant at a point in time?

speedfreek
2008-Apr-30, 09:31 PM
Are you absolutely certain that as mass moves beyond the comoving distance that this effect, the lessening of the total mass of the observable universe, isn't responsible for the accelerating expansion?

I find this article, (http://www.astrophysicsspectator.com/topics/cosmology/BasicTheory.html) written by an astrophysicist with a B.S. in Physics from the University of Notre Dame and a Ph.D. in Astronomy from Harvard University, sums up the differences between a universe that is matter dominated and one with a cosmological constant very well.

In a purely matter dominated universe "the universe can be bound, so that expansion eventually halts and then contracts; the universe can be at the escape velocity, so that the rate of expansion of the universe is always decreasing, but the expansion never halts; and the universe can be unbound, so that it eventually reaches a point where it expands at a constant rate forever." He shows the maths too, although his numbers are out of date. But the principles stand.

Only if you introduce a cosmological constant, a "repelling force", do you find that the rate of expansion can accelerate.


Also, I'm unschooled with respect to how we know that "the rate of expansion was only decelerating until around 6 billion years ago."

Galaxy Clusters and Dark Energy: Chandra Opens New Line of Investigation on Dark Energy (http://chandra.harvard.edu/photo/2004/darkenergy/)

"Chandra images of multimillion degree Celsius gas in galaxy clusters have provided astronomers with a powerful new method to probe the mass and energy content of the universe. A recent study of 26 clusters of galaxies confirms that the expansion of the universe stopped slowing down about 6 billion years ago, and began to accelerate."