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stitt29
2008-Sep-26, 09:35 PM
I am a little puzzled about big bang theory. It seems to predict that all mass was created in a millisecond 13.7 billion years ago. Is there any formula for this or is everyone meant to accept this on good faith (very unscientific).
From what I can gather The universe "expanded" from a singularity of infinite density. This singularity is sometimes described as a dimentionless point and infinite density seems a little far fetched.
I'm hoping someone can give me concrete mathematical formula that fits the claim that all the mass in the universe came out of nothing. Also BBT claims this creation of mass happened before time and space existed!
I'm hoping someone can convince me the No.1 theory in Cosmology is not a creation myth.

nutant gene 71
2008-Sep-26, 11:08 PM
I'm hoping someone can convince me the No.1 theory in Cosmology is not a creation myth.
Hopeless, because it really is a 'creation myth'! Though, having said that, there's some pretty good math behind all of it. :)

But I'd let the true BBT 'creationists' explain it here, those who are truly comfortable with it.

Neverfly
2008-Sep-27, 12:00 AM
Hopeless, because it really is a 'creation myth'! Though, having said that, there's some pretty good math behind all of it. :)

But I'd let the true BBT 'creationists' explain it here, those who are truly comfortable with it.

You're both wrong.

There are a few takes, but the BBT istelf does NOT cover the CREATION of the Universe. It only deals with the time AFTER.

Currently, we do not have any good theories as to the actual creation.

Stitt, you have several misconceptions right off the bat.
For one, the BBT does not discuss any kind of matter creation prior to the existence of time and space.

Neverfly
2008-Sep-27, 12:02 AM
As an after thought- I did a quick google of Big Bang and can see from the first three (not including YouTube or creation sites) hits where these misconceptions come from.

It's very hard to describe the BBT considering how our brains are wired for our current environment.

JTankers
2008-Sep-27, 12:21 AM
Evidence supports some aspects of big bang theory, other aspects are wild conjecture.

We have strong evidense the matter in the universe exploded from a single location about 14 billion years ago.

What caused the explotion is pure conjecture. The "cause" I find presently most compelling is the theory at bigcrash.org.

Jeff Root
2008-Sep-27, 12:59 AM
I am a little puzzled about big bang theory. It seems to predict that all
mass was created in a millisecond 13.7 billion years ago. Is there any
formula for this or is everyone meant to accept this on good faith
There is no formula and no-one is expected to accept it on faith.

The Big Bang theory has no explanation for the initial creation event.

What it does is apply known physics to what we see, and extrapolate
backward in time. Part of what we see is that the more distant a
galaxy, the more redshifted its light. We interpret that as a variation
of Doppler shift, caused by widely-separated clusters of galaxies
moving away from each other. The greater the separation between
two galaxy clusters, the faster they are moving apart. Extrapolating
back in time, there is a point in time at which everything we can see
must have all been squished together very tightly. At the limit of
the extrapolation, everything was in the same place, and the density
was infinite.

But that is just an extrapolation using known physics. We don't know
what happened to create the Universe in the first place, so we don't
know what the actual evolution of the Universe was at the beginning.
From the relative abundances of the nuclear isotopes of hydrogen,
deuterium, helium, and lithium, we can be confident that we know the
density, temperature, and composition of the Universe at the time
those isotopes were created, and how long those conditions lasted.

What we can't do is extrapolate all the way back to time zero and
expect the extrapolation to have any reality. We know that it does
not. We just don't know what to replace the extrapolation with.
That is not a secret. But it isn't very well-known, either.

On the other hand, consider this: Energy is relative, not absolute.
That means, for example, that I can measure the kinetic energy of
a flying rock as having one value relative to me and you can measure
it as having a different value relative to you, and we will both be
completely right. (This is simple Newtonian physics. Einsteinian
physics is the same, but isn't required for this particular relativity.)
The point is, since every measurement of energy is relative, it is
possible to have both positive and negative values. The values
only depend on what you choose as your reference point, or "zero"
value.

If you define gravitational potential energy to be negative, then the
sum of all the (negative) gravitational potential energy in the Universe
appears to be exactly equal to all the (positive) other forms of energy
in the Universe. In other words, the overall total mass/energy of the
Universe is zero, the positive energy of matter being exactly balanced
by the negative gravitational potential energy. So whatever the
creation event was, it didn't create any mass/energy, it left the total
at zero, but separated the different kinds of energy from one another.

-- Jeff, in Minneapolis

stitt29
2008-Sep-27, 01:10 PM
Yes got you. There was no Space and Time (nor matter, Or could there have been matter?), then a creation event(of which nothing can be known about), then what is known as big bang when subatomic particles turned into the matter of the Universe. So we can never know where the particles came from. Have I got this right "the Universe was created out of nothing, prior to this there was no Time, Space or matter."
I am truly puzzled how this is taken seriously by anyone. Is it because there are no alternatives?

Jeff Root
2008-Sep-27, 02:52 PM
Until someone comes up with a theory for the origin of the Universe that
is logically consistent and explains all the observed facts, we won't know
whether such a theory is possible or not.

We can say confidently that 13.7 billion years ago the Universe was
very hot, very dense, and expanding rapidly. What it was doing a few
seconds earlier is still largely a matter of speculation.

-- Jeff, in Minneapolis

Spaceman Spiff
2008-Sep-27, 07:57 PM
Yes got you. There was no Space and Time (nor matter, Or could there have been matter?), then a creation event(of which nothing can be known about), then what is known as big bang when subatomic particles turned into the matter of the Universe. So we can never know where the particles came from. Have I got this right "the Universe was created out of nothing, prior to this there was no Time, Space or matter."
I am truly puzzled how this is taken seriously by anyone. Is it because there are no alternatives?

The statements regarding the standard big bang theory (http://en.wikipedia.org/wiki/Big_Bang) above are generally accurate in so much that the model does not address initial conditions. But they're a little lackadaisical about how much we do understand or at least are beginning to address scientifically.

There is a great deal of interwoven evidence for what happened going back to the first microsecond after the current expansion began (because we don't yet know what larger stage of space-time "our universe" operates within). The physics governing physical processes back to this point is based on the well-established (yet incomplete) standard model of particles (http://en.wikipedia.org/wiki/Standard_Model) and general relativity (http://en.wikipedia.org/wiki/Introduction_to_general_relativity). This includes how energy eventually 'condensed' into matter (protons, electrons, neutrons, but leaving behind photons and neutrinos with a ratio of these particles to the nucleons and electrons of about 2 billion to 1) via E = mc^2 as the temperature (energy density) dropped due to expansion. There are several, more speculative, yet well-developed, models taking us back to VERY early times, and generally the earlier, the more speculative. The leading one taking us back to within an epsilon of initial conditions is a class of models known as Inflation (see here (http://en.wikipedia.org/wiki/Cosmic_inflation) for a starter; be sure to read about the observational status of Inflation). There are competing models, such as this (http://www.physics.princeton.edu/%7Esteinh/npr/) one.

And just forget about the idea of an initial singularity. The BBT does not address it, not to mention that it isn't useful because it just refers to what happens when you extrapolate General Relativity Theory beyond its physical validity. e.g., It's just plain silly to extrapolate geometric optics to the realm of sub-micron sized particles (we have better models which explain in a unified manner both small and large particles' interactions with light), or it's just plain silly to use Newton's laws in a scenario involving speeds a fair fraction of the speed of light or in the presence of strong gravitational fields -- we have a better model that explains in a unified manner how nature behaves (General Relativity).

Going forward in time, we know where all of the hydrogen and helium came about, and for that matter, much later, how stars within galaxies created (and are still creating) all the rest of the elements on the periodic table.

Here (http://www.astro.ucla.edu/%7Ewright/BBhistory.html) is a brief time-line. For much more information, go here (http://en.wikipedia.org/wiki/Physical_cosmology) and be sure to read the articles in the Physical Cosmology sidebar.

At some point, we expect that the BBT will be incorporated into a much larger framework, but whether that will be some form of inflation within a multiverse landscape or what, no one yet knows. Amazingly, most of these yet speculative models are already able to make many testable predictions.

I also suggest this gentle introduction (http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf) to the BBT. Actually, it's an article that sets straight (as much as any written at this level can) popular misconceptions of what the BBT is all about). And if you've still more time to burn, I've got a collection of links (http://homepages.wmich.edu/~korista/cosmology.html) pertaining to cosmology.

Lepton
2008-Sep-27, 09:10 PM
Going forward in time, we know where all of the hydrogen and helium came aboutExplain that one to me please. I have heard that said many times yet i don't quite follow. If Helium was from the BB along with Hydrogen how did the first star "ignite"? Doesn't the nuclear process in a star have to start with Hydrogen?

Hornblower
2008-Sep-27, 11:52 PM
Explain that one to me please. I have heard that said many times yet i don't quite follow. If Helium was from the BB along with Hydrogen how did the first star "ignite"? Doesn't the nuclear process in a star have to start with Hydrogen?
The onset of hydrogen fusion is not going to be prevented by having some primordial helium mixed with the hydrogen.

Yes indeed, hydrogen is the first element to start undergoing fusion, because it can do so at a much lower temperature. If by some fluke we got a contracting mass of nothing but helium, it would contract gravitationally to a much hotter and more compact state before the onset of fusion. My educated guess is that the resulting star would be more like a red giant than a main sequence star.

Lepton
2008-Sep-28, 12:34 AM
The onset of hydrogen fusion is not going to be prevented by having some primordial helium mixed with the hydrogen.

Yes indeed, hydrogen is the first element to start undergoing fusion, because it can do so at a much lower temperature. If by some fluke we got a contracting mass of nothing but helium, it would contract gravitationally to a much hotter and more compact state before the onset of fusion. My educated guess is that the resulting star would be more like a red giant than a main sequence star.
Thanks.

Spaceman Spiff
2008-Sep-28, 01:40 AM
Explain that one to me please. I have heard that said many times yet i don't quite follow. If Helium was from the BB along with Hydrogen how did the first star "ignite"? Doesn't the nuclear process in a star have to start with Hydrogen?

Yes indeed, that is true. Hydrogen is the lightest element, with the fewest number of protons (just 1), and has the lowest threshold in temperature requirements for fusion and so it goes first.

The mix of protons (hydrogen nuclei) and neutrons that resulted from matter/energy interactions in the first minute or so and in the next 15 minutes or so (http://www.astro.ucla.edu/%7Ewright/BBNS.html) converted them to a mix of 76% H and 24% helium, along with minuscule %s of heavy hydrogen (2H), light helium (3He) and some lithium.

Over the billions of years of stellar evolution since that epoch, a small fraction of hydrogen has been converted into helium, so that today roughly 10% of the helium in the Milky Way has been produced by stars, but the rest came from the first 1000 seconds of the hot, dense, early universe.

Spaceman Spiff
2008-Sep-28, 02:54 AM
If by some fluke we got a contracting mass of nothing but helium, it would contract gravitationally to a much hotter and more compact state before the onset of fusion. My educated guess is that the resulting star would be more like a red giant than a main sequence star.


Yes, but the helium MS (for stars whose composition is dominated by helium) lies to higher surface temperatures and luminosities on the H-R diagram. It also has a higher minimum mass star, ~0.5 solar masses instead of 0.08 solar masses for the hydrogen main sequence.

Thanatos
2008-Sep-28, 06:15 AM
Helium was a byproduct of the BB, no stars required.

stitt29
2008-Sep-28, 02:52 PM
spaceman spiff, thanks for your links.

I've just read the in the expanding Universe the space between galaxies expand but the galaxies themselves do not expand. First of all why would anyone think the vacuum of space can push galaxies apart? and 2 how does the vacuum know that it mustn't expand the space between stars and planets within the galaxy apart but it must expand between galaxies? Are we seriously being asked to believe the the vacuum of space can make decisions?

Also if we expanded from a single point everything would be redshifted. If we look at local galaxies as many are blueshifted as redshifted. So the vacuum energy can also contract space between galaxies. This theory is seriously flawed.

Lepton
2008-Sep-28, 03:02 PM
This theory is seriously flawed.

If you have an idea that you think is better, post in ATM for everybody to see and discuss it's merits/demerits.

Jeff Root
2008-Sep-28, 08:18 PM
I've just read the in the expanding Universe the space between galaxies
expand but the galaxies themselves do not expand. First of all why would
anyone think the vacuum of space can push galaxies apart? and 2 how
does the vacuum know that it mustn't expand the space between stars
and planets within the galaxy apart but it must expand between galaxies?
The mechanism of the expansion is unknown. In particular, the
mechanism of the acceleration of the expansion, discovered just
ten years ago, is unknown.

Widely-separated clusters of galaxies are observed to move away
from each other. The geometry of this expansion can be described
as an expansion of spacetime.

There is no evidence one way or another that the expansion takes
place within individual galaxies, or even within individual clusters of
galaxies. Depending on what causes the expansion, it might be
happening everywhere, or it might only be happening between
clusters.

Even if spacetime is expanding equally everywhere, we can't detect
it within galaxies because nuclear, electromagnetic, and gravitational
forces are much stronger than the expansion "force" on scales the
size of a galactic cluster and smaller. My atoms are held together by
nuclear and electromagnetic forces. The cells of my body are held
together by electromagnetic forces. I am held to the Earth by gravity.
The Earth is held in orbit around the Sun by gravity. The Solar System
is held in orbit within the Galaxy by gravity. The Galaxy is bound to
other galaxies in the local group by gravity. Only when you look far
enough away that the galaxies are not gravitationally bound to each
other is it possible to detect the expansion.

A fast-moving galaxy on the edge of a cluster might be marginally
bound to the cluster. In that case, the expansion of spacetime might
be able to pull it away from the cluster, unbinding it -- depending on
the cause of the expansion.

If the expansion is caused by some kind of force, we might eventually
be able to measure the force directly. If it is caused by spacetime
expanding equally everywhere, we might be able to measure redshift
even over relatively short distances. But the expansion is so weak
that such measurements may never be possible.



Also if we expanded from a single point everything would be redshifted.
If we look at local galaxies as many are blueshifted as redshifted.
There are only two or three major galaxies whose light is blueshifted
rather than redshifted. One is the Andromeda Galaxy. Of the 40
brightest galaxies in Earth's skies, it is the only one whose light is
blueshifted. Some of the very small galaxies in the local group are
moving toward the Milky Way and are blueshifted. I also know of a
galaxy in a nearby cluster which has such a high proper velocity that
it is moving toward us rather than away. Like a fast-flying bee in a
truck full of bees which is driving away from you, it happens to be
flying toward us faster than the cluster it is part of is moving away.

-- Jeff, in Minneapolis

Spaceman Spiff
2008-Sep-28, 10:14 PM
spaceman spiff, thanks for your links.

I've just read the in the expanding Universe the space between galaxies expand but the galaxies themselves do not expand. First of all why would anyone think the vacuum of space can push galaxies apart? and 2 how does the vacuum know that it mustn't expand the space between stars and planets within the galaxy apart but it must expand between galaxies? Are we seriously being asked to believe the the vacuum of space can make decisions?

Also if we expanded from a single point everything would be redshifted. If we look at local galaxies as many are blueshifted as redshifted. So the vacuum energy can also contract space between galaxies. This theory is seriously flawed.

This is nothing more than an argument from ignorance. Your statements demonstrate that you do not have any understanding of general relativity (and you needn't dive deep into the maths to have a working conceptual understanding, although the maths are necessary for deeper understanding), and that you have not bothered to understand the theory of science (BBT) that you ridicule with your snide questions. Rather you create a strawman based on your ignorance, and then point out how silly he looks -- pointless.

The vacuum of space does not make decisions, anymore than a ball does when tossed into the air or the moon in orbiting the earth. And while we do not have an understanding of initial conditions, including what triggered the initial expansion (although there are testable models), General Relativity makes testable predictions of how the universe should behave as a function of time, given the matter/energy content and its distribution.

After reading some of the links I've listed, above (especially this one for starters (http://www.mso.anu.edu.au/%7Echarley/papers/LineweaverDavisSciAm.pdf)), try this one: Expanding Space: the Root of All Evil? (http://arxiv.org/abs/0707.0380). Jeff Root, and anyone else who wants to understand what is meant by an "expanding universe" (and why you and I don't expand - at least not for cosmological reasons :whistle:), might want to read it as well. You might also want to try reading some elementary explanations of General Relativity.

Cougar
2008-Sep-29, 03:28 PM
This is nothing more than an argument from ignorance.Yes, with a heavy dose of incredulity on top.


...how does the vacuum know that it mustn't expand the space between stars and planets within the galaxy apart but it must expand between galaxies?.... This theory is seriously flawed.

As Spaceman Spiff intimated, it seems presumptuous, inappropriate, and just plain odd of you, Stitt, to admit you don't understand what the theory says, and then to claim the theory is flawed. It makes it very difficult not to speculate about the source of such prejudicial conclusion-reaching. But we try to stick to the issues here, rather than the apparent agenda....


...how does the vacuum know that it mustn't expand the space between stars and planets... but it must expand between galaxies?

Do you have any idea about how much space there is between galaxy clusters compared to how much space there is between stars and their planets?

Spaceman Spiff
2008-Sep-29, 09:59 PM
Even if spacetime is expanding equally everywhere, we can't detect
it within galaxies because nuclear, electromagnetic, and gravitational
forces are much stronger than the expansion "force" on scales the
size of a galactic cluster and smaller. My atoms are held together by
nuclear and electromagnetic forces. The cells of my body are held
together by electromagnetic forces. I am held to the Earth by gravity.
The Earth is held in orbit around the Sun by gravity. The Solar System
is held in orbit within the Galaxy by gravity. The Galaxy is bound to
other galaxies in the local group by gravity. Only when you look far
enough away that the galaxies are not gravitationally bound to each
other is it possible to detect the expansion.

A fast-moving galaxy on the edge of a cluster might be marginally
bound to the cluster. In that case, the expansion of spacetime might
be able to pull it away from the cluster, unbinding it -- depending on
the cause of the expansion.


Some of this is 'ok', but for the most part these explanations are misleading. Please read this paper carefully, Expanding Space: the Root of All Evil? (http://arxiv.org/abs/0707.0380)

Jerry
2008-Sep-29, 10:59 PM
There is a great deal of interwoven evidence for what happened going back to the first microsecond after the current expansion began (because we don't yet know what larger stage of space-time "our universe" operates within). The physics governing physical processes back to this point is based on the well-established (yet incomplete).
The 'missing links' are not trivial. They truly represent the difference between what was expected in this 'backward extrapolation', and what we are observing. The standard cosmology added as few as parameters as possible to deal with unexpected realities; and the search is on to round-out the evidence. The standard model today is less-than-convincing to many who do understand GR and most of the relevant assumptions.

Suggesting that a skeptic produce a 'new and better model' before criticizing is disingenuous. We should all be excited by the fact that there is much to learn.

Spaceman Spiff
2008-Sep-30, 02:49 AM
The standard model today is less-than-convincing to many who do understand GR and most of the relevant assumptions.


Incomplete, yes, by far -- we have a lot to learn. Less than convincing to many who do understand GR and most of the relevant assumptions (and let's throw in the observational data) -- give me a break. :lol:



Suggesting that a skeptic produce a 'new and better model' before criticizing is disingenuous.

Nobody has suggested that. Did you actually read what was written above by me and Cougar? It was the person who began this thread who was being "disingenuous". Standing up on a soapbox to make statements such as,


I am truly puzzled how this is taken seriously by anyone. Is it because there are no alternatives? and


This theory is seriously flawed. when stitt29 was so obviously arguing from ignorance and incredulity, was a mistake of stitt29, not mine.



We should all be excited by the fact that there is much to learn.


Agreed there.

Jerry
2008-Sep-30, 03:05 AM
Nobody has suggested that. Did you actually read what was written above by me and Cougar? It was the person who began this thread who was being "disingenuous". Standing up on a soapbox to make statements such as...

Sorry, it was a statement by Lepton I took minor exception to. Both you and Cougar have been very patient.

Jeff Root
2008-Sep-30, 03:59 AM
Even if spacetime is expanding equally everywhere, we can't detect
it within galaxies because nuclear, electromagnetic, and gravitational
forces are much stronger than the expansion "force" on scales the
size of a galactic cluster and smaller. My atoms are held together by
nuclear and electromagnetic forces. The cells of my body are held
together by electromagnetic forces. I am held to the Earth by gravity.
The Earth is held in orbit around the Sun by gravity. The Solar System
is held in orbit within the Galaxy by gravity. The Galaxy is bound to
other galaxies in the local group by gravity. Only when you look far
enough away that the galaxies are not gravitationally bound to each
other is it possible to detect the expansion.

A fast-moving galaxy on the edge of a cluster might be marginally
bound to the cluster. In that case, the expansion of spacetime might
be able to pull it away from the cluster, unbinding it -- depending on
the cause of the expansion.
Some of this is 'ok', but for the most part these explanations are misleading.
Please read this paper carefully, Expanding Space: the Root of All Evil? (http://arxiv.org/abs/0707.0380)
Could you describe what you consider to be misleading in what I said?

-- Jeff, in Minneapolis

Spaceman Spiff
2008-Sep-30, 02:33 PM
Could you describe what you consider to be misleading in what I said?

-- Jeff, in Minneapolis

Sure thing. I'm not saying that what you said was totally incorrect, but that your explanation requires some clarification, because without it we end up helping to propagate misconceptions that leaves the average consumer confused. You'll know exactly what I am talking about immediately after reading the paper I linked above (http://arxiv.org/abs/0707.0380). It's not a long paper, nor is it highly technical, but it is one that blows away a good deal of the fog of confusion. This general topic has been discussed in at least one other thread (http://www.bautforum.com/questions-answers/71102-prove-me-space-expands-i-think-galaxies-move-static-space-4.html#post1196082); see also here (http://www.bautforum.com/questions-answers/77359-recent-expansion-space-papers-implications-3.html#post1302031).

Here is another key passage from the paper (section 2.2: Local Expansions; I hope it's not too long):


At the global level, Peacock (2006) suggests that the expansion of space is uncontroversial since,

"the total volume of a closed universe is a well-defined quantity that increases with time, so of course space is expanding."

but questions

"whether this concept has a meaningful local counterpart? Is the space in my bedroom expanding, and what would this mean?".

Retaining the relativistic picture of expanding space, it is easy to address the question of what happens to Peacock’s bedroom, namely it will evolve as determined by the relativistic equations. But as ever, knowledge of the scenario, and particularly the initial conditions, is vital; the walls of the bedroom are held together by electromagnetic forces and hence are not following geodesics, and the distribution of matter has collapsed and is not uniform, and so the underlying geometry of spacetime in this region needs to be calculated; it would not be represented by the FRW spacetime of the homogeneous and isotropic universe. Clearly, if the universe were homogeneous on scales smaller than Peacock’s bedroom, and the walls were not held together by electromagnetic or other forces, and the particles making up the wall were at rest with the cosmological fluid which, importantly, requires that they not be initially at rest with respect to one another, then indeed as the universe expands the total volume of the bedroom would increase. The many conditions listed above are (at least approximately) true for galaxies not bound in common groups and hence they behave in ways that can be understood and predicted via the framework of expanding space.

This leads to an important point, namely that we should not expect the global behaviour of a perfectly homogeneous and isotropic model to be applicable when these conditions are not even approximately met. The expansion of space fails to have a ‘meaningful local counterpart’ not because there is some sleight of hand involved in considering the two regimes but because the physical conditions that manifest the effects described as the expansion of space are not met in the average suburban bedroom.

Lepton
2008-Sep-30, 03:07 PM
Sorry, it was a statement by Lepton I took minor exception to. Both you and Cougar have been very patient.

And what was that statement?

Quickshift
2008-Sep-30, 03:53 PM
I have a question. After the big bang would there be momentary suckback as in an atomic explosion demonstrates then it all ripples outward as expansion stabilizes?

antoniseb
2008-Sep-30, 04:30 PM
I have a question. After the big bang would there be momentary suckback as in an atomic explosion demonstrates then it all ripples outward as expansion stabilizes?

I don't think I've ever seen something like that demonstrated. Can you point to a reference?

Quickshift
2008-Sep-30, 05:14 PM
I don't think I've ever seen something like that demonstrated. Can you point to a reference?

I come from a long line of army engineers. My son is a bomb specialist. I've been exposed to a lot kaboom talk. Check out this link and you will see what I'm asking about.

http://www.metacafe.com/watch/1657985/the_atomic_cannon/

Quickshift
2008-Sep-30, 05:25 PM
BTW The tripods you see contained pigs to study the effect o radiation on human skin. There are a lot of good explosion film footage at the attached site.

http://www.metacafe.com/watch/1657985/the_atomic_cannon/

speedfreek
2008-Sep-30, 06:20 PM
Sure thing. I'm not saying that what you said was totally incorrect, but that your explanation requires some clarification, because without it we end up helping to propagate misconceptions that leaves the average consumer confused. You'll know exactly what I am talking about immediately after reading the paper I linked above (http://arxiv.org/abs/0707.0380). It's not a long paper, nor is it highly technical, but it is one that blows away a good deal of the fog of confusion. This general topic has been discussed in at least one other thread (http://www.bautforum.com/questions-answers/71102-prove-me-space-expands-i-think-galaxies-move-static-space-4.html#post1196082); see also here (http://www.bautforum.com/questions-answers/77359-recent-expansion-space-papers-implications-3.html#post1302031).

Here is another key passage from the paper (section 2.2: Local Expansions; I hope it's not too long):

I understand where you are coming from and I know that paper well but do not see how the way Jeff described the expansion could be misleading or could lead to misconceptions.

Jeff Root
2008-Sep-30, 06:36 PM
Spaceman Spiff,

The passage you quoted from the article describes a situation in which
there is no expansion force within galaxy clusters. The two paragraphs
of my post that you quoted deal with the suggested plausible situation
in which there is some kind of expansion force within galaxy clusters.

I said that even if spacetime expanded everywhere, we would not
be able to detect it within galaxy clusters because other, stronger
forces prevent it from having any effect there. "Even if" were the first
two words of the first of those paragraphs. I think those two words
should satisfy your concern. Yes? No?

-- Jeff, in Minneapolis

Jeff Root
2008-Sep-30, 06:56 PM
I have a question. After the big bang would there be momentary
suckback as in an atomic explosion demonstrates then it all ripples
outward as expansion stabilizes?
I think I know what you are referring to. I don't know for sure what
causes it, though. But I have a guess which I think is pretty likely
to be correct.

The heated products of a fusion bomb explosion and the surrounding
heated air expand explosively for about a second, and then begin to
cool. The initial expansion is so extreme that the initial cooling is also
extreme. The air shrinks again almost as violently as it expanded, so
that the rest of Earth's atmosphere, vastly larger than the volume
affected by the explosion, rapidly forces the cooling air back part of
the distance it just moved.

It is the atmosphere shoving the heated, expanded air nearly back to
where it started that I think we see in those films. Hurricanes have
far lower pressure differentials than fusion explosions, but are able to
produce very high wind speeds. The weight of the atmosphere is
definitely a force to respect.

Since everything was equally hot and equally expanding in the Big
Bang, there was no shoveback possible.

-- Jeff, in Minneapolis

Quickshift
2008-Sep-30, 07:14 PM
Jeff,
Thanks for the reply. So to be consistant with your statement wouldn't there be a huge void at big bang's ground zero ?

Spaceman Spiff
2008-Sep-30, 08:57 PM
I understand where you are coming from and I know that paper well but do not see how the way Jeff described the expansion could be misleading or could lead to misconceptions.

By saying that the reason you and I (or the MW or individual galaxy clusters) don't follow uniform expansion because of locally stronger forces, and leaving it at that, is to ignore an important part of what GR says what's going on -- and then begs the question as to what "force" is represented by space-time expansion.

From the paper (my bold):

The expansion of space is no more extant than magnetic fields are, and exists only as a tool for understanding the unambiguous predictions of GR, not a force-like term in a dynamical equation.What is missing is a statement which says that uniform "expansion" or "contraction" is a prediction strictly speaking for an isotropic, homogeneous universe. Look at this passage, again (my bold):


But as ever, knowledge of the scenario, and particularly the initial conditions, is vital; the walls of the bedroom are held together by electromagnetic forces and hence are not following geodesics, and the distribution of matter has collapsed and is not uniform, and so the underlying geometry of spacetime in this region needs to be calculated; it would not be represented by the FRW spacetime of the homogeneous and isotropic universe. Clearly, if the universe were homogeneous on scales smaller than Peacock’s bedroom, and the walls were not held together by electromagnetic or other forces, and the particles making up the wall were at rest with the cosmological fluid which, importantly, requires that they not be initially at rest with respect to one another, then indeed as the universe expands the total volume of the bedroom would increase. The many conditions listed above are (at least approximately) true for galaxies not bound in common groups and hence they behave in ways that can be understood and predicted via the framework of expanding space.The authors drive the point home here:


...we now turn to objects held together by gravitational ‘force’. One response to the question of galaxies and expansion is that their self gravity is sufficient to ‘overcome’ the global expansion. However, this suggests that on the one hand we have the global expansion of space acting as the cause, driving matter apart, and on the other hand we have gravity fighting this expansion. This hybrid explanation treats gravity globally in general relativistic terms and locally as Newtonian, or at best a four force tacked onto the FRW metric. Unsurprisingly then, the resulting picture the student comes away with is is somewhat murky and incoherent, with the expansion of the Universe having mystical properties. They then end this discussion with (my emphases):

A clearer explanation is simply that on the scales of galaxies the cosmological principle does not hold, even approximately, and the FRW metric is not valid.......There is no expansion for the galaxy to overcome, since the metric of the local universe has already been altered by the presence of the mass of the galaxy.....The expansion of space is global but not universal, since we know the FRW metric is only a large scale approximation.
So, speedfreek and Jeff, perhaps I am being a little pedantic, but I think that these explanations deserve a good deal of attention and should be part of any explanation of questions pertaining to the "expanding universe" and to "why don't we expand?", and related. It's not that local forces have no role, but I am suggesting that there is more to the explanation than that.

What do you think?

Spaceman Spiff
2008-Sep-30, 09:00 PM
It is a mistake to compare the "expanding universe" to an explosion. There were (virtually) no pressure or temperature gradients in the early universe, as witnessed by the cosmic background radiation. Explosions are governed by pressure gradients. These are different phenomena, so drawing any conclusions by way of comparison is pretty risky and likely leads to misconceptions.

And there is no "ground zero" wrt to the big bang. There are no special locations in space! (or at least that's what GR says wrt to the BBT)

Jeff Root
2008-Sep-30, 09:29 PM
So to be consistant with your statement wouldn't there be a huge void
at big bang's ground zero ?
Yes, and it is still opening up. It is the expansion. The expansion
involves the entire Universe. The void is everywhere. Ground zero
is everywhere. And everywhere that matter hasn't collapsed again
locally, the matter is far less densely packed than it was early in
the first moments of the Big Bang.

-- Jeff, in Minneapolis

speedfreek
2008-Oct-01, 12:32 AM
What do you think?

I think that all adds up to saying that the universe expands only in the places where there are no gravitationally bound galaxies, in the large relatively smooth and empty voids between the unbound clusters. The reason given is that the distribution of matter has collapsed and is not uniform and thus the local metric has been altered. The leads to the question of why the distribution of matter is not uniform...

Whilst the notion that the forces that bind things together overcomes the force of expansion may seem misleading, consider which of these forces we have any understanding of. We know of the stuff that binds matter, and we see and feel the effects of gravity, but the reason for the expansion of the universe is still a complete mystery. It is one of the initial conditions and therefore does have mysterious qualities. The waters are muddied even further with the accelerating expansion.

The different mathematical methods required to describe the conditions at different scales leave a lot to be desired when it comes to understanding the overall concept. Someone who completely understands General Relativity is no closer to knowing the reason why different mathematical methods are required than the rest of us, when that why is the question of the mechanism behind the expansion of the universe.

It is all very well saying the metric is different and therefore there is no force of expansion where galaxies are, but why is the metric different? Why is the matter non uniform? Why is there a galaxy here?

The metric was changed due to the local gravitational conditions pulling stuff together. The reason the stuff wasn't uniform to begin with goes all the way back to the inflationary epoch and beyond.

And yet the mainstream view is that the universe was expanding right from the beginning, when it was opaque, filled with quarks and gluons. It was expanding later, when it became transparent and filled with atoms and photons. As the universe continued to expand, gravity caused the non-uniformly distributed matter to clump into what eventually became stars and then clusters of galaxies. Should I be saying here that the local metric around concentrations of matter was changing in relation to the global metric (which is an approximation)?

We use different metrics depending on whether we are describing an empty universe, an homogeneous and isotropic universe, or the conditions around massive objects. How do we stitch them together?

Quickshift
2008-Oct-01, 03:06 AM
It is a mistake to compare the "expanding universe" to an explosion. There were (virtually) no pressure or temperature gradients in the early universe, as witnessed by the cosmic background radiation. Explosions are governed by pressure gradients. These are different phenomena, so drawing any conclusions by way of comparison is pretty risky and likely leads to misconceptions.

And there is no "ground zero" wrt to the big bang. There are no special locations in space! (or at least that's what GR says wrt to the BBT)

By this statement I would say the name Big Bang was a mistake and misleading. It's difficult to wrap my head around a time where current laws of physics dont apply knowing it's part of the theory. The explosion or expansion the result of a monumental change in it's being. Albeit it's all theory, but very interesting.
Most science is achieved by comparison of a control group and experimental group, unfortunatly we only have one universe we hardly know. My question is of possibility without any conclusion. To draw any conclusion is a mistake that closes all possibilities.

Quickshift
2008-Oct-01, 03:36 AM
Yes, and it is still opening up. It is the expansion. The expansion
involves the entire Universe. The void is everywhere. Ground zero
is everywhere. And everywhere that matter hasn't collapsed again
locally, the matter is far less densely packed than it was early in
the first moments of the Big Bang.

-- Jeff, in Minneapolis

I'm unfamilar with the concept of callapsed matter could you expand on that.

Thanks

Jeff Root
2008-Oct-01, 06:53 AM
I'm unfamilar with the concept of collapsed matter could you
expand on that.
I wasn't referring to anything you aren't familiar with: Just the collapse
of matter to stars and galaxies, caused mostly by gravity and friction.

If the hot gas in the very early universe had been distributed perfectly
evenly throughout space, it would all have remained gas. But there
were small variations in density. Those variations started a runaway
positive feedback in which self-gravitation led to the formation of
stars and galaxies, with enormous voids of almost completely empty
space between them.

I'm not up on the latest research, but as far as I can tell, formation
of the first stars and the first galaxies seem to have been pretty much
concurrent. Galaxies did not have to reach any particular stage of
evoution for stars to form in them, and stars did not need to reach
any particular stage of evolution to organize into galaxies.

Formation of stars and of galaxies was essentially one phenomenon
on different scales. The two big differences are that hydrogen atoms
bumped into each other and piled up to make stars, while stars did
not need to bump into each other to make galaxies (instead, they
"virialize", with stars exchanging momentum via gravity so that the
slowst-moving stars end up near the center, and the faster-moving
stars spend most of their time far from the center); and that stars
fuse hydrogen nuclei, giving off light and particle radiation which blow
away additional gas molecules, stopping the stars' growth, while
galaxies continue to grow as long as there is matter nearby to be
picked up.

-- Jeff, in Minneapolis

Jeff Root
2008-Oct-01, 07:18 AM
speedfreek,

My impression is the same as yours.

-- Jeff, in Minneapolis

Jeff Root
2008-Oct-01, 08:11 AM
Spaceman Spiff,

The PDF article makes lots of good, important points. I won't ignore
it, but I'm not swayed by all of its arguments.

The Universe is clearly "expanding" overall. Before the discovery of the
acceleration of the expansion, we could say the expansion appears to
be ballistic: Widely-separated galaxy clusters move away from one
another ballistically. What caused that is unknown. What causes the
acceleration is also unknown. The two could have one cause. It could
be some kind of force, but it doesn't have to be. If it is a force, it might
operate everywhere to the same degree, or it might be stronger between
clusters than within clusters, or it might only exist between clusters and
not within clusters at all.

Speeedfreek mentioned one important question that is being studied:
How (quantitatively) the distribution of matter came to be nonuniform.
I have a question that I've never seen asked by anyone else: How long
did the creation event take? I'm pretty much convinced that it took
some amount of time, but since I have no suggested mechanism, I have
no hint of what the time might have been. Nanoseconds? Milliseconds?
Seconds? Minutes? Six days? Six million years? I think it is a question
which can be helpful in working out a creation mechanism.



It is a mistake to compare the "expanding universe" to an explosion.
There were (virtually) no pressure or temperature gradients in the early
universe, as witnessed by the cosmic background radiation. Explosions
are governed by pressure gradients.
I'll take it as a given that there were virtually no pressure gradients in
the early universe. I'll also take it as a given that explosions we deal
with on Earth are governed by pressure gradients. Still, the Big Bang
looks very much like an explosion. I don't think the posibility can be
dismissed that it was an explosion not governed by pressure gradients.

On the other hand, my question about the time the creation event
took suggests the possibility of a non-explosive beginning -- perhaps
one which started out slowly and built up speed over time.

-- Jeff, in Minneapolis

Spaceman Spiff
2008-Oct-01, 01:02 PM
By this statement I would say the name Big Bang was a mistake and misleading. It's difficult to wrap my head around a time where current laws of physics dont apply knowing it's part of the theory. The explosion or expansion the result of a monumental change in it's being. Albeit it's all theory, but very interesting.
Most science is achieved by comparison of a control group and experimental group, unfortunatly we only have one universe we hardly know. My question is of possibility without any conclusion. To draw any conclusion is a mistake that closes all possibilities.

Not our idea. The name "big bang" was coined flippantly if not derisively by Sir Fred Hoyle, some 55 years ago during an on-air BBC interview (he was a detractor until the end of his life). A reasonably serious attempt was made to choose a different name a few years back, but got no traction.

As for the part that I have placed in bold --- where current "physical laws" don't hold means it's not part of a scientific theory. The standard BBT (or BB model, if you will) does not include situations where current physical understanding is shaky or lacking altogether. There are models (at various levels of informed speculation) which try to incorporate quantum physics into the space-time physics of GR (loop quantum gravity, string theory, etc), or suggest that known quantum processes should have operated in a very early universe and set it expanding initially exponentially (e.g., Inflation), etc. And as I said somewhere, when we have a better grasp as to the physics operating well before the first microsecond, a larger more general model will incorporate the standard BBT into it.


Albeit it's all theory

I don't know what you mean by that -- or rather I suggest that you might not know what you mean by that. Try reading this (http://homepages.wmich.edu/~korista/fact-hypot-theory.pdf) and this (http://www.notjustatheory.com/index.html), and then this one (http://www.talkorigins.org/faqs/evolution-fact.html) if you've a bit more time.

Quickshift
2008-Oct-01, 02:08 PM
Not our idea. The name "big bang" was coined flippantly if not derisively by Sir Fred Hoyle, some 55 years ago during an on-air BBC interview (he was a detractor until the end of his life). A reasonably serious attempt was made to choose a different name a few years back, but got no traction.

As for the part that I have placed in bold --- where current "physical laws" don't hold means it's not part of a scientific theory. The standard BBT (or BB model, if you will) does not include situations where current physical understanding is shaky or lacking altogether. There are models (at various levels of informed speculation) which try to incorporate quantum physics into the space-time physics of GR (loop quantum gravity, string theory, etc), or suggest that known quantum processes should have operated in a very early universe and set it expanding initially exponentially (e.g., Inflation), etc. And as I said somewhere, when we have a better grasp as to the physics operating well before the first microsecond, a larger more general model will incorporate the standard BBT into it.



I don't know what you mean by that -- or rather I suggest that you might not know what you mean by that. Try reading this (http://homepages.wmich.edu/~korista/fact-hypot-theory.pdf) and this (http://www.notjustatheory.com/index.html), and then this one (http://www.talkorigins.org/faqs/evolution-fact.html) if you've a bit more time.

Are there any clues, theories (imperfect facts my definition) or hypothesis on the physics.
before the first microseconds.
please include links you might have for me to check out.
Thanks

Spaceman Spiff
2008-Oct-02, 12:39 AM
Are there any clues, theories (imperfect facts my definition) or hypothesis on the physics.
before the first microseconds.
please include links you might have for me to check out.
Thanks

Yes, there are. Before about 10^-6 s, the energy densities (everywhere, nearly uniformly so) were so enormous that protons and neutron did not exist as such. We don't have a full understanding of this state of matter (although it certainly exists within neutron stars (http://en.wikipedia.org/wiki/Neutron_stars)), but the leading model is the quark-gluon plasma (http://en.wikipedia.org/wiki/Quark-gluon_plasma). Without a standard model of this equation of state of matter and those ever more energetic, our understanding becomes ever more speculative/extrapolative as the energy densities soar at yet earlier epochs. The Electro-weak (http://en.wikipedia.org/wiki/Electro-weak) unification theory, which includes the Higgs field (http://en.wikipedia.org/wiki/Higgs_mechanism) and the Higgs boson (http://en.wikipedia.org/wiki/Higgs_boson) (the last major experimentally unconfirmed predictions thereof), takes us back to about 10^-12 seconds. Extrapolating further we're on thin ice, although the physics of the Inflaton field (http://en.wikipedia.org/wiki/Inflaton) during the Inflation epoch (http://en.wikipedia.org/wiki/Cosmic_inflation) (10^-34 s) is on firmer ground physically. Between those two epochs our ideas are quite speculative. For a summary of the complete time-line, go here (http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang).

Spaceman Spiff
2008-Oct-03, 03:34 PM
Spaceman Spiff,

The PDF article makes lots of good, important points. I won't ignore
it, but I'm not swayed by all of its arguments.

The Universe is clearly "expanding" overall. Before the discovery of the acceleration of the expansion, we could say the expansion appears to be ballistic: Widely-separated galaxy clusters move away from one another ballistically.

What do you mean by ballistically? Normally, what is meant is that a mass is suddenly accelerated and then "falls" freely in a newtonian (i.e., attractive) gravitational field. The "stuff" of the universe on large scales is evolving in response to the energy density of matter (normal and the presumed non-baryonic), of radiation, of whatever the source of the accelerated expansion is, and whatever else lies out there that we haven't yet discovered. Stuff that obeys different equations of state do different things to the metric, evolve differently in time, and the mass-energy density of the universe is reacting to the sum total.

Let me provide a concrete example. During the first ~10,000 years of expansion, radiation dominated over matter, and the universe evolved differently then than it later did when matter dominated the energy density budget after ~30,000 years had gone by. Matter continued to dominate the energy density, apparently, until about 7 Gyrs ago, and since that time the evolution of the universe has taken another turn. All contributions to the stress-energy tensor have their effects on the evolving geometry; there is no point singling out the one causing the accelerated expansion.



What caused that is unknown. What causes the acceleration is also unknown. The two could have one cause. It could be some kind of force, but it doesn't have to be. If it is a force, it might operate everywhere to the same degree, or it might be stronger between clusters than within clusters, or it might only exist between clusters and not within clusters at all.


Ok, so we don't yet know the origin of the term in the stress-energy tensor that is apparently now dominating the other terms and causing the accelerated expansion. But that doesn't mean that anyone is arguing that the evolution of mass-energy density on large scales is being directed by kind of "expansion force". If you want to believe that, fine, but don't confuse that belief with what GR coupled with the current data say about the "expanding universe".

Or maybe you're thinking about "quintessence (http://en.wikipedia.org/wiki/Quintessence_%28physics%29)"? This is one of the proposed models to explain the accelerative expansion behavior, and is one in which a contributor to the stress-energy tensor has an equation of state that evolves in time (unlike that of matter and radiation). In that way it is a dynamic term in the equations of motion, but it's not a "force" of expansion in the way you seem to be describing. Or maybe I have misinterpreted what you mean by your reference to "force".



Speeedfreek mentioned one important question that is being studied: How (quantitatively) the distribution of matter came to be nonuniform.


Well, if you and Speedfreek are asking what were the conditions that allowed for tiny fluctuations in energy density in the very early universe, which then allowed gravity and then other forces to then "collapse" matter into locally non-uniform distribution and whole lot of empty space in between -- the Inflationary model, for one, addresses that and makes specific predictions on what the fluctuation spectrum (how much fluctuation on what size scale) should be. These were due to quantum fluctuations of the scalar potential field that drove Inflation (the field did not decay uniformly everywhere). Try reading this paper (http://arxiv.org/abs/astro-ph/0502328).




I have a question that I've never seen asked by anyone else: How long did the creation event take? I'm pretty much convinced that it took some amount of time, but since I have no suggested mechanism, I have no hint of what the time might have been. Nanoseconds? Milliseconds? Seconds? Minutes? Six days? Six million years? I think it is a question which can be helpful in working out a creation mechanism.


Not too sure what you're asking here. All of the protons, neutrons, and electrons, nearly all of the photons and neutrinos in our universe had come into existence in the first second of the initial expansion from some initial ultra dense state. Helium and Lithium followed in the next several minutes as protons and neutrons combined when the temperature fell below 1 billion K. The first neutral atoms of hydrogen began forming en mass about 380,000 years later (neutral atomic helium earlier). Again, we don't yet understand what led to the initial expansion, but GR + matter/energy contents predict the path forward.



I'll take it as a given that there were virtually no pressure gradients in the early universe.


Well, that's what the cosmic background radiation tells us (or rather that there were, but these were tiny in amplitude (http://background.uchicago.edu/%7Ewhu/beginners/introduction.html) and of well-understood causes (http://background.uchicago.edu/%7Ewhu/Papers/HuWhi04.pdf); see also here (http://cmb.as.arizona.edu/%7Eeisenste/acousticpeak/acoustic_physics.html)).



I'll also take it as a given that explosions we deal with on Earth are governed by pressure gradients.

Well, because they are.



Still, the Big Bang looks very much like an explosion. I don't think the posibility can be dismissed that it was an explosion not governed by pressure gradients.


How so? Let's see, spatial pressure and temperature gradients which diminish with time as the material spreads through pre-existing space on the one hand, nothing of the kind at all on the other. Nope, the "big bang" is nothing at all like what is normally defined to be an explosion (and what every person on the street pictures in their heads as an explosion).



On the other hand, my question about the time the creation event took suggests the possibility of a non-explosive beginning -- perhaps one which started out slowly and built up speed over time.

-- Jeff, in Minneapolis

Again, we don't yet understand the initial conditions that set the "universe expanding" (although various versions of inflation and loop quantum gravity models have been proposed), but in any case it did not involve stuff flying out from and out into pre-existing space. Or if you believe it did, don't confuse that notion with what GR coupled with the current data say about the "expanding universe".

Spaceman Spiff
2008-Oct-03, 04:49 PM
I think that all adds up to saying that the universe expands only in the places where there are no gravitationally bound galaxies, in the large relatively smooth and empty voids between the unbound clusters. The reason given is that the distribution of matter has collapsed and is not uniform and thus the local metric has been altered.

Not quite that restrictive. It says that the expansion appears to well approximate that of the predictions of the FRW equations of motion on sufficiently large distance scales such that the assumptions of the FRW equations of motion (isotropic, homogeneous) are valid at varying levels of approximation. That is, they are not at all valid on this scale (http://www.atlasoftheuniverse.com/galaxy.html) or even this one (http://www.atlasoftheuniverse.com/localgr.html), but expansion definitely becomes important if distorted (galaxy peculiar velocities) somewhat on this scale (http://www.atlasoftheuniverse.com/virgo.html), completely dominates (with possible minor perturbations - role of voids still being debated in the data) at this scale (http://www.atlasoftheuniverse.com/superc.html), and is the name of the game on this scale (http://www.atlasoftheuniverse.com/universe.html).

stitt29
2008-Oct-04, 10:16 AM
My point about expanding space is this: vacuum energy density predicts that the vacuum of space has an energy that can expand space between galaxies. As some galaxies are blueshifted that means the space between observer and galaxy is contracting i.e. the complete opposite of what is predicted. How can this be?

Hornblower
2008-Oct-04, 11:19 AM
My point about expanding space is this: vacuum energy density predicts that the vacuum of space has an energy that can expand space between galaxies. As some galaxies are blueshifted that means the space between observer and galaxy is contracting i.e. the complete opposite of what is predicted. How can this be?
My educated guess is that the galaxies in question are close enough together and massive enough that their mutual gravitation overpowers the natural tendency of expanding space. Please note that only a handful of nearby galaxies, of which M31 is most noteworthy, are blueshifted from our point of view.

speedfreek
2008-Oct-04, 11:28 AM
My point about expanding space is this: vacuum energy density predicts that the vacuum of space has an energy that can expand space between galaxies. As some galaxies are blueshifted that means the space between observer and galaxy is contracting i.e. the complete opposite of what is predicted. How can this be?

The galaxies that are blue-shifted are either relatively close to us and part of our super-cluster, or are in a cluster that is close enough to us that their peculiar motion towards us is greater than the rate of expansion over that distance.

The classic answer as to why galaxies cluster together is gravity, the gravitational interaction between galaxies that are relatively close to each other. The greater the distance between objects, the less gravitational interaction between those objects. Locally to us, the gravitational conditions cause the Andromeda galaxy to have a peculiar motion towards us (of course from Andromedas point of view, we have a peculiar motion towards them).

A cluster of galaxies across the universe will be apparently receding from us far in excess of any of the peculiar motions of the galaxies within that cluster, but as we look closer to home we might find a neighbouring cluster where the peculiar motion of a galaxy in that cluster is moving towards us, whilst the cluster itself is, on average, moving away from us.

The point that Spaceman Spiff was making is that we should not necessarily attribute space itself with the properties of expansion, as this can lead to possible misconceptions about the mechanism behind expansion. The idea that the gravity between galaxies might "hold them in place" against expanding space, like a rope that ties two boats together in a lake with a rising water level (where, without the rope, the boats would move apart), may not reflect the true picture accurately enough, but I still think it is a reasonable analogy.

Spaceman Spiff
2008-Oct-04, 04:13 PM
The galaxies that are blue-shifted are either relatively close to us and part of our super-cluster, or are in a cluster that is close enough to us that their peculiar motion towards us is greater than the rate of expansion over that distance.

The classic answer as to why galaxies cluster together is gravity, the gravitational interaction between galaxies that are relatively close to each other. The greater the distance between objects, the less gravitational interaction between those objects. Locally to us, the gravitational conditions cause the Andromeda galaxy to have a peculiar motion towards us (of course from Andromedas point of view, we have a peculiar motion towards them).

A cluster of galaxies across the universe will be apparently receding from us far in excess of any of the peculiar motions of the galaxies within that cluster, but as we look closer to home we might find a neighbouring cluster where the peculiar motion of a galaxy in that cluster is moving towards us, whilst the cluster itself is, on average, moving away from us.


Minor quibble: both evidence and simulations suggest that superclusters of galaxies are not bound entities, and if the accelerated expansion is real, then these structures (along with the interior voids) will in the distant future dissolve back into near 'homogeneity'. Superclusters, walls and voids are apparently the most recent structures to form, and they may not be long for the universe.




The point that Spaceman Spiff was making is that we should not necessarily attribute space itself with the properties of expansion, as this can lead to possible misconceptions about the mechanism behind expansion. The idea that the gravity between galaxies might "hold them in place" against expanding space, like a rope that ties two boats together in a lake with a rising water level (where, without the rope, the boats would move apart), may not reflect the true picture accurately enough, but I still think it is a reasonable analogy.

:dance: Thanks! That was a really nice summary. And this brings to mind the "tethered galaxy (http://arxiv.org/abs/astro-ph/0609271)" problem (also discussed in section 2.5 of the Francis et al. paper (http://arxiv.org/abs/0707.0380)), which from my reading doesn't yet have a solution that everyone agrees upon.
(http://arxiv.org/abs/astro-ph/0609271)