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please
2007-Feb-08, 12:21 PM
Hello people, I'm new, I like this icon :boohoo: and let's get to the question:

How come expansion is detectable?

Should not space expand within us and our measurement tools the same way it does between galaxies? And if it does, how can we possibly notice expansion.

Kwalish Kid
2007-Feb-08, 01:37 PM
Hello people, I'm new, I like this icon :boohoo: and let's get to the question:

How come expansion is detectable?

Should not space expand within us and our measurement tools the same way it does between galaxies? And if it does, how can we possibly notice expansion.
Expansion is effected by gravity. As such, in the area near us, there is no expansion. Expansion only takes place between galaxies or galaxy clusters, or galaxy superclusters that are far enough apart not to be bound together by the gravitational effects that we normally think of.

If we look at distant objects that meet this criterion, then there is really one big clue about this expansion: redshift. If the distance between us and a distant star is increasing, then there will be a relativistic effect that introduces a difference in the way that we caclulate the time at that star (and vice versa). The light is emitted from that distant star has a certain wavelength determined by the time bewteen the peaks in the waveform. To us, this time seems longer than it is in its own frame of reference. This means that the wavelength is longer, and thus the light appears redder than it otherwise would.

If the universe is roughly the same in all directions (on a large scale), this sort of behaviour is predicted by general relativity (though it wasn't immediately obvious).

please
2007-Feb-08, 02:19 PM
are you saying that atoms somehow pull space around them so that it does not run away?

satori
2007-Feb-08, 02:24 PM
my understanding would be that it (expansion) indeed happens everywhere albeit to such smal extent, that any solid body easily resists.
It would be nice if somebody on this forum could give a concrete number as to how long it takes currently to double a given space volume in size. It should be around some billion years I think ! Now you have a measure of the minuteness of this phenomenon - undetectable in the lab!

Kwalish Kid
2007-Feb-08, 02:37 PM
are you saying that atoms somehow pull space around them so that it does not run away?
Well, I guess so.

Gravity is mass pulling at other mass. (Well, more technically, mass/energy pulling at mass/energy.) So if there is a general expansion, it doesn't have to also be a local expansion.

If there is a component to the universe that accelerates this expansion, it would be felt everywhere, but its effect over small distances (e.i., within a galaxy) would be minuscule, and so not result in actual expansion on the non-cosmological scale.

please
2007-Feb-08, 02:52 PM
my understanding would be that it (expansion) indeed happens everywhere albeit to such smal extent, that any solid body easily resists.But why should it resist at all? Why particles themselves are not equally stretched with the space they occupy, or why their interaction does not get less?

Cougar
2007-Feb-08, 03:19 PM
But why should it resist at all? Why particles themselves are not equally stretched with the space they occupy, or why their interaction does not get less?
The electromagnetic and strong nuclear forces keeping atoms together are so much stronger than the "force" of expanding space, it isn't even funny. :lol: And as Kwalish Kid says, the force of gravity, between earth and sun for example, or even between galaxy nucleus and orbiting stars, is much much greater than the cosmic expansion, which is only evident on very, very large scales.

please
2007-Feb-08, 03:32 PM
but is space stretching a force for nuclear forces to fight with? I thought it was just that - space getting bigger.

Jeff Root
2007-Feb-08, 03:42 PM
Pick up a solid object, such as a pencil. Pull very, very gently on
its ends. It stretches by an imperceptible amount. It doesn't pull
apart, and it doesn't keep on stretching, getting longer and longer.

There are electrochemical forces in the pencil holding the atoms
close together. Those forces are many, many, many quintillions
of times stronger than any possible force that might be applied by
the expansion of space within the pencil. (Ken, can you state the
actual value?) The electrochemical forces could hold the pencil
together forever against such an expansion.

There are at least two different possible ways that the expansion
of the Universe might work. Different people favor one or the other.
Both can be described using the balloon analogy.

Imagine a toy rubber balloon being inflated. There are many little
metallic sequins stuck to the surface of the balloon with grease.
They stick well enough that they normally do not slide across the
balloon's surface. However, they are magnetized, so they attract
each other, forming little clusters. The attraction prevents them
from moving apart as the balloon is inflated, so instead they slide
across the surface of the balloon and remain clustered together.

The sequins represent galaxies. The magnetism represents gravity.
Galaxies attract each other gravitationally into clusters, so the
expansion of the Universe doesn't drive them apart.

The second way the expansion might work also has magnetized
sequins on the balloon, but this time the sequins are glued to the
balloon instead of being stuck on with grease. So it is impossible
for the sequins to move around on the surface of the balloon.
However, the magnetic attraction between the sequins is strong
enough to tug the rubber that the sequins are glued to, preventing
the rubber from expanding wherever there is a cluster of sequins.

This would represent space not expanding within galaxy clusters.

A third possibility is that space itself does not expand. In this
case, galaxy clusters would simply be coasting apart from each
other through space after having been given an initial motion
relative to their neighbors. Or they could be being pushed apart
by some kind of repulsion force. Or both.

The acceleration of the expansion, discovered in 1998, could be
due to a repulsion force, rather than expansion of space.

There may be other possibilities.

-- Jeff, in Minneapolis

Cougar
2007-Feb-08, 03:51 PM
but is space stretching a force for nuclear forces to fight with? I thought it was just that - space getting bigger.
Well, this analogy is not very good, but it may be instructive: Your questions are a little like you are pouring a glass of water into the Pacific Ocean, and you're asking why doesn't the sea level rise?

Ken G
2007-Feb-08, 03:57 PM
but is space stretching a force for nuclear forces to fight with? I thought it was just that - space getting bigger.

Your confusion stems from thinking that space is a real thing that can expand and pull other "stuff" with it. Space is just a way to coordinatize (i.e., enumerate) the concept of distance. So if distances are increasing, we may say that space is expanding, if we choose that coordinatization of things. The reason distances are increasing is much simpler-- on the large scale, it just dates back to the initial conditions of the universe, in ways that are not predicted by merely observed. On shorter scales, distances are not increasing, due to the binding effects of gravity (as has been pointed out above). Thus gravity is not the cause of expansion, indeed it tends to limit it (except for the recent acceleration, but that's kind of a detail to your question).

Note also that some of the "dierent" possible descriptions above are actually the sae thing, just different coordinatizations of space. The most common, and sensible, way to coordinatize space has galaxy clusters at the same pace, and space expanding-- this is in the spirit of letting all observers use their own local observations to coordinaize space.

trinitree88
2007-Feb-08, 03:59 PM
Well, this analogy is not very good, but it may be instructive: Your questions are a little like you are pouring a glass of water into the Pacific Ocean, and you're asking why doesn't the sea level rise?

Cougar. It does. It adds ~ 8 molecules of water for every glass in the oceans.:D pete

please
2007-Feb-08, 04:06 PM
Your questions are a little like you are pouring a glass of water into the Pacific Ocean, and you're asking why doesn't the sea level rise?shouldn't expansion of space in our measurement tools, in meters per meters per second, be the same as expantion of space between galaxies?


thinking that space is a real thing that can expand and pull other "stuff" with it.doesnt it pull galaxies apart? edit: i mean all interactions like gravity should continue to happen wether space is expanding or not. this is not the question why our bodies are not ripped apart. this is a question why expanding space equally in every point should be detectable.

satori
2007-Feb-08, 04:39 PM
It would be nice if somebody on this forum could give a concrete number as to how long it takes currently to double a given space volume in size. It should be around some billion years I think ! Now you have a measure of the minuteness of this phenomenon - undetectable in the lab!
Could anyone give an answer to the question i raised?

Ken G
2007-Feb-08, 04:41 PM
shouldn't expansion of space in our measurement tools, in meters per meters per second, be the same as expantion of space between galaxies?The issue is that the local forces are much stronger.


doesnt it pull galaxies apart? edit: i mean all interactions like gravity should continue to happen wether space is expanding or not. this is not the question why our bodies are not ripped apart. this is a question why expanding space equally in every point should be detectable.Space doesn't expand equally, that's the point-- the matter is subject to different forces, and the expansion only occurs in areas that "remember" the initial conditions of the Big Bang (or are subject to dark energy effects).

jamini
2007-Feb-08, 06:20 PM
shouldn't expansion of space in our measurement tools, in meters per meters per second, be the same as expantion of space between galaxies? doesnt it pull galaxies apart? edit: i mean all interactions like gravity should continue to happen wether space is expanding or not. this is not the question why our bodies are not ripped apart. this is a question why expanding space equally in every point should be detectable.

Hi Please, welcome to the forum and to the fascinating and intriguing world of astronomy and physics. I can tell by your questions that you are new to physics and astronomy and I admire your interest in gaining a better understanding of the workings of our universe. I can’t answer your questions directly any better than the excellent information that has already been provided; I just want to say that most people new to physics have a difficult time understanding why things seem to work differently in space than they do here on earth. While the same laws of physics apply everywhere in the universe, reconciling them here on earth is often confusing and even frustrating; this can be attributed largely to the laws of gravity and atmospheric conditions we experience here on earth, which cause things to behave much differently than they do in the vacuum of space. I know it’s difficult to visualize (and even harder to explain) but I do have a few suggestions for reading material that was very helpful to me:

The Universe in a Nutshell – Stephen Hawkings

A Brief (or the newer, Briefer) History of Time – Stephen Hawkings

Mr. Hawkings has dedicated his life to physics and astronomy and has a remarkable talent for being able to explain even the most difficult of concepts in very clear and simple layman’s terms. His books are rich in illustrations and very easy to digest – i.e., not a lot of math formulas to decipher.

You can learn more about Mr. Hawkings at his web site:
http://www.hawking.org.uk/home/hindex.html


I also highly recommend “The Elegant Universe” by Brian Greene. While much of the book explains a relatively new multi-dimensional theory called String Theory, Mr. Greene devotes a great deal of the book to general relativity and quantum physics, which he ultimately suggests may unify into a single theory. Much like Mr. Hawkings, Mr. Greene is extremely adept at expressing even the most complex of theories, laws and properties of our universe in extremely simple and easy to understand terms.

Reading some or all of this material will not only answer all of your questions about expansion but also give you a very clear understanding of how our universe works.

Happy Reading!

astromark
2007-Feb-08, 06:48 PM
If you could make a list of the forces that are at work on you right now gravity although relentless is not the strongest force acting on you.
Local gravity is what is holding you down. The nuclear forces are holding you together. Electromagnetic forces are what drive your brain. This list is not complete, just the beginning of a larger subject. The force of gravity is holding this Galaxy, solar system. Together.
Now ask what force is driving the expansion. . . I have no idea.

Bjoern
2007-Feb-08, 07:04 PM
It would be nice if somebody on this forum could give a concrete number as to how long it takes currently to double a given space volume in size. It should be around some billion years I think !


The calculation isn't hard. The Hubble "constant" gives the rate of change. The currently accepted value is around H = 70 km/s/Mpc. You merely have to convert this to more practical units...

Convert the Mpc to km: 1 Mpc = 3 260 000 light-years = 3 260 000 * 365.25 * 24 * 60 * 60 light-seconds = 3 260 000 * 365.25 * 24 * 60 * 60 * 300 000 km; this gives about 3*10^19 km. Hence we have H = 70 km/s/3*10^19 km, which gives about H = 2.3 * 10^(-18) 1/s.

Now convert the seconds to years: 1 s = 1 yr / 365.25 / 24 / 60 / 60, which gives about 3.2 * 10^(-8). Insert this above, then we have approximately: H = 7 * 10^(-11) 1/yr.

Finally, convert the years to billion years (Gyr); this gives another factor of 10^9. Hence finally we get: H = 0.07 1/Gyr. I. e., what I said: 7 percent per billion year.

Obviously, this isn't exactly right, since during this one billion year, H is not really constant, but changes (decreases) - but it gives at least a rough idea.

Additionally, the calculation above was done in a rather clumsy way (in order to make the steps comprehensible); a much fast way would be to first multiply the s and the Mpc and convert this: 1 s * Mpc = ... = 978 Gyr * km. Insert this above, and you get immediately: H = 70 km / 978 Gyr * km = 0.07 1/Gyr.

HTH.

jamini
2007-Feb-08, 07:09 PM
Could anyone give an answer to the question i raised?

Using Hubble's constant it’s estimated to range from 70 - 90 km/s Mpc (megaparsec) = 3.26 x 10^6 light years.

[edit] Bjoern we must have posted at the same time. Your calculations are obviously far more detailed and precise than mine.

satori
2007-Feb-08, 07:34 PM
thanks a lot, Bjoer , but i am not satisfied yet.
my understanding of the Hubble const. is, that it informs me about the increase of escape velocity as a function of distance.
So if i look to Andromeda (m31 I mean) and do away with attractor eigen movement this object should flee me with roughly 45km/sec as its distance would roughly be 2/3 Mpc.
This translates obviously into a extension of space...
but what I would need for the answer to my question would be the ratio ...
No, no I can't see why you get away without an integration!
You are obviously not giving me the TIME it takes to blow one liter of space now to double its volume .... it can't be that straight forward! Am I blind?

satori
2007-Feb-08, 07:37 PM
Ok, Bjoern and jamini , BATTLE it OUT !
I am too stupid...!

satori
2007-Feb-08, 07:58 PM
Oh yes i have the solution but i have no calculator handy
it surely must involve a cubic-operation it is a x^3 operation every else is merde...

Bjoern
2007-Feb-08, 08:32 PM
thanks a lot, Bjoer , but i am not satisfied yet.
my understanding of the Hubble const. is, that it informs me about the increase of escape velocity as a function of distance.


That's essentially right (although "escape" velocity is a strange term to use here).
v = H * d
(v: velocity, d: distance)

But the velocity of a galaxy with respect to us can also be written as the first time derivative of the scale factor (adot) times its coordinate, let's call it x:
v = adot * x
And the distance is the scale factor times the coordinate:
d = a * x
Insert these two relations above, then you have:
adot * x = H * a * x
And that gives (x cancels out):
H = adot / a
Which is the relation I used: H gives the rate of expansion.




So if i look to Andromeda (m31 I mean) and do away with attractor eigen movement this object should flee me with roughly 45km/sec as its distance would roughly be 2/3 Mpc.


The expansion happens only on scales where matter looks essentially to be homogeneously distributed. 2/3 Mpc is much too small for that. So there is no expansion on that scale, but "contraction": things get in general closer to each other.




but what I would need for the answer to my question would be the ratio ...


Which I provided. What more do you want to know?




No, no I can't see why you get away without an integration! You are obviously not giving me the TIME it takes to blow one liter of space now to double its volume .... it can't be that straight forward!


Well, if you really want the time needed for doubling, true, the calculation would be more complicated - because H changes this time. As I said, in my calculation, I assumed that H remains constant during this billion year.

If you really want to have required for doubling, the easiest way would be to use an explicit expression for a. You can find one e. g. in this short paper, which I wrote once for myself:
http://www.rzuser.uni-heidelberg.de/~bfeuerba/universe.pdf

Use equation (1.4), set a(t) equal to 2, insert the values for Omega_lambda,0 and H_0, and solve for t. Not that hard to do.

satori
2007-Feb-08, 09:18 PM
Bjoern,
what you gave after some procedures namely
H = adot / a
I used as my starting point.
what you call "scale factor" here (note: you have not introduced this term formaly) would be My distance (or not?)
H=adot/a <=> adot=H*a !
Now you go on :

Quote:
Originally Posted by satori View Post
So if i look to Andromeda (m31 I mean) and do away with attractor eigen movement this object should flee me with roughly 45km/sec as its distance would roughly be 2/3 Mpc.
The expansion happens only on scales where matter looks essentially to be homogeneously distributed. 2/3 Mpc is much too small for that. So there is no expansion on that scale, but "contraction": things get in general closer to each other.
If you look in my post you will see that I allready took into consideration the attraction going on !
I simply choose to choose Andromeda to have a nice mark.
I you wish so, do away with this damn thing and look only at empty space but look at a point roughly 2/3 Mpc away!
If you smooth every thing out, space expandes on even the smalest scale! where is the problem?
I still can't see how you could provide me an answer with your ansatz! No I am not speaking now about this integration thing... in my very last post i made the point that there would have to be involved an x^3-operation to come up with the very concrete question how long it roughly takes to pump up 1l to 2l .
Don't you have this x^3 thing on your radar, is this not necessary?

korjik
2007-Feb-08, 09:31 PM
The industrious student now takes the 7% per billion years expansion in distance and uses that to solve his problem

satori
2007-Feb-08, 09:42 PM
nice remark korjik,
but here i am back from my TimeMashine!
Hear Earthling Bjoern, I come to a volume change rate of roughly 10^-53 /sec

satori
2007-Feb-08, 10:08 PM
derivation: I take your measure for one Mpc =3*10^19km
now I take the Hubble constant H=70km/sec/Mpc
this equation means : a point in the distance of 1Mpc flees us at a rate of 70km/sec or in other terms a lengh of 1Mpc grows at a rate of (70km/sec)/3*10^19 km= 23*10^-19/sec
so this should now be the change rate of any sufficently smal length per sec .
so a meter would grow 2.3*10^-18 meter per sec
now take that to the third power and you get 10^-53 m^3/sec as the change of your unit cube of one cubic meter per second or the volume change rate of 10^-53/sec.

satori
2007-Feb-08, 10:27 PM
Ehyyyy **** !
I withdraw that. It's only valid for just those 1Mpc !
On the small scale the effect is way smaller

please
2007-Feb-09, 09:18 AM
Space doesn't expand equally, that's the pointYes it seem so. But:
The Hubble "constant" gives the rate of change. The currently accepted value is around H = 70 km/s/Mpc.this seem to suggest otherwise?

Let me draw my problem for you. Here is standard "baloon" analogy they tell in classrooms and all over - let's draw a man and a galaxy on baloon, and expand it: http://img206.imageshack.us/img206/1371/expyb8.th.jpg (http://img206.imageshack.us/my.php?image=expyb8.jpg)Obviously, there's something wrong with this picture. Hence the question.

If other "forces" are relevant, then baloon analogy is wrong, and space expands not with, but relative to matter in it, or otherwise matter just would not know that space has been expanded.

satori
2007-Feb-09, 02:44 PM
hello please( I mean you the member above!( I must say you have choosen a problematic name)), nice to meet you!
I don't know whether you will trust me in this after my painful derailment upstairs....anyway:
I think this simple baloon model is just exelent!
It is funny to see how much complication is caused even to very well educated people (Bjoern is a veritable Ph.D in physics), by the fact that real space time is a little lumpy.
It would sofore be best to first smooth all matter with their peculiar movements out! So that you come to a situation, where there are no galaxies etc. at all. Fact is, that no much harm is done, if we do so. It is an universaly accepted fact, that our real universe is homogenous and isotropic to a very high degree anyway. All the modeling and data evaluations done for WMAP etc. recurr to this idealized situation and then give the corrections as found by observing.
So you are right in stating, that in deed H tells about a uniformly expanding universe !
So if Andromeda (M31) was just a kind of a holographic projection moving along the uniformly spread out matter right there at a distance of roughly 2/3 Mpc, then H=70km/sec/Mpc would rightly inform us, that it should move away from us at roughly 45km/sec.
In truth of course you have this lumpyness in matter and an attraction going on between us and M31. This all acounts for some buckles in real space time curvature vice versa buckles in the baloon.
The aim of my privious posts was to get a feel of how strong this expansion thing really is. And so I brought the question, how long it would take for a liter-volume of space at our time now to double in size. This caused some complications because you have to integrate over time and second you may have a time dependence of H=H(t) (remember all that fuss about far out super novae in 2000 ?). So you should prefere the linear, differantial picture, that is you should ask for example : how far will two "typical" points at distance 1km move away from each other due to space expansion (right now in the history of the univers) in say one second ?
And you can indeed take now H=70km/sec/Mpsec =2.3*10^-18/sec ( as provided thankfuly by Bjoern) and multiply this with 1km and you get 1km*2.3*10^-18/sec =2.3*10^-15m/sec and you see that your two test particles will separate with just one or two proton-diameters per second!
But what me irritated in my previous post was, that this isn't really all that small ! The ongoing gravitational-wave experiments are trying (to the best of my knowledge) to measure even smaller length changes! That they are still optimistic must have something to do with periodicity in those signals....

Ken G
2007-Feb-09, 03:31 PM
If other "forces" are relevant, then baloon analogy is wrong, and space expands not with, but relative to matter in it, or otherwise matter just would not know that space has been expanded.

That is true, but you're not really saying anything here, because the balloon analogy is only meant to convey the situation on the largest scales where there are no important forces other than a very smooth and gradual evolution due to gravity and the existing dynamics. On the scales of actual matter features, many other effects come into play. Another analogy that is probably better is the raisin bread analogy-- space is the rising dough, and galaxy clusters are the raisins-- the raisins don't expand with the dough. Again, the "expansion of space" is merely a picture you obtain if you choose to coordinatize the motion of the universe in terms of spatial coordinates that are locally fixed to the matter (not including the dark energy effects, which do appear more and more to be a real gravity effect). Put differently, the distance between the galaxy clusters doesn't increase because it's "stuck to" the space, the space expands because it is "stuck to" the matter. The increasing distance between the matter in the galaxy clusters is a ramification of the dynamics of the universe, as described by Einstein's equations of general relativity. Do not mistake an analogy with balloons for real physics.

Cougar
2007-Feb-09, 03:44 PM
...dark energy effects... do appear more and more to be a real gravity effect.
I'm not sure what you're saying here. Please clarify.

satori
2007-Feb-09, 04:46 PM
Ken G,
firstly : yes please expand!
secondly :

Do not mistake an analogy with balloons for real physics.
I would think the baloon picture is exactly Robinson-Walker just projected, just minus the differential geometric "details"! Isn't RW the dominant model and just quite perfect after homogenisation done?

Kwalish Kid
2007-Feb-09, 05:39 PM
Ken G,
firstly : yes please expand!
secondly :

I would think the baloon picture is exactly Robinson-Walker just projected, just minus the differential geometric "details"! Isn't RW the dominant model and just quite perfect after homogenisation done?
The big difference between R-W and the balloon analogy is that in the R-W model there is not really another dimension that explains the expansion. There is another parameter related to spatial distance, but it is not associated with an actual dimension (even though it is sometimes referred to as a radius).

The "force" driving acceleration has a natural place in General Relativity as a part of the action of gravity. It's just that, relative to the ordinary activity of gravity that we notice, the new activity is larger at a distance.

daxloves
2007-Feb-09, 07:44 PM
Should not space expand within us and our measurement tools the same way it does between galaxies? And if it does, how can we possibly notice expansion.

Layman examples: No higher education required!

Dont think of it as space expanding, only the universe expanding within space.

Set off a fire cracker and the gasses inside expand. Pieces of it fly out in an expanding circle but the pieces themselves dont keep expanding too.

Think of pure space as nothing with limitless distance and our universe is expanding within that space, not with it but within it.

Or if ya like the redneck example: Fire a shotgun in the air an the shot dont get no bigger, they jes get further apart, then land on yer head.

satori
2007-Feb-09, 08:09 PM
Kwalish Kid,
It is quite true that from its out set GR was meant provide a observer independent (often called covariant) description of space and time. Einstein found such a description allready existent ( or at least a prefabricated formalism plus theorems concerning curvature, covariant derivative etc.) in the work of Riemann (a certain Marcell X. (mathematican) was actualy helping him there as Einstein himself owned quite freely a less than extensive knowledge of math.). So the object taking center stage now is the (pseudo) Riemann manifold with "metric" and curvature form. The coordinates would in the pure mathematicans view of the thing have to vanish alltogether(!). Indeed are they ubiquitus in the physicists way of dealings because this nearer to their education and also nearer to the doing of actual concrete(!) computations. Mathematicans tend to shun away from such mundane activities...
Yes indeed the object of interrest is "selfcontained" independent of a "bedding space"....BUT:
The actual universe is of this well behaved kind as to be describable by a RW metric and this thing looks to me as naturaly embedded in lR^4. So if in general embedding reduces elegance of description, RW would lend itself quite naturaly to this and I personaly see the "understandability" (Anschaulichkeit) greatly enhanced by so doing! So why abstain from it ( even if one knows that this embedding space has no "physical meaning of its own" )!!!!

Your second point is more tricky and I will need some time to reflect. Thanks....

buzgz
2007-Feb-09, 08:56 PM
Hello people, I'm new, I like this icon :boohoo: and let's get to the question:

How come expansion is detectable?

Should not space expand within us and our measurement tools the same way it does between galaxies? And if it does, how can we possibly notice expansion.

I'll probably regret saying this, but I don't think we're really measuring distances to distant galaxies with real distance measuring tools, so it wouldn't matter whether or not those tools also expanded.

Rather, I think what we're really calling distance measurements are mathematical calculations explaining the redshift in light from distant galaxies.

Then we say the universe MUST have expanded to yield those results.

Ken G
2007-Feb-09, 09:13 PM
I'll probably regret saying this, but I don't think we're really measuring distances to distant galaxies with real distance measuring tools, so it wouldn't matter whether or not those tools also expanded.
We are never "really" doing anything in physics, beyond seeking a mental picture that is effective. That is what science is all about.


Rather, I think what we're really calling distance measurements are mathematical calculations explaining the redshift in light from distant galaxies.
I agree, but would repeat that this is all science ever does (and note it is more than just redshifts, there are other ways to get distance, like looking at what happens to the angular size, or observing what happens to the intensity. That there are several different ways all giving different answers underscores your point, but physics understands these differences, it's all in there). The test is that it works, not that it is "right"-- or put differently, to be "right" is to pass the test, pure and simple. That's what makes physics objective rather than philosophical (and you'd never get the world's scientists to agree if it were the latter!).

Ken G
2007-Feb-09, 09:18 PM
I would think the baloon picture is exactly Robinson-Walker just projected, just minus the differential geometric "details"! Isn't RW the dominant model and just quite perfect after homogenisation done?
Actually, the balloon is more general than RW, as the latter does not include dark energy but the former can. What it really requires is called the "cosmological principle" that everywhere is the same as everywhere else, on the largest scales, at the same local age. This is the key property possessed by the balloon, the curvature is more a confusing detail that doesn't even have much merit in light of recent observations.

buzgz
2007-Feb-09, 09:33 PM
We are never "really" doing anything in physics, beyond seeking a mental picture that is effective. That is what science is all about.


O.K., but the questioner was asking whether the measuring tools also expanded. It seemed to me that everyone jumped on assuring him/her that they did not, as though those tools were actually being used in some sort of measurement.

I thought that pointing out what was really going on might be helpful.

Ken G
2007-Feb-09, 09:37 PM
O.K., but the questioner was asking whether the measuring tools also expanded. It seemed to me that everyone jumped on assuring him/her that they did not, as though those tools were actually being used in some sort of measurement.There is a tool being used, and that's a clock (say a light clock of fixed width)-- measuring the redshift, and an imager, measuring, say, the angular size of the source. So one does need to recognize that expansion is indeed relative to local measuring devices, and if those devices expanded also, then you would see nothing at all. But it is also true that the actual nonlocal distance to the distant object is not directly measured, it is inferred in a self-consistent way by the basic assumptions about how physics works. That last part is inescapable, and is part of the limitations of using science to probe truth. Nevertheless, when you do that, you find that matter does not expand with the universe.

buzgz
2007-Feb-09, 10:05 PM
There is a tool being used, and that's a clock (say a light clock of fixed width)-- measuring the redshift, and an imager, measuring, say, the angular size of the source. So one does need to recognize that expansion is indeed relative to local measuring devices, and if those devices expanded also, then you would see nothing at all. But it is also true that the actual nonlocal distance to the distant object is not directly measured, it is inferred in a self-consistent way by the basic assumptions about how physics works. That last part is inescapable, and is part of the limitations of using science to probe truth. Nevertheless, when you do that, you find that matter does not expand with the universe.

It seems to me that every part of every measurement made is done locally.

I repeat, the big thing is to explain that redshift. Therefore expansion is postulated as the answer.

Ken G
2007-Feb-10, 02:04 AM
Yes, all measurements are purely local, as I've repeated many times. And every answer in physics is "postulated", you aren't saying anything here. What is it you are trying to say?

buzgz
2007-Feb-10, 06:19 PM
Assume we measure redshift of light collected from a distant galaxy by passing light through a prism and recording the resulting spectrum on a photograph. I'm trying to see where the answer we get could in any way depend on whether or not our measuring instruments expanded with the universe.

It seems to me that the aperture of the telescope is unimportant, in that many different size apertures would suffice. Similarly, the focal length of the telescope is unimportant. The size of the prism is unimportant. The size of the film we record the resulting spectrum on is unimportant, and we could use a piece of string of unknown length to measure redshift by proportionality.

Thus we arrive at a distance to the galaxy independent of whether or not local sizes of things expand with the universe or not. Similarly, we calculate the rate of recession of that particular galaxy. Where in all of this would it matter one whit whether or not measuring instruments also expand ?

Ken G
2007-Feb-11, 03:34 AM
The size of the film we record the resulting spectrum on is unimportant, and we could use a piece of string of unknown length to measure redshift by proportionality.Huh? How do we measure "by proportionality" if our standard is of "unknown length"? If all length scales increase, nothing happens at all. The universe has no absolute scale, only relative scale. The universe is a scale model of itself, our physics has no information to the contrary (perhaps it will someday!). The expansion of the universe is entirely relative to what the bound systems are doing, no other definition has any physical meaning.

Where in all of this would it matter one whit whether or not measuring instruments also expand ?
Think about it some more. If everything expands, nothing does.

buzgz
2007-Feb-11, 08:55 PM
Huh? How do we measure "by proportionality" if our standard is of "unknown length"? If all length scales increase, nothing happens at all.

Think about it some more. If everything expands, nothing does.

I'm really confused by this line of thought. One could calibrate the film in the above example by recording the Balmer series from the sun and comparing it to the corresponding Balmer series from the distant galaxy.

Now, are you saying that if the earth and sun were moving apart at the same rate as the universe expands, we wouldn't get an answer ? I think we would not only get an answer, but we would pretty much get the same answer as we would have gotten if the sun and earth had not been moving apart, since the time and distances involved are insignificant compared to the corresponding time (light travel time) and distances involved with the distant galaxy.

Jeff Root
2007-Feb-11, 10:16 PM
Gads!
There are two different and incompatible notions of "expansion" at
play in this thread, and it seems like nobody can see both-- just
one or the other.

The way the actual expansion works is that galaxy clusters are
moving apart from each other.

The other kind of expansion, probably a fantasy of every geeky
teenager since Henry Hasse wrote "He Who Shrank" in 1936, is
where everything-- every galaxy, every star, every planet, every
rock, every speck of dust, every molecule, every atom, every
proton and electron-- everything expands (or shrinks!) equally.
If everything expands or shrinks in unison, the effect is exactly
the same as if no change takes place at all.

-- Jeff, in Minneapolis

buzgz
2007-Feb-11, 11:12 PM
Gads!.


-- everything expands (or shrinks!) equally.

-- Jeff, in Minneapolis


We are discussing proportional expansion as if everything were expanding at the rate of universal (Hubble) expansion.

Expanding equally has nothing to do with this discussion.

buzgz
2007-Feb-11, 11:14 PM
Think about it some more. If everything expands, nothing does.

I've thought about it some more.

If everything expands, everything does.

Ken G
2007-Feb-12, 01:26 AM
I've thought about it some more.

If everything expands, everything does.

Keep thinking. The two statements are perfectly indistinguishable.
One could calibrate the film in the above example by recording the Balmer series from the sun and comparing it to the corresponding Balmer series from the distant galaxy.Absolutely, and since everything expands, the Balmer lines from the Sun have expanded wavelengths, as do the Balmer lines from the early universe due to the expansion itself. So if both Balmer lines expand equally, as I've stated is the requirement we are considering, then exactly how do you plan on not getting a null result?

Bjoern
2007-Feb-12, 06:54 PM
Bjoern,
what you gave after some procedures namely
I used as my starting point.
what you call "scale factor" here (note: you have not introduced this term formaly) would be My distance (or not?)
H=adot/a <=> adot=H*a !


"scale factor" is a very basic term in cosmology, so I didn't saw the need to explain the meaning of this term. It is not the same as "distance". Actually, it measures the way in which distances change. Or, more to the point: if you have two objects with coordinates x1, y1, z1 and x2, y2, z2 (coordinates which stay constant with time! such objects are called "co-moving with the expansion"), then the distance d between them is:
d(t) = a(t) * sqrt((x1-x2)^2 + (y1-y2^) + z1-z2^2),
where sqrt means the square root, the whole square root term is just the usual result from Pythagoras' theorem, and t is time.

BTW, do you understand what I mean with adot?




I you wish so, do away with this damn thing and look only at empty space but look at a point roughly 2/3 Mpc away!
If you smooth every thing out, space expandes on even the smalest scale! where is the problem?
I still can't see how you could provide me an answer with your ansatz!


But I already told you how I arrived at my answer. What exactly did you not understand?




No I am not speaking now about this integration thing... in my very last post i made the point that there would have to be involved an x^3-operation to come up with the very concrete question how long it roughly takes to pump up 1l to 2l .
Don't you have this x^3 thing on your radar, is this not necessary?


Sorry, I have no clue where you got that x^3 from.

BTW, did you look at my short paper, especially at formula (1.4), as I suggested? I did the calculation myself in the meantime, and what I came up with for the time it will take all distances to double is:
t_double = 2/[3 sqrt(Omega_lambda,0) H_0]
* [arsinh sqrt(8 Omega_lamda,0/(1-Omega_lambda,0))
- arsinh sqrt(Omega_lamda,0/(1-Omega_lambda,0))]
Inserting standard values for Omega_lambda,0 and H_0 (both = 0.7), I get:
t_double = 10.7 Gyr.

Are you satisfied now?

Bjoern
2007-Feb-12, 06:56 PM
derivation: I take your measure for one Mpc =3*10^19km
now I take the Hubble constant H=70km/sec/Mpc
this equation means : a point in the distance of 1Mpc flees us at a rate of 70km/sec or in other terms a lengh of 1Mpc grows at a rate of (70km/sec)/3*10^19 km= 23*10^-19/sec
so this should now be the change rate of any sufficently smal length per sec .
so a meter would grow 2.3*10^-18 meter per sec
now take that to the third power and you get 10^-53 m^3/sec as the change of your unit cube of one cubic meter per second or the volume change rate of 10^-53/sec.

Yes, that's essentially the calculation I also used. This leads to a doubling time (for distances - if you want volume, the calculation is obviously different) of about 10.2 Gyr. Note that my more precise calculation, in which I did not assume that H is constant, lead to a result of 10.7 Gyr. So this approximation is not too bad...

satori
2007-Feb-12, 08:18 PM
thank you Bjoern for the pains you took to enlighten me, our previous conversation must rightly have frustrated you...
In a real life conversation things would not have derailed in this awkward manner !
Let me note that the (allmost) same results we got points to a glitch in communication as the main cause for our misunderstanding.
Let me also note that I indeed halfway studied your paper and I must confess that we are not on an equal plain!
I could surely have made more sense of it, if I had choosen to invest more time...alas.....
The main reservation I had, to make use of the formulas you provided, were though, that I wished to go the pedestrian way, where I felt that easily possible...
You would certainly agree, that in our wellbehaved space time H plays a very dominant role and should lend itself to very straight foreward computations which would shortcut the more correct but also more cumbersome discussions based on full Rob-Walker...
Did you read that I came to a space expansion of 2 (or so) proton diameters per second for a test length of 1km (idealy smoothed out matter situation) ? Isn't that roughly the value they expect for space distortions due to grav. waves ?

Bjoern
2007-Feb-12, 08:28 PM
You would certainly agree, that in our wellbehaved space time H plays a very dominant role and should lend itself to very straight foreward computations which would shortcut the more correct but also more cumbersome discussions based on full Rob-Walker...


In order to answer your question, we have to know how H changes with time (if we don't want to use the (actually rather good) approximation that it is constant). But in order to know this, we unfortunately indeed need "the full Rob-Walker" (actually, the Friedmann-Lemaitre equations, which are derived from that)




Did you read that I came to a space expansion of 2 (or so) proton diameters per second for a test length of 1km (idealy smoothed out matter situation) ? Isn't that roughly the value they expect for space distortions due to grav. waves ?

Yes, I think that's the same order of magnitude (IIRC, the relative distortions expected due to gravity waves are on the order of 10^-18).

Kwalish Kid
2007-Feb-13, 03:16 AM
Actually, the balloon is more general than RW, as the latter does not include dark energy but the former can.
The R-W metric is used in Lemaitre models (that is, those with a cosmological constant) as well as in Friedman models.

Ken G
2007-Feb-13, 06:25 AM
I stand corrected, thanks. The main thing about the balloon analogy is that it is intended to convey the idea of a single degree of freedom, the "scale parameter", in describing the expansion. So it's really about the "cosmological principle", which is appropriate to the RW metric. That it has a closed spatial geometry is a pretty strong limitation for using the balloon analogy, I generally prefer the "unbounded raisin bread" approach.

sethoscope
2007-Feb-14, 03:06 PM
I've thought about it some more.

If everything expands, everything does.

So if everything was expanding then there would be no measuring stick by which to detect that expansion so it would appear not to expand. When we are traveling in a car at 60 mph we know that we are moving because the road we are on is stationary (relatively speaking). Thus we have a measuring stick. However if the road and everything else around us was also moving along at the same speed then we would be stationary. Just like how we are moving around the sun very fast but don't feel like we are moving at all.

So the only reason we can detect red shift is because gravity causes some matter (ex. our earth from which we observe the universe) to not expand with space thus we have a measuring stick to compare or detect this expansion. Otherwise there would be no red shift and thus not possible to use this method to detect this expansion.

Ken G
2007-Feb-14, 03:25 PM
Exactly.

sethoscope
2007-Feb-14, 03:58 PM
Exactly.

Here's another question:

Since everything in universe is affected by the gravitational pull from everything else. At what point is gravity strong enough to stop the expansion of individual bodies?

I don't know if that makes since so let me refer back to your raisin bread analogy. If two raisins are close enough to each other and their gravitational effect on each other strong enough could the force of gravity counter the force of expansion causing the two raisins to become one body? Why or why not?

Bjoern
2007-Feb-14, 04:29 PM
Here's another question:

Since everything in universe is affected by the gravitational pull from everything else. At what point is gravity strong enough to stop the expansion of individual bodies?

I don't know if that makes since so let me refer back to your raisin bread analogy. If two raisins are close enough to each other and their gravitational effect on each other strong enough could the force of gravity counter the force of expansion causing the two raisins to become one body? Why or why not?


First, a very rough answer: AFAIK, on scales of approximately up to 10 Mpc, the local forces of gravity can lead to an approach of two objects, counter to the cosmic expansion.

Second, a more-to-the point answer: the distribution of objects in the universe isn't the same everywhere; there are galaxy (super) clusters, voids, field galaxies etc. And beside galaxies, you also have to include intergalactic gas clouds etc. All in all, this leads to a rather inhomogenous density distribution - and the answer to your question isn't straightforward anymore, but depends crucially on the (mean) density in the part of the universe you are interested in. I. e., inside a void, the effect of cosmic expansion will be observable on much shorter length scales than inside a huge galaxy cluster.

Third, I don't like this picture of "local gravitational forces work counter to the cosmic expansion" at all. After all, we use GR to describe the cosmic expansion - and in GR, there are no gravitational forces! More to the point is the following: GR says only for a homogeneous universe that it has to expand (or contract, but let's leave this subtlety out here). But the universe is anything but homogeneous on "small" scales (say, on scales smaller than 100 Mpc). So we have to look what GR has to say about an inhomogeneous universe.

And guess what? It tells us that the inhomogenities have to grow with time, i. e. regions of higher density become even more dense, and regions of lower density become even less dense. That can be interpreted as local forces of gravity working counter to the cosmic expansion - but IMO, it shouldn't, because this merely tends to obfuscate what really happens.

buzgz
2007-Feb-14, 08:45 PM
So the only reason we can detect red shift is because gravity causes some matter (ex. our earth from which we observe the universe) to not expand with space thus we have a measuring stick to compare or detect this expansion. Otherwise there would be no red shift and thus not possible to use this method to detect this expansion.

I still don't think this is correct.

Let's assume that everything is expanding at 70km/s/Mpc. If I look up the definition of a meter or look up the wavelengths in the Balmer series when I am 20 years old, by the time I'm 65 by just how much has the meter or the wavelengths expanded ?

Not by a whole bunch, I'll (conservatively) guess.

And you guys are saying this insignificant ( and probably not even measurable) expansion here on earth will keep me from detecting redshift in a distant galaxy ?

Please explain in some detail.

Ken G
2007-Feb-14, 09:44 PM
And you guys are saying this insignificant ( and probably not even measurable) expansion here on earth will keep me from detecting redshift in a distant galaxy ?

Please explain in some detail.
Your lifetime has nothing to do with this issue-- do you think the light you are seeing from that distant galaxy was emitted in your lifetime? What matters is how long a meter was when the light was emitted (because the wavelength of the light is some fraction of that meter), compared to how long it is now (because that's also how much the wavelength you see now will have increased). If the two are in proportion, then the wavelength will still be the same fraction of a meter as it started out. Hence you'll have no way to identify any redshift at all.

Jeff Root
2007-Feb-15, 09:16 PM
I still don't think this is correct.

Let's assume that everything is expanding at 70km/s/Mpc. If I look
up the definition of a meter or look up the wavelengths in the Balmer
series when I am 20 years old, by the time I'm 65 by just how much
has the meter or the wavelengths expanded ?
Which expansion are you talking about? The one which is actually
observed, in which distant galaxy clusters are moving apart from
each other at 70km/s/Mpc, or a fantasy expansion in which
everything in the Universe is expanding at 70km/s/Mpc?

In both situations, the definition of the meter and the measured
wavelengths of the Balmer series never change.

But in the first situation (which is reality) we see distant galaxies
moving away from us, currently at a rate of 70km/s/Mpc, while in
the fantasy situation the expansion would be undetectable.
Nobody could ever know it was happening.

Which one are you talking about?

-- Jeff, in Minneapolis

transreality
2007-Feb-15, 10:21 PM
That can be interpreted as local forces of gravity working counter to the cosmic expansion - but IMO, it shouldn't, because this merely tends to obfuscate what really happens.

Which is that expansion manifests in vaccuum fluctuations and that local density inhibits vaccuum. So the solid meter long ruler does not expand at all but surrounding rarified space does. At some low measure of density that rate would approach this 70km/s/Mpc, maybe even exceed it out in a void...

buzgz
2007-Feb-16, 05:19 PM
What matters is how long a meter was when the light was emitted (because the wavelength of the light is some fraction of that meter), compared to how long it is now (because that's also how much the wavelength you see now will have increased). If the two are in proportion, then the wavelength will still be the same fraction of a meter as it started out. Hence you'll have no way to identify any redshift at all.

I understand this now. Thanks for your patience.

Can you comment on what drives the expansion ? This is another thing that I have difficulty comprehending.

jamini
2007-Feb-16, 06:04 PM
I understand this now. Thanks for your patience.

Can you comment on what drives the expansion ? This is another thing that I have difficulty comprehending.
This question seems to get asked a lot around here.

The most legitimate explanation for our observed accelerated expansion - as evidenced by Hubble red shift and SN1data, is a particular value of negative pressure, which is expressed as a cosmological constant in the mathematics of General Relativity. It is more commonly referred to as Dark Energy, which seems to cause a lot of phobia among many who do not truly understand the mathematical parameters of the formulas used to describe it; some see a word like Dark Energy and think of all kinds of ominous mysterious things, when in fact it’s simply an appropriate label for a well-established concept based on our most recent observations.

Ken G
2007-Feb-16, 06:34 PM
That is true, but the answer to "what accelerates the expansion" is different from "what drives the expansion". We don't need the former to have the latter, the former is kind of a detail. As for the "driving", it's important to realize that expansion does not need to be driven, that is kind of like asking "what drives a hockey puck sliding along the ice". There needs to be an initial condition that involves motion, but when we follow time forward, there is no requirement that the expansion be "driven"-- it is because it was. Physics often requires initial conditions to make sense of observed behavior, and note there is no "science of initial conditions", it's just the way we set up problems in physics.

sethoscope
2007-Feb-17, 12:54 AM
So what exactly was the initial condition that started the motion of the expansion? The hockey puck slide because of the initial puch of the hockey stick.

Ken G
2007-Feb-17, 01:08 AM
Nobody knows.