1. Jul 11, 2006

### Born2Perform

I don't understand what you mean with: universe is expanding.

Is space that -as an elastic membrane- is expanding or galaxies have a proper velocity?

And however, shouldn't all become bigger during the expansion?

for example, in a sphere-universe of diameter 100 metres there is a galaxy of diameter 5 metres.
If the diameter of the universe become 10,000 meters also the diameter of the galaxy must become 500 metres; because space inside the galaxy would grow too.
If you deny this, you also must deny that space does not drag along galaxies, so they don't move.

I send back to my first question.
Thanks

Last edited: Jul 11, 2006
2. Jul 11, 2006

### EL

This has been discussed in some threads earlier, and when you dig deep into the problem it's really not that simple at all. However, I'll try to give some intuitive "first step" answers:
The expanding universe solution really only holds on large scales where the universe can be seen as homogenious and isotropic. As soon as you get down to scales small enough that you start to see "clumpsiness" in the matter distribution, the equations of General Relativity give you other spacetime solutions.
For example our solar system is well described by the Robertson-Walker metric in which space does not expand with time. The solar system itself lives in our galaxy, which must be described by some other new metric...and so on...It's first when we reach scales large enough that the universe can be well described by the FLRW-metric, which predicts the expansion of space we talk about, that we really can see the expansion.

Maybe an even more intuitive (although not as "correct") way to look at it is like this:
In a galaxy the stars are gravitationally bound to each other, and the expansion of space at those scales are not "fast enough" to overwin the gravitational attraction. Only clumps of matter that are far enough from each other will move away from each other due to the expansion.

Last edited: Jul 11, 2006
3. Jul 11, 2006

### Born2Perform

under "expansion" there are 497 results.. can you link me just the more significant thread?

4. Jul 11, 2006

### EL

5. Jul 11, 2006

### MeJennifer

Cosmologists come up with the most stange conclusions, at least to me, of course for them it is perfectly "explainable".

For instance they claim that objects are apparently travelling away from us faster than the speed of light.
Then if you ask them how that is possible, they answer, well it is because space is expanding. So much for Einstein's "nothing that has mass can reach the speed of light".
Or when cosmologists speak about the age of the universe as if there suddenly is some sense of absolute space and time afteral because of the background radiation. There is supposed to be no prefered frame of reference, and there is covariance and so, but everything cosmological now seems to be measured from the new absolute space and time reference frame.
Or am I misinformed?

Last edited: Jul 11, 2006
6. Jul 11, 2006

### EL

Yes you are.
You are drawing false conclusions due to a lack of understanding of the differences between Special and General Relativity.
If you tell us what level you are at, maybe you could have some books recomended.

7. Jul 11, 2006

### MeJennifer

So are you agreeing that objects can travel faster than light because space is expanding?
Edited:
Never mind, I will open a topic on this, seems like a question that can improve my understanding.

Last edited: Jul 11, 2006
8. Jul 23, 2006

### Jenny

I read somewhere a funny joke about the expansion of space. (it isn't lame, a clever funny joke)
If the atom, the person, the Earth and the Solar system expanded just as the rest of space expanded (ie. space between each elementary particle expanded and the space each elementary particle occupies expanded in the same ratio etc etc... how this happens I dont' know...) then the human wouldn't even be able to observe the expansion of space!
The author went so far as to joke: maybe space isn't expanding, merely matter shrinking within it =) It would produce the same observed affect (given it's ideal homogenous shrinking of gravitationally bound stuff, where all distance ratios within a system remain eternally equal. The ratio of distances between systems to distances witin the system would increase =)

Last edited: Jul 23, 2006
9. Jul 23, 2006

### chronon

Cosmologists have chosen a particular coordinate system, which disagrees with the coordinate system suggested by Special relativity. This choice is often to be presented as the only one possible, but it isn't. See Stretchy Space? for more of my argument against this choice.

10. Jul 24, 2006

### MeJennifer

To me it seems that scientists are simply plugging in data by measurements they make in the different regions and making it work with GR. How can we say we can derive all this from GR when we have constants, scale factors, dark matter, de Sitter spaces, energy vacuums etc?

So you are saying it is not possible that we simply have no clue as to why the numbers at very large distance are so different, and that instead we simply plug in some new metric that makes the numbers fit?

So you are saying that gravitational attraction stops expansion?

If we take a simple example in GR of a sphere with a certain volume and mass we indeed see that the volume is reduced.
However from within the sphere no such conclusion is made, it is only outside the sphere that the reduction is visible.
Now we live in the universe, how could we possibly see a volume reduction effect on the expansion?

What about the time part of space-time?
Is only space expanding or is time expanding as well?
So did time run faster in the past?

Last edited: Jul 24, 2006
11. Jul 25, 2006

### Parlyne

The geometric model of cosmology comes from applying two observations about the universe on large scales to the field equations of general relativity. Astronomers have known for some time that the universe on large scales looks pretty much the same in every direction (it's isotropic) and in every place (it's homogenous).

It can be shown that the only geometries allowed by general relativity in the case that the sources of gravity are homogenous and isotropic have line elements that look like:

$$ds^2 = - dt^2 + a^2(t) \left (\frac{dr^2}{1-kr^2} + r^2 (d\theta^2 + \sin^2\theta d\phi^2) \right )$$,

up to a general change of coordinates. The value of k (which can only be 0, 1, or -1 to begin with) and the time dependance of a(t) are determined by the specific properties of the sources of gravity. This is where things like dark matter and dark energy become relevant. They are necessary to make a(t) and k behave as we observe them to behave.

As I implied above, the cosmological metric really only applies on large scales, because it is only on large scales that the universe is homogenous and isotropic. When we look as smaller scales things are pretty clumpy; so, the metric must deviate from the above in response to that .

The simple answer about small scale structures is that on such distance scales the effects of expansion are very small - much smaller than the gravitational attraction between objects composed of normal or dark matter. So, gravitationally bound systems remain unaffected by the expansion.

As for the nature of expansion itself, as you can see in the line element above, the scale factor, a(t), only plays a role in the spacial part of the geometry; so, space is expanding over time.

12. Jul 3, 2008

### robheus

Yes, and that is of course true by definition, because all physical phenomena are independent of the ruler (units of measurements) you choose. If tomorrow we choose a new meter unit as 0,5 of the old meter unit, we would have to rewrite all our physics books to adjust to that, but apart from that, nothing in the universe would change. The physics laws and phenomena stay the same.
This is even true if the ruler we choose is time variant.

So physically this outlook is perfectly ok, yet it is a little unpractical to choose the distance between two far away galaxies as your unit of measurement.

13. Jul 29, 2008

### azzkika

If it expands over time, what kind of time would it be when it's expanding >c as is observed at present. and i repeat a question from another post, what causes the reduction of speed of space expansion to <c thus allowing galaxies which were once moving >c to be seen.

14. Jul 30, 2008

### JimJast

Actually you may say that the time run slower in the past and that's why we see the Hubble redshift. Yet the cosmologists don't consider such a possibility despite that it is the only scenario consistent with the global conservation of energy and supplies the means of calculating theoretically the Hubble constant. For some reason the cosmologists prefer expanding distances and "dark energy" to conservation of energy and theoretical predictions of parametrs of this (apparent in such a case) expansion.

Last edited: Jul 30, 2008
15. Jul 30, 2008

### JimJast

Nothing in nature can expand faster than c since whenever some distance from us gets increasing with high speed the time at the moving end of this distance starts running slower in relation to us and the result is that it can never crosses the speed of light. It is elementary relativistic physics and our world happens to be relativistic. Physicists don't worry about >c stuff so you don't need neither. If some of your calculations (not observation since there is even no way to observe >c) give you >c result your have to check your calculations since they are obviously false.

16. Jul 30, 2008

### marcus

could you be more explicit about what you mean by "It".

the universe does not have a definite speed of expansion so I suppose you are not talking about the universe expanding >c.

but a lot of the distances beween stationary points have always been increasing at rates >c, so by "It" maybe you mean some particular distance, like, to some galaxy? A galaxy which is approximately at rest relative to CMB and the distance to it increasing >c?

the matter density causes slowing. there is a term in one of the two Friedman equations that gives the second time-derivative a''(t) of the scalefactor a(t) in terms of the matter density and also positive pressure (if there is any measurable positive pressure)

============
we are able to see galaxies which have always been receding >c. We see them all the time. they are a large part of the galaxies available for study. You just point the telescope at them and look. How this happens is explained in the Lineweaver SciAm article whose link is in my signature. check it out. good article. they use it to teach with at Princeton. simple and lots of picture.

what you said there shows some misunderstanding because a galaxy's recession speed does not have to slow down to <c in order for it to be seen. the distance to it does not have to increase at a rate lower than <c to allow (as you say) the light to get here.

we've been over this a lot at the forum but at the moment I can't get you a link to a thread. have a look at Lineweaver, that draws pictures of how it happens and makes it clear.
================

Kikkah, check this out.
http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html
put in the standard 3 parameters (.27, .73, 71) and try redshift z=2.4

You will see that any galaxy we are now looking at that has redshift of 2.4 or more has ALWAYS been receding faster than c. the distance to it has been increasing >c. Not that the thing has been moving relative CMB. I am talking about recession, not local motion.

But we still see the suckers. We see hundreds of thousands of galaxies with redshift 2.4 and up. the fact that they are receding at such a big rate doesn't prevent this.

Last edited: Jul 30, 2008
17. Jul 31, 2008

### Garth

That is not a joke, it is one way of describing a conformal gravity theory such as Fred Hoyle's mass field theory, "On the Origin of the Microwave Background", Ap.J. 196:661-670 1975 March 15, in which the masses of fundamental particles varied from event to event.

As particle masses varied so would their size with the result that our interpretation of an expanding universe with fixed rigid rulers would be reinterpreted as a static universe with shrinking rulers.

Hoyle proposed this theory to resurrect some idea of his Steady State Theory in the light of the discovery of the CMB radiation.

In this theory the mass field went negative beyond a zero mass field surface. He postulated that as photons went from a -mass field to a +mass field region they were thermalised and thus became the microwave background, which would then be simply the light from galaxies beyond that zero-mass field surface.

A similar reinterpretation of the expanding universe is also found in the Jordan Conformal frame of http://en.wikipedia.org/wiki/Self-creation_cosmology [Broken].

Garth

Last edited by a moderator: May 3, 2017
18. Jul 31, 2008

### mysearch

Query on the timeline of similar threads
This thread appears to have been originally opened 07.11.06 and had an interesting exchange up to 07.25.06. Strangely, a ‘joke’ about the expansion of space was raised on 07.23.06 (#8), which then got a response on 07.03.08 (#12). This appears to be quite a delay between the punchline and audience laughter! Subsequently, another thread The physical meaning of expansion in cosmology opened on 07.24.08, which in parts seems to parallel the same issues. Was just curious!
As somebody relatively new to the details of cosmology, I am only trying to build a framework around the standard model built on accepted physics rather any suggestion of any ‘alternative’ theory. So, as a general statement, much of modern cosmology seems to be built on the assumption of relativity, especially GR, in the form of Friedmann’s equations. Now it is said that Friedmann’s solution is derived from Einstein’s field equations, but today much of this theory is shrouded in the complexity of Riemann geometry, differential geometry, conformal geometry etc, much of which I am assuming was not available to Friedmann. It is also highlighted that the basic form of Friedmann equation can still be derived from the assumption of the conservation of energy, although aspects of GR are said to question this basic axiom of classical physics:

[1] $$H^2 = \frac {8}{3}\pi G \rho + \frac{2 E_T}{mr^2}$$

The last term may look a bit unfamiliar because it is usually substituted as

[2] $$- \frac {kc^2}{a^2}$$

Which might suggest that:

[3] $$k = \frac{2E_T}{mc^2}$$

Of course, measurements to-date suggests that [k=0], at least, in approximation. As such, equation [1] would reduce to

[4] $$H^2 = \frac {8}{3}\pi G \rho$$

Now the value of [H] appears to be based on measurements of redshift, which are then linked to assumptions about luminosity of distant objects, from which it has been concluded that H=v/d, where [v] is the recessional velocity with distance [d]. Therefore, knowing the value of (G), we can calculate the critic density $$\rho_c$$. As understood, the standard model of cosmology assumes this density contains all forms of mass-energy, e.g. matter (4%), dark matter (23%) and dark energy (73%). However, only matter and dark matter can be linked to gravitational attraction, because dark energy corresponds to a ‘force’ linked with the observed expansion of the universe. As such, it appears that we have a model that describes expansion in terms of a balance between some unverified expansion source, e.g. dark energy, and gravity. This is simply a statement of my current understanding of the basic model, which is open to correction. So my first question is:

How does gravity slow H in this model?

What I referring to is the classical concept of a centre of gravity, which a homogeneous and isotropic universe is said not to have. If so, I am finding it difficult to resolve how the net effects of a gravitational slow down works within this model. I accept that I am no expert of GR either, but the concept of a model of a homogeneous universe, where the matter density is analogous to ‘dust’ suggests that much of the complexity of GR theory is confined to relatively small sections of the universe, where gravitational potential is higher, e.g. galaxies.

However, in contrast to all this apparent definite talk about expansion, the thread The physical meaning of expansion in cosmology appears to highlight some level of both philosophical and technical doubt about the reality of any expansion. However, referencing the 1st post in the current thread, the premise seems to assume expansion of the universe and everything in it. Whereas the standard model only seems to assume a relative ‘expansion’ of the universe. As such nuclei don’t expand, atoms don’t expand, neither do solar system or entire galaxies, only the large-scale space between galaxies. If so:

Can we say there must be, at least, some relative expansion of the universe with respect to smaller objects not apparently subject to any net expansion; otherwise the standard model itself would be inconsistent?

Does this imply there is a threshold where expansion, if it exists, overcomes the internal forces that hold any given structure in place?

I recognise that some of these questions may seem naive to the experts, but if so, they will hopefully not have to resort to tensor notation and relatively obscure ideas about 4-D manifolds to outline how the basic model works, at least, in general principle. Thanks

Last edited: Jul 31, 2008
19. Jul 31, 2008

### Garth

As Parlyne has already said, the FRW equation can be derived without GR, simply from the condition of maximally symmetry space, i.e. from the Cosmological Principle of a homogeneous and isotropic universe.

$$ds^2 = - dt^2 + a^2(t) \left (\frac{dr^2}{1-kr^2} + r^2 (d\theta^2 + \sin^2\theta d\phi^2) \right )$$,

The condition of maximally symmetry space is then applied to the GR field equation to obtain its cosmological solution that determines a(t). The specific solution of a(t) and k depends on the content, matter (dark and otherwise), radiation and DE that you put into the field equation.

1. Gravity does not "slow H", the gravitational field of positive mass and energy within the universe decelerates its expansion, just as gravity slows a rising rocket, which may or may not escape the Earth's gravitational field depending on its initial velocity.

H is determined by a(t) and its time derivative and those are determined by the cosmological gravitational field. If the positive mass and energy content of the universe were to be increased then the value of H would increase, perhaps this is what you are thinking of....

2. There is a real increase of the distances between distant galaxies as measured by a ruler constructed of atoms, a steel rule. All measurements are relative to the standard by which they are being compared....

3. The standard model is consistent with the principles upon which it is based.

4. In the standard model cosmological expansion does not apply on 'local' structures which are gravitationally bound.

Garth

Last edited: Jul 31, 2008
20. Jul 31, 2008

### mysearch

Response to #19

Garth: Many thank for the helpful and concise summary. By way of response, I was trying to rationalise a basic model in which the FRW metric, as defined by Parlyne (#11), reduces to the form:

$$ds^2 = - dt^2 + a^2(t) dr^2$$

This form only looks at the equatorial expansion along a radial path and assumes k=0. As also pointed out by Parlyne (#11),

To which you have added the qualification:

In other words, the expansion may have varied in time depending on the makeup of the energy density $$[\rho]$$. While I am taking a deliberately simplistic approach, as far as I can see my basic model doesn’t violate anything being implied above or current measurements. However, I am less clear about the following statement:

Again, from a basic approach, classical physics only defines 4 fundamental forces, 3 of which are effectively neutralised at the atomic level leaving only gravity to operate on the cosmic level. I would define ‘pressure’ as an aggregate ‘force’ caused by individual kinetic collisions between components within the makeup of the energy density being considered.

Now if pressure is an expansive ‘force’ what is slowing down the expansion?

I know GR prefers not to describe gravity as a force, but rather a geodesic path or a gravitational field gradient. However, I assume that these two concepts can be transposed and a 'force' can be used to described a geodesic path.

So where is this path? Is the implication that each unit volume of space has a gravitational curvature or field that slows the expansion due to dark energy pressure?

As I pointed out, I don’t understand how the concept of a net gravitational field within a homogenous universe can be considered without some form of centre of gravity. Therefore I would appreciate any further clarification of the mechanism that is used to explain the slow down of the expansion within the standard large-scale homogeneous universe model.