# Cosmic time and a cosmic location.

1. Jul 11, 2009

### keepitmoving

doesnt the fact that we are moving through space resulting in a red shift and a blue shift of the CMB prove that there is such a thing as cosmic time and a cosmic location.

2. Jul 11, 2009

### Rymer

Re: cmb

From our perspective - yes. But who is to say that is the only prospective.

3. Jul 11, 2009

### Staff: Mentor

Re: cmb

You can use it as a reference frame, sure, but why is it any more scientifically valid of a reference frame than my living room?

Last edited: Jul 11, 2009
4. Jul 11, 2009

### keepitmoving

Re: cmb

not being argumentative here but would using a cosmic time eliminate some of the paradoxes in SR? I suppose it would be hard to keep track of cosmic time and location, after all theres not great GPS in the sky, is there.

5. Jul 12, 2009

### Chalnoth

Re: cmb

There are no paradoxes in special relativity.

6. Jul 12, 2009

### Staff: Mentor

Re: cmb

Similar to my previous response: I'm considering putting a beacon in my living room that sends out a time coded signal for use as a reference frame for anyone who can receive it. Can you tell me why it would have any less scientific validity than your proposed "cosmic time"?

7. Jul 12, 2009

### Rymer

Re: cmb

Do you live in outer-space with no gravity?

8. Jul 12, 2009

### Staff: Mentor

Re: cmb

No. How does that answer my question? One need not be in outer space, with no gravity (no such thing anyway) for such a beacon to work/be useful.

9. Jul 12, 2009

### Rymer

Re: cmb

True -- but it would be a better beacon if its properties were known. The less gravity the better.

10. Jul 12, 2009

### marcus

Re: cmb

I didn't understand what you meant by "and a cosmic location" so I'll focus on your question about cosmic time. Cosmic time is very useful and important in cosmology. The Hubble Law depends on cosmic time for its statement and meaning. The basic differential equation model of the universe which all cosmologists use depends on cosmic time. It tells how the scale factor a(t) increases as a function of cosmic time.

So cosmic time is a practical reality we have to acknowledge, and it can be defined using among other things the CMB, and (as you say) the idea of an observer at rest relative to CM Background.

If I just take the general spirit of your question, and don't pick nits about it, then I have to say Yes. But the practical and constantly used idea of CMB rest does not by itself prove the validity of cosmic time, and it is not the only piece of supporting evidence. Obviously since we assume an approximately uniform universe (homogeneous isotropic) and believe General Relativity (at least until a quantum theory of gravity gets estabished) we therefore have the Friedman model and we already have cosmic time. We had that even before we observed the CMB.
The Friedman model dates back to 1922, was rather much confirmed by Hubble's observations in the 1930s, and the CMB was only first observed in the 1960s.
Before we had CMB-rest, we had the idea of being at rest with respect to the Hubble flow. You can do something analogous just by observing the recession rates in different directions (if it's symmetric then you are at rest.)
The existence of the CMB-rest criterion does not (entirely by itself) prove anything. You have to take it together with a bunch of other stuff that is conventional standard practice in cosmo.

Last edited: Jul 12, 2009
11. Jul 12, 2009

### Rymer

Re: cmb

Agree with the above.

Does this mean that the acceptance and use of this scale factor is a requirement to be a cosmologist?

Not trying to cause trouble -- trying to understand why this specific requirement and what it is supposed to mean.

The scale factor relation was introduced in the FRW metric of General Relativity. As far as I know it has no significance outside that. Is there more here that I need to know?

12. Jul 12, 2009

### marcus

Re: cmb

Rymer, I don't know about requirement. What I have noticed is just a statistical rule of thumb. They nearly all use the FRW metric. They nearly all use the Friedman equations. I don't think there is any rule that they HAVE to, they just do. Even the ones who are trying to critique the standard model use the standard as a point of departure.

My impression of scientists in any healthy branch of science (and cosmo is vital and flourishing right now) is that they are often skeptical and rebellious against their own conventions.
If someone doesnt understand the standard model and the conventions then they brand him as a crackpot and ostracize him---boot him out and slam the door. But they are always on the lookout for some observation that will challenge the standard model. They are self-critical of their own discipline.
So you can make a career (David Wiltshire has, I believe) and a name for yourself by challenging assumptions like Homogeneity and Isotropy, and, in effect challenging the Friedman model, and that would get rid of a(t) because it would no longer be welldefined.
And you can propose alternative mathematical models to try to explain the supernova data and the apparent acceleration based on INhomogeneity. People do this.
Im not sure but I think Wiltshire is a prominent example and IIRC there are others.
Revolutionaries are good. Yes they are allowed. But there is a fine line---the pig-headed ones who don't understand the common point of departure are branded crax and out the door.

I cant give you a onesentence summary. Hope this discursive woolgathering is helpful.

13. Jul 12, 2009

### Rymer

Re: cmb

Yes, it is. I suspect it is as much a long established habit as anything. Understandable I guess, things can get that way in 35 years.

Just for the record, I see nothing wrong with General Relativity as a way to mathematically model a given situation. Its just that that is all GR is -- a math model. It does not in itself provide much insight (at least not to me) on exactly how to use it. Along with this general acceptance as the 'gold standard' there seems to now be a 'gold standard' on the acceptable metric -- an isotropic one as you mention.

I will simply note in passing -- not to harp on -- that spherical symmetry (as used in FRW) does not give the lowest energy solutions. Lower ones are found with cylindrical symmetry. This is important because that indicates a higher entropy -- and therefore favored by thermodynamics.
This may or may not be of any importance in a given situation since most astrophysical ones are not in thermodynamic equilibrium.