# How to probe the homogeneity of the universe ?

1. May 15, 2006

### brahma

We can probe the isotropy of the universe by observing the universe just from a single place. However, as I know, that is not true for homogeneity.

What type of expriments we can carry out on ground or spcae, for probing the homogeneity of the universe.

2. May 15, 2006

### Garth

If the universe is homogeneous then it will look isotropic.

However, if the universe looks isotropic then it does not necessarily mean it is homogeneous, so this is a good question as you rightly said we can only observe the universe from a single location.

If the universe is non-homogeneous it would look isotropic at only one, or a few, very special places within it. For example, the density could be distributed in concentric spherical shells but if so then it would only look isotropic from the very centre.

If we now apply the Coperican Principle, that we are not situated at a special place within the universe, then as the universe does looks isotropic, and back at the Surface of Last Scattering was so to one part in 105, then we can say it is homogeneous as well.

Note, we might violate the Coperican Principle with the Anthropic Principle if we want to and pose the hypothesis that only at the very centre of such a series of concentric shells is life possible. Then we are here at such a centre and observe an isotropic universe because we can be nowhere else, however, this would appear to be very contrived.

Garth

Last edited: May 15, 2006
3. May 15, 2006

### SpaceTiger

Staff Emeritus
Not necessarily. See here:

For example, a uniform vector field is homogeneous, but anisotropic. Isotropy about every point implies homogeneity, but you're right that isotropy about one point does not.

4. May 15, 2006

### brahma

Garth, you are absolutely right ! I know it is diificult, but still I want to know if there is any way to check homogeneity. Yes of course theoretical considerations like Compernical principle etc., are there... l

5. May 15, 2006

### Garth

Yes, that is an possible added complication. I was talking from within the standard Friedmann R-W assumptions of the equation of state being a perfect fluid with homogeneous pressure and density.

Your illustration in that link of the three forests was very instructive, thank you. Do you have a specific theory or example in mind that suggests a cosmological uniform vector field?

brahma the problem in testing homogeneity, which has been seriously suggested by others, is that in an isotropic universe curvature effects become convoluted with any isotropic (from Earth) inhomogenities, and you cannot separate the two.

For example, it has been seriously suggested that cosmic acceleration may be explained by our observable universe being located in a relative void in a much larger universe of overall greater density. The outer regions, which we see at high z and are therefore early, are also being attracted outwards isotropically from the Milky Way.

That does not explain why we should be in the centre of such a void of course, unless you invoke the Anthropic Principle....

Garth

6. May 15, 2006

### SpaceTiger

Staff Emeritus
Most theories that I'm familiar make an attempt to satisfy the cosmological principle, but some have considered using a uniform vector field (rather than a scalar field) to induce inflation:

Inflation driven by a vector field

7. May 15, 2006

### Garth

Yes, thank you, so the universe as it exits the Inflation epoch is anisotropic, but then this anisotropy is smoothed out and dies away either during the reheating period or later, if collisionless particles are created, during reheating.

Garth

Last edited: May 16, 2006
8. May 23, 2006

### Nereid

Staff Emeritus
Depending upon just what sort of 'inhomogeneity' you wish to probe, there are several observational programmes you could undertake.

For example, the 3D distribution of luminous baryonic matter - are there regions of the observable universe where galaxies, clusters, superclusters, walls, voids etc are distributed differently than in the rest of the observable universe? To some extent, the 2dF and SDSS surveys answer these questions, as does 2MASS (AFAIK, no inhomogeneity found).

The CMB can be used as another probe, both what we see from Earth, and the (past, distant) temperature we can infer from spectroscopic studies of distant gas clouds.

A different probe might be the 3D distribution of GRBs, assuming that one day we can measure their distance to within +/- 30% (say).

Finally, there is the in principle probe of the neutrino equivalent of the CRB. Sadly there appears to be no way we can even detect this, let alone use it to probe inhomogenities, without some major developments in particle physics.

9. May 24, 2006