Cosmological principle.

Main Question or Discussion Point

How do we know that the universe is isotropic and homogeneous and why do we succumb to accepting that there is no special place in the universe that is the center? Since if everything is expanding from everywhere else at the same rate, there is still going to be a point center where everything came from. For example if we use the balloon analogy where you mark a point on the balloon and you watch it get further and further away from another marked point, there is still a center of the balloon where you first started pumping air into it.

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Because everywhere we look it looks the same. Large scale distribution of galaxies, isotropic redshift (implying uniform expansion), CMBR......

Furthermore, LCDM model, which is standard model of BB cosmology, and is based on the assumption of isotropy and homogeneity is highly successfull in matching predictions with observational results.

Balloon analogy relates to the surface of the balloon.

nicksauce
Homework Helper
For example if we use the balloon analogy where you mark a point on the balloon and you watch it get further and further away from another marked point, there is still a center of the balloon where you first started pumping air into it.
Yeah, but the universe doesn't need anything/anyone to blow it up.

BillSaltLake
Gold Member
The Cosmological Principle is assumed to be correct but it's only an assumption. Observations verify it to ~experimental error at least within the observable Universe.

One seldom-discussed feature pertaining to the Cosmological Principle has to do with the consequences if there were an "edge". Ignore inflation for the moment because it complicates things, but I think the results would be the same with or without inflation. Also ignore the effect of dark energy, so this treatment applies most accurately to an earlier era.

Suppose the Universe is spherical with average density equal to the critical density (a function only of time) out to some radius r in cosmological coordinates (non-expanding coordinates, at least during the matter-dominated era), where r is large enough that it is outside the current observable Universe. Assume the density is zero at > r.

If we solve the "heavy dust" problem for a sphere we find that the first time derivative of the Hubble constant (H') is isotropic regardless of the observer's location and is proportional to the average density at the time. This is consistent with observation of course.

Now suppose that the Universe is not spherical. For example, assume it is ellipsoid with two minor axes = r and one major axis = nr (n > 1). The heavy dust solution now gives an anisotropic H' which depends on location. At the center, H'minor/H'major ~ n. (More explicitly, the minor axis deceleration is n times the major axis deceleration.)

This suggests that if the Universe is finite it must be spherical (although we could be anywhere in the sphere).

However if we extend r to infinity, the "heavy dust" solution gives us a weird solution: that the Universe cannot slow itself down because of the finite speed of gravitation. (Maybe Inflation fixes this problem.)

skippy1729
The Cosmological Principle is assumed to be correct but it's only an assumption. Observations verify it to ~experimental error at least within the observable Universe.
Experimental error? What is the standard by which homogeneity is measured at a given scale?

"When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge of it is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced it to the stage of science." Sir William Thompson, Lord Kelvin

Skippy

well new evidence re the "dark flow" may suggest the universe is not as homogenous as previously thought. Hundreds or thousands (maybe more) of galaxies being dragged in one direction towards something in the unobservable universe should *not* be happening if the distribution of matter is even throughout the whole universe.

BillSaltLake
Gold Member
I put the "~" symbol before "experimental error" because the Universe appears at least grossly uniform from our vantage point. Carefully done experiments may find discrepancies however.