I Does the Cosmological Principle in any way underpin Big Bang Theory?

[PeterDonis]

what laws of physics are in question

The example I referred to violates the law of gravity.

The cosmological principle is based upon not a single law of physics that I am aware

The post of mine that you quoted was not about the cosmological principle. It was about the specific example proposed by the poster I responded to and the invalid claim he made based on it.

Nobody is claiming that the cosmological principle itself violates any law of physics.

The cosmological Principle AFAIK does not state that there is not a centre to the universe, only the implication that the universe should appear to be the same for an observer no matter where located.

You're contradicting yourself. "There is a center to the universe" means that the universe does not appear the same from everywhere.

if an actual centre does exist

Then the cosmological principle would be false. So your reasoning that follows, which assumes the cosmological principle is true, is obviously wrong.

 

[Jaime Rudas]

No, it's not any such example, because it contradicts the laws of physics: it assumes velocities are constant when they can't be because of gravity. An "example" that contradicts the laws of physics can't prove anything.

But if a distribution of dark energy is established such that it counteracts gravity, then it wouldn't be contradicting physical laws.

 

[PeterDonis]

if a distribution of dark energy is established such that it counteracts gravity, then it wouldn't be contradicting physical laws

There is no expanding solution with this property: the solution where dark energy just "counteracts" the gravity of the matter is the Einstein static universe, which does not expand.

Please do not hijack this thread any further with these claims. If you do you will receive a warning and a thread ban.

 

[vanhees71]

Yes, that's right, but what is in dispute isn't whether the cosmological principle holds. What is in dispute is whether or not the constancy of the velocity-distance relationship implies the cosmological principle, as seen here:

https://www.physicsforums.com/threa...underpin-big-bang-theory.1055179/post-6926218

The cosmological principle is a conjectured. The conceptual problem of cosmology is that you have to work with conjectures, derive predictions for observable effects and compare observations with these predictions, and one should be aware that all our observations are "local", i.e., we can observe only a tiny neighborhood of spacetime around the Earth.

Nevertheless through observing farther and farther away objects or rather the em. waves (and recently also the gravitational waves) we also "look to the past".

The cosmological principle is the statement that on a coarse-grained large-scale level the universe is homogeneeous and isotropic, leading to the Friedmann-Lemaitre-Robertson-Walker spacetime. The Einstein equations then tell us that the energy-momentum tensor of matter and radiation takes the form of an ideal-fluid energy-momentum tensor (+the cosmological-constant term, also dubbed "dark energy"). In standard "co-moving coordinates" with the coordinate time chosen as the proper time of an observer co-moving with this fluid,
$$\mathrm{d}s^2 =\mathrm{d} t^2 -a^2(t) \left [\frac{\mathrm{d} r^2}{1-K r^2} - r^2 (\mathrm{d} \vartheta^2 + \sin^2 \vartheta \mathrm{d} \varphi^2) \right],$$
the spatial coordinate system is chosen similar to spherical coordinates, and this of coarse "hides" the translation invariance of space, but that's only due to the coordinates. The chosen "origin" of these spherical coordinates can be an arbitrary point in this space, because no point is distinguished from any other. The same holds for the "polar axis" of the coordinate system, which can be chosen to point in any direction you like since space doesn't distinguish any direction in this spacetime model.

An observer in the origin at rest wrt. this reference frame (a "fundamental observer") sees the distance between him and a distant galaxy increasing with the scale factor $a$ and he also sees a red-shift of light emitted from a far distant galaxy. As should be clear, the interpretation of this Hubble-Lemaitre redshift as a "Doppler shift" is only approximately right for not too far-distant objects.

For a nice pedagogical paper, see

https://doi.org/10.1119/1.1446856