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Mordred

Cosmological Principle

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Posted (edited)
On 5/30/2015 at 7:05 AM, Mordred said:

The Cosmological principle

 

is defined as "at sufficiently large scales, the universe appears as homogeneous and isotropic."

 

Homogenous

 

is oft defined as " no preferred location"

 

Isotropic

 

is oft defined as "no preferred direction"

I see no significant mistake in the enunciation of the principle. I wouldn't include time to it though, nor do I know of any formulation that does.

Another hopefully useful observation is that isotropy everywhere implies homogeneity, which is kind of more economic to me, but not really a big deal.

As to current limits to its application/validity/solidity, I hope you find interesting my comments below:

The whole issue of the universe being homogeneous and isotropic at 'large' scales is, in my opinion, a very suspect hypothesis. It looks kind of reasonable, though, and allows you to gain access to the big picture of what goes on. But 1) from the theoretical perspective we do know that quantum field theory (QFT), when combined with the general theory of relativity (GTR) in inflationary models, predicts a universe that is more like a fractal, meaning a scale-independent series of embedded structures that may look clustering depending on what scale you look at it. And 2) from the observational point of view, the universe does seem to display huge voids in its structure, very strongly resembling that fractal that QFT+GTR predicts. It's more like the caustics in a swimming pool in 3D (this is a numerical simulation):

Eulerian_sigma=3_a=1.png

About isotropy, a very recent piece of news from the experimental front is this:

https://phys.org/news/2020-04-laws-nature-downright-weird-constant.html?fbclid=IwAR3_NdXDNfcNU05E8khtN1pnshucr-gr7KoJO5OTh6OAuDDX19Z5yUBPD_c

The headline reads, "New findings suggest laws of nature 'downright weird,' not as constant as previously thought". 

UNSW --Sidney-- professor John Webb: "We found a hint that that number of the fine structure constant was different in certain regions of the universe. Not just as a function of time, but actually also in direction in the universe, which is really quite odd if it's correct... but that's what we found."

If that's true, not only the universe wouldn't be homogeneous; it wouldn't be isotropic either, and at the deepest level, because what's different is the electromagnetic coupling constant itself. Now this would really be amazing and we should take it with a grain of salt.

The statement that the universe is homogeneous in time is tantamount to saying that it looked pretty much the same in the past than now or in the future.

It was obviously not the same in the past, as it looked like a singularity, then opaque to radiation and neutrinos (plasma), then radiation dominated, then matter dominated, and today it's considered to be dark-energy dominated.

So it doesn't really look like it's going to be the same in the future, as it will exponentially expand.

 

Edited by joigus

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