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):

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 May 3, 20205 yr by joigus

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Moderator Note

Hijack has been split to the appropriate thread

https://www.scienceforums.net/topic/72263-is-nature-playing-fair-with-krauss-object/

 

 

11 hours ago, swansont said:

!

Moderator Note

Hijack has been split to the appropriate thread

https://www.scienceforums.net/topic/72263-is-nature-playing-fair-with-krauss-object/

 

 

It was a continuation of post  #4 in this same thread but whatever you say.

Mr. Mordred ,

True , the Cosmological Principle has become ever more ensconced in mainstream physics as empirical evidence for it mounts . However , the anisotropic and inhomogenous effects engendered by the budding theory of Cosmological Coupling have yet to be quantified .

*Contributions to this knowledge base would be greatly appreciated .

^ Read "Cosmological Coupling" in the Section "Speculations" .

https://scienceforums.net/topic/136535-cosmological-coupling-split-from-dark-matterenergy-hubble-tension-solved/?&do=findComment&comment=1294783

Edited July 31Jul 31 by Professor-M
Referencing .

58 minutes ago, Professor-M said:

Mr. Mordred ,

True , the Cosmological Principle has become ever more ensconced in mainstream physics as empirical evidence for it mounts . However , the anisotropic and inhomogenous effects engendered by the budding theory of Cosmological Coupling have yet to be quantified .

*Contributions to this knowledge base would be greatly appreciated .

^ Read "Cosmological Coupling" in the Section "Speculations" .

https://scienceforums.net/topic/136535-cosmological-coupling-split-from-dark-matterenergy-hubble-tension-solved/?&do=findComment&comment=1294783

Moderator Note

Rule 2.5 says “Stay on topic. Posts should be relevant to the discussion at hand. This means that you shouldn't use scientific threads to advertise your own personal theory”

That means not linking to your thread in speculations

(Not to mention resurrecting a five-year-old thread)

I hear you , but...

Is not the question related to the subject in Mordred's thread ?

Cosmological-Coupling is actually a rising physics theory and much about it is unclear, but the discussions in the thread could definitely help define it for me .

Also , five years is like yesterday to me . I'll bet that many of the participants in the abovementioned thread would actually find contrasting it with C-C quite enervating .

Seriously now , these Threads are like a living library of interesting books . Does it not make sense to let the readers peruse ALL of the books , then dive into the ones that actually catch their interest ?

Personally , I'd bet big bucks that the books themselves would rather be utilized once in a while , rather than moldering away even though they still have much to offer .

*Newer is not always better , so how's about liberalizing the strictures here , and perhaps upping the enjoyment factor of the participants, and increasing their numbers ? 🤓

The reason for the isotropy and homogeneity of the Cosmological Principle is quite simple.
It is the only universe we can describe.

If it were anisotropic and non-homogenous ( as this notion of cosmological coupling suggests ) we wouldn't have a hope in hell of describing it.

Hallo MigL ! 🤓

I think that there would be "method to the madness" with this ; the characteristics you are describing would have averages and predictable patterns , even while appearing random and chaotic on the surface. This would be analogous to the overall manifestation of gravity within the Universe ; fairly constant when seen from the largest perspective , but seeming to be unpredictably intense when examined from many local ones .

The behavior of Dark-Energy is now under more scrutiny than ever before , primarily because of advancing technology in the field of astronomy . This particularly applies to the dynamic history of .D-E. throughout the history of the Universe .

*Reference Article below :

https://www.astronomy.com/science/dark-energy-may-be-changing-over-time/

Edited August 1Aug 1 by Professor-M

12 hours ago, MigL said:

The reason for the isotropy and homogeneity of the Cosmological Principle is quite simple.
It is the only universe we can describe.

If it were anisotropic and non-homogenous ( as this notion of cosmological coupling suggests ) we wouldn't have a hope in hell of describing it.

Agreed. It's perhaps worth nothing that whenever you have a "bulk" behaviour in physics that's complicated (or in fact non-solvable), a first-order approximation that manages to qualitatively describe things reasonably well is some kind of coarse graining that averages over any local features. One example is mean-field approximation in magnetism.

🤓 Universal-Gentlemen ,

I am attempting to sleuth out which of two paradigms applies here ; is Dark-Energy created by "super-compression" of mass-energy , as with black-holes , this implying that it then spreads out to the regions of lowest density in the universe . OR is D-E preferentially directly generated within low-density regions , or even generated throughout ALL of space , then slipping-out through the areas of high density on it's way to those of low-density ?

8 minutes ago, Professor-M said:

🤓 Universal-Gentlemen ,

I am attempting to sleuth out which of two paradigms applies here ; is Dark-Energy created by "super-compression" of mass-energy , as with black-holes , this implying that it then spreads out to the regions of lowest density in the universe . OR is D-E preferentially directly generated within low-density regions , or even generated throughout ALL of space , then slipping-out through the areas of high density on it's way to those of low-density ?

Why would it do any of those things? There is a good rationale at present to ascribe DE to the energy of the vacuum in QFT. The cosmological constant in Einstein's field equations is independent of the distribution of matter in the universe, and does not depend on time or place --thereby the name: cosmological constant. AKA dark energy.

🤓 Indeed Mr. Jolgus , that would place it in the category of "introduced throughout all of space"" , and begets the question of anisotropy examined in the link above .

Personally , I wonder if such an introduction would really be enough to account for the apparent black-hole mass overages referenced in the theory of Cosmological-Coupling .

Edited August 1Aug 1 by Professor-M

The word I'm having problems with is "indeed"... 🙂

18 hours ago, Professor-M said:

I hear you , but...

Is not the question related to the subject in Mordred's thread ?

Cosmological-Coupling is actually a rising physics theory and much about it is unclear, but the discussions in the thread could definitely help define it for me .

This thread is in a mainstream section. Yours was placed in speculations, because your version/characterization of the science is decidedly not mainstream. As I pointed out elsewhere, there didn’t seem to be much overlap between what you said and what a supporting link said. You might be better served asking questions

One must also note that C-C is a proposal, not accepted science.

16 hours ago, MigL said:

The reason for the isotropy and homogeneity of the Cosmological Principle is quite simple.
It is the only universe we can describe.

If it were anisotropic and non-homogenous ( as this notion of cosmological coupling suggests ) we wouldn't have a hope in hell of describing it.

I’m not sure that’s true. It would depend on what kind of patterns we observed.

36 minutes ago, swansont said:

I’m not sure that’s true. It would depend on what kind of patterns we observed.

Agreed.
But there is a possibly huge unobservable part of the universe.
If we don't assume that these vast unobservable parts are the same as our observable part, then we cannot say they follow the same Physical laws.

2 hours ago, swansont said:

I’m not sure that’s true. It would depend on what kind of patterns we observed.

That's a fair point.

If you just dropped a solar system somewhere in the universe at random (equal probability of dropping it anywhere), I suppose there would be instances (like, eg, if you do it close to the rim of a big cosmic void) where the mass, radiation, etc distribution would not look isotropic, nor would it look homogeneous. In fact, it would look highly "non-so".

In that sense, that vantage-point factor in it cannot be overlooked. We might have been fortunate (or unfortunate, as the case may be) that things look more or less uniform from here.

1 hour ago, joigus said:

That's a fair point.

If you just dropped a solar system somewhere in the universe at random (equal probability of dropping it anywhere), I suppose there would be instances (like, eg, if you do it close to the rim of a big cosmic void) where the mass, radiation, etc distribution would not look isotropic, nor would it look homogeneous. In fact, it would look highly "non-so".

In that sense, that vantage-point factor in it cannot be overlooked. We might have been fortunate (or unfortunate, as the case may be) that things look more or less uniform from here.

Of course, if we noticed certain kinds of anisotropy it might be because certain symmetries aren’t there and our laws of physics would be different. We assume isotropy and homogeneity because it’s reasonable to do so, based on what we know and observe. Which is the best we can do.

Put another way, different cosmic behavior might have implications on behavior we observe nearby, so we can at least test for that.