# Thread: What phenomenon promote/impulse the isotropic expansion of the Universe?

1. Originally Posted by Jeff Root
I see that the density *can* be greater at the center of a gas-like expansion, but nothing requires it to be greater there. ...
Show me any way in which in less than infinite time gas expanding from a finite container into infinite space will not always be densest at the release point?

2. Originally Posted by antoniseb
Show me any way in which in less than infinite time gas expanding from a finite container into infinite space will not always be densest at the release point?
Then we can state the following differences:

- That Isotropic means that the same observational evidence is available by looking in any direction by an observer...and this is not possible in a gas expanding.
- The high speed of isotropic expanding of the Universe (faster than light speed) is not possible for gas expanding.
- In the gas expanding...there will be a concentration of matter at the release point, and the density as seen by the fastest particles will be highly different looking out vs. looking in....while in the isotropic expanding there will be not concentration of matter in any place.....and it will be homogeneous looking out vs. looking in.

Does the isotropic expanding of the Universe an unic phenomena?....or there are other isotropic expanding cases ??

3. Originally Posted by dapifo
But I donīt see that homogeneity and isotropically will be excluding or contradictory concepts.

Something colud be isotropically but also homogeneity,....and also vice versa.
Isotropic - look up, look down, look left, look right, every direction looks the same.
Homogeneous - looking in one direction only, come here, go there, go anywhere it all looks the same in that direction.

Isotropic and homogeneous - look in any direction from any place and it will look the same.

So your expanding ball of gas is isotropic from one location only, at the centre of expansion. (As Shaula mentioned several posts ago). It is not homogeneous.
The universe on the other hand is both.

Homogeneity and Isotropy are a consequence of the Cosmological Principle, which is the base assumption of all science. We assume that we are not in a special place or time and that all laws hold the same everywhere. Therefore we assume we are not at the centre of expansion, which is the centre of the universe. These assumptions have been tested and hold up well at cosmological scales.

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Originally Posted by loglo
Homogeneity and Isotropy are a consequence of the
Cosmological Principle, which is the base
assumption of all science.
The cosmological principle is derived from the observed
apparent overall approximate homogeneity and isotropy
of the Universe. It applies only to cosmology, not all of
science. It is a useful assumption in cosmology, but is
a guiding principle rather than a base assumption.

Overall approximate homogeneity and isotropy of the
Universe are a consequence of how the Universe formed,
not a consequence of a principle.

If there is a base assumption of all science, it may be
that attempting to understand nature is a good thing.

-- Jeff, in Minneapolis

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I think loglo phrased it quite well. The Cosmological Principle, suitably modified by the Anthropic Principle (something along the lines of "the conditions and relations we observe here and now are representative of those that apply everywhere and everywhen, except in as far as they favour the emergence of intelligent life, which seems to be a rarity and thus introduces an observer bias", perhaps) is as close to an axiom as natural science gets. Without that axiom, one comes up with epicycles and brains-in-jars and young-earth creationism and all sorts of other models which may (or may not) produce metaphysically appealing explanations but don't have any predictive power whatsoever.

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loglo's statement was backward, the opposite of how
it actually is. He said "Homogeneity and Isotropy are a
consequence of the Cosmological Principle", but they are
not. The cosmological principle is a consequence of
homogeneity and isotropy. The Universe did not become
homogenous and isotropic because somebody decided to
assume that we are not in a special place or time.

If assumption of the cosmological principle were the only
thing that weeded out ideas like epicycles and the Earth
being only 6000 years old, then science would be in a very
sorry state. Opposition to such ideas may be motivated
by assumption of the cosmological principle, or by belief
in the cosmological principle, but the cosmological principle
is no more essential to getting right answers in science
than is the assumption that heavenly bodies must travel
in perfect circles or the assumption that descriptions of
the passage of time in Genesis must be accurate and
complete. The cosmological principle is a useful guide,
like the weak anthropic principle or Ockham's razor, not
the base assumption of all science, and only applies
specifically to cosmology, not to "all science".

-- Jeff, in Minneapolis

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Originally Posted by Jeff Root
loglo's statement was backward, the opposite of how it actually is.
Ah, I see that this kind of shorthand is a pet peeve of yours. Yes, of course, the observed isotropy doesn't really result from any philosophical idea, and processes aren't really governed by conservation laws, and genes don't really have survival instincts. I don't really see the harm in putting things that way for the sake of brevity, since anybody who actually understands what one is talking about isn't going to be misled. But you may be right that this sort of forum requires a more careful use of language, I can't speak to that.

Originally Posted by Jeff Root
If assumption of the cosmological principle were the only thing that weeded out ideas like epicycles and the Earth being only 6000 years old, then science would be in a very sorry state.
That's how it is, though. "The universe is billions of years old" and "six thousand years ago, God created a universe that looks like it is billions of years old" and "the computer simulation into which my brain is plugged makes it look like there is a universe which is billions of years old" can't be distinguished by any observational means. You need philosophical axioms like the Cosmological Principle and Occam's Razor to choose between those. I wouldn't say that this translates into a sorry state of affairs: it's just something that may trouble the philosophically-minded for a bit when they first think about it, but that they'll just have to get over to then get on with "real stuff" like life and science.

Originally Posted by Jeff Root
The cosmological principle [...] only applies specifically to cosmology, not to "all science".
It depends on how you phrase it. "Assume that you are not a privileged observer" is an acceptable phrasing of the Principle, IMO, and applies way beyond cosmology.

8. Originally Posted by Jeff Root
loglo's statement was backward, the opposite of how
it actually is. He said "Homogeneity and Isotropy are a
consequence of the Cosmological Principle", but they are
not. The cosmological principle is a consequence of
homogeneity and isotropy. The Universe did not become
homogenous and isotropic because somebody decided to
assume that we are not in a special place or time.

If assumption of the cosmological principle were the only
thing that weeded out ideas like epicycles and the Earth
being only 6000 years old, then science would be in a very
sorry state. Opposition to such ideas may be motivated
by assumption of the cosmological principle, or by belief
in the cosmological principle, but the cosmological principle
is no more essential to getting right answers in science
than is the assumption that heavenly bodies must travel
in perfect circles or the assumption that descriptions of
the passage of time in Genesis must be accurate and
complete. The cosmological principle is a useful guide,
like the weak anthropic principle or Ockham's razor, not
the base assumption of all science, and only applies
specifically to cosmology, not to "all science".

-- Jeff, in Minneapolis
No, it is not backwards. The cosmological principle was generalised from the Copernican Principle and the properties of homogeneity and isotropy are the consequences.

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Originally Posted by loglo
Originally Posted by Jeff Root
loglo's statement was backward, the opposite of how
it actually is. He said "Homogeneity and Isotropy are a
consequence of the Cosmological Principle", but they are
not. The cosmological principle is a consequence of
homogeneity and isotropy. The Universe did not become
homogenous and isotropic because somebody decided to
assume that we are not in a special place or time.
No, it is not backwards.
It sure is.

Originally Posted by loglo
The cosmological principle was generalised from the
Copernican Principle and the properties of homogeneity
and isotropy are the consequences.
The first clause of that sentence is fine, the second is
just silly. Maybe you're dyslexic and don't notice that
your statement is backward. In that case, examine it
more carefully.

-- Jeff, in Minneapolis

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Originally Posted by onomatomanic
Originally Posted by Jeff Root
loglo's statement was backward, the opposite of how it
actually is.
Ah, I see that this kind of shorthand is a pet peeve of yours.
I don't believe that "A and B resulted from C" is shorthand
for "A and B resulted in C".

-- Jeff, in Minneapolis

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Originally Posted by Jeff Root
I don't believe that "A and B resulted from C" is shorthand for "A and B resulted in C".
In that case, your aversion seems very natural.

I think the shorthand in question arises or is at least related to our tendency to anthropomorphize: Human laws govern human behavior because humans choose to follow them (or not, in some cases), because it seems like the right/prudent thing to do. By analogy, scientists will often say things like "this entity acted this way in order to follow that physical law", when what they really mean is "this entity was subject to environmental conditions which made it behave this way, and we know that that physical law generally describes the effect any such conditions have on any such entity". Shorthand, as I said.

It is tentatively expected that the audience of the anthropomorphized statement will be familiar with this use of language and able to extract the long-hand version from it.

12. Originally Posted by Jeff Root
The first clause of that sentence is fine, the second is
just silly. Maybe you're dyslexic and don't notice that
your statement is backward. In that case, examine it
more carefully.
Isn't homogeneity, at least, a consequence of the cosmological principle?

Referring to loglo's earlier definition:
"Homogeneous - looking in one direction only, come here, go there, go anywhere it all looks the same in that direction"

We can only look at the universe from one place. We assume that looking at it from other places it would look the same; e.g. that alien astronomers a few billion light years away would see that everything in universe appears to be receding from their (not ours) galaxy with a Hubble-like relationship.

p.s. I don't think you know what dyslexia means.

13. Originally Posted by Jeff Root
Maybe you're dyslexic and don't notice that your statement is backward.
Jeff Root,

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Strange,

Dyslexia covers a rather broad range of difficulties in
interpreting written language. A typical problem for a
person with dyslexia is to confuse the letters 'b' and 'd'.

Dyslexia is also called "developmental reading disorder"

http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002379/

In the section titled "Symptoms", the first symptom listed is:

In general, symptoms of DRD may include:

• Difficulty determining the meaning (idea content) of a
simple sentence
Since people with dyslexia often reverse the order of letters
in a word or of words in a sentence, it seems likely that they
might also reverse the order of phrases in a sentence, which
is exactly what happened here.

Swift,

If loglo is dyslexic, then he should re-read what he wrote
to see how it is reversed. If that isn't the problem, then he
needs to explain how generalizing the Copernican principle
to the cosmological principle resulted in homogeneity and
isotropy of the Universe.

-- Jeff, in Minneapolis

15. Jeff Root,

If you take this thread any further off topic, if you mention the word "dyslexia" again in this thread, or if you discuss moderation in this thread again, you will be infracted strongly enough to be suspended. There will be no further warnings.

16. Originally Posted by Strange
Isn't homogeneity, at least, a consequence of the cosmological principle?Referring to loglo's earlier definition:
"Homogeneous - looking in one direction only, come here, go there, go anywhere it all looks the same in that direction"

We can only look at the universe from one place. We assume that looking at it from other places it would look the same; e.g. that alien astronomers a few billion light years away would see that everything in universe appears to be receding from their (not ours) galaxy with a Hubble-like relationship.

p.s. I don't think you know what dyslexia means.
My bold. No, I would say that homogeneity is a consequence of fundamental properties. As I think I understand it, the cosmological principle is a line of thought describing a characteristic of the cosmos. Lines of thought do not cause the physical properties they describe.

17. Originally Posted by Hornblower
My bold. No, I would say that homogeneity is a consequence of fundamental properties. As I think I understand it, the cosmological principle is a line of thought describing a characteristic of the cosmos. Lines of thought do not cause the physical properties they describe.
That sounds like Jeff's sort of pickiness. I'm certainly not suggesting that a principle is responsible for the structure of the universe...

Perhaps it would be better to say: is the fact we interpret our observations as a homogeneous universe a result of the cosmological principle? (I'm also not suggesting there is anything wrong with the principle or the interpretation.)

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Originally Posted by Strange
I'm certainly not suggesting that a principle is responsible for the structure of the universe...
If a principle is embedded in the physical laws that are responsible for the structure of the universe and we uncover that principle, we could say that the principle is responsible for the structure of the universe. (Not saying that this is so for the cosmological principle - it isn't embedded in the currently known laws of physics, but there are examples that fit such as the equivalence principle).

19. Semantic matters aside, it would seem that homogeneity is also closely related with symmetry. The precise description of the relationship between the properties (see edit) of a physical system and the laws of conservation are provided by Noether's first theorem.

Isometry of space gives rise to conservation of linear momentum, and isometry of time gives rise to the conservation of energy.

It could be argued that the Principle extends beyond the field of Cosmology, and is well and truly woven into the physical laws of nature

Regards
Last edited by Selfsim; 2012-Aug-28 at 10:03 PM. Reason: Symmetry properties, that is

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The cosmological principle does not rule out the possibility that gradients may exist in measures. We could see that the observable universe is more dense in one direction, without needing to assume some from of preferred location.

Such an observation would violate both homogeneity and isotropy. Thus the cosmological principle does not imply homogeneity and isotropy.

Other than a possible preferred axis for galaxy orientation, and a slight difference between the CMBR frame and observational motions of our velocity with the matter we see, both of which are a stretch, there is no evidence against homogeneity and isotropy. In fact, we have very tight bounds on the amount of violation an competing theory can allow.

This strongly favors a uniform dynamic over a radial explosive dynamic such as an expanding gas.

A gas expanding into a vacuum will have a density gradient increasing towards the center of expansion. This is not observed.

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