Comments about "Cosmological principle" in Wikipedia

This document contains comments about the document "Cosmological principle" in Wikipedia

• The text in italics is copied from that url
  
• Immediate followed by some comments
  

In the last paragraph I explain my own opinion.

Contents

• 1. Implications
• 2. Justification
• 3. Criticism
• 4. See also
• 5. References

Reflection

• Reflection 1 - Cosmological Principle
• Reflection 2 -
• Reflection 3 -

Introduction

The article starts with the following sentence.

In modern physical cosmology, the cosmological principle is the notion that the distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throughout the universe, and should, therefore, produce no observable irregularities in the large scale structuring over the course of evolution of the matter field that was initially laid down by the Big Bang.

The first question to answer is: was and is the universe finite?
When the answer on this question is Yes then there is no reason to assume that all the forces acted uniformly throughout the universe and that the entire is homogeneous.
"when viewed on a large enough scale" the universe can easily be homogeneous but on a still larger scale this maybe not the case,

The cosmological principle contains three implicit qualifications and two testable consequences. The first implicit qualification is that "observers" means any observer at any location in the universe, not simply any human observer at any location on Earth: as Andrew Liddle puts it, "the cosmological principle [means that] the universe looks the same whoever and wherever you are."

The problem is we on earth can not validate or invalidate this statement. As such we cannot say that it is right or wrong. The most logical assumption is that this statement is wrong.

The third qualification, related to the second, is that variation in physical structures can be overlooked, provided this does not imperil the uniformity of conclusions drawn from observation: the sun is different from the Earth, our galaxy is different from a black hole, some galaxies advance toward rather than recede from us, and the universe has a "foamy" texture of galaxy clusters and voids, but none of these different structures appears to violate the basic laws of physics.

The fact that there are different physical structures like planets, stars, galaxies and black holes etc this fact alone says nothing if the universe as a whole is either homogeneous or heterogeneous.

The two testable structural consequences of the cosmological principle are homogeneity and isotropy. Homogeneity means that the same observational evidence is available to observers at different locations in the universe ("the part of the Universe which we can see is a fair sample").

The fact that we can only see a small part makes the whole concept not verifiable.
Next we read.

Isotropy means that the same observational evidence is available by looking in any direction in the universe ("the same physical laws apply throughout"). The principles are distinct but closely related, because a universe that appears isotropic from any two (for a spherical geometry, three) locations must also be homogeneous.

This so called observational evidence is no evidence that the universe is either homogeneous or isotropic. The fact that "the same physical laws apply throughout" also says nothing. For example: the assumption that Newton's Law applies throughout for the whole universe at present says nothing if the whole universe is homogeneous.

The cosmological principle is first clearly asserted in the Philosophić Naturalis Principia Mathematica (1687) of Isaac Newton.

clearly asserted I accept. clearly verfied ?.

That is, he asserted the equivalent material nature of all bodies within the solar system, the identical nature of the sun and distant stars ("the light of the fixed stars is of the same nature with the light of the sun, ... and lest the systems of the fixed stars should, by their gravity, fall on each other, [God] hath placed those systems at immense distances from one another"), and thus the uniform extension of the physical laws of motion to a great distance beyond the observational location of earth itself.

Correct. But that does not mean that the whole universe always was (in the past) and is (at present) homogeneous.

1. Implications

For example, the original discovery that far galaxies appeared to have higher spectral redshifts than near galaxies (an apparent violation of homogeneity) led to the discovery of Hubble flow, the metric expansion of space that occurs equally in all locations (restoring homogeneity).

The fact that we measure different redshift i.e. z values, is not a violation of homogeneity nor a restoration of homogeneity.
The fact that the "expansion occurs equally" has to be verified by observations.

Because this history is currently described (after the first fraction of a second after the origin) almost entirely in terms of known physical processes and particle physics, the cosmological principle is extended to assert the homogeneity of cosmological evolution across the anisotropy of time:

The evolution of the universe is mainly described as a cooling process in which the universe more or less synchronuous went to a sequences of phases. This sequence is more or less known but that does not mean that the whole universe at each moment was in the same phase.
It is also no prove that the entire universe is homogeneous.

2 Justification

Although the universe can seem inhomogeneous at smaller scales, it is statistically homogeneous on scales larger than 250 million light years. The cosmic microwave background is isotropic, that is to say that its intensity is about the same whichever direction we look at.

The size of the entire universe at of this moment is roughly 35 billion-lightyears. See Friedmann Lambda=0.01155 for details.
The fact the CMB radiation at this moment is isotropic does not imply that the universe at the time of emmision (380000 years after the BB) was homogeneous.

A related implication of the cosmological principle is that the largest discrete structures in the universe are in mechanical equilibrium.

The discussion should go the other way around.
First you should demonstrate that the local universe is in mechanical equilibrium (what it means), then global.
The problem ofcourse is how can you be in equilibrium and at the same time expanding.

3. Background

Karl Popper criticized the cosmological principle on the grounds that it makes "our lack of knowledge a principle of knowing something". He summarized his position as follows:

the “cosmological principles” were, I fear, dogmas that should not have been proposed.

You can ofcourse propose the cosmological principle, but is has to be demonstrated independently.
At the same time anyone proposing the fact that the universe at present is homogeneous should indicate how difficult this is to prove.

In November 2013, a new structure 10 billion light years wide has been discovered, the Hercules–Corona Borealis Great Wall, putting further doubt on the validity of the cosmological principle.

This structure demonstrates inhomogeneities existing 3.5 billion year after the BB. This is just outside the Hubble sphere. At that moment the structure was 6 billion-light years away from us and the size of the universe was 13 billion light-years.
The most important region to investigate is outside this structure.

4. See also

Following is a list with "Comments in Wikipedia" about related subjects

• Friedmann equations Comments

5. References

Reflection 1 - Cosmological Principle