Is there any evidence against the Cosmological Principle?

I have been slowly getting more into math, but haven't gone into differential geometry or anything like that yet, so this question might be basic. Trying to get a deeper understanding of the Cosmological Principle, and why we assume it without question.

Physics/cosmology books make the assumption that the universe is isotropic, because (to summarize), "every observation we have made with telescopes show that the universe looks the same in all directions".

Because of that assumption, and the second assumption that the Earth/Sun aren't the center of the universe (the Copernican Principle), we make a final assumption that the universe must be homogeneous. Together creating the Cosmological Principle.

Has there been any evidence that these assumptions could be invalid? In terms of isotropy, do our observations that "the universe looks pretty much the same no matter where you look with a telescope" agree with a mathematical definition of isotropy? (Not sure what that, haven't dug much into algebraic/differential geometry yet). If it doesn't match exactly, what are some of the edge cases?

• There are some (inconclusive?) observations that put some doubt on the cosmological principle, some of which are given in the Criticisms section of the Wikipedia entry you linked. Jul 3, 2015 at 3:00
• Also, as written, I feel that this post reads more like troll-flame than an honest query (e.g., why we assume it without question and That seems vague for starters). You might want to consider toning down your criticism/disbelief in the cosmological principle. Jul 3, 2015 at 3:02
• And for more on the "homogeneity" aspect of cosmology, see this Physics.SE post. Jul 3, 2015 at 3:06
• In science you are welcome to present other assumptions that are equally good or better to explain ALL the observations. These hypotheses should also lead to testable differences from the cosmological principle, i.e. they have to be more than just a change of coordinates. Until someone does that, of course, the cosmological principle stays around. Just to be clear, I am not that certain that even the observable universe is completely homogeneous and isotropic (beyond the fluctuations) but lacking any data and suitable instrumentation that's not an actionable scientific idea for me. Jul 3, 2015 at 3:34
• Hi Lance. A good answer would in effect be a review article and I think that makes the question too broad. I found an interesting paper on the Arxiv that includes a literature survey. This would be a good start. A brief answer is that there is some tantalising evidence that the universe may not be as homogeneous and isotropic as we have assumed, but nothing concrete yet. Jul 3, 2015 at 6:06