Is the universe truly isotropic?

Question

I am a second year undergrad student, so forgive me if this question is a waste of time. In my cosmology class, we've been discussing the Cosmological Principle and the fact that the universe is homogeneous and isotropic on large scales. Immediately, this seems to me to be nothing more than an assumption made for mathematical ease. That is not the point, though. I found out about Hemispherical Power Asymmetry from a Sabine Hossenfelder video and skimmed through the paper that was linked in the description (https://arxiv.org/abs/2411.15786). Although the results of this paper are not necessarily statistically significant, what would this mean for (an)isotropy of the universe if it were?

Edit (important): I apologize because my original question was unclear. Suppose the evidence of statistical anisotropy is significant, what are possible explanations? Is it possible that there is an explanation for this that doesn't violate the CP (in the same way the CMB dipole has an explanation that fits the CP)? If the CP ought to be thrown out, what leading theories might there be?

Thank you for all the replies so far. I understand this is an open ended question, but I want to get as many ideas as I can about this.

Answer 1

I am having trouble understanding if you're asking if cosmological models can be anisotropic, which they can be and have been investigated for quite a while. When people say "an FLRW model is isotropic and homogeneous" it is sort of a cosmological model for early times when anisotropies were low. A little more concretely, one expects that there is a gravitational entropy estimator (like the Weyl estimator $W=W_{abcd}W^{abcd}$ or other Weyl invariants) which become non-zero and trend like $\delta W\geq 0$. There's of course issues with purely the Weyl invariant based gravitational entropy, so you use something like the Clifton-Ellis-Tavakol proposal which works well for type D and N spacetimes. See https://arxiv.org/abs/1303.5612 or other stuff. Further, when cosmologists say "we have issues with the cosmological principle", it is in a wildly different sense than what general relativists say it. Isotropy is not a condition at all for a universe to be physically plausible. And I would take Sabine's words with a grain of salt.