Cosmologists assume that the universe is homogeneous on large scales and, in particular, that the unobservable parts are similar to the observable universe. As I understand it, one important reason for this assumption is simply something along the lines of “hey, it would be really strange and surprising if our corner of the universe turned out to be special.” While this does seem very reasonable to me, I was wondering what other reasons (if any) we have for the assumption. Is it, for instance, predicted by the big bang model?


In the standard big bang model, homogeneity is an assumption - therefore it cannot predict homogeneity. Homogeneity is not a requirement of the model and indeed you can build "inhomogeneous big bang models" - though they must still be consistent with the observational evidence of a high degree of homogeneity on large scales within the observable universe.

The obvious observational evidence for homogeneity on large scales (and of course, in the observable universe) is the smoothness of the cosmic microwave background (CMB). The CMB is very, very uniform - with perturbations (bar the large-scale dipole) at the level of 1 part in $10^5$.

We can say very little about the unobservable universe, since it is... unobservable. And because it is unobservable, it has no influence on us and cannot affect the utility of the models we use to explain the observable universe.

NB: the big bang is just a model of how the universe works, with adjustable parameters including dark matter, vacuum energy density and homogeneity, not a theory.

  • $\begingroup$ I'm sorry, but I need some elaboration. The question is why we are justified in extrapolating uniformity from the observable to the unobservable. Your answer seems to be "because the uniformity is (or was) strong." If I can only see a small part of a wall, and it is uniformly green, and I wonder whether the entire wall is green, it doesn't seem to help that the small part is very uniformly green. This must be a bad analogy, somehow, but I don't understand why. $\endgroup$ – Casper Feb 5 at 14:57
  • $\begingroup$ I also don't understand why the distinction between "baked in" and "predicts" is relevant. In the sense of "predict" I intended, I don't think it makes a difference whether uniformity is a basic assumption of the big bang theory or is derived from basic assumptions. If uniformity is implied by the theory, and we have empirical evidence for it, isn't the evidence also evidence for uniformity? $\endgroup$ – Casper Feb 5 at 15:01
  • $\begingroup$ You are asking why we assume that there are no pink elephants $\endgroup$ – planetmaker Feb 5 at 16:45
  • $\begingroup$ @planetmaker … which is an excellent question to ask, and may get one started on studying the problem of induction. $\endgroup$ – Casper Feb 6 at 4:30
  • $\begingroup$ @ProfRob Thank you for expanding your answer! $\endgroup$ – Casper Feb 6 at 4:31

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