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the observable Universe [Q&A]


Supriya of Poornaprajna College asks, what does the Universe look like beyond the limits to our visible Universe?

It is a good point to make that the Universe is actually bigger than what we can see. According to the standard cosmological model, in the early stages of the Big Bang, the Universe underwent a rapid exponential inflationary epoch, when space inflated by a factor of 1078 in less than 10-32 seconds. It has continued to expand since then, but at a much slower pace. What this inflation did was to radically shrink the horizon of any point in the Universe, since it put vast regions beyond light travel time distances from each other. What we can see now is just those things whose light can reach us. This is called the Comoving volume, and it is now a sphere of about 45 billion light years in radius, and it seems that it can at most be about 68 billion light years. There is a wonderful paper by Gott et al. (2005, Astrophysical Journal, v.624, p.463) that lays out a detailed map of the Universe all the way from the Earth to the unreachable limit (at about 10 billion light years, this is the limit to which light emitted at Earth can get to before the accelerating expansion puts points beyond out of reach), and beyond that to the visibility limit.

What could it be like beyond that? In truth, we don’t know. We can’t know, because we have no information from those regions. But what we can see at the outer reaches of our telescopes suggests that physics still works. As far as we can tell, the Universe there looks pretty much what it looks like here, with differences in degree and detail, and not in anything fundamental.