An oft used estimate for the number of stars in the universe is 10[sup]22[/sup] (i.e. 10 billion trillion). This figure is derived by assuming there are about 10[sup]11[/sup] (i.e. 100 billion) stars in a typical galaxy, and around 10[sup]11[/sup] galaxies in the universe.
My question is whether this estimate and, in particular, the estimate for the number of galaxies, is referring to the ENTIRE universe or only to those parts of it which can be observed through the EM spectrum. In other words, as I understand it, subsequent to (hyper)inflation in the very early universe, some parts of it were “hurled” so far away as to be forever inaccessible to our telescopes, radio antennae, etc. Are such “forever inaccesible” galaxies included in the 10[sup]22[/sup] estimate or not?
I think it refers to the observable universe, meaning looking outward to the beginning of time. What limits the universe in this sense isn’t that it’s in the wrong part of the electromagnetic spectrum, it’s that what we see at “the edge” predates star formation.
The “entire universe”, without qualification, is believed to be infinite.
Any figure for the number of stars must be for the observable universe. So far as we know, the Universe as a whole is infinite, which would of course imply that there are an infinite number of stars in the Universe. Or, at least, if the Universe is finite, it’s sufficiently large that we have no way whatsoever of even estimating its size.
But I thought there were folks estimating the size by observing the expansion rate, or sumthin… and saying that there needs to be “x” amount of dark matter to get the effects we observe… (I bet I am misremembering something here.)
That’s estimating the density, not the size. Density is fairly straightforward to measure. Difficult, for the Universe as a whole, but straightforward.
Actually, I think density and size are related in the case of the universe. That is, understanding the way the expansion rate evolves depends in part on understanding whether the density is sufficient to overcome the expansion, or on the other hand if the density is insufficient and the expansion will slow forever but not stop or reverse.
If the density is high enough to reverse the expansion, the universe is also finite in size. However, if the expansion is fast enough that the density (which of course is also going down) is too low to reverse it, the universe is infinite in size.
I think the current understanding is that the universe is infinite but interestingly close to the balance point.
I hope I remember this right. I actually got a degree in physics and astronomy, but more than a quarter century ago. And maybe some of what I said is obsolete…
Well, half-obsolete. Your quarter-century old education probably neglected the cosmological constant, now known to be a very significant part of cosmology, and it almost certainly neglected the possibility of a nontrivial topology on the Universe. With a sufficiently large positive cosmological constant (such as we seem to have), it is possible for a universe with density greater than critical to expand forever, and with a sufficiently large negative cosmological constant, it’s possible for a universe with sub-critical density to recollapse. If the density is greater than critical, then the universe is positively curved, and guaranteed to be finite (and an upper bound on the size can be determined by measuring the curvature). If the density is critical (and hence space is flat) or subcritical (and hence negatively curved), though, there’s no way of knowing whether it’s finite, since even in a flat or negatively curved space, the “edges” could be identified, like in the world of many video games where going off the top of the map brings you back in on the bottom. It might be possible to measure the size of such a universe, if one happened to see features repeating within the observable portion, and indeed studies have been done looking for such features. Unfortunately, though, they didn’t find any, and so far as we can determine, the Universe is flat, so all we can say is that if it does repeat, it does so on scales larger than we can see.
