# Is it possible to estimate the number of galaxies in the universe?

According to this article the observable universe may contain 2 trillion galaxies.

Assuming we know the large-scale curvature of the universe thanks to standard model of cosmology, can we estimate the number of galaxies in the entire universe based on the number of galaxies in the observable universe?

To put it another way, if you can only observe a small patch of an FLRW spacetime, but enough to estimate the scale factor, you should be able to estimate the total "volume" of the universe, right?

• Possibly migrate to Astronomy SE. – StephenG Sep 1 '17 at 3:47
• @StephenG: For the nth time: please stop requesting perfectly on-topic questions to be migrated to another site. This site is for physics, astronomy & astrophysics, so every question (even loosely) about astronomy need not be requested migration away to another site when it's on-topic at this site. – Kyle Kanos Sep 1 '17 at 12:06
• @kyle-kanos "For the n-time..." stop telling other users to not make perfectly reasonable suggestions. Just because the question can be on this forum does not mean it might not be even more appropriate on the Astronomy SE. – StephenG Sep 1 '17 at 12:19
• @StephenG: if it were a reasonably suggestion, sure. But it's an unreasonable suggestion because it's on topic here. At best, it's as appropriate to post on Astro.SE than here (rather than "more appropriate"), but since OP put it here (where it's on topic), it's unreasonable to suggest it be put to another site (even if it'd also be on topic there). – Kyle Kanos Sep 1 '17 at 12:35
• Kyle-kanos We clearly do not agree about this, so could you confine yourself to just making a simple comment to state you don't agree with my suggestion to migrate to Astronomy SE, rather than ordering me not to make the suggestion at all ? This is called agreeing to disagree and no one needs to here a debate on this subject (again). If you want Astronomy SE closed (and you seem to) then discuss that on the appropriate Meta pages and not here. – StephenG Sep 1 '17 at 12:39

Well, we know that universe may be infinite, and this is compatible with the standard model. If that is true, and if the cosmological principle is correct (so, for instance, the universe is not mostly empty with only the bit we can see containing galaxies), then the number of galaxies is not finite. So, no, you can't estimate the number: the best you can hope for is to estimate the number we can see.

• But he was asking about the number of galaxies in the observable universe. Which is most definitely finite. – Anders Sandberg Sep 1 '17 at 7:40
• @AndersSandberg The question states '... in the entire universe ...' so no, it was not asking about the number in the observable universe. – tfb Sep 1 '17 at 7:42

It is incredibly difficult to answer this. First and foremost, there are many factors that need to be assumed such as the life time of a galaxy, the number of stars in a galaxy (correlated to its mass), and what one considers a galaxy. The first thing one can do is look at the CMB and find the energy of the universe by tracing it back to the moment of the big bang. With that energy, you must assume a certain amount is matter and then with that amount, partition that mass into each galaxy. That would give an incredibly rough answer but scientists have not mapped the entire visible universe yet so we cannot yet count the galaxies.

• There is no well-defined notion of the total energy of the universe. See physics.stackexchange.com/q/2838 – Ben Crowell Sep 1 '17 at 3:41
• There's no need to count the galaxies in the whole (observable) Universe, since it's extremely close to being homogenous on scales much smaller than that. Also, the life time of a galaxy is not an important factor. But you mention one important factor, namely the importance of "what one considers a galaxy". – pela Sep 1 '17 at 6:46

Just to add a couple of comments: 1) The latest data from supernova type Ia measurements indicates that the total density parameter Ωt is slightly greater than 1, about 1.02, indicating a closed universe. This would be consistent with a finite and unbounded topology. 2) The notion that the universe began from a point singularity, the "Big Bang", 13.8 Gyrs ago, is now widely accepted. Thus it was obviously finite in extent at that time. In order for the universe to have evolved to a state of infinite volume requires expansion at infinite velocity, which I maintain is a logical impossibility. For v = dx/dt to be infinite, either dx must be infinite for any value of dt, or dt must be exactly zero and neither one, IMHO, has a reasonable basis in physical reality. Thus to evolve to an infinite universe would mandate a second "creation event" --- no matter how rapid an expansion may be, the resulting volume will ALWAYS be finite.

• The universe was not "obviously finite in extent at the time" of the Big Bang. The Big Bang is defined as the point where the scale factor $a(t)$ in the FRW metric formally becomes zero. Any finite volume on a time-slice after the Bang shrinks to a point if you run the tape backward, but it is certainly possible that the "total volume of space" was infinite both before and after the Bang. – tparker Sep 5 '17 at 18:16