Yes. Take the age of the universe, multiple by the rate of expansion to get the total size of the universe, then multiple by the average density of galaxies in the observable universe. There are some further complications, but at the root it is basic algebra.
Replying to the other replies here - this regards the observable universe. Speed of light limits and all that. Of course we have no reason to believe the universe just stops at the point where we happen to lack the ability to observe.
Well, no. The density in the observed universe is used to extrapolate the number of galaxies in the non-observed universe. The size of that universe is extrapolated from the rate of expansion and the time since the big bang.
The size and shape of the observable universe also changes. A moving observer, say someone doing 30% of lightspeed, will see further in one direction than another. Accelerate quickly enough and the "dark" side of your custom observable universe might catch up with you, causing all sorts of havoc.
You’re assuming that space was compressed into a single point at the Big Bang. However, this is not implied by the Big Bang or cosmology. All we can truly infer is that the universe was very hot and dense and that spacetime experienced rapid expansion. We do not know the size, extent, or shape of space at that time, and we don’t even know how much matter and energy were present. We only have a notion of the density.
We may not know the exact size at the start, but we know it was infinitesimally smaller than it is today. So the size of the initial universe isn't a big factor in the equations about how big it likely is today. Weather it started as a few centimeters across or a few thousand light years across, both are functionally zero compared to the current size.
Yeah this is a difficult concept, and I think the way the big bang is commonly portrayed in media often leads to this misconception of the big bang as starting at a point in space rather than a density.
I uncovered this for myself when asking, "where is that point now?" and discovering it was never a point at all, space is expanding from all points simultaneously.
> The density in the observed universe is used to extrapolate the number of galaxies in the non-observed universe.
The unobserved universe is likely to be many orders of magnitude larger than the observed universe. It is possible that it is unimaginably larger.
Technically, it is possible that the unobserved universe is infinite, however whether that is a credible option depends on individual scientists informed intuitions. We simply have no experimental or theoretical evidence either way at this point.
So there is no estimate of how many galaxies there are in the universe in toto.
> The density in the observed universe is used to extrapolate the number of galaxies in the non-observed universe.
As has already been pointed out, our best current model of our universe is that it is spatially infinite. That means an infinite number of galaxies.
The finite galaxy numbers that astronomers give are for the observable universe.
> The size and shape of the observable universe also changes.
Not the way you are describing, no. The observable universe does increase in size as time goes on, because there is more time for light to travel so the light we see can come from objects further distant. Its shape, however, does not change.
A good reference is Davis & Lineweaver's 2003 paper:
> A moving observer, say someone doing 30% of lightspeed, will see further in one direction than another.
I don't know where you're getting this from. What part of the universe you can observe from a given point does not depend on your state of motion.
> Accelerate quickly enough and the "dark" side of your custom observable universe might catch up with you, causing all sorts of havoc.
This is nonsense. The Unruh effect is (a) nothing like what you are describing, and (b) irrelevant to this discussion anyway, since the Unruh effect only applies to objects which have nonzero proper acceleration, which is not the case for any galaxies, stars, or planets in the universe.
Is that really the way to see it? As I understand it, the Big Bang didn't happen in "one place". The Universe is expanding from an compressed state - the Big Bang state. But there is no center point. We can only see that there's expansion but it's not from a single point. The only known "center point" is us. And the only reason it's a center point is because we can only see as far away as light has traveled since the Big Bang.
This theory of multiple points supports the big ring and other structures outside the “this shouldn’t exist” bubble. The bubble is the Big Bang + rate of expansion. It was thought that there was nothing outside of the farthest point… but there is!
>Take the age of the universe, multiple by the rate of expansion to get the total size of the universe, then multiple by the average density of galaxies in the observable universe
My understanding is that, at the largest scales, clusters of galaxies are organized along a series of gravitationally bound filaments, sometimes called the cosmic web.
So they aren't distributed like random noise, but more like a web. I have no reason to think this changes anything about calculating average densities, but it is notable that there's the general density but probably a significantly different density within that structure.
So if the universe has a size then what do you see if you are on the edge of it? Do you see stars to the left and nothing to the right? I mean given the speed of light and the age of the universe and the rate of expansion there are regions inaccessible to us but that doesn't quite mean the universe has a finite size.
The observable universe has a size, the cosmic microwave background is what we 'see' at the 'edge' in terms of photons (~400k years after the big bang). We could see further if we could map out the gravitational wave or neutrino backgrounds (1 sec after the big bang).
But for now we can't really say if the universe in its entirety has a finite size.
For the gravitational wave background, maybe with LISA we might be able to get a glimpse, but the neutrino background seems like it'd take some truly unprecedented breakthroughs in our ability to detect neutrinos to have any chance of mapping it out.
Finite size doesn’t require an edge. Consider the surface of a balloon for a 2-D case (or the perimeter of a sphere, for a 1-D case): it has finite extent, but no edge.
It has a surface, though, which is what PP was asking about.An answer to the question is, yes, nesr the edge/face, one side is dark. But relativity and expansion makes the situation a bit more complicated.
> Isn't the rate of the expansion of the universe increasing?
It is now, but up until a few billion years ago, it wasn't, it was decreasing. Many of the objects we currently see are far enough away that the light we are now seeing from them was emitted while the universe's expansion was still decelerating.
> that assumes the observable universe is homogeneous, which it isn't
No, the models cosmologists use do not assume the universe is homogeneous period. They only assume it is homogeneous on average, on large distance scales (roughly scales larger than the size of the largest galaxy clusters).
Yes. I find it amusing I sparked a debate on what might be beyond the observable universe, when my point was entirely about what you could theoretically observe in the night sky.