[sandworm101]

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.

[causal]

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.

[sandworm101]

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.

https://en.wikipedia.org/wiki/Unruh_effect

[nilkn]

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.

[sandworm101]

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.

[irjustin]

> 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.

> 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.

Most things you're saying are correctly rooted except for what's beyond the observable universe. I'm not sure why the staunch belief that you can confidently claim this. To be clear, you aren't provably wrong - likewise not provably right either.

The replies to you are just fine, they represent a significant portion of the scientific community that says our universe is likely infinitely big and that, possibly, the big bang was infinitely small, yet still, still infinitely large. An infinite expanding into infinite still results not knowing what's out there.

PBS Space time talks about it in terms of "scale factor"[0] instead of absolute diameter.

Still, these are all debatable theories, so your take _could_ be valid, but generally, it points infinitely large.

[0] https://youtu.be/K8gV05nS7mc?t=271

[nilkn]

We don’t know that, though. Consider an evolution of a flat coordinate plane given by (x,y) -> (e^t * x, e^t * y). This process can run forever and has the property that all points appear to move away from all other points through time, yet the size of the plane never changes.

It’s better to think of the Big Bang as describing a point in time rather than a point in space.

[thaumasiotes]

> Consider an evolution of a flat coordinate plane given by (x,y) -> (e^t * x, e^t * y). This process can run forever and has the property that all points appear to move away from all other points through time, yet the size of the plane never changes.

What do you mean by that last claim? Any observable region is bigger at later times than it is at earlier times. The reason all points always appear to be moving away from all other points is that that is in fact happening.

What's the significance of claiming that the size of the infinite plane never changes? It's just as true that if you start with the unit interval [0, 1] and let it evolve under the transformation f(x) = tx, the size of the interval will never change -- every interval calculated at any point in time will be in perfect 1:1 correspondence with the original (except at t=0). But this doesn't mean that the measured length of the interval at different times isn't changing; it is.

[mynameishere]

Does anyone know why wolframalpha is plotting this with cute little arrows?

https://www.wolframalpha.com/input?i=plot+%28x%2Cy%29+%3D+%2...

[BearOso]

We know the observable universe was part of the big bang and is expanding, maybe even because we're observing it. We have no concept of whether that dense spot was all there was, and there are a whole slew of other caveats, so it could even be orders of magnitude larger.

Our current knowledge is functionally zero in the grand scheme of things.

[causal]

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.

[sandworm101]

The easy answer to the hard concept is that the big bang is not the increase in size of a thing. It is an increase in dimensions, including time. Our notions of size, of dimension, might not exist outside the bubble. We would therefore never perceive an edge, but that doesn't mean that one does not exist nor that there may be a finite size.

[Baeocystin]

I explain it to folks as if one was trying to go further south than the south pole. There's nothing physically impeding you; it's simply that once on the pole, all directions are north.

[regularfry]

Even that's not especially easy, because you then need to deal with "if the dimensions themselves are changing, why aren't protons the size of planets?"

[Nevermark]

> 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.

[pdonis]

> 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:

https://arxiv.org/abs/astro-ph/0310808

> 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.

[layer8]

As far as we know, the total universe may have infinite size, and thus contain infinitely many galaxies.

[an-honest-moose]

That doesn't necessarily follow - the universe can be infinite in size, but contain a finite amount of matter.

[pdonis]

Not the universe we observe, no. There is no valid model in GR that has this property and matches our observations of the universe as a whole. Models with a finite amount of matter surrounded by an infinite region of vacuum exist in GR, but they are not homogeneous and isotropic on large scales, while our observations indicate that our universe is.

[layer8]

True, I was keeping the reasoning about the average density. A homogeneous universe is still the null hypothesis.

[worldsayshi]

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.

[pdonis]

> As I understand it, the Big Bang didn't happen in "one place"...there is no center point.

That is correct. The only tenable answer to "where did the Big Bang take place" is "everywhere".

[reactordev]

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!

[reactordev]

What about the big ring [0]? Or other mega structures of galaxies outside that “bubble”?

[0] https://en.m.wikipedia.org/wiki/Big_Ring

[glenstein]

>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.

[injidup]

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.

[dotnet00]

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.

[mvkel]

Fascinating. Do you think it's possible that we can map these out in the next 50 years?

[dotnet00]

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.

[mvkel]

Funny, in reading up on both, I had higher hopes for the gravitational waves.

It seems like GWB is a superposition of infinite overlapping waves that would be impossible to single out and "unwind" in order to form a map.

And big bang neutrinos are very weak, which makes them undetectable. My assumption was we'd need a breakthrough in measurement sensitivity but is there more to it?

[datameta]

Naive thought - can a subsurface detector on the moon serve as an ultracold shielded scenario?

[dragonwriter]

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.

[lupire]

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.

[mvkel]

Isn't the rate of the expansion of the universe increasing?

And that assumes the observable universe is homogeneous, which it isn't

[pdonis]

> 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).