[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...

[itishappy]

It's a vector field! It has 2 dimensional inputs and 2 dimensional outputs, so it doesn't fit on your traditional graph.

    f(x,y) = (c * x, c * y)
    f(x,y) = c * (x,y)
    f(P) = c * P

If you give some thought to what `c` is doing to each point of your plane (start with the origin!), I bet that graph might make a bit more sense. :)

[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.