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Comment by yalogin | Hacker News Reader

I know and understand nothing about this topic but given the fact that the universe is infinite by definition makes the conclusive/definitive statement very surprising.

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The universe is not infinite by definition and anyway they are only talking about the visible universe.

I really dislike when they say "universe" and really mean "visible universe" or when they say infinite but don't really mean it.

I'm not really good at physics at this level so it throws me off. It makes it very difficult to really understand what they are talking about.

You would think physicists would be very precise with their language, by I guess they mostly write for people who are know what they are talking about.

The problem is that there's no generic way to describe the universe without contextualizing the characteristic you're interested in. But as a layman (not an astronomer, not a physicist, not a topologist), I'd argue that it's fair to say that the universe is infinite. I say that not only because of where the evidence regarding expansion and geometry of the universe points, but because even in a discussion of the visible universe you have to address the fact that the visible universe is shrinking as objects at the edge disappear due to expansion. Those objects don't cease to exist (at least not unless you make some highly contentious metaphysical arguments), which means there's no avoiding the inference that there can be (and likely are) an infinite number of objects which exist but which are not visible.

Objects becoming invisible does not infer infinite objects. When you observe shopping carts leaving the store do you assume there are infinite shopping carts outside of the store?

True, but my point was that you already have to contend with the reality of objects that exist but which are beyond the visible universe. Where are all those shopping carts going? You can't hide the ball there without eliciting even more questions than you answer.

Limiting discussion to the visible universe (to a "finite" universe by eliding messy details) can mislead by creating seeming contradictions. It's sort of like saying that evolution doesn't exist (as a first order approximation) because the lineage from ape to man is just too complex and doesn't really matter; let's simplify things by eliding that lineage so we have an easier time analogizing human morphology and genetics as it relates to practical questions. It can work superficially but even laymen will have a sense that things don't add up, not to mention that it doesn't help resolve the more important "big" questions often implicit in any discussion.

The fact that the universe is likely infinite stems from experimental results confirming topological characteristics that reflect infinite space. Fortunately, when you try to conceptualize phenomena like the Big Bang, a flat, open infinite universe actually makes things simpler, IMO.

I'm probably missing something, but my understanding of the "visible universe" is that the light from further out has yet to reach us, and this the visible portion should be constantly expanding (at the speed of light). Then, how would objects at the edge disappear unless the expansion is faster than light?

The expansion is FTL.

Objects don't outright disappear: As with other event horizons, they freeze in time and become redshifted as they approach the cosmological one (they also become fainter due to an increase in proper distance).

The cosmological event horizon is the light cone at future infinity and the asymptotic boundary of the observable universe: Light emitted within the horizon will take a finite time to reach us, whereas light emitted right at the horizon would take an infinte amount of time to arrive; in a way, light emitted beyond the horizon still moves towards us in the sense that the comoving distance decreases, but we'd have to wait a longer-than-infinte amount of time for it to arrive...

Because they are beginning to move faster than c, when these objects are at the very edge, due to the space between us and the edge expanding faster than c. This is called the cosmic horizon of the universe - sort of like an inside out event horizon of a black hole. A particle inside the horizon with a speed of c can still reach us (albeit very strongly redshifted), a particle outside the horizon would have to travel faster than c to reach us. The thing is, our own universe is still not old enough, that it can have a proper event horizon. It is estimeted, according to a model that assumes dark energy is a cosmological constant, that the universe must be at least 16GY old for a cosmic horizon to develop.

Oh, and by the way, an expansion faster than the speed of light is consistent with relativity. Special relativity only describes local laws of physics -- you and the edge are not "local". And general relativity doesn't have a constraint on a maximum velocity between 2 arbitrary points in spacetime.

Huh? Whether the universe is infinite or finite is an open question. See https://en.wikipedia.org/wiki/Shape_of_the_universe#Infinite...

I guess the phrase could be parsed two ways but in context it should be clear - the universe is not "infinite by definition". The definition doesn't mention size.

This is why we should all speak like Yoda. There is no ambiguity between:

By definition, infinite the universe is not.

And

Infinite by definition, the universe is not.

If the universe were "infinite by definition", it wouldn't be an open question.

Who is telling you that the universe is infinite? Astronomers/cosmologists generally all agree that our current universe (the matter-containing 4 dimensions bit we live in) had a defined beginning from which is has grown. There is an outer edge,. defined by the rate of expansion starting from the big bang.

Then what do physicists call what is beyond the edge of the universe?

edit: excuse my french ;)

It may mean beyond the observable universe.

The universe may or may not be infinite, for some particular definition of infinity. Our light cone, which essentially defines our universe from our point of view, is distinct and measurably finite.

I don't think my physician calls the edge of the universe anything unless he's a hobbyist physicist as well.

Imagine you are in inside of a giant beach ball, walking on the surface. You will never find an edge, and it is meaningless to talk about such a thing. Yet the space is finite.

Conversely, the surface of the beach ball has no "center". It's also fun to realize if you blow that beach ball up, from the point of view of any point on the ball, all other points grow further away.

A beach ball has no "edge", but it definitely has a boundary surface, and everything on the other side of this surface is "not beach ball".

Now what lies beyond the universe's boundary surface? Is such a surface even present?

That is indeed where the analogy starts to crumble. The reason is that space itself is curved. It is curved by the matter inside it. Still the property holds that if you travelled far enough fast enough you’d come back to where you started.

The Big Bang supposedly encompassed all of space and had no edges.

Has no edges, and therefore unbounded, but still finite in size.

How can it be both finite in size, and have no edges?

To me, it would seem that were it finite, there would be a point at which one would look back, and see the galaxy and clusters that compose the universe; forward would be an expanse of nothingness. But if this isn't the case, then how I can keep progressing forward (presumably forever, as I can't hit an edge) through space, encountering galaxy after galaxy, but it is still finite?

Unless this is like RPG games where the edges wrap.

How can it be both finite in size, and have no edges?

Good question, but find me the edge of an idealized balloon. Where is the edge of a sphere? As to why you won’t come back, remember that spacetime is expanding faster an faster, and you can only travel below light speed. Mind you that’s just one possibility. The universe at large could be a lot of different things, but as humans were causally disconnected from anything beyond the shrinking observable universe. Shrinking from our perspective at least, because of the aforementioned expansion and speed limit.

It is exactly the case that the edges wrap (presuming that I understand correctly, of course).

It is a running theory that space time could be curved and wrap, or be more traditionally flat, or even be some kind of saddle that means it is still curved but never meets itself again like a sphere does. Recent studies point to it being flat though, if I remember correctly.

The surface of the Earth is finite in size and has no edges. You can keep progressing forward without hitting an edge.

There have been recent studies which show that universe is probably "flat" though.

Or very slightly curved - measurements come with error bars, so we can't be sure if it's exactly flat. Also note that flatness and finite size are compatible in case of non-trivial topology (think of the flat torus - pacman world - which, however, is not isotropic).

All "space" ie the matter-containing 4d part we live in. There are things before and perhaps outside our universe, brains and such, but that is outside "our universe".

But what if your Big Bang was only a local bang among an infinite number of bangs so far apart from one another that the light from any of them would take 100s of billions of your earth years to reach its nearest neighbor. Then, even now, they are all expanding toward each other with no way for any of their passengers to know it. You won't need to worry about a heat death or a cold death, you will have a death by collision and probably some kind of rebirth to follow.

What you're describing is the 'visible universe'. There may well be other parts of the cosmos outside of our light cone. Einstein's equations allow it, and we have no way of knowing.

No, visible universe is what we can see, it's defined as all the points close enough in spacetime that light from there had enough time to get to us. Visible universe is centered on Earth, much smaller than what GP was talking about, and decreasing steadily (because at the edge of it expansion of space pushes stuff outside faster than speed of light).

at the edge of it expansion of space pushes stuff outside faster than speed of light

Not quite: According to the cosmological standard model, the visible universe will continue to grow (ie new galaxies will continue to come into view) - but only asymptotically, ie until a maximum size given by the comological event horizon is reached. However, the parts of the universe that aren't gravitationally bound to us will become fainter and fainter and increasingly resdhifted, and eventually, we'll be unable to detect other theoretically visible galaxies due to technological limitations.

If they are moving away from us faster than speed of light because of expansion of space - wouldn't it mean at some point no new light from them reaches us? Even ignoring the limitation of equipment?

In my opinion, that whole 'moving away from us faster than c' business is not really a good way to think about this: For one, we can see to a redshift of about 10, corresponding to a comoving distance of about 30Gly, and a recession velocity of about 4c (four times the speed of light!) at time of emission.

There's a cosmological event horizon. Light emitted from within will reach us in finite time, light emitted from without won't. Similar to how a distant observer will never see on object falling into a (stationary) black hole cross the Schwarzschild horizon, we won't see galaxies crossing the cosmological horizon.

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