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The Last of the Universe’s Ordinary Matter Has Been Found

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Re: The Last of the Universe’s Ordinary Matter Has Been Found

#51

Post by cygx »

ajuc wrote: post #47
auntienomen wrote: post #37

Earlier quoted context omitted.

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.

Re: The Last of the Universe’s Ordinary Matter Has Been Found

#52

Post by Neeek »

AnimalMuppet wrote: post #35
deathanatos wrote: post #33

Earlier quoted context omitted.

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…

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.

Re: The Last of the Universe’s Ordinary Matter Has Been Found

#53

Post by cygx »

Neeek wrote: post #50
mecameron wrote: post #39

Earlier quoted context omitted.

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).

Re: The Last of the Universe’s Ordinary Matter Has Been Found

#54

Post by TheCapeGreek »

wahern wrote: post #46
jnordwick wrote: post #38

Earlier quoted context omitted.

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 abo…

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 visib…

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?

Re: The Last of the Universe’s Ordinary Matter Has Been Found

#55

Post by akvadrako »

wahern wrote: post #45
akvadrako wrote: post #13

Earlier quoted context omitted.

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

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.

Re: The Last of the Universe’s Ordinary Matter Has Been Found

#56

Post by akvadrako »

TheCapeGreek wrote: post #54
wahern wrote: post #46

Earlier quoted context omitted.

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 visib…

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.

Re: The Last of the Universe’s Ordinary Matter Has Been Found

#57

Post by cygx »

TheCapeGreek wrote: post #54
wahern wrote: post #46

Earlier quoted context omitted.

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 visib…

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?

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...

Re: The Last of the Universe’s Ordinary Matter Has Been Found

#58

Post by okonomiyaki3000 »

akvadrako wrote: post #55
wahern wrote: post #45

Earlier quoted context omitted.

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.

Re: The Last of the Universe’s Ordinary Matter Has Been Found

#59

Post by okonomiyaki3000 »

thisismyswamp wrote: post #26
sandworm101 wrote: post #15

Earlier quoted context omitted.

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.

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

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.

Re: The Last of the Universe’s Ordinary Matter Has Been Found

#60

Post by maze-le »

TheCapeGreek wrote: post #54
wahern wrote: post #46

Earlier quoted context omitted.

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 visib…

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?

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.

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