Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

quantamagazine.org

Post reply on HN
Print view View original

31–40 of 65 posts

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

#31

Post by azernik »

coldtea wrote: post #18
TomMckenny wrote: post #8

Slightly related, I wonder if anyone has figured out the density of interstellar comet or asteroid like objects. I notice that Oumuamua happened to pass within some 20 million km of earth within a decade or so of having systems in place to spot it. Wouldn't this imply there are an awful lot of them?

> I notice that Oumuamua happened to pass within some 20 million km of earth within a decade or so of having systems in place to spot it. Wouldn't this imply there are an awful lot of them? Finding one in a decade's span within 20 million km would imply there are "an awful lot of them"?

That's a pretty small volume and a pretty short timespan, all things considered.

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

#32

Post by sandworm101 »

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.

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".

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

#33

Post by deathanatos »

AnimalMuppet wrote: post #27
thisismyswamp wrote: post #26

Earlier quoted context omitted.

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.

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

#34

Post by GW150914 »

deathanatos wrote: post #33
AnimalMuppet wrote: post #27

Earlier quoted context omitted.

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…

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.

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

#35

Post by AnimalMuppet »

deathanatos wrote: post #33
AnimalMuppet wrote: post #27

Earlier quoted context omitted.

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…

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

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

#36

Post by privong »

Firerouge wrote: post #17
TomMckenny wrote: post #8

Slightly related, I wonder if anyone has figured out the density of interstellar comet or asteroid like objects. I notice that Oumuamua happened to pass within some 20 million km of earth within a decade or so of having systems in place to spot it. Wouldn't this imply there are an awful lot of them?

I'm not qualified to answer your question, but the observation of Oumuamua alone doesn't give a large enough sample size to estimate how often large interstellar comets or asteroids pass through our solar system.

> I'm not qualified to answer your question, but the observation of Oumuamua alone doesn't give a large enough sample size to estimate how often large interstellar comets or asteroids pass through our solar system.

It's an observation, so it sets some level of constraints on the rate. Though it's true that an estimate of that rate would have large uncertainties.

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

#37

Post by auntienomen »

sandworm101 wrote: post #15
yalogin wrote: post #12

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.

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.

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.

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

#38

Post by jnordwick »

akvadrako wrote: post #13
yalogin wrote: post #12

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.

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.

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

#39

Post by mecameron »

deathanatos wrote: post #33
AnimalMuppet wrote: post #27

Earlier quoted context omitted.

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…

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