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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

#41

Post by jaipilot747 »

jeremyjh wrote: post #28
aikah wrote: post #19

Earlier quoted context omitted.

Then what do physicists call what is beyond the edge of the universe? edit: excuse my french ;)

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.

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?

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

#43

Post by wahern »

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've wondered the same thing. Also consider that

  [t]he energy released by a cosmic collision increases as the 
  square of the incoming object's speed, so a comet could pack
  nine times more destructive power than an asteroid of the
  same mass. (https://www.space.com/26264-asteroids-comets-earth-impact-risks.html)
ʻOumuamua reached a barycentric speed of 87.71 km/s. The tables on Wikipedia's Impact event article (https://en.wikipedia.org/wiki/Impact_event) assume a speed of 17 km/s relative to Earth. The energy of objects local to our solar system is limited in a way that interstellar objects are not.

With a single observation we can't deduce much of anything concrete except to floor the incidence of these interstellar objects at greater than 0. I'm no astronomer, but I assume models of interstellar objects as they reflect actual risk to Earth wouldn't be very useful without more observations. Whatever the average density in galactic space, I'm betting they're not uniformly distributed. Our solar system is speeding through space that could be littered with clouds of objects.[1] Are we entering a cloud? Leaving a cloud? We can't know without more observations.

[1] There are theories that posit that the ~30- and ~225-million year cycles we see in extinction events are a function of our solar system's orbit in the galaxy, which takes about 200-250 million years. Shorter cycles could relate to the inclination of our orbit (and other stars' orbits) relative to the galactic plane.

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

#44

Post by ChuckMcM »

wahern wrote: post #43
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've wondered the same thing. Also consider that [t]he energy released by a cosmic collision increases as the square of the incoming object's speed, so a comet could pack nine times more destructive power than an asteroid of the same mass. (https://www.space.com/26264-asteroids-comets-earth-impact-risks.html) ʻOumuamua reached a barycentric speed of 87.71 km/s. The tables on Wikipedia's Impact event article ( https:/…

One of the interesting things about the galactic orbit is that our system is not exactly in the galactic plane, so as it orbits it passes through the plane to the other side and then back again.

The kinetic energy of objects is proportional to the square of the velocity (Ke = (mv^2)/2), so an object going 4 times faster than a solar system object has 16 times the energy for the same mass. This makes it possible to have extinction level events from rocks that are 1/4 the size of planet killing asteroids.

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

#45

Post by wahern »

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.

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

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

#46

Post by wahern »

jnordwick wrote: post #38
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.

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 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.

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

#47

Post by ajuc »

auntienomen wrote: post #37
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.

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).

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

#48

Post by ubershmekel »

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…

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?

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

#49

Post by wahern »

ubershmekel wrote: post #48
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…

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.

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

#50

Post by Neeek »

mecameron wrote: post #39
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…

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.
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