The Last of the Universe’s Ordinary Matter Has Been Found • Hacker News Board

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

#61

Post by ajuc » Tue, Sep 11, 2018, 10:36 AM UTC

cygx wrote: post #51

ajuc wrote: post #47

Earlier quoted context omitted.

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 bec…

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?

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

#62

Post by cygx » Tue, Sep 11, 2018, 11:50 AM UTC

ajuc wrote: post #61

cygx wrote: post #51

Earlier quoted context omitted.

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 bec…

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.

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

#63

Post by jeremyjh » Tue, Sep 11, 2018, 4:59 PM UTC

jaipilot747 wrote: post #41

jeremyjh wrote: post #28

Earlier quoted context omitted.

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?

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.

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

#64

Post by alanbernstein » Tue, Sep 11, 2018, 7:05 PM UTC

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...

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

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

#65

Post by mhandley » Tue, Sep 11, 2018, 10:29 PM UTC

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?

Ok, here's an extremely rough back-of-the-envelope calculation. As you'll see, these numbers can be out by orders of magnitude, and it doesn't greatly change the conclusion.

Oumuamua interstellar asteroid. 230x35x35m, ~= 280000 m^3

Density assumption: 2 x water. => mass is ~500,000 metric tonnes.

Spotted only after passing the Sun. Assume we'd spot such objects only if they came within the orbit of mercury so are well illuminated. Assume one such object every 10 years (we've not been searching very long with automated telescopes), and we spot all of them.

Mean mercury orbit radius ~ 60,000,000 km

Area of mercury's orbit: 1.1 x 10^16 km^2

Mercury's orbital area x path length in 10 years = volume swept by one visible object in 10 years.

Asteroid velocity ~100,000 km/h

Path length in 10 years = 100,000 x 10 x 24x365. Swept volume ~ 10 x 10^25 km^3

Distance to Alpha Centauri: 4.37 light years = 4.37 x 9.5 x 10^12 km = 4.15 x 10^13km

Sol's "cube of influence" ~= 7 x 10^40 km^3

Cube of influence / swept volume = rough estimate of number of asteroids in cube of influence. Number of asteroids: 7 x 10^14

Mass of asteroids: 3.5 x 10^20 tonnes. Mass of sun: 2 x 10^27 tonnes.

Conclusion: dark interstellar asteroids like Oumuamua are a tiny fraction of the visible mass of the galaxy.