Why is there a normal galaxy sitting at the edge of the Universe? • Hacker News Board

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#161

Post by skissane » Wed, Sep 09, 2020, 9:27 PM UTC

pvarangot wrote: post #109

s1dev wrote: post #61

Earlier quoted context omitted.

How do you propose to power a fleet of spacecraft at 550AU? Would you be able to acquire enough RTGs? What about DSN time for the downlink?

That webpage says a tech demo is due 2024, with launch in 2034 and the constellation would reach 550AU by 2064, so probably a lot of how this will work is still WIP.

The problem with dates like that is they are pure best wishes. They might have funding for a demo in 2024 – although it isn't even clear they've got that yet, it sounds like they may just have funding to develop their proposal further. Even assuming the tech demo is successful, it sounds like they don't even yet have a detailed costed proposal for a 2034 launch, and no idea how likely that it will actually get funded.

Politicians and bureaucrats want immediate results, so a "launch now and see results in 30 years" mission – by which time many of the senior decision-makers will likely be dead – is going to be less attractive than competing missions offering results much sooner.

If you go back through the history of space exploration, it is full of proposals that "by year X we will be doing Y!" which never came to pass, because the money never turned up.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#162

Post by nelsonenzo » Wed, Sep 09, 2020, 9:32 PM UTC

cgriswald wrote: post #105

zemnmez wrote: post #59

Earlier quoted context omitted.

this is because things aren’t ‘moving apart’— space itself is expanding (into what? nobody knows). it’s not that the galaxies are being propelled apart from a force in a particular place either; everything is expanding away from everything else . On the traditional balloon model, the surface of the balloon represents space itself. Those dots on the surface of an expanding balloon are not moving relative to space (the…

> On the traditional balloon model, the surface of the balloon represents space itself... For any still confused, there is also a 'raisin bread' model (which I prefer because it's 3D, unlike the surface of a balloon). The raisins are galaxies, and the bread is the space between them. As it bakes, the raisins get further apart, even though the raisins aren't moving through the dough. (Although galaxies also have movem…

Omg, thank you for this! The balloon model always confused me because I could never understand what was in the middle of the balloon, and why we were on top of it. It's an odd 2d example using an object that is 3d, the balloon.

Raisin bread, oh I love thee even more today.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#163

Post by chowells » Wed, Sep 09, 2020, 9:34 PM UTC

hashmymustache wrote: post #151

chowells wrote: post #91

Earlier quoted context omitted.

Not in practice. In practice, this provides a very tiny force pulling subatomic particles apart in an atom's nucleus, or pulling atoms away from each other in a molecule, or pulling a celestial body's molecules away from each other. It makes no difference at all, because that force is absolutely dwarfed by the strong nuclear force, electromagnetism, or gravity, respectively. Systems that are coupled together by those…

Does that expansion provide potential energy by creating space between attractive bodies? If so how do you account for conservation of energy?

Yes, it will create some additional potential energy.

That doesn't contradict strict conservation of energy - it just adds another energy source that has to be accounted for when balancing everything.

It might contradict the aphorism "energy is never created or destroyed", but I'm not sure that was ever science.

It's more fun to think about it in terms of thermodynamics. Can that extra energy be used to reverse the second law of thermodynamics? I'm guessing not - it's so diffuse that it's probably impossible to use it to reduce entropy. The math for actually doing those calculations is well beyond what I know, though, so I'll just say that one's no more than a guess.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#164

Post by ddevault » Wed, Sep 09, 2020, 9:35 PM UTC

joshspankit wrote: post #157

ddevault wrote: post #103

Earlier quoted context omitted.

I didn't really understand mekkkkkk's question, but I wrote up a longer reply here at your prompting: https://news.ycombinator.com/item?id=24424626

Thank you. If I understand what you’ve written already, the essence of the answer in terms of mekkkkkk’s question (for reference: “ Sorry for also asking a space 101 question, but since the fabric of space is expanding, does that mean that things are getting bigger as well? Naively, it would seem that way. In your balloon example, the dots would get bigger.”), I think the answer would be: Anything held together (for…

>This does produce an interesting side question though: aren’t we (humans) shrinking in relation to the universe? Or, without the click-baity phrasing: isn’t the universe growing without us?

Yep.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#165

Post by rosstex » Wed, Sep 09, 2020, 9:38 PM UTC

cgriswald wrote: post #105

zemnmez wrote: post #59

Earlier quoted context omitted.

this is because things aren’t ‘moving apart’— space itself is expanding (into what? nobody knows). it’s not that the galaxies are being propelled apart from a force in a particular place either; everything is expanding away from everything else . On the traditional balloon model, the surface of the balloon represents space itself. Those dots on the surface of an expanding balloon are not moving relative to space (the…

> On the traditional balloon model, the surface of the balloon represents space itself... For any still confused, there is also a 'raisin bread' model (which I prefer because it's 3D, unlike the surface of a balloon). The raisins are galaxies, and the bread is the space between them. As it bakes, the raisins get further apart, even though the raisins aren't moving through the dough. (Although galaxies also have movem…

Why not chocolate chip bread? I suppose raisins more closely represent galaxies.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#166

Post by [deleted] » Wed, Sep 09, 2020, 9:40 PM UTC

ddevault wrote: post #116

noizejoy wrote: post #111

Earlier quoted context omitted.

So will expansion accelerate enough to eventually be a stronger “force” than local gravity?

We don't understand why expansion is occuring, so we can't say for certain. It would be a very long time in the future if that were to come to pass, and that's a whole lot of time for poorly-understood physics to happen in. But yes, it is accellerating, and if it continues to do so, eventually it will overcome gravity. But like I said, this depends on an incomplete understanding of physics, and the current evidence s…

[deleted]

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#167

Post by [deleted] » Wed, Sep 09, 2020, 9:48 PM UTC

zemnmez wrote: post #59

iwonderwhy wrote: post #55

All right hackernews. Genuine question, not sure where else to ask. If galaxies and stars at the other “end” formed out of matter resulting from the big bang, just as the matter we are made of, how did we get here at the same time that that matter got there? Is it because speeds faster than light are possible, and around the big bang all matter was spread near instantly at that speed? I don't understand how matter of…

this is because things aren’t ‘moving apart’— space itself is expanding (into what? nobody knows). it’s not that the galaxies are being propelled apart from a force in a particular place either; everything is expanding away from everything else . On the traditional balloon model, the surface of the balloon represents space itself. Those dots on the surface of an expanding balloon are not moving relative to space (the…

[deleted]

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#168

Post by reubenswartz » Wed, Sep 09, 2020, 9:53 PM UTC

ben_w wrote: post #93

mekkkkkk wrote: post #74

Earlier quoted context omitted.

Sorry for also asking a space 101 question, but since the fabric of space is expanding, does that mean that things are getting bigger as well? Naively, it would seem that way. In your balloon example, the dots would get bigger.

Just to add to your question: The usual response of “it only affects space between galaxies not inside galaxies” is deeply unsatisfying, and feels like it must be an oversimplification. What I think is meant, but I absolutely want someone wiser to confirm or refute , is that the rate of expansion is proportional to distance, and therefore so small inside a single galaxy it can be ignored. What I don’t know is: can an…

Yes, because the "Hubble Constant" is expressed as km/s/Mpc (that's Megaparsec), and the value is thought to be around 70. 70km/s seems crazy fast, until you divide by the 3,300,000ish light years (each light year being about 6,000,000,000,000 miles).

Of course, if you calculate out far enough time, you would eventually end up with the observable universe getting smaller (because the stretching of spacetime would take away more space than was added by light reaching us from further away), down to our galaxy, and eventually everything getting torn to shreds, even down to protons...

In the "near" term, the effect is so small that we have much more to worry about from the sun expanding in a few billion years than the Big Rip.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#169

Post by ckosidows » Wed, Sep 09, 2020, 9:58 PM UTC

tectonic wrote: post #2

Nice gravitational lens observation! We're hoping to use the same technique with the Sun to observe exoplanets by launching a fleet of small satellites to 547 AU (80 billion km / ~3 light days). The craft would sample the distorted Einstein ring around our Sun from that vantage point, then combine and reconstruct an image of the disk of an Earth-like exoplanet up to 100 light-years beyond, at a resolution of 25 kilom…

20 AU per year is a tremendous distance. Pluto is apparently right around 25 AU. Is this speed actually feasible even with solar sails?

Big layman here. I only skimmed this link but this stood out to me.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#170

Post by meowface » Wed, Sep 09, 2020, 10:02 PM UTC

jquery wrote: post #152

meowface wrote: post #137

Earlier quoted context omitted.

From my layman's perspective, I think this is plausible, and many physicists seem to agree it's plausible. Maybe another plausible explanation could be the reverse: maybe before the Big Bang, our/the universe was in the state you describe, and the Big Bang is our perception of it "snapping back" and exploding violently outward (aka a white hole). Carlo Rovelli and others propose that black holes actually only appear…

> Carlo Rovelli and others propose that black holes actually only appear as they do to us due to the extreme time dilation, and similar to how a photon's "perspective" is that it's experiencing all of time at once, a black hole's perspective is that it's basically a massive explosion that occurs at essentially the same moment it forms. So under this hypothesis, a black hole never reaches a true singularity, but as cl…

Don't know why you're being downvoted. It's definitely not too crazy of an idea, given what's been known since general relativity was confirmed and the discovery that black holes do indeed exist. I haven't looked into it, but I wouldn't be surprised if many other people have proposed it in the past. I haven't heard or thought of it before, but when I first read it it definitely made a lot of sense, intuitively.

Maybe the downvotes (which seem to no longer outweigh the upvotes?) are because you said "is actually" rather than "might actually be". Humility and uncertainty is crucial in science. Even for a field that's much more testable, and even for a leading scientist in a field, let alone some random speculator on the internet. It's fun to think and talk about these things, but especially as total amateurs, we should never claim anything with even a little bit of confidence.

My own speculative stuff I posted above is probably all dead wrong and hopelessly confused; I'm just throwing it out there as stuff that's popped into my head. And Rovelli may be wrong, too, though he's a highly-respected theoretical physicist and has published several papers about it. And even if the Planck star and black hole/white hole theory is true, it may not necessarily mean it has anything to do with how our universe started; that's just additional unsubstantiated speculation by me and some other people, and not anything Rovelli has claimed, though his theory seems to have increased discussion about that topic.

Here're some resources:

https://phys.org/news/2014-02-astrophysicists-duo-planck-sta...

https://en.wikipedia.org/wiki/Planck_star

https://en.wikipedia.org/wiki/White_hole#Big_Bang/Supermassi...