Cosmic voids | Hacker News Reader

Cosmic voids

askamathematician.com

80 pointsCarolineW29 comments

A not-really-spoiler-alert for Iain M. Banks' "Against a Dark Background", whose title turns out to be a bit of foreshadowing itself: The planet system where the novel takes place is extragalactic, surrounded by no stars. If I remember correctly, they can see their nearby galaxy, but nothing nearer.

An extrasolar (if not extra-galactic) planet is a subplot in Becky Chambers' kickstarted[1] _The Long Way to a Small Angry Planet_[2], too.

1: https://www.kickstarter.com/projects/beckychambers/the-long-...

2: http://www.tor.com/2015/08/14/excerpts-the-long-way-to-a-sma...

I thought the titular planet was just near the core, not extra-galactic? Or am I forgetting one?

The existential loneliness of a void is weird. We're privileged to look up at the stars and ask who's out there.

A species living in a large void would have to develop extremely (compariable to modern humans) technology to ever prove there was another galaxy within the universe.

As the universe expands, there will come a time when no feasible technology will be able to tell you that there's anything outside of your galaxy. (Maybe if you wait long enough, there won't be anything outside of your solar system.)

That's an interesting idea. I knew the universe is expanding relative to galaxies, but I didn't realize that the expansion of the universe was overcoming gravity within the scope of a galaxy. Does this also mean that the distance between planets within a solar system will increase? Is the rate of expansion more than or less than the expansion of the sun's diameter? Does this mean that the distance from the Earth to the moon is increasing, and does it mean the planet itself is expanding? Is the expansion of the universe a destabilizing effect on orbital mechanics?

The dark energy phenomenon driving accelerated expansion is poorly described by theory, so predictions don't mean much. But yes: if expansion continues accelerating as it has been then eventually it will overpower intra-galaxy gravitation, then intra-system, and eventually intra--celestial-bodies and so on. Eventually planets and stars (and atoms and protons!) would be ripped apart.

> if expansion continues accelerating as it has been then eventually it will overpower intra-galaxy gravitation, then intra-system, and eventually intra--celestial-bodies and so on. Eventually planets and stars (and atoms and protons!) would be ripped apart

This is not true for dark energy, i.e., for accelerated expansion per se. It is only true for a "Big Rip" scenario in which the acceleration itself accelerates. The "stuff" that drives such a scenario is called "phantom energy" and such a scenario is currently believed to be extremely low probability.

IIRC the expansion is accelerating according to the best observations. Which means in the far future galaxies should find themselves in an endless sea of voids. Eventually stars, planets, living beings and atoms will all get torn apart unless something counters the accelerating expansion.

> Eventually stars, planets, living beings and atoms will all get torn apart unless something counters the accelerating expansion.

See my reply to ajross. Dark energy is not the same as a "Big Rip" scenario, which is what you are describing.

Thanks for the correction.

As far as I know, the Sun will be long gone by the time that level of expansion is reached; at best, it'll be a tiny white dwarf, having long ceased its expansion.

It can really be described as a problem that doesn't concern humanity, because by the time it hits, we won't be recognizably human anymore :P

> I didn't realize that the expansion of the universe was overcoming gravity within the scope of a galaxy.

It isn't, at least not according to our best current models, which do not involve a "Big Rip" scenario. See my replies to others downthread.

> Does this also mean that the distance between planets within a solar system will increase? Does this mean that the distance from the Earth to the moon is increasing, and does it mean the planet itself is expanding? Is the expansion of the universe a destabilizing effect on orbital mechanics?

According to our best current models (i.e., in the absence of a "Big Rip"), no to all of these.

Imagining how exciting the first galaxy detection would be though is quite a thing.

On the other hand, the probability of life arising in the middle of a void seems relatively small. We are made of dead stars.

This reminds me of Douglas Adams' Krikkiters, whose home planet is completely surrounded by black dust. It blocks out everything in the sky and so they had no concept of anything at all existing beyond their world.

http://aliens.wikia.com/wiki/Krikkiter

It'll have to go...

I found the video linked at the bottom interesting too: https://www.youtube.com/watch?v=rENyyRwxpHo

I'd never heard of Laniakea before.

I found the answer disappointing because the argument has a huge hole in it: it discounts the atmosphere. The article says the farthest objects earth-based naked-eye observations can detect are ~70 million light years away, so if you're in space in the middle of a billion light year void, you won't see anything. But if the atmosphere has just one order of magnitude attenuation (or rather you could see 10 times further without it), then maybe you could see to the 500 million light year edge of your void bubble.

That's what bothers me about these pseudo-scientific columns: they aren't rigorous and the reader doesn't know it. In this particular case, the author even changed the reader's question, and then didn't even give a satisfactory answer, just wanted to write about "cool" things.

These voids change moving up the distance scale - while our star system is visibly not inside a void, there is evidence that the Milky Way is inside a galactic-scale void.

http://news.wisc.edu/celestial-boondocks-study-supports-the-...

https://www.universetoday.com/135954/largest-scales-milky-wa...

Horrifyingly lonesome - almost complete sensory deprivation. It would be interesting to experience though.

If you were here in a space suit, would you be able to see anything at all? I would think not, but am not sure.

The linked article answers this. If you had no other sources of light, so that your eyes could fully acclimate, you might be able to see the faintest smudges if there were galaxies within 70 million light years.

I think we underestimate the importance of a starry sky for the development of intelligence. A species looking up at complete blackness will probably never develop higher reasoning or curiosity.

Due to light pollution, many people on Earth rarely see a starry sky.

Would it? There are plenty of curiosity to be had on the ground, in the air and under water. Birds flying, insects digging, fishes swimming.

Sure, they might not feel the need to visit space if they had no stars, no moon and no sun but who knows? They might end up going there anyway due to population and create their own superstructures.

The existence of stars gives us (humans at least) an idea that we actually live in a vast and varied universe and that I think is crucial psychological motivation to push boundaries and explore. If we lived within a void and believed that our one planet (and its star) were the universe, we would be content to develop only those skill & intelligence needed to survive on the planet. It might stagnate after a point. Of course, even a small telescope would reveal the distant galaxies, but would a hypothetical intelligent species get to that point at all?

And even if they could glimpse the distant galaxies, what could conceivably motivate them to develop space travel?

Am just wondering aloud. Astronomy was (and is) a pretty fundamental pillar of our scientific development and all that it has brought us. An inky black sky would have interesting effects upon nascent, intelligent species.

The Hitchhiker's Guide to the Galaxy actually explored the concept.

One planet is in a dust cloud. The sky was completely black, and thus the people of that planet led insular lives and never realised the existence of the Universe. They eventually built a small ship and managed to exit the dust cloud. Upon first witnessing the glory and splendor of the Universe, they casually, whimsically, decided to destroy it, remarking, "It'll have to go."

The race changed from a peaceful one to the most violent warfaring race of the galaxy.

I'm skeptical of your theory. Is there reason to believe that humans didn't push boundaries and explore prior to understanding the nature of stars and the vast and varied universe?

I suspect that early humans found plenty of mysterious phenomena around them, and would continue to do so even with a perfectly black sky.

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