Comment by dheera | Hacker News Reader

Comment by dheera | Hacker News Reader

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The biggest "concern" I have about the expanding universe is that although I have a lot of hope for interstellar travel at some point in the coming few millenia, intergalactic travel poses a whole another set of impossibilities to face, that I'm worried it will never be possible. The space between galaxies is pretty devoid of anything you can use as fuel, repair materials, or anything else. That space is expanding, and at some point around 100-150 billion years from now, galaxies that are not gravitationally bound to the Local Group will be no longer reachable or observable [1].

You need ten times the mass of Mt. Everest in e=mc^2 energy (i.e. 100% efficiency) per 1 kg of mass to get to Andromeda at 1g acceleration/deceleration. [0]

For this reason I sincerely hope that conservation of mass/energy is not necessarily true. Nothing says it needs to be true other than that it empirically seems to be the case, and there is no known process that violates it, other than the Big Bang itself. If it can be violated, we have hope for the long long long term future.

Scary to think about this stuff.

[0] https://en.wikipedia.org/wiki/Intergalactic_travel

[1] https://en.wikipedia.org/wiki/Timeline_of_the_far_future

Well, that puts some perspective on 2020.

I mean, it's bleak perspective, but surely if we all write to our congressperson we can act now to prevent the heat death of the universe, right?

However, after the universe finishes expanding, wouldn't it start contracting again?

Btw, Andromeda will collide with the Milky Way in about 4 billion years, so at least we won't need ten times the mass of Mt. Everest to get there...

If you subscribe to Roger Penrose's theory, at the point of infinite expansion, the current "Aeon" will end and a new big bang will happen. As best I understand it, at the point of infinite expansion, there are only photons. Mass no longer exists therefore scale no longer exists. So an infinitely huge universe becomes indistinguishable from an infinitely small universe.

What causes the Big Bang in this scenario?

Basically, the way I understand it, is the probability of a Big Bang spontaneously occurring (due to the quantum fluctuations) is some godawful infinitesimally low number 0.00000...1 that for all intents and purposes is 0, but is NOT zero.

So given ungodly eons of time (googleplex ^ googleplex * graham's number, etc. of years) something with a very, very, very low probability becomes inevitable.

> However, after the universe finishes expanding, wouldn't it start contracting again?

Unless the answer has changed in the last several years, no, the consensus is that it's expanding too fast and has too little mass (and thus gravity) to collapse again.

(Disclaimer: not even an amateur at this, at least not for 40 years.)

500 years from now - a shockingly small slice of time in the history of human evolution, our understanding of the universe has changed a lot. Folks, if you’re reading this from the future - we are currently mostly clueless but have made some solid progress. GIFs load 10x faster than they did just 10 years ago! This virus thing has been a real pain in the ass though.

I really wish people would stop using GIFs for video. Use at least H.264 for those cat videos -- 24-bit color and much better compression than GIF.

Energy isn’t really conserved in general relativity. See this Sean Carroll article for more (1). I don’t think this helps with your concerns, though.

(1) https://www.preposterousuniverse.com/blog/2010/02/22/energy-...

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