What Energy Sources Could Sustain a Refuge in an Expanding Universe?

[kriku]

Dear All

As I have understood, there will be only cooling chunks of matter like stray planets, neutron stars, brown dwarves and all other kind of debris left of our Universe in 10^20 years, which continue drifting apart as the Universe expands on (let us assume that it does). They will continue their journey forever if the protons are stable (let us assume that they are).

If some intelligent race built a refuge, it will drift along until it it will run off energy and freeze. The question is, what energy sources will be available by then? It is longer than half-lives of most of unstable elements and I guess that one does not draw much from Bismuth decay anyway. Would it be possible to harvest the background radiation? Any other ideas?

Let us assume that they live in a kind of virtual reality and their energy needs are thus low compared to our present civilization (to keep their computer ticking and maintained).

Thanks in advance.

 

[marcus]

... The question is, what energy sources will be available by then?
Thanks in advance.

Well you certainly are an advance thinker! Small black holes, if you feed them any kind of matter (ground up rock, trash, dust-bunnies, fingernail clippings, excess gas) convert it to highenergy radiation at near 100% MC² efficiency

Much more efficient than nuclear fission power plants, or fusion in stars. They only convert on the order of 1% of the mass into energy. This method converts it totally 100%.

The thing is they are very hot, the smaller the hotter, and they radiate intensely and tend to evaporate. So you have to be feeding them so they don't evaporate altogether.

I recall there is a paper by Louis Crane, a math prof at U Kansas, about using small black holes to drive rockets for space travel. He got a handsome research grant from a private foundation to study this and other energy conversion stuff. So he is not as crazy as you might at first suppose.

He is also an important contributor to Loop quantum gravity. Co-author of the Barrett-Crane spin-foam model. Creative guy.

 

[kriku]

Good point.

Does this also happen when dark matter falls in? If that's a feasible way to convert dark matter into ordinary energy, the refugees should not experience shortage if they manage to pet a black hole.

 

[marcus]

It sounds strange to me when you say "to pet a black hole". Unless you really mean that, it might sound better to say

"to tame a black hole"
"to domesticate a black hole"

Petting involves touching with your hand, as for example you pet a dog or cat.
A pet is an animal that lives in the house with you. Suppose you catch a wild animal, if you know how you can TAME the wild animal and maybe even make a pet of it.
When the wild animal becomes used to humans, and not afraid, and trained to be useful, or trained to be a house-pet, then we say the animal is DOMESTICATED.

If you catch a wild horse, you may be able to tame it and train it. Maybe you can HARNESS the horse so it can pull a wagon for you. Then the horse is domesticated. It learns not to be afraid of humans.
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Dark matter should work.
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there are several ways to get energy from a BH.

I am thinking of just one possible way, which only works if the BH is very small. Small holes are hot, the smaller the hotter. They glow with intense heat radiation. they lose energy and therefore mass by this. So if you do not feed them they will shrink smaller and smaller and finally disappear. (We are not sure about the end state. Probably disappear, in any case stop being useful.) So you have to feed them just to keep them a constant size.

Big BH are cold so their heat radiation is useless. But you can get energy from them in other ways. The gravitational potential energy that is given up by stuff falling in. It can be captured in various ways.

Quasars are powered by stuff spiraling into a big BH. I am not talking about that. I only wanted to discuss using the light and heat radiated from a very small BH.

 

[kriku]

Yes, "domesticate" is certainly better.

They will have to rely on stocked up matter eventually as everything gets wider and wider apart and new rocks become harder to find. It might be more convenient to convert the best part of it to antimatter and annihilate it in small quantities than to domesticate a small black hole and feed it. In the end of the day, it runs down to the best means to conserve energy.

A more general question - if the black holes consume dark matter, would it be possible that they stop or reverse the repulsion and expansion in long turn? I mean the real monsters, not the small ones.

 

[chronon]

If you're interested in the long term fate of the universe, then you might like The Five Ages of the Universe by Fred Adams and Greg Laughlin. Alternatively if you think it's better to get out of this universe and find a new one then there's Michio Kaku's Parallel worlds

 

[marcus]

A more general question - if the black holes consume dark matter, would it be possible that they stop or reverse the repulsion and expansion in long turn? I mean the real monsters, not the small ones.

Be careful! Don't confuse dark matter with "dark energy".

You could start one or two new threads asking questions about "dark energy".

In popular books and magazines it is often said that the content of the U is
4% ordinary matter (including a tiny percentage of ordinary radiation, starlight etc)
23% dark matter (which seems to be some kind of particle and can collect in clouds and can fall into BH)
73% "dark energy" (which seems to be uniformly distributed, and accelerates expansion)

This is a simplification.

The usual mainstream cosmo model is called LambdaCDM because of two major assumptions. CDM is "cold dark matter". It does not cause expansion. It does not accelerate expansion. Like other matter its gravity slows down expansion. It collects into clouds and helps galaxies form, by helping gather other matter. It helps to hold galaxies (and clusters of galaxies together).
Its existence is hypothetical, but it seems very likely that it exists and there is some particle, because of many different pieces of evidence. Clouds of CDM have even been mapped by the way their gravity distorts the light from galaxies in background coming to us through the clouds, by an effect called "weak gravitational lensing".
So it is quite iikely that CDM is real, and some kind of matter.

But Lambda is so far just a constant term in the Einstein GR equation. We have no evidence that it is anything but constant (in both space and time).

The equation has curvature on the Lefthand Side and matter-energy on the Righthand Side.

Originally Lambda was on the LHS as a constant intrinsic curvature of spacetime. However it is mathematically convenient to be able to move it over to the RHS where it becomes an effective energy, uniformly distributed, which CAUSES the constant uniform curvature.

If one wants one can then fantasize that this energy term on the RHS can change its properties over time, and perhaps even change into something else. But so far there is no evidence that there is any change (either in space or time) of the Lambda term.

If some evidence were discovered that would be very exciting. But so far it simply seems to be a constant term which WOULD represent 73% of the energy in the U if it actually were an energy field.

The expansion of distances began in the early universe--as part of the initial conditions of the LambdaCMD model. The model says that this initial expansion was so strong that expansion would still be continuing today even if Lambda were zero. In that sense, Lambda is not needed as a cause of expansion.

So we should not think of Lambda as the cause of expansion. It is responsible for a slight acceleration, which was noticed in 1998. Even if Lambda could change and decay to zero, expansion would continue indefinitely, according to the model. But it would slow down because of ordinary matter and dark matter gravity. That is, as far as we know, according to the prevailing model.
Nothing is certain. you just have to go with the simplest model that gives the best fit to the most data you can gather. Ideas will change as we find out more but that is the tentative picture we have for the time being.

 

[kriku]

Thanks a lot, this was indeed unclear to me.