[Ace]

Well that would be the case if dark matter consists of WIMPs.

[Bobboau]

anyway, this thread is about dark energy, not dark matter.

[Herra Tohtori]

Well that would be the case if dark matter consists of WIMPs.

...And it probably does, at least according to WMAP observations, which tend to block out the possibility that the dark matter would consists of MACHO's. That is, it the interpretation of the measurements is correct...


By the way, am I only one who finds it profoundly hilarious that a bunch of scientists names the candidates for dark matter MACHO's and WIMP's - and even more, that WIMP's are the likely winner for the job?

Seriously, those abbreviations are like directly from The Junior Woodchucks titles...

anyway, this thread is about dark energy, not dark matter.

Well, the way I see it is that energy is by definition a potential to do work. Hence, because space expands at accelerating speed and it is assumed that the accelerating expansion demands energy, it is also assumed that there is some form of energy potential that provides the energy for the expansion. It's dark only because we don't know what that energy source is.

It's kinda similar definition as matter has. If energy has gravitational mass, it is matter. If it doesn't have it, it's just energy. Relativistic energy forms don't (as far as I know) have effect on gravitation; these energy forms include kinetic energy, electromagnetic radiation and stuff like that. They are relative to observer. Gravitational mass, however, has an energy potential on its own inertial frame (ie. at rest). That's what separates matter from random energy. But that also means that dark matter is just a form of matter because it has gravitational mass; it's just our definition to call if "dark" because we don't know what it is.

The same thing as with X-rays, by the way. They were named X-rays because the founder of them had no fricking idea what they were, and only later it was found out that it's actually same stuff as light, only sligtly tuned-up version of it... but I digress.

The same thing is with dark energy. It's just a name. It'll stop being dark as soon as it's figured out. But the main thing is that it wouldn't be energy at all if it couldn't do work. Also, as it's obviously doing work (accelerating the expansion) that means that at some point the energy source will be depleted. That will most likely occur some time billions of years after the thermal death of the universe.

Unless the conservation laws are completely b0rked, which I don't think is the case.


So. Perhaps it would indeed be possible to power up a generator attached to this mega-string. It would obviously be practically impossible, but maybe not theoretically. Could be interesting to find out a way to harness this energy source. Subspace power generator, anyone...?

[Bobboau]

"at some point the energy source will be depleted. That will most likely occur some time billions of years after the thermal death of the universe.

Unless the conservation laws are completely b0rked, which I don't think is the case."

which was my initial point, if dark energy works like we think it does then conservation of energy does not hold.

[Herra Tohtori]

Not necessarily, if the [dark] energy ends some day (ie. is entirely converted to work as expansion of space). Then the acceleration of expansion will halt, and what happens after that is not really our headache.

The net amount of energy in universe (known matter+dark matter+known energy+dark energy) will remain the same, regardless of where the dark energy comes from or what it is. In the process of expanding (according to our assumption that energy is required to accelerate the expansion of space) the space, the amount of dark energy is converted into more familiar forms of energy - mostly seen as kinetic energy of fastly escaping distant galaxies (although again, there can be several interpretations whether or not their kinetic energy actually increases or not, or does it just look like that to us).

Eventually, as the thermal energy differences of universe are depleted and maximum entropy is slowly creeping closer and closer, probably some day also the source of dark energy will be depleted, at which point there is no dark energy any more.

The assumption that the accelerated expansion will go on for infinite time is just as ridiculous as assumption of string of infinite length. Whatever is accelerating universe (if energy is required to that - I have my doubts) - will not last forever, because there is no infinite energy in the universe.


What is interesting is that in the inflation of universe the expansion was first accelerated insanely much, but then it's speed decreased substantially, and now the speed is again increasing. So if we look at the acceleration curve of the speed of expansion, it starts at zero at t=0, and goes almost instantly way up and stays there for a while. Then it comes down below zero while the speed of expansion decreases. Now, it's above zero again, increasing the speed of universe.

You know what it sounds like to me? Damped harmonic oscillation. Which is not surprizing, considering that harmonic oscillation is rather common phenomenon in universe. Though I have no idea on how many periods have there been already, or what the frequency is, or what the initial conditions were and how the oscillation is dampening, it certainly seems to have at least some characteristic properties of oscillation. Itäs also probable that this has been thought before, but it would feel somehow intuitive to me.

Then again, intuitive is not always what physics is...

[Bobboau]

but as far as I know our current understanding of dark energy is that it will not run out some day.

[Herra Tohtori]

...current understanding of dark energy...

...is modest at best. We know something is there accelerating the expansion. What we do not know are (to mention a few things):

-Why did the expansion accelerate very fast at the inflation?
-What made the inflation speed decrease so much after the initial inflation phase?
-What caused the acceleration speed to rise to current level again?

The acceleration speed hasn't been a constant thorough the history of universe. We don't know how it will proceed in the future. These are only few things we don't know about the nature of dark energy.

I don't personally know any reason why the dark energy couldn't run out.


Then again, we actually know that conservation of energy is not entirely valid principle considering cosmology. We know that things (a whole universe of them) can literally pop out of nothingness.

We just have this assumption that in current universe, energy cannot be born. It's a good principle but not necessarily always applicable, as our existence proves. So perhaps more energy is constantly generated into the universe from... nothing. Perhaps the birth of our universe is a continuous process. I don't know. But the idea of energy coming out of nothing is rather interesting.

Find a way to manipulate the speed at which the energy is created, and we're in the stars.

[Ghostavo]

Then again, we actually know that conservation of energy is not entirely valid principle considering cosmology. We know that things (a whole universe of them) can literally pop out of nothingness.

We just have this assumption that in current universe, energy cannot be born. It's a good principle but not necessarily always applicable, as our existence proves. So perhaps more energy is constantly generated into the universe from... nothing. Perhaps the birth of our universe is a continuous process. I don't know. But the idea of energy coming out of nothing is rather interesting.

Find a way to manipulate the speed at which the energy is created, and we're in the stars.

The energy of the universe is (according modern understanding) constant. At the big bang energy gave birth to matter and anti-matter but for some reason matter didn't decay as fast as anti-matter and then when the universe was calm enough to have matter/anti-matter reactions there was an imbalance in the ratio that allowed some matter continue to exist.

According to this theory energy is constant, but where the "inicial" energy came from is anyone's guess. Most people don't question what came before so...

[Ace]

We just have this assumption that in current universe, energy cannot be born. It's a good principle but not necessarily always applicable, as our existence proves. So perhaps more energy is constantly generated into the universe from... nothing. Perhaps the birth of our universe is a continuous process. I don't know. But the idea of energy coming out of nothing is rather interesting.

Which sounds a bit like steady state theory.

[Herra Tohtori]

Not quite.

Steady state assumes that the universe remains similar, ie. either the universe doesn't expand or more mass is continuously generated to fill the gaps in the expanding universe. Neither has been observed, but instead there is actual proof of a "static" energy level of the vacuum of space itself. Its existence can be proved by Casimir-effect, for example, but as it's just a static, and for all practical purposes uniform, energy level, it cannot be used as energy source, as that requires differences in energy potential.

But the point is that vacuum's quantum fluctuation keeps the space itself at certain energy level. And there's a lot of space. And as energy has a tendency to dissipate and spread, but in this case it is bound to vacuum... perhaps that causes a "pressure" that expands the space itself and the energy yield of quantum fluctuation along with it.

So perhaps it is the energy of those fluctations that is accelerating the expansion. That would certainly explain the inflation of the universe - as the universe itself is likely result of a gargantual quantum fluctuation, the enery yield of vacuum in the beginning would have likely been much bigger than it is now. Perhaps. If this is indeed the case here, then the acceleration of the expansion will continue, approaching zero but never reaching it. That will result in universe that forever expands with accelerating rate, but the acceleration itself would decrease as time passes.

That in itself doesn't explain the changes in the speed of expansion during inflation. But this all is just speculation, of course.

[Agent_Koopa]

Then again, we actually know that conservation of energy is not entirely valid principle considering cosmology. We know that things (a whole universe of them) can literally pop out of nothingness.

Well, "nothingness" is relative. It is so impossible to think about what came before the universe existed that we shouldn't try. And, besides, conservation of energy only holds for an isolated system, so it hardly works with the creation of the universe. In addition to that, particles pop in and out of existence all the time, an existence so brief as to escape the jurisdiction of the principle of conservation of energy.

Find a way to manipulate the speed at which the energy is created, and we're in the stars.

That, or *BOOM*! 

The energy of the universe is (according modern understanding) constant. At the big bang energy gave birth to matter and anti-matter but for some reason matter didn't decay as fast as anti-matter and then when the universe was calm enough to have matter/anti-matter reactions there was an imbalance in the ratio that allowed some matter continue to exist.

According to this theory energy is constant, but where the "inicial" energy came from is anyone's guess. Most people don't question what came before so...

That's because it's almost impossible to question. Outside our space-time, time has no meaning and space has no meaning. This obviously makes it rather hard to define.