Twice as much dark energy as dark matter

[aguycalledwil]

Okay, so being fourteen, I know that I'm way out of my league on this forum. I do my best to understand anything to do with physics and to at least get a vague idea of theories and ideas, both old and new. Please forgive me for my ignorance.
Aside from one topic a while back, I havn't done anything on this forum really, but I plan to become a regular. Anyway, now I'm rambling, so onto my question.
Recently, I read of loop quantum cosmology, and how it indicates an eventual constriction of the universe to a point (CRUNCH), and then a sudden expansion (BANG). What I don't get is, how does LQC overcome the fact that there is twice as much dark energy as dark matter in the universe. Surely this ratio would continually accelerate the universe's expansion, leading to a Big Rip, not a Big Bounce? How is the big rip contradicted by LQC? or have I completely got the wrong idea?
Thanks guys.

 

[marcus]

Recently, I read of loop quantum cosmology, and how it indicates an eventual constriction of the universe to a point (CRUNCH), and then a sudden expansion (BANG).

Aguy,
I follow LQC research, check out many of the papers as they appear. I've never seen any LQC paper that indicates what you say.

So I am curious where you got the idea that LQC indicates a crunch in our future. If you can provide a link to a source, I'll check it out.

You could have been confused by seeing a discussion of various cases which were not intended to predict our universe behavior. LQC researchers run computer models of many different cases to study what happens depending on how you set the model up. Sometimes what they get is a one-time bounce.
Sometimes they get an endless repeating series of bounces (but that would not be talking about our universe, it would be a different set-up).

==========================

There is a side issue that may interest you. This has to do with classic cosmology but it carries over. Look up "deSitter space" in the Wikiped.
The deSitter universe is a classic 1917 solution to Gen Rel. that sketches the behavior of a simple one-time bounce universe containing nothing but dark energy----a universe with positive cosmo constant. It has no singularity.

You see collapse is compatible with dark energy----you have an infinitely long decelerating contraction which turns around and is followed by an infinitely long accelerating expansion.

Visualize it as two infinite cones, joined point to point but with a little smoothed-out waist (the equations do not predict a singularity in this case).

The deSitter universe is a simplified picture of our own, or what our own universe is gradually becoming, that is comparatively empty and dominated by dark energy. Einstein eqn is time reversible and what we see is an endless expansion in future. So you could just as well have an infinitely long contraction in the past. Equally realistic possibility. Just think of our presumed future, but run backwards.
Joining the two cones, or the two horns, back to back, happens naturally without a singularity in the original deSitter case (1917) because he has no ordinary matter, just dark energy. Matter dominates at high enough densities.

The new feature that LQC brings to this is that you get a bounce even with matter included. Because of quantum corrections that become significant at high density, making gravity repellent.

In other words you already had a one-time bounce universe with dark energy---already in 1917, classic---so what LQC brings you that's new is that you can include matter in that one-time bounce picture and it still works.

In itself the one-time bounce picture is an old familiar one to cosmologists. A collapsing phase extending infinitely back into the past has always been one of the models on the table.

My personal view is that it is ridiculous to try to pick winners at this point. The thing is to keep several pictures in mind as possibilities. We don't yet actually know for sure that dark energy is constant. An endless oscillating universe is still possible. We need to understand dark energy better. But the one-time bounce (with constant dark energy) is also possible.

 

[aguycalledwil]

Ah alright, I get what you mean. So dark energy won't always expand our universe. It will at some point trigger the collapse?

 

[marcus]

That is not what I said.
We could be in a situation where dark energy is perfectly constant and always has the same effect: it slows down contraction and it accelerates expansion. That is the simplest model of dark energy corresponding to the cosmo constant Lambda which Einstein put in the 1916 equation
and which deSitter used to get a one-time bounce.

In that situation we can only look forward to endless expansion. If we only have constant dark energy then there is nothing that will "trigger" a contraction.

 

[aguycalledwil]

Okay, thanks Marcus. I get what you're saying now. I appreciate your patients with me. I don't want to annoy people on the forums by getting the wrong idea of things, but I'm just so interested, I can't leave it without getting it :)

 

[marcus]

I get what you're saying...

I'm glad. What you asked is a question that more people than you might think get sidetracked by.
You got it comparatively quickly.

It is a bit like the parabola arc of a ball tossed into the air. Earth's pull decelerates it on the way up and accelerates it on the way down.

So a positive cosmo constant (or a constant dark energy) decelerates collapse and accelerates expansion. And what still impresses me a bit is that Willem deSitter, a Dutchman with a pointed beard, figured out an eternal one-bounce universe already in 1917.

And it's a model we are hearing more and more about. There was just a Scientific American article about by a group of researchers in Utrecht (Jan Ambjorn, Renate Loll...) who got deSitter universe to emerge as an average from a kind of quantum chaos of randomly self-assembling building blocks. Here's a link to the SciAm article:

http://www.signallake.com/innovation/SelfOrganizingQuantumJul08.pdf

The Utrecht group were excited because the deSitter universe is exactly the type that ought to appear. It is the simple universe that most closely resembles what we see in present reality and expect in the future.

As our universe continues to expand, matter will get so thinned out that virtually 100 percent of the total mix will be dark energy---so our universe will be approximately the same as the simple deSitter (in which there is no matter at all, only dark energy).
That is, assuming the cosmo constant---the darkenergy density--really is constant, which sofar it appears to be. So our more complicated reality will approximate more and more the simple deSitter model.

Since 1998, when people discovered there was dark energy, the deSitter model has become a focus of awareness. One sees more and more references.

It is good to know something about Willem de Sitter, I think.
http://en.wikipedia.org/wiki/Willem_de_Sitter
He is the guy in the upper right corner of the photograph. Einstein on the upper left.
BTW he and Einstein seem to have been the first people to propose that there might be a large amount of dark matter, which is a different issue from dark energy. I don't know much about this but I gather they wrote the first paper suggesting dark matter in 1932*.
Then Fritz Zwicky came out with observational evidence for dark matter the following year 1933.
http://en.wikipedia.org/wiki/Dark_matter

*This was when they proposed the universe model which is spatially flat and (just barely) keeps on expanding indefinitely. It is the k=0 Friedmann model. For that model to fit there has to be a lot more matter than what we can observe---actually some 20 or 25 times more, I think. So Einstein and deSitter suggested there might be all this dark matter we couldn't see. Zwicky then found evidence for it by measuring the speeds of galaxies in a swarm of galaxies. He found that there wasn't enough visible mass in the cluster to hold the galaxies in a bunch, they were going so fast they would escape. For the cluster to be permanent there had to be invisible matter adding to its mass. It is strange how the things we are most intensely curious about today---dark energy and dark matter---were already thought of and on the table back in 1917 and 1932.

 

[Nereid]

[...]

What still impresses me a bit is that Willem deSitter, a Dutchman with a pointed beard, figured out an eternal one-bounce universe already in 1917.
It's a model we are hearing more and more about. There was just a Scientific American article about by a group of researchers in Utrecht (Jan Ambjorn, Renate Loll...) who got deSitter universe to emerge as an average from a kind of quantum chaos of randomly self-assembling building blocks. Here's a link to the SciAm article:

http://www.signallake.com/innovation/SelfOrganizingQuantumJul08.pdf

The Utrecht group were excited because the deSitter universe is exactly the type that ought to appear. It is the simple universe that most closely resembles what we see in present reality and expect in the future.

[...]

This result from Loll et al. must surely be one of the most impressive to come from all the BTSM (beyond the standard model) work in physics!

Simple, elegant, ... and seems to be consistent with (all) the relevant observations!

 

[marcus]

Nereid thanks for the comment. I absolutely agree!

Okay, so being fourteen, I know that I'm way out of my league

Heh heh, at this point we are all just a bunch of monkeys, whether 14 or 40. Dark energy is a big mystery to everybody. Quantum gravity (the resolution of the big bang) is also big mystery. Why don't you read the Renate Loll SciAm article. Nereid said she liked it too.

I'm delighted you started a LQC thread in cosmology forum. The field of cosmology right now very much needs quantum gravity inputs. They are getting into resolving the singularity and figuring what might have been actually happening, and consequences to look for as tests.

... the fact that there is twice as much dark energy as dark matter in the universe. Surely this ratio would continually accelerate the universe's expansion, leading to a Big Rip, ...

Aguy, here is another thing for you to learn. A cosmo constant does not lead to a Big Rip.

Big rip scenarios are exotic fantasies based on a imagined type of dark energy that changes its physical nature (e.g. its "equation of state")----something the weight of evidence is increasingly against. The observation data increasingly points to the kind of dark energy that deSitter was originally talking about---constant density, constant physical nature. That kind of dark energy has very mild results.

The dark energy we observe appears likely to be a constant 0.6 nanojoules per cubic meter. That doesn't pull galaxies apart. It even allows Andromeda and Milky and some others to merge into a larger permanent galaxy.
So 100 billion years down the pike we Milkyway animals are simply living in a somewhat larger galaxy. Planets are not being pulled apart
The problems to be faced are more mundane like keeping food in the fridge and a breathable atmosphere (assuming we still like oxygen and macaroni salad.)

Larry Krauss is a real cosmologist and he recently did a most-likely standard model future piece that people could read to counteract the more sensational speculation that gets into the popular media. I'll get the link to it as time permits. Yeah, here:
http://arxiv.org/abs/0704.0221
No big deal, it just says if you take the current standard dark energy model universe (the LambdaCDM) and extrapolate forward in time all evidence of expansion disappears. Future people, unless they have access to data from the distant past, have no way of discovering the the universe is expanding. They live in their own (Milkyway-plus) galaxy and all other galaxies are too far away to see and the CMB is too redshifted to see and they
just watch television and dwell in innocent cluelessness forever. Very different from the Big Rip scenarios.

 

[xantox]

Big rip scenarios are exotic fantasies based on a imagined type of dark energy that changes its physical nature (e.g. its "equation of state")----something the weight of evidence is increasingly against.

Big rip happens with a constant equation of state w, but with a value inferior to -1. Since the scale factor of a flat universe evolves approximately as t^(2/3(1+w)) it follows that when w<-1 the evolution is exponential and diverging in finite time. Observation points to -0.5 < w < -1 and leaning towards the -1 boundary – so while this scenario is not the most likely one (as it implies a violation of the dominant energy condition), it still cannot be excluded. Of course being favored or not by observation is not a reason to not study big rip singularities, as big crunch singularities are even less favored by observation (though they are relevant for the study of the big bang singularity). In all cases LQC also avoids big rip singularities.

 

[marcus]

Big rip happens with a constant equation of state w, but with a value inferior to -1.

Quite right! thanks for the correction. w doesn't have to vary, it only has to be something less than -1. And -1 is what it is in the simple case where dark energy is just another name for cosmological constant Lambda. As you may have indicated the recent observational data does tend to favor a straightforward w = -1 Lambda interpretation, confidence intervals seem to be narrowing down on that case.

But even if w < -1 is unrealistic there are still reasons (as you say) to study it.