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## Could Dark Energy be Dark Matter cooling?

This is amazing - I posted this question 2 years ago and then forgot about it - and just found it in my bookmarks... I love that a simple question has generated 3 pages of good debate from clever people... Job done!

Thanks all for the replies.

In the career's thread I always mention that I've been fascinated with the politics and economics of astrophysics, and the HET is one of the things that got me interested in that.  Let's go back to the 1980's.  Oil was super expensive, Texas was getting flooded with money, and people at the universities there were talking about using oil money to make some big enormous telescopes.  Then the boom collapsed, and people looked at the money available and there wasn't enough to build the perfect telescope.  So then what people did was to figure out how to maximize science and minimize cost.  Most of the cost of a telescope turns out to be in the mount.  You have a multi-ton piece of metal and then you have this big machine to point the telescope at what you want to point it at.  Another big cost is the internal mirrors.  In order to get an image you need to mount a giant mirror inside that that makes thing expensive.  So people figure out that instead of making a big expensive general telescope, you could with very little cost make a telescope that's really cheap but it good at one thing, which is doing spectroscopic surveys.  Now if you are doing galaxy surveys, the cool thing is that you don't care where you point your telescope at.  You point your telescope at some random part of the sky and if it's not getting blocked by the Milky Way, then any random part of the sky is as good as any other random part.  So you remove the ability to point the telescope everywhere and that saves \$ since you don't have huge motors.  Now you are doing spectroscopy.  That means no images.  Without images you can then use fiber optics to move light from the top of the telescope down to the instruments  Again lots of  savings.  Since you don't have to spend money pointing the telescope or dealing with internal mirrors you can use some of that to make the big light bucket at the bottom bigger.  What's really cool is that all of these decisions were being made in the late-1980's and early-1990's before people had even dreamed of dark energy.  It turns out that HET is the perfect instrument for studying dark energy, but none of the designers realized that.

Gosh. It's really not *that* complicated.  We already know one "dark matter" particle: it's neutrino. It has mass, but does not interact electromagnetically. If a narrow beam of one solar mass of neutrinos would fly through Solar System, we wouldn't see it, but sure as hell we will feel its attraction. (Thankfully, neutrinos never travel in such humongous massive tight beams)  So dark matter not merely CAN exist, we know it DOES EXIST, at least a part of it, and we are pretty confident what it is. It is not mysterious.  But since we also are pretty sure that (known) neutrinos alone can't explain what we see, it's not too difficult to postulate that there exist other similar particles: ones which have mass, but do not interact electromagnetically. To match observations, we postulate that these particles are more massive than (known) neutrinos.  So why are you guys so freaked out by "mysterious" dark matter? Are you feeling the same about neutrinos? I think not...

Quote by nikkkom 					 				 				But since we also are pretty sure that (known) neutrinos alone can't explain what we see, it's not too difficult to postulate that there exist other similar particles: ones which have mass, but do not interact electromagnetically. To match observations, we postulate that these particles are more massive than (known) neutrinos.
It is in fact *very* difficult to postulate that there are other similar particles.

Basically you start off with one equation that describes how particles behave. It's very, very difficult to add a new particle without causing the equation to either predict things that we don't see or be inconsistent theoretically.

One particular problem with massive particles is that massive particles will decay into less massive particles unless there is something that prevents that from happening. For example, there is something called baryon number. Because the proton is the lightest particle with a non-zero baryon number, it can't decay to something lighter. However heavier particles can and do decay into the proton.

So if you invent a "heavy particle" you are going to have to mathematically describe how that particle interacts with other particles, and this is rather difficult to do without tripping of something that we already know

So why are you guys so freaked out by "mysterious" dark matter? Are you feeling the same about neutrinos? I think not...
Everything is easy until you know why it's hard. The bottom line is that you just can't randomly add a particle. Adding a particle is like adding an element in the periodic table. If you want to add something at the end, no problem. If you want to add something between carbon and nitrogen, big problem.

You can graph the known particles and the form a nice chart. There is no obvious place to put another particle. You can assume that there is a heavier neutrino, but that means you need a heavy quark and all of that violates experiments that say that you have only three families of particles.

Quote by twofish-quant 					 				 				It is in fact *very* difficult to postulate that there are other similar particles.  Basically you start off with one equation that describes how particles behave.  It's very, very difficult to add a new particle without causing the equation to either predict things that we don't see or be inconsistent theoretically.
You did not understand me.

I am not saying that to add a particle to the Standard Model is very easy. I am somewhat familiar with the math involved, I know that it's not trivial.

I am saying that some people seem to think that postulated dark matter is a very unusual kind of matter we never saw before, and thus they find it hard to believe it may be a viable theory. But dark matter is not something unlike we ever saw before - neutrinos are similar to it, and we know about neutrinos for what, 80 years already.

Quote by twofish-quant 					 				 				You can graph the known particles and the form a nice chart.  There is no obvious place to put another particle.  You can assume that there is a heavier neutrino, but that means you need a heavy quark and all of that violates experiments that say that you have only three families of particles.
Well, how about right-handed, so-called "sterile neutrinos"? Seesaw mechanism which gives them large mass? People are working on such models right now...

Quote by nikkkom 					 				 				I am saying that some people seem to think that postulated dark matter is a very unusual kind of matter we never saw before, and thus they find it hard to believe it may be a viable theory.
Dark matter passes the principle of "least weirdness". It's weird but everything else is even weirder.

But dark matter is not something unlike we ever saw before - neutrinos are similar to it, and we know about neutrinos for what, 80 years already.
We know that ordinary neutrinos are *not* similar to dark matter. You can invent something about weird neutrinos. One other thing is that even if you restrict yourself to baryonic matter, most of that material is dark.

Well, how about right-handed, so-called "sterile neutrinos"? Seesaw mechanism which gives them large mass? People are working on such models right now...
Yes, exactly....

http://arxiv.org/pdf/1102.4774.pdf

http://arxiv.org/abs/1204.3902

But the point here is that you just can't invoke a new particle. Every time you invoke a new particle you have to do a ton of work to justify that new particle.

Quote by twofish-quant 					 				 				We know that ordinary neutrinos are *not* similar to dark matter.
Are you argumentative a-hole or something?

I am not going to argue what level of similarity is required to qualify for word "similar".

If you think neutrinos are sufficiently different from hypothetical dark matter particles (they have different mass! wooo hooo) so that word "similar" can't be applied, feel free to think that way. I don't care.

One other thing is that even if you restrict yourself to baryonic matter, most of that material is dark.
"Dark matter" is a misnomer. It is not in fact dark, it seems to be transparent. Baryonic matter is not. Even as a dilute gas, it is detectable by observations in EM.

Recognitions:      Gold Member        Science  Advisor  Neutrinos are very 'hot' compared to dark matter. The only DM models that appear to work require non-relativistic velocities.

Calm down.

Quote by nikkkom 					 				 				If you think neutrinos are sufficiently different from hypothetical dark matter particles (they have different mass! wooo hooo) so that word "similar" can't be applied, feel free to think that way. I don't care.
This is the problem with these sorts of arguments. You see two things and they are "similar". I see the same two things and they aren't.

Now there are reasons why neutrinos don't look the same as other particles to me is that I did a lot of research on neutrino radiation hydrodynamics. To me saying that the dark matter particle and neutrinos are similar because they both interact with the weak force only is like saying that a bowling ball and an orange are similar because they are both round. This doesn't make much sense to a professional bowler or an orange grower.

Now I'm not going to get annoyed if someone says that things look similar. Just don't get too annoyed at me if I tell you that they don't look similar to me, because they don't.

Remember ***you*** are the one that asked:

So why are you guys so freaked out by "mysterious" dark matter? Are you feeling the same about neutrinos? I think not..
Now I answer the question by saying, WIMP's may look like neutrinos to you, but to someone that has researched neutrinos for a decade, they look very different to me. Now if they look the same to you, that's fine, but you asked the question.

"Dark matter" is a misnomer. It is not in fact dark, it seems to be transparent. Baryonic matter is not. Even as a dilute gas, it is detectable by observations in EM.
If it's not ionized or not in compact bodies. Ionized hydrogen is quite difficult to detect. One interesting thing is that if you add up all of the baryonic matter that we can account for, it's still much, much less than the amount that we infer is out there from cosmology.

Quote by JcX 					 				 				The idea of dark matter comes from mathematical model of gravitational force. Scientist suggests that there must be matter that we don't see in our universe, maybe between galaxies that exert the gravitational effects that we feel. The content/percentage of dark matter is chosen to fit and balance all the gravitational effects that has been observed in the universe (expansion of universe, perturbation of orbits, etc). We predicted the existence of them, but we can't be really sure of what dark matter is.  Some theories such one those who suggest multi-dimensional universe says that dark matter is actually some matters in other universe. String Theory says that only gravitons are able to escape from the membrane of the dimension, hence only gravitational forces are able to penetrate between dimensions. Living in this dimension, we can feel the gravitational effects from other dimension, but we don't see them, that's why we called them Dark Matter. That's one of the explanation that I liked a lot... although...... it's hard to imagine.
Is it ?
I was led to believe that it was more of an observational find rather than resolving a mathematical discrepancy.

We are trying to answer the accountability of mass surrounding the center of galaxies. Basically , Astronomers assumed that mass density (hence radial speed) would decrease as the distance from the galactic radius increases , however, to their surprise it did not and stayed fairly constant.

There are various contenders of DM: Mainly the WIMPs , MACHOS however recent studies highlight towards WIMPs.

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dark energy, dark matter

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