# Dark matter and the Standard Model

1. Feb 4, 2008

### jnorman

At this point, i am not yet convinced that enough data is in on the subject of dark matter/dark energy to support expansion of the standard model to include them, but, then, i am just a lowly layperson...

How do you guys see dark matter and dark energy fitting into the standard model? how likely do you think it is that those concepts could actually be real, as opposed to some aspects of gravitational theory not yet being completely understood? (yes, heresy, heresy...) thanks.

2. Feb 4, 2008

### Coin

I wrote a long post here about different "non-matter dark matter" candidates I'd heard about and what in my amateur's opinion seemed to be their current status with regard to the scientific consensus. Then I accidentally deleted it. Arrgh..

I guess the only really important thing is, the dark matter debate seems to have been really seriously tipped by the bullet cluster observation from 2006. In that case an astronomical observation was made which astronomers consider to have been an actual direct observation of dark matter.

In the bullet cluster, two galaxies were seen to collide. When the two galaxies collided, the normal matter in the galaxies slowed down as it all collided with each other, but the dark matter in each galaxy just kept going. The dark matter in this case cannot be anything that interacts, or else it would have been caught up in the collision too. And it can't interact with or emit light, because we can't see it. But it must be there, since we can observe its presence by the gravitational lensing it causes. And it must be "stuff"-- matter, I guess-- because it behaves like it, because it was traveling along with the galaxies until they collided and the dark matter was flung out. So this seems to demonstrate there is something which acts like matter but interacts only by the gravitational force. Since nothing in the standard model fits this description, it must be that we will need to extend the standard model to accommodate it. (One interesting thing to look up here is the minimal dark matter model, which shows us that we don't need to extend the standard model much!) This observation seems to have convinced almost all physicists that dark matter is explained by some specific kind or kinds of as-of-yet unobserved particle.

I personally have confused philosophical issues with the whole dark matter idea (it's practically everywhere, right? so where is it? shouldn't there be, like, some here, like in the solar system?), but I don't see any other way to explain the bullet cluster data.

3. Feb 6, 2008

### Barmecides

Hello

- for me, one of the key measurement is that baryonic dark matter is < 3% (deuterium measurement, how far can we trush this measurement ?) => dark matter has to be something new.

- Maybe dark matter is also a mixture a different new particles.
For example, it seems that SUSY LSP has a too small cross-section to explain current dark matter constrains.

So, what are the candidates ?
- RH neutrinos ?
- SUSY LSP and NLSP
- new SU2 doublet (like inert Higgs doublet)

ps : a subsidiary question, why not plenty of small black holes ?

4. Feb 7, 2008

### John Richard

A good link to the WMAP page on dark matter / dark energy is:
http://map.gsfc.nasa.gov/m_uni/uni_101matter.html
They seem to suggest there is a hell of a lot more of that than anything else in the universe. They have calculated that 96% of the energy density of the universe is a form of substance that has never been detected in the laboratory.

23% cold dark matter and 73% dark energy.

Since this is a section on ideas beyond the standard model, what do you folks think about the posibility that the dark matter / energy is gravity?

5. Feb 8, 2008

### Barmecides

I think this is the MOND hypothesis what you mean which is not very supported alternative.

Concerning dark energy, it has to be clear, this just means that the $$\Lambda CDM$$ does not fit observations without introducing Einstein constant. So you either have to had a new energy term (like Einstein constant or a field) or change the geometric term or remove one of the hypothesis (like Universe described by Robertson-Walker metrics).

6. Feb 8, 2008

### John Richard

Thanks for the clues Barmecides, I will seek the MOND hypothesis out for sure.
I understand that dark energy is just a way of saying we have observations we can't properly account for yet. It has to be agreed that this is quite an exciting period for cosmology though, given the recent and ongoing WMAP observations.

John

7. Feb 8, 2008

### ZapperZ

Staff Emeritus
One should also take note that Dark Energy is still a relatively NEW concept. We really need a lot more observations before we can say anything more on it. So let's try to be patient and wait until the Dark Energy Survey project can get off the ground and tells us more about its properties.

Zz.

8. Feb 8, 2008

### John Richard

Ha, ha, ha, ha, that's a tall order ZapperZ. Do camels have humps? Do laymen speculate when they really 'should' wait?
Point taken though!

9. Feb 8, 2008

### jnorman

NO - i cant wait!! i need to completely understand reality RIGHT NOW!!!

i guess part of my original question was:
Why do the theorists think it is more realistic to conceive of, and search for, undetectable entities like dark matter/dark energy, than to assume there might be some problem with our understanding of gravity on a extra-macro level, or perhaps a need to re-introduce a variation of einstein's constant?

10. Feb 8, 2008

### jal

There is a lot of speculation about dark matter.

When reading about the research concerning solid hydrogen and solid neutrons, it appears that there is expectation that the next experiment could be the turning point in creating a solid phase. This would change the explanations of the big bang and dark matter.
See my blog for the info that I have gathered.
jal

11. Feb 8, 2008

### John Richard

Your question is answered by the fact that the theorists, in fairness, are looking at all the angles, one of which is dark matter. Try this link, it deals with just the issue you are questioning: http://cosmicvariance.com/2006/08/21/dark-matter-exists/

John

12. Feb 8, 2008

### Coin

Would you agree though that we know rather a lot about dark matter? At least insofar as knowing where it is in the universe goes.

13. Feb 8, 2008

### EL

At first sight, introducing more parameters into the laws of gravity, or introducing more parameters into the standard model in order to solve the dark matter problem may seem on equal footing. The reason why particle dark matter has become the major candidate is because it is capable of fitting observations, something modified gravity has big problems with. If you want to read a long discussion about this, you may look here: https://www.physicsforums.com/showthread.php?t=194931

The reintroduction of the cosmological constant is indeed the main model for dark energy.

14. Feb 8, 2008

### EL

15. Feb 8, 2008

### John Richard

Hasn't the path of physics been one of complementarity of investigation. The smaller gets smaller, the bigger gets bigger. Microscopes cracked open the molecule and telescopes broke the bounds of space. Partical accelerators smashed the atom and rockets took radio telescopes into space.

With each step, the new observations demanded a modification, the options are always the same:
1) New substances are discovered that explain the new observations and allow the old model to continue working.
2) New theories are developed allowing new observations to find a place in a new or modified model.
3) New substances are discovered that explain the new observations but still demand a new or modified model.

Emergent phenomena allow us the unique and privileged opportunity to play the investigation game, and we do it by mixing and matching the three options with further observations and experimentation.

Just as there are no failed experiments because we always gain something from the results, even if it is just to tell us that our search was fruitless. So there are no wasted ideas, provided they are testable. Because by this process of reduction we zero in on the more likely.

Although this is beyond my level, perhaps we should be speculating on possible methods for locally testing the various explanations put forward to explain the new observations. As ZapperZ indicated earlier in the thread, we need more information before we draw conclusions and getting that information is perhaps what we should be focussing on.

Remember, we only know about gravity because we observe its effects. It is very local, but still we don't know what its fundamental properties are. One of the possible solutions to understanding gravity is the theorised existence of the graviton partical. I have no idea if the existence of the graviton, (if it was found), would contradict general relativity. But it is a very similar form of debate to the one about dark matter or not dark matter

16. Feb 9, 2008

### jal

John Richard!
A very good post.

"... getting that information is perhaps what we should be focussing on."

The problem is that if a person considers him/herself an expert and has made up his/her mind then more info is irrelevant especially if it would be in disagreement with their faith/conclusions.

Fortunately, this forum, contains a majority of "seekers" willing to evaluate new information.
jal

17. Feb 11, 2008

### Barmecides

I read it in some arXiv. Recently I have observed quite a lot of theoritical activity (I'm experimentalist in HEP) in this domain. It is less elegant than SUSY, but much more simpler. And it can fullfil electroweak constains.

Last edited: Feb 11, 2008
18. Feb 19, 2008