What if dark matter is unrelated to WIMPs?

[Carlos L. Janer]

Is it conceivable that dark matter only interacts gravitationally? Is SUSY losing her charm? I truly want to know what the experts think. I'm not qualified to have an informed opinion about this subject.

 

[Orodruin]

Is it conceivable that dark matter only interacts gravitationally?

If it only interacts gravitationally it would be very difficult to find a theory for how it is created in the right amount in the early Universe. This does not mean that it needs to interact weakly or even with a strength that is of the same order as the weak interactions. There are many models of dark matter where its creation is not attributed to thermal freeze-out.

Is SUSY losing her charm?

To some extent yes in the sense that the more simple and "natural" SUSY models have been ruled out. However, there still remain large parts of parameter space that is still fine so it is certainly possible that there is SUSY. It may just be that it is not so wonderfully elegant as may have been portrayed by SUSY proponents before the LHC turned on.

 

[Carlos L. Janer]

There are many models of dark matter where its creation is not attributed to thermal freeze-out.

Cosmological models or particle physics models?

 

[mfb]

For the big bang, both are relevant.

Dark matter that only interacts gravitationally is the worst case for our attempts to find it on Earth.

 

[Carlos L. Janer]

For the big bang, both are relevant.

I apologyze in advance for what I am about to ask. You have already tried to explain it to me before, but I seem to be unable to get it:

- What is the point of using the standard model of particle physics in cosmology? The cosmos is intrinsically curved in space-time, the Poincare group is not a symmetry of the universe. How can it be a good description or even relevant of whatever happened in the early universe?

- How did Hawking managed to combine general relativity and particle physics to predict that a black hole emits radiation?

 

[mfb]

We don't know what happened at the very early phase of the universe, but at the energy scale where our theories work spacetime was extremely flat. QFT in flat spacetime works well there. You don't take the gravity of Earth into account to model LHC collisions for the same reason.

- How did Hawking managed to combine general relativity and particle physics to predict that a black hole emits radiation?

It is not a full combination, as he did not quantize gravity. QFT on curved spacetime is tricky, but some simple results (like the emission of radiation) can be done there as well.

 

[Carlos L. Janer]

It is not a full combination, as he did not quantize gravity. QFT on curved spacetime is tricky, but some simple results (like the emission of radiation) can be done there as well.

Is there any consistent formulation of QFT in a curved space-time? (I'm not thinking about quantum gravity). How dou you describe a fermion field in a curved space-time? Can it be done?

 

[mfb]

Is there any consistent formulation of QFT in a curved space-time?

You can write it down. The question is can you also calculate it.

But it does not matter for the early universe, as I said (repeatedly). So where is the point?

 

[Carlos L. Janer]

You can write it down.

Can you? Loosely speaking, in the standard model of particle physics, you start with two irreducible representations of the Poincare group. Then you realize that if the two internal global symmetries of these two representations are forced to be local, you end up with a Lagrangian which looks beautiful and, when second quantization is applied, describes particles that interact and have acquired mass.

How do you do that with a set of generalized coordinates that change from one 4-dimensional point to the next? Poincare's symmetry group is essential in QFT. What do you do if you don't have that symmetry group to start with?

 

[Orodruin]

I suggest starting by reading up on a set of lecture notes regarding QFT in curved spacetime, eg, https://arxiv.org/abs/gr-qc/9707062

 

[Carlos L. Janer]

I suggest starting by reading up on a set of lecture notes regarding QFT in curved spacetime, eg, https://arxiv.org/abs/gr-qc/9707062

Thank you very much for the tip. I just have one question: Am I talking nonsense? Is this a dead-end path?

 

[fresh_42]

Thank you very much for the tip. I just have one question: Am I talking nonsense? Is this a dead-end path?

That's a question about personality. To those being on the hunt for quick and short answers? Probably.
However, those who gets inflamed by the idea and want to find out, what's behind it, it might as well be the start of a career in physics.

 

[Carlos L. Janer]

That's a question about personality. To those being on the hunt for quick and short answers? Probably.
However, those who gets inflamed by the idea and want to find out, what's behind it, it might as well be the start of a career in physics.

OK, thank you very much. You've given me a honest answer that I really appreciate.

 

[Gerhard Mueller]

Is there any hint, apart from its possible cosmological origin, that dark matter can't be normal matter?

 

[ChrisVer]

hint ... that dark matter can't be normal matter?

if it was "normal" matter we would be able to see it.

 

[mfb]

It does not interact with the electromagnetic interaction.
Its distribution is different from the distribution of normal matter.
It influences the CMB in different ways, and we can see the difference.
We know the amount of regular matter.

 

[Gerhard Mueller]

If it is cold and dense matter like the matter of objects in the Kuiper Belt how would we be able to see it?
What kind of electromagnetic interaction could we expect from far distant cold objects, which are also far away from sources of electromagnetic energy?
What kind of influence on the cosmic microwave background can we expect from such objects?
We know the amount of regular matter...This is related to the question of the possible cosmological origin of such kind of objects.

 

[ChrisVer]

.This is related to the question of the possible cosmological origin of such kind of objects.

still they don't add up...
https://en.wikipedia.org/wiki/Massive_compact_halo_object

 

[ChrisVer]

also when we say that they don't interact electromagnetically it doesn't mean that they have to somehow radiate something away- but they are more or maybe less transparent to the light that goes through them... When light for example pass through a cloud of gas, its spectrum will be altered (eg some frequencies of the light are going to be absorbed)

 

[Gerhard Mueller]

Yes, but this is exactly what I mean, according to the current theory, enough "Halo Objects" to explain galaxy dynamics can't exist.
But it seems that there is no direct proof, that they do not exist.

 

[mfb]

There are not enough heavy elements around to form so many solid objects. And gas is interacting with radiation in a detectable way.

What is a "direct" proof, apart from going there and looking around with a flashlight?

 

[Chronos]

Actually, CMB and other data offer overwhelming evidence that the mass 'missing' in the universe cannot consist of objects too faint to be seen with telescopes [i.e., asteroids, planets, dim/failed stars, black holes, etc., aka MACHOS]. For a discussion that does not require an advanced degree in mathematical physics to understand how and why dark matter was tried and convicted for crimes against physics and human intuition, see https://ned.ipac.caltech.edu/level5/March10/Garrett/Garrett3.html

 

[Gerhard Mueller]

The text in the link says:
...
B. Cosmological Evidence
BBN is a period from a few seconds to a few minutes after the Big Bang...
......
The conclusion is that based on the Big Bang theory, the mass 'missing' in the universe cannot consist of objects too faint to be seen with telescope.
But the question is: Are there any observations, which directly proof the existence of dark matter.
The observation could be the presence of a gravitational lense in a region penetrated by intense radiation. If the cause of the gravitational lense is real matter, it should become visible in such a region.

 

[Chronos]

The Bullet cluster and similar examples are peculiar in one outstanding respect: gravitational lensing shows the center of mass of colliding galaxies is not located where the apparent center of visible mass in the clusters is located according to x-ray observations. This is generally considered smoking gun evidence that dark matter sailed through unhindered by the collision between galaxies, whereas gas associated with these galaxies was left behind where the collision occurred like derelict cars from a titanic traffic crash. That's about as close to direct evidence of dark matter offered by nature as it gets.

 

[Gerhard Mueller]

From extended clouds of larger cold objects, which are only weakly coupled via gravitation with their own galaxy , such a behaviour "sailing through without noticeable collision interaction" also could be expected. The fact that remaining gas can be observed at the collision point, rather indicates that real matter has been involved in the collisions.
Aren't there any more obvious hints for the existence of something which even dominates the gravitational behaviour of galaxies, but never has been detected within our near environment?
May be some smoking gun like evidence can be derived as follows:
After a very long time, the trajectories of all objects in a glaxy must reach some kind of an equilibrium with mass concentrated to attractors. This kind of equilibrium should be accessible via simulations and then be comparable to observations. The simulations can start with equally distributed clouds of real matter objects of random mass (sparsely distributed in the cloud) or with an equally distributed cloud of dark matter.

 

[Chronos]

Such simulations have already been conducted: The Illustris project is one that might be of interest http://www.illustris-project.org/about/

 

[mfb]

Universe simulations first simulate the dark matter part only, and then let regular matter follow the gravitational potential given by dark matter. The evolution of dark matter is exactly what leads to the observed structure formation. Simulating regular matter without dark matter does not lead to reasonable results.

 

[rootone]

Yet I recently read that more NEOs of around 50m or so are being discovered, which was not expected.
This does not imply a greater risk for one of them impacting Earth in the near future.
It does imply though that there is more normal baryronic matter near Earth than had thought to be.
So why not everywhere else as well as the solar system, or just everywhere?

 

[mfb]

If you discover more dust than expected under your bed, it does not mean the mass estimate of Earth increased. It just means the distribution was not exactly as expected.
The methods to estimate the number of near-Earth objects of a given size in the solar system and the methods to estimate the overall amount of baryonic matter are completely independent. The overall mass of all NEOs is negligible anyway.

 

[Gerhard Mueller]

You wrote:
"The evolution of dark matter is exactly what leads to the observed structure formation. Simulating regular matter without dark matter does not lead to reasonable results."
Do you have a source of this reasonable/not reasonable simulation results?
The "Illustris-Project" simulation results seem to be still far from matching to observations:
(Future Directions ..."Although the general trend is captured, galaxies with stellar masses below approximately 10exp(10.5) solar masses are a factor of 2-3 too old when compared to observations."...)

 

[mfb]

The millenium simulations are the most prominent example, see the external links there for more details.

Regular matter without dark matter is in conflict with all the independent observations of dark matter, the last point there is about the formation of structures in the universe.

The simulations don't agree perfectly, but there is a huge difference between "some details are not so accurate" and "we don't get any structure formation".

 

[nikkkom]

Yes. If anything, Illustris achieved an amazing level of detail and matches pretty closely.