# Dark Matter : Is it science?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by The God, Mar 8, 2017.

1. ### The GodValued Senior Member

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Is the gravitational interaction between DM and baryonic matter same as gravitational interaction between DM and DM?

3. ### Xelasnave.1947Valued Senior Member

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I dont know enough about the matter to comment however it seems the observations point to a vast amount of hidden mass and professionals provide theories consistent with our best models.
What more can they do?
I do recall in an old astronomy magazine Vera Rubin saying she would rather see our, then, theories ammended rather than introduce a new particle but I suspect she had to move past her "feeling" and lets face it you can bet she understood the su ject better than either of us. That was a long time ago yet the need for dark matter has not gone away.
I think even MOND needs dark matter, less appparently but even that approach requires something more.
I think you expect science to have all the answers which is probably unreasonable.
We know what we know but we do not know what we dont know, we can only proceed upon what we know if something is wrong we will eventually find out where we are wrong.
I am still unsure of your exact critisizm of the science maybe as this thread unfolds I will understand your possition better than I do now.
Alex

5. ### The GodValued Senior Member

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There is no criticism here. It is just a question if DM concept qualify as scientific hypothesis.

7. ### Xelasnave.1947Valued Senior Member

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I think it does but I am not a scientist
Perhaps you could review the definition and defermine if it fits.
Alex

8. ### spidergoatValued Senior Member

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I don't know what self sufficiency means in this context.
What are the minimum requirements for a scientific hypothesis?

9. ### rpennerFully WiredStaff Member

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That's why
1) DM is clumpy, so clearly has gravitational self-attraction
2) Many probes of cosmology alone can't distinguish between DM and baryonic (normal) matter in that they have the same relation with GR.

So the big constant is
$h \Omega_m \equiv h \Omega_{\textrm{matter}} = h \left( \Omega_{\textrm{baryonic matter}} + \Omega_{\textrm{cold dark matter}} \right) \equiv h \left( \Omega_{b} + \Omega_{c} \right)$​
See 25.3.2, 25.3.4, 25.3.6, and 25.4 of my first technical reference. $\Omega_{b}$ is precisely determined by the CMB studies because $\Omega_{b}$ is the part of $\Omega_{m}$ which couples to the CMB. See paragraph 5 of 25.3.3.

When everything that checks $\Omega_{m}$ checks out, and everything that checks $\Omega_{b}$ checks out, and $\Omega_{\textrm{hot dark matter}}$ doesn't fit observations (see paragraph 2 of 25.2.2), then $\Omega_{c} = \Omega_{m} - \Omega_{b}$ is pretty trivial math for cosmology.

In addition, direct observation of galaxy clusters requires $\Omega_{c} \geq 0.1$ which is, again, consistent with observation. See 26.1.1. of the second technical reference.

10. ### SchmelzerValued Senior Member

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No. Dark matter is something which has, in its actual version, well-defined properties: It consists of massive particles which interact only gravitationally.

Dark energy is in its most popular mainstream version simply a name for the cosmological term in the Einstein equations.

So, above are well-defined as physical things, have observable effects.
We already have matter which interacts gravitationally but not electromagnetically, neutrinos. Anyway any theory which unifies them has to be able to handle different EM charges, which includes the EM charge 0.

Of course, one can name DM, in the way it appeared in modern cosmology, ad hoc. But today it has become simply something we observe, in galaxies, by observing their gravitational field.
Yes.

11. ### The GodValued Senior Member

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Schmelzer, Rpenner..

There is no doubt that DM interacts with itself gravitationally. My question was more on the quantum of gravitational attraction among DM. Why would it be same as between DM and baryonic matter? And what's holding it from not collapsing to form a BH?

12. ### SchmelzerValued Senior Member

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Gravitational interaction is, according to GR, universal. There is no such animal as a gravitational charge, which may differ between DM and usual matter. So, this is simply GR, nothing more.

Then, you should not forget that the other interactions, like EM, play a role in the formation of stars. Say, a usual piece of matter hits the Earth. What happens? It becomes part of the Earth, increases its mass. What would happen if a similar piece of dark matter would hit the Earth? It would simply go through, come out down under with the same velocity it has hit the surface here, and fly away.

And this is what happens on the very large scale too. Remember that picture of the collision of two galaxy clusters? They hit each other, with their galaxies, their intergalactic H clouds, and their dark matter clouds around them. The final picture showed that the H clouds where unable to go through each other, so that there remained a big unified H cloud, which showed itself by radiation. Instead, the galaxy clasters have gone through each other, simply because the probability that some of the galaxies hit each other was too small. And, then, they measured, by looking how light was distorted by it, what happened with the dark matter, and the result was similar, it stayed together with the clusters themselves.

Some attempts to observe dark matter simply hope that it is not completely dark. So, if it goes through like neutrinos, which have no EM charge, but interact when they hit an atomic kernel because of weak interaction, so that they travel undistorted until they hit a kernel. We may not see it, but a few of them would stop to travel, and, in particular, leave, together with their mass, also some energy here, the energy transferred to the kernels they hit. It seems, one can actually exclude this, we would have to be able to see such a temperature effect if it would exist, and if most of the dark matter would be of this type.

13. ### The GodValued Senior Member

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The kernet agitation by DM experiment is not fruitful yet.

Basically you are talking about motion (temperature) of DM particle, without qualifying how it acquires. My point is, what prevents continuous clumping and subsequent formation of DM BH?

14. ### SchmelzerValued Senior Member

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Nothing but that this needs time. Much more time than for usual matter to clump and form a BH. The point is that clumping is much easier for particles which interact. Gravitation is, last but not least, the weakest of the forces, many orders weaker than the usual forces.

Of course, dark matter clumps. The size of the clumps are that of galaxy clusters and galaxies.

15. ### The GodValued Senior Member

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I do not know when DM formed, along with baryonic matter or later..but was these billions of years not sufficient to produce a single DM BH? Gravity is very weak, no doubt, but this very gravity overcomes NDP and BH forms. So either there must be some unknown force physics for DM which prevents massive compaction of DM clump or something is amiss about the very concept of DM.

16. ### SchmelzerValued Senior Member

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DM formed probably along with baryonic matter, and some but far from all baryonic matter is yet in BHs. Now compare with dark matter, which, as explained, will create BHs only after a much longer time, once the forces which help to create it are much smaller. So where is no need of anything preventing BH formation, not enough time yet is enough for an excuse.

17. ### The GodValued Senior Member

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Much longer time?

We have a baryonic BH detection 3 billion years old (ago not age). So if baryonic BH can form in 10 billion years after BB, why cannot DM BH form in 13 billion years? Or it needs still more time? How much more, any data, any forecast?

18. ### SchmelzerValued Senior Member

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Think about the example I have given about a piece of normal matter hitting the Earth and a piece of dark matter. Then think about how much time the Earth would need to become a BH if it catches all the dark matter which hits it, and how much if it catches only usual matter. I have not made computations, but I would expect that the difference would be a large factor. How large? I don't know, but I would guess something of order of 1000 or 1 000 000, certainly not as small as 1.3.

19. ### NachoRegistered Senior Member

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At our present level of capabilities in detecting BHs, how could you tell any difference between a regular one and a DM one? Couldn't we have already found a DM BH and not even know it?

20. ### The GodValued Senior Member

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Good question. I have no answer. Schmelzer guess is that time has yet not come for DM BH.

21. ### exchemistValued Senior Member

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At the risk of exposing my shallow knowledge of this subject, I had thought the point about a black hole was that the matter comprising it had effectively vanished and the only influence it had on the rest of the universe was its gravitation.

If that is right, then I would have thought a black hole formed from dark matter would be indistinguishable from one formed of ordinary matter.

22. ### rpennerFully WiredStaff Member

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You can't ask "why" questions without stopping at fundamental physical theory. According to GR all mass is treated the same. According to observation, what we see of Dark Matter's behavior makes it look like it obeys GR.
Conservation of angular momentum and the very darkness of dark matter makes it difficult for clouds of it to collapse. Collapsing dark matter would speed up = heat up = resist further collapse and unlike a dusty mostly-hydrogen cloud, there is no mechanism to radiate away that heat.

Last edited: Mar 9, 2017
23. ### The GodValued Senior Member

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you specifically talk of DM heating and subsequent lack of radiating away mechanism, suggesting that radiation indeed forms during collapse. What is radiation for DM, can it produce radiation? Or it is due to collateral collapse of surrounding baryonic matter?