# NASA pictures of dark matter collisions

1. Mar 27, 2015

### Staff: Mentor

http://www.nasa.gov/press/2015/marc...that-may-help-identify-dark-matter/index.html

The take away seems to be that some ideas about dark matter may have problems. The possibilities checklist of dark matter interactions has been shortened.

There are some really great pictures of galactic halos in colliding galaxies. Which make looking at the link a must. IMO.

2. Mar 27, 2015

### Greg Bernhardt

What about the hubble telescope makes the image blue?

3. Mar 27, 2015

### Chalnoth

The blue blobs are estimates of the mass of the galaxy cluster by examining the distortions of galaxies behind the cluster due to the cluster's gravity.

4. Mar 27, 2015

### marcus

http://arxiv.org/abs/1503.07675
The non-gravitational interactions of dark matter in colliding galaxy clusters
David Harvey, Richard Massey, Thomas Kitching, Andy Taylor, Eric Tittley
(Submitted on 26 Mar 2015)
Collisions between galaxy clusters provide a test of the non-gravitational forces acting on dark matter. Dark matter's lack of deceleration in the bullet cluster collision' constrained its self-interaction cross-section \sigma_DM/m < 1.25cm2/g (68% confidence limit) for long-ranged forces. Using the Chandra and Hubble Space Telescopes we have now observed 72 collisions, including both major' and `minor' mergers. Combining these measurements statistically, we detect the existence of dark mass at 7.6\sigma significance. The position of the dark mass has remained closely aligned within 5.8+/-8.2 kpc of associated stars: implying a self-interaction cross-section \sigma_DM/m < 0.47 cm2/g (95% CL) and disfavoring some proposed extensions to the standard model.
5 Pages, 4 Figures and 18 pages supplementary information
Science, Vol 347, Issue 6229 (2015)

5. Mar 27, 2015

### wabbit

6. Mar 27, 2015

### Staff: Mentor

What exactly does this mean?

7. Mar 27, 2015

### marcus

Hi! It's interesting how they map invisible mass concentrations using so called "weak lensing" of background shapes. I know you're familiar with this but someone new to it might not be.

Background shapes get "squashed" in the direction of increasing mass. Circles become ellipses elongated in the direction perpendicular to where the mass is. So the short axis of the ellipse, the "minor axis" will tend to be aligned along the mass gradient. So they can actually produce contour maps of the distribution of invisible mass.

Statistical methods are needed because the background shapes are not perfect circles. They are roughly circular galaxies but tilted randomly so that they appear ellipses oriented in random directions. As their light comes to us, passing the mass concentration, there is a further elongation or, to put it another way, squashing of the shapes.

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8. Mar 27, 2015

### Chalnoth

If you have one particle of dark matter passing through a cloud of dark matter particles, this cross section gives the expected distance before the dark matter particle is deflected.

The mass is a part of the calculation because the particle mass determines how many particles there are (we know how dense the dark matter is, but a dark matter particle with half the mass would require twice as many particles to make up that same density).

9. Mar 27, 2015

### marcus

If DM particles had a substantial collision cross section (non gravitational interaction) then two clouds could bump, and cancel each others momentum. So a larger cloud would remain at the site of the collision.
But for example in the "bullet cluster" collision where two clusters collided the two DM clouds basically just passed through each other and came out the other side. The ordinary matter galaxies did likewise because they were scattered so sparsely in the cluster that they had very little chance of colliding.

But the collision left a cloud of hot hydrogen gas in the middle, radiating Xray. Because the intergalactic medium hydrogen did have a substantial interaction cross section. Those clouds could collide and cancel each other's momentum and accumulate at the site of the collision.

When clusters collide the stars and dark matter particles pass through freely. Only the intergalactic medium, the ordinary (hot, partly ionized) gas, actually crashes.

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10. Mar 27, 2015

### wabbit

I was impressed by what seems to be very precise post-treatment of the image to correct for residual aberrations (off-axis astigmatism it seems from how they describe it) in Hubble's optics (figure S4, p.8, supplementary material). This might otherwise interfere with their interpretation of the image I presume, for smaller galaxy images.

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11. Mar 27, 2015

### wabbit

I was aware there was distortion but not really of its precise shape, thanks for the explanation.

12. Mar 29, 2015

### SpiderET

13. Mar 29, 2015

### wabbit

I would read it more as a measurement of dark matter properties (well, that's not much of a stretch given that this is how the experiment is constructed and reported). There aren't so many of these as far as I know, and more are needed to narrow down the search for what it might be. Excluding some of the proposed models sounds like excellent news for this search.

I see arstechnica titles "strongest case yet for dark matter". I need to read it now, not seeing yet why it adds so much to the case given that the evidence was already pretty strong. Edit : not sure the article really supports the title, but it's a well done piece, thanks for the link.

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14. Mar 29, 2015

### Chalnoth

15. Mar 29, 2015

### SpiderET

Somehow I cant copy text from the pdf, but in the preprint on page 1 and 2 is written, that interacting particles would solve some problems of incorrect predictions of Lambda CDM model. But this paper has shown that the particles dont interact.

But personally I would go even further. This dark matter has strange properties, it interacts gravitationally with normal matter but it doesnt interact gravitationally with other dark matter? Seems like another sign that we need new theory which will include some extended or modified gravity theory.

16. Mar 29, 2015

### Chalnoth

It hasn't shown that they don't interact. It's placed an upper limit on how strongly they can possibly interact. While there are models where dark matter particles have no interactions except through gravity, most models have dark matter that interacts weakly with itself and with normal matter.

Yes, dark matter interacts gravitationally with other dark matter. By "doesn't interact" they're talking about the fact that dark matter particles rarely collide with one another.

17. Mar 30, 2015

### PWiz

Very interesting. It would appear that even collisions between entire galaxies cannot induce dipole moments in the dark matter and "light it up". I think it's reasonable to suspect that dark matter does not have internal organization like atoms, which makes me suspect that the absence of any electromagnetic interaction is due to the presence of uncharged hadrons that are simply held together by the gravitational force. Anyone care to tell me how right/wrong I am?

18. Mar 30, 2015

### wabbit

Not an expert in any way here - but hadrons are standard model particles. As such, they are as far as I know pretty much excluded as a significant component of dark matter defined in a broad sense (which does include hadrons in "baryonic dark matter"), and I'm quite sure they're not a component of cold dark matter as currenly modelled in LCDM.

Also, hadrons are held together by the strong nuclear force, not by gravitation which affects them far more weakly. And they do interact with EM - free neutrons quicly decay to proton + electron + photon for instance (https://en.wikipedia.org/wiki/Neutron#Free_neutron_decay)

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19. Mar 30, 2015

### Jimster41

Would "uncharged Hadrons held together by g..." be little clumps of neutrons?

I'd appreciate a clarification of just how likely it is, given this new upper constraint on self interaction, that DM is truly non self interacting. I mean is this number truly small/large or just bounding off the normal end. If this stuff is at once non self interacting, though differentiated in space time (identifiably somewhere and not somewhere) then one implication is that it is everywhere in space time only itself - entangled, non-local.

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20. Mar 30, 2015

### PWiz

I don't see any reason to completely rule out neutron stars with particularly thin ionized gas layers.
@wabbit I wasn't saying that the "inside" of the hadron is held together by gravity, I was saying that individual hadrons must be held my gravity (no graviton in the standard model yet ). It's like comparing intramolecular forces with intermolecular ones (the analogy is not a brilliant one, but serves the purpose). Thanks for the reference of CDM.

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21. Mar 30, 2015

### Jimster41

Clearly your theory (or rather "theery") is more grounded than mine... Now I need to look up neutron stars today. My cartoon is that in such a star remnant the neutrons are stable only because they got pushed down into a deep enough gravity well during stellar death, their decay is inhibited?

If so, seems like the required number of these could be ruled in or out given the age of the universe and rates of stellar death. And I would think that such gravity wells would interact with each other....

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22. Mar 30, 2015

### Orodruin

Staff Emeritus
What you are thinking about are essentially massive compact halo object (or MACHOs for short). There are large problems in explaining the dark matter using MACHOs, including, but not limited to, producing enough baryons in the early Universe while still being ok with the primordial abundance of the elements, as well as studies of the large scale structure of the Universe.

23. Mar 30, 2015

### PWiz

I just want to get one thing out of the way - I'm not making/advertising a theory. I'm merely speculating and I want to know if my reasoning is correct (in accordance with current physics of course).
Secondly, please don't place so much trust in my ideas. I'm just in HS and I could be hopelessly wrong, so let's not give any special credibility to what I have to say
A neutron star is simply the end result of a massive star (mass>1.39 solar masses) after all it's fuel has been "burnt", where the electron degeneracy pressure has been overcome (no outward radiative pressure from nuclear fusion), forcing electrons and protons to fuse together to form neutrons, which (being half spin fermions) by Pauli's exclusion principle can't occupy the same quantum state. The star shrinks in size until the neutron degeneracy pressure equals the gravitational pull. If the gravity is strong enough though, the total number of microstates sky rockets and the neutrons gain more "liberty" as to which state they can occupy (the wavelength that the neutron can have is approximately given by $\frac{N^{1/3}}{V^{1/3}}$, where N is the number of neutrons and V is the volume of the neutron star), and the volume shrinks below the Schwarzschild radius. The thing is that neutrons are electrically neutral hadrons, and do not interact with photons (a gauge boson which mediates the electromagnetic force), so the bulk of the neutron star is actually invisible. Only ionized gas near the surface interacts with light.
@Orodruin I guess I was jumping to an erraneous conclusion

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24. Mar 30, 2015

### wabbit

I'm not sure what the impact of that is here though. Hadrons in a nucleus are bound together by the strong force, then atoms are bound together by the EM force - then of course every large scale structure is held by gravity, be it formed of dark matter or ordinary matter.

25. Mar 30, 2015

### PWiz

But if we're only talking about electrically neutral hadrons, there won't be any EM force, so you won't have atoms, just a soup of hadrons.

26. Mar 30, 2015

### wabbit

@PWiz, about your idea of neutron stars, I see a few issues here. First, neutrons do interact with EM though I m not sure it's relevant here. Then, neutron stars do radiate EM (X-rays mostly I believe). AFAIK neutron stars are not counted as dark matter, but as ordinary matter - but perhaps that depends on what one calls dark matter.