Some questions regarding WIMPs as a DM candidate

[matt.o]

Whenever someone brings up MOND, I always like to post a link to this paper;
"Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E0657-56"
http://arxiv.org/abs/astro-ph/0309303"

The basic gist is the comparison of a Chandra x-ray image and a lensing map of the "bullet cluster". The MOND people agree that the majority of baryonic matter in a cluster is in the intra-cluster medium , so this begs the question; Why do we see a lensing map consisitent with the majority of the mass being offset from the gas in the merging subcluster? Interestingly, mass contours overlay an optically detected galaxy overdensity, presumably the infalling cluster galaxies, which lies in front of the gas which has been ram-pressure stripped from the galaxies by its interaction with the main cluster gas (the galaxies are collisionless). Fair evidence against MOND?

 

[kmarinas86]

Whenever someone brings up MOND, I always like to post a link to this paper;
"Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E0657-56"
http://arxiv.org/abs/astro-ph/0309303"

The basic gist is the comparison of a Chandra x6ray image and a lensing map of the "bullet cluster". The MOND people agree that the majority of baryonic matter in a cluster is in the intra-cluster medium , so this begs the question; Why do we see a lensing map consisitent with the majority of the mass being offset from the gas in the merging subcluster? Interestingly, mass contours overlay an optically detected galaxy overdensity, presumably the infalling cluster galaxies, which lies in front of the gas which has been ram-pressure stripped from the galaxies by its interaction with the main cluster gas (the galaxies are collisionless). Fair evidence against MOND?

What about Lagrange points at the Galactic scale (I'm not sure if they exist - there's objects from many angles)? What happens to weak lensing at Lagrange points where orbits are strong (probably nothing right)? Could the Lagrange points have an affect on how the gas and matter behave?

 

[Garth]

Whenever someone brings up MOND, I always like to post a link to this paper;
"Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E0657-56"
http://arxiv.org/abs/astro-ph/0309303"

The basic gist is the comparison of a Chandra x-ray image and a lensing map of the "bullet cluster". The MOND people agree that the majority of baryonic matter in a cluster is in the intra-cluster medium , so this begs the question; Why do we see a lensing map consisitent with the majority of the mass being offset from the gas in the merging subcluster? Interestingly, mass contours overlay an optically detected galaxy overdensity, presumably the infalling cluster galaxies, which lies in front of the gas which has been ram-pressure stripped from the galaxies by its interaction with the main cluster gas (the galaxies are collisionless). Fair evidence against MOND?

Yes, thank you matt-o for that link. From that paper:

2.1. The gas — dark matter offset
The most remarkable feature in Fig. 1b is a ∼ 23′′ offset between the subcluster’s DM centroid and the gas bullet, which is at least 2$\sigma$-significant (C04). C04 use this fact as a direct proof of dark matter existence, as opposed to modified gravity hypotheses (Milgrom 1983 and later works) in which one would expect the lensing mass peak to be associated with the gas — the dominant visible mass component.

but note it concludes:

Finally, we note that our limit, $\sigma$/m < 1 cm² g^-1, excludes most of the 0.5 - 5 cm² g^-1 interval proposed to explain the flat mass profiles in galaxies. Within the SIDM paradigm, the galaxy profiles and the tight cross-section limits coming from clusters can still be reconciled if the cross-section were velocity-dependent, so that it would be smaller on average in clusters than in galaxies (e.g., Firmani et al. 2000, 2001;Hennawi & Ostriker 2002; Colín et al. 2002). However, it is difficult to justify this additional degree of freedom in the model until a nonzero cross-section is detected at any velocity.

(emphasis mine)

How does this square with weakly interacting DM required to fit non-cuspy halo centres?

Garth

 

[Garth]

But I thought the neutrino mass has been established to only allow $\Omega_\nu$ < 0.01

I think that limit is assuming $\Lambda CDM$.

Isn't that just assuming h ~ 0.73, i.e. $\Omega_\nu h^2 \sim 0.02$, rather than the rest of the model?

Garth

 

[SpaceTiger]

Isn't that just assuming h ~ 0.73, i.e. $\Omega_\nu h^2 \sim 0.02$, rather than the rest of the model?

I believe the limit is obtained by constraining the effect of neutrinos on the CMB power spectrum (as well as the growth of large-scale structure). They couldn't make this constraint without simultaneously fitting other parameters and I assume they would have used the usual ones (with $\Lambda$, CDM, etc.).

How does this square with weakly interacting DM required to fit non-cuspy halo centres?

It's still not entirely clear whether or not the "cusp" problem is actually a problem. If it is, self-interacting dark matter is one possible solution, but it's not the only one.

 

[Garth]

I believe the limit is obtained by constraining the effect of neutrinos on the CMB power spectrum (as well as the growth of large-scale structure). They couldn't make this constraint without simultaneously fitting other parameters and I assume they would have used the usual ones (with $\Lambda$, CDM, etc.).

Yes, the $\Lambda$CDM limit for an individual neutrino mass is about 0.3 eV, but I was thinking of direct laboratory measurement such as by the Super-Kamiokande neutrino detector & the http://ej.iop.org/links/q81/ftMTkHWSKK1+w6fGYyfRdg/jhep022003009.pdf & MINOS experiments which were consistent with the heaviest neutrino being at least 0.05 electron volts, but no more than 0.3 electron volts.

Garth

 

[SpaceTiger]

Yes, the LCDM limit for an individual neutrino mass is about 0.3 eV

LCDM gives a limit on the sum of the neutrino masses (all flavors).

experiments which were consistent with the heaviest neutrino being at least 0.05 electron volts, but no more than 0.3 electron volts.

Where in the papers do they say that? I was under the impression that these experiments measured the difference in neutrino masses. I know there are upper limits on the neutrino mass at around 0.3 electron volts from other experiments, but I'm not aware of any lower limits.

 

[Garth]

LCDM gives a limit on the sum of the neutrino masses (all flavors).

experiments which were consistent with the heaviest neutrino being at least 0.05 electron volts, but no more than 0.3 electron volts.

Where in the papers do they say that? I was under the impression that these experiments measured the difference in neutrino masses. I know there are upper limits on the neutrino mass at around 0.3 electron volts from other experiments, but I'm not aware of any lower limits.

From Neutrino Masses and Mixing: Evidence and Implications

The evidence for neutrino masses implies that the SM cannot be a complete picture of Nature. In particular, if the SM is only a low energy effective theory, very light neutrino masses are expected. The scale at which the SM picture is not valid anymore is inversely proportional to the scale of neutrino masses. Specifically
m_ν >~ sqrt {\triangle m²_atm} ∼ 0.05 eV

(Page 87)

Garth

 

[SpaceTiger]

It looks like that's a mass scale for the breakdown of the standard model, not an experimental limit on the neutrino mass.

 

[Chronos]

I think you are dead wrong on this count, Garth. MOND is just plain wrong IMO. Any attempt to apply MOND to WMAP3 gives horrid results. I have attempted to give MOND the benefit of the doubt, but not anymore. It looks like grasping at straws to me.

 

[Garth]

I think you are dead wrong on this count, Garth. MOND is just plain wrong IMO. Any attempt to apply MOND to WMAP3 gives horrid results. I have attempted to give MOND the benefit of the doubt, but not anymore. It looks like grasping at straws to me.

You are mis-reading me, having reported the NS article I disagree with it and agree with you Chronos - as I posted above from matt-o's paper "Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E0657-56" http://arxiv.org/abs/astro-ph/0309303

C04 use this fact as a direct proof of dark matter existence, as opposed to modified gravity hypotheses

however they then say

excludes most of the 0.5 - 5 cm² g^-1 interval proposed to explain the flat mass profiles in galaxies

which then might leave a problem for the standard model of DM as well.

Garth

 

[SpaceTiger]

however they then say which then might leave a problem for the standard model of DM as well.

Interacting dark matter is not part of the standard model, by any means. The standard for CDM simulations is always non-interacting dark matter.

 

[Garth]

It looks like that's a mass scale for the breakdown of the standard model, not an experimental limit on the neutrino mass.

Well, they measure the difference in mass between the different types of neutrinos in various oscillation experiments and use theory to predict their absolute masses, so that lower mass detection is theory dependent on the standard particle model as is the upper mass determination from the WMAP data dependent on the standard cosmological model.

Garth

 

[SpaceTiger]

Well, they measure the difference in mass between the different types of neutrinos in various oscillation experiments and use theory to predict their absolute masses

Actually, I think they're just assuming that none of the neutrinos can have a mass < 0 and taking the measured mass difference to be the lower limit on the neutrino mass. It's the inferred difference in mass that depends on the standard model mixing-matrix formalism.

Anyway, I agree that the lower limit would have to be in that ballpark. I had forgotten that atmospheric neutrinos were giving such a large mass difference.

 

[Garth]

Interacting dark matter is not part of the standard model, by any means. The standard for CDM simulations is always non-interacting dark matter.

Not even to solve the 'cuspy halo problem? http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v547n2/52142/52142.text.html ?

We find, in agreement with various authors, that CDM (collisionless cold dark matter) yields cuspy halos that are too centrally concentrated as compared to observations

(Italics mine)

Garth

 

[SpaceTiger]

Not even to solve the 'cuspy halo problem?

As I said before, there's no standard solution to that problem and the fact that it's getting less severe with time suggests that it may have just been an artifact of imperfect cosmological simulations. We shouldn't ignore it, but that doesn't mean that evidence against self-interacting dark matter is evidence against the standard model, by any stretch of the imagination.

 

[Garth]

Are we back to the OP? Do these constraints rule out WIMPs?

If so what alternatives are left that solve the mass cluster distribution and flat galactic centre distribution?

Garth

 

[SpaceTiger]

Are we back to the OP? Do these constraints rule out WIMPs?

If so what alternatives are left that solve the mass cluster distribution and flat galactic centre distribution?

Why would they rule out WIMPs?

 

[Garth]

Why would they rule out WIMPs?

Your statement:

The standard for CDM simulations is always non-interacting dark matter.

Perhaps I am confused. Does you use of the term 'non-interacting' particles include 'weakly interacting' WIMPS?

Garth

 

[SpaceTiger]

Perhaps I am confused. Does you use of the term 'non-interacting' particles include 'weakly interacting' WIMPS?

"Non-interacting" basically means "interacting so weakly (with both itself and its environment) that the effects are not noticable". Self-interacting dark matter, on the other hand, hypothesizes relatively strong interactions between dark matter particles, but weak interactions with everything else.

 

[Garth]

"Non-interacting" basically means "interacting so weakly (with both itself and its environment) that the effects are not noticable". Self-interacting dark matter, on the other hand, hypothesizes relatively strong interactions between dark matter particles, but weak interactions with everything else.

Thank you for that clarification, so WIMPS are non-interacting then.

Garth

 

[SpaceTiger]

Thank you for that clarification, so WIMPS are non-interacting then.

For cosmological purposes, yes. They still have non-zero cross sections, they're just small enough as to be dynamically negligible.

 

[Chaos' lil bro Order]

Its a pleasure watching Garth talk with ST, this is such a vibrant conversation between two brilliant people. I love this.