I don't believe the claimed gamma ray excess from the galactic centre will turn out to be dark matter. First of all, the diffuse gamma ray background from these regions was assumed to have a certain form, which might not be right. Secondly, neutron stars (especially millisecond pulsars) generate gamma rays and it's not obvious to me that fermi would resolve these as extended sources. Thirdly, such a particle at that cross section would probably show up when looking at the supposedly dark matter dominated satellite galaxies of the Milky Way. In fact, a paper titled:

Stringent constraints on the DM annihilation cross section from subhalo searches with Fermi

suggests that such a mass and cross section for decay via those channels is unlikely to be consistent with dark matter. More worrying perhaps is that a 35 GeV particle has a mass that allows for rather easy investigation, so the LHC would likely have produced events missing 35 GeV of energy (and lots of momentum).

From a historical perspective, how many times have people claimed to have found the dark matter?

I would like to draw your attention to pictures of structure formation simulations.

The following site at Cambridge university displays comparison between

L-CDM and warm dark matter simulations. This pictures shows structures

and voids in these two different models.

http://www.kicc.cam.ac.uk/news/kavli-scientists-set-strictest-limits-yet-warm-dark-matter

It can be understood that a massive cold dark matter does not have

relativistic speed. So a dark matter halo will be surrounded by

smaller halos. But such surrounding halos are not found in

good enough numbers. Thus we may be tempted to think that

dark matter should not have entirely non-relativistic speed.

A light sterile neutrino in the 1 KeV range is thus a candidate

of warm dark matter.

Biswajoy,

Interesting - thanks!

Regarding galactic halos, the question is whether WDM or HDM can be gravitationally bound to galaxies. This question might also depend on whether they could have been bound to protogalaxies in the early universe. As I understand, the simulation study referenced did not actually evaluate that question.

As I understand, http://arxiv.org/abs/1402.2301 and http://arxiv.org/abs/1402.4119 discuss a sterile neutrino whose mass ~7.1 keV, based on excess spectral lines thought to represent sterile neutrino decay products.

The Kavil Institute web page at your link includes an image that specifies "WDM 0.5 keV". However, it's not clearly indicated that these images are actually related to the referenced research. The web page refers to the simulation study paper http://arxiv.org/abs/1306.2314, which indicates that they evaluated _all_ dark matter being represented by WDM with neutrino masses of 1, 2 and 4 keV. It's abstract concludes:

"... Thermal relics of masses 1 keV, 2 keV and 2.5 keV are disfavoured by the data at about the 9sigma, 4sigma and 3sigma C.L., respectively. Our analysis disfavours WDM models where there is a suppression in the linear matter power spectrum at (non-linear) scales corresponding to k=10h/Mpc which deviates more than 10% from a LCDM model. Given this limit, the corresponding "free-streaming mass" below which the mass function may be suppressed is 2x10^8 Msun/h. There is thus very little room for a contribution of the free-streaming of WDM to the solution of what has been termed the small scale crisis of cold dark matter."

Critically, this simulation of the effects of WDM as a complete replacement for _all_ CDM did not evaluate the many potential configurations of mixed DM. If for example, it's considered that both sterile neutrinos with mass of ~7 keV and some more massive CDM particles exist, some mix of the two could likely be fit to resolve the 'small scale crisis of cold dark matter'. Whether that same mix could also produce the desired galactic gravitational effects (which would likely involve only CDM) - including resolution of the 'missing satellite problem', also mentioned in the Kavil link, is a more difficult question.

I'm unclear at to what particle masses might qualify as CDM, WDM or HDM, which may vary as the universe evolves. The best definition I've found is at http://en.wikipedia.org/wiki/Dark_matter#History_of_the_search_for_its_composition - which states:

"Dark matter candidates can be approximately divided into three classes, called cold, warm and hot dark matter. [73]

"These categories do not correspond to an actual temperature, but instead refer to how fast the particles were moving, thus how far they moved due to random motions in the early universe, before they slowed down due to the expansion of the Universe – this is an important distance called the "free streaming length". Primordial density fluctuations smaller than this free-streaming length get washed out as particles move from overdense to underdense regions, while fluctuations larger than the free-streaming length are unaffected; therefore this free-streaming length sets a minimum scale for structure formation."

- Cold dark matter - objects with a free-streaming length much smaller than a protogalaxy.[74]

- Warm dark matter - particles with a free-streaming length similar to a protogalaxy.

- Hot dark matter - particles with a free-streaming length much larger than a protogalaxy.[75]

I conclude from this that neither WDM or HDM would likely be bound within extended galactic DM halos.

I did a google search on how free streaming length depends on

neutrino mass, and found a lecture note by A.D. Dolgov, where following

formula for free streaming length is given,

L_FS = 250 Mpc (eV/m_nu)

Then for a 1000 eV neutrino or a 1 keV neutrino

L_FS= 1/4 Mpc.= 250 Kpc

Size of milky way galaxy is 30 Kpc. Then for a ten

keV neutrino the free streaming length is of

the order of the size of Milky way galaxy. This

is perhaps a rough estimate for warm dark matter

mass.

James,

I am a little confused now. Perhaps Dolgov is using

a slightly different terminology. He is referring to a

free streaming horizon for the neutrinos. The horizon

size for the neutrinos in an equivalent concept to the

commonly referred horizon size of the universe by which

we mean the horizon of massless photons that

travel with the velocity of light. Now if you use the

formulas of special relativity and find out the maximum

possible velocity of a 10 keV object and then multiply

that velocity with the age of the universe, you will

probably get the size of the milky way.

BTW - a separate news article, focusing more on a revised, more selective independent reanalysis of the Fermi Gamma Ray satellite data reported to be inconclusive last year, mentions:

"... If the glimmer of gamma rays from the inner galaxy is the afterglow from such annihilations, then their detected energy levels indicate that they most likely originate from WIMPs with a mass of 35 giga-electron-volts (GeV) annihilating into quarks, or 10-GeV WIMPs annihilating into tau particles."

See https://www.simonsfoundation.org/quanta/20140303-case-for-dark-matter-signal-strengthens/ and http://arxiv.org/abs/1402.6703.

Also, http://www.preposterousuniverse.com/blog/2014/03/07/guest-post-katherine-freese-on-dark-matter-developments/ discusses the prior reports of candidate direct detections of 10 GeV 'light' WIMPS, among other dark matters...

I've had some very nice complements on this question and some of the comments - thank you all very much. I've also had some anonymous downvotes with no explanatory comments - I presume this is a natural result of asking somewhat controversial questions that may upset someone's sensibilities. However, when I receive a downvote for a comment that merely references published studies and reports - I really think that's a little too much! Thanks all for your interest and consideration!

I don't believe the claimed gamma ray excess from the galactic centre will turn out to be dark matter. First of all, the diffuse gamma ray background from these regions was assumed to have a certain form, which might not be right. Secondly, neutron stars (especially millisecond pulsars) generate gamma rays and it's not obvious to me that fermi would resolve these as extended sources. Thirdly, such a particle at that cross section would probably show up when looking at the supposedly dark matter dominated satellite galaxies of the Milky Way. In fact, a paper titled:

Stringent constraints on the DM annihilation cross section from subhalo searches with Fermi

suggests that such a mass and cross section for decay via those channels is unlikely to be consistent with dark matter. More worrying perhaps is that a 35 GeV particle has a mass that allows for rather easy investigation, so the LHC would likely have produced events missing 35 GeV of energy (and lots of momentum).

From a historical perspective, how many times have people claimed to have found the dark matter?

Indranil,

I agree - the expectation of increased dark matter annihilations at the center of the Milky Way seem to presume that the galactic dark matter halo has a dense, 'cuspy' inner region, increasing the potential for CDM particle interactions. As I understand, however, there is no evidence that the MW actually has the cuspy DM halo predicted by CDM simulations. Selected dwarf galaxies that do exhibit cuspy DM halo characteristics might be a better place to look, except that the total amount of DM contained within their halos would be much less than that thought to be contained within the MW DM halo - thereby reducing the potential for annihilation detections...

I do not find a paper entitled "Stringent constraints on the DM annihilation cross section from subhalo searches with Fermi" - can you provide a reference or link?

Thanks!

Great information and wonderful Discussion

I am just curios, how would sterile neutrino solve the cuspy halo problem?

Heleri,

Good question! In very general terms, gravitationally interacting heavier, slower cold dark matter particles would tend to congregate at the center of a large halo, whereas somewhat lighter, faster warm dark matter particles (which might be composed of sterile neutrinos) would tend to move around more, reducing and perhaps preventing any tendency to congregate at the center of gravitational potential.

I did find one study focused on galactic halos that only considered complete replacement of cold dark matter with warm dark matter. There are a number of differences identified, especially that halos tended to have low-density central cores rather than the high density central cusps predicted for cold dark matter halos. As I understand, cored halos better fit with the kinematics of disk galaxies (but not the lensing studies of elliptical galaxies)... Again, I suspect that complete replacement of CDM with WDM would not solve all LCDM problems - additional fine tuning might be necessary... Please see http://dx.doi.org/10.1086/321541.

Thanks James,

I wonder if black holes in the centre of spiral galaxies would have some effect on those predicted cusps? Are they even taken into account in the models?

Apparently, the central black hole is likely not going to affect the dark matter distriubtion substantially. A quick way to find the size of the region much affected by the black hole is to find the radius within which the enclosed mass is no longer dominated by the black hole - this is only a few light years.

http://arxiv.org/abs/astro-ph/0210658

On the other hand, there are other baryonic processes which can affect the dark matter density profile and destroy the cusps. Especially important is galactic winds. There was an article on this recently, but I expect more work is still required to show if cusps can actually be destroyed in this way.

http://www.nature.com/nature/journal/v506/n7487/full/nature12953.html

However, the highly complicated processes alluded to by these authors leave little chance of being able to account for tight correlations between the dark matter and the baryons in galaxies, such as the well-known Baryonic Tully-Fisher Relation between the flatline level of a rotation curve and the total baryonic mass (even if dark matter is dominant). This was a clear prediction of MOND in 1983 (v_f = (GMa_0)^(1/4)) and has no natural explanation within LCDM. You will have to decide if the large scatter in properties of galaxies that is inevitably the outcome of such complicated baryonic processes is compatible with relations like the BTFR and more generally the mass discrepancy-acceleration correlation in disk galaxies.

LCDM has made very few successful predictions in galaxies and constantly needs adjustments to fit the data better. While that is not necessarily bad science, it is perhaps not the best model given that most data on galactic dynamics was predicted in advance based on Modified Newtonian Dynamics, a theory with very few free parameters that were basically fixed decades ago. It must be remembered that the presence of a dominant invisible component to the mass will likely prevent one predicting forces based on the visible mass, especially when it obeys fundamentally different physics to the invisible mass. The only trouble with this is if there are some interactions between the invisible and the visible mass, which may be. This would be some really non-standard physics. As would modifying gravity.

The lack of direct evidence for dark matter and the ruling out of wide ranges of possible particle parameters over three decades of intensive searches for it must also surely count against the LCDM model.

Heleri,

Also an interesting point, but I suspect not. I doubt that the supermassive black hole's gravitational influence would extend far enough to have much influence by itself (it would encourage cusp development), but I wonder if the relativistic jets produced by periodically accreting SMBHs might have some disruptive effect as a result of their momentum (E=mc^2). While dark matter would not directly interact with jet material I wonder if the relativistic velocities would produce some spacetime distortions above and below the galactic bulge that might disrupt the halo... Large gas bubbles have been identified there, just to illustrate the effects of those jets... See http://www.nasa.gov/mission_pages/GLAST/news/new-structure.html.

Thanks James, For the reference of Neil Turok's work with

his collaborators on warm dark matter.

I found the arXiv number astro-ph/0010389

This paper was written ten years ago, therefore the idea

of WDM is around for a while I would guess.

Neil Turok is quite famous for his work with Hawking.

"The Hawking-Turok Instanton theory" on origins of

Big Bang.

Thank you, Biswajoy - I was quite unaware of Neil Turok's other work (although the name does seem familiar, now that you mention it).

BTW - the IOP ApJ link happens to be free for this article!

Thank you James,

Or maybe the properties of galaxies, (e.g gas and stars in them ) should be studied thoroughly first ?

http://arxiv.org/abs/1402.1764

Heleri,

Very curious reference! I'll have to read more thoroughly, but it appears that the referenced study proposes that cold dark matter heats up through gravitational interactions between clumps of gas individual CDM particles. As I understand, even an energized CDM particle would remain massive and only vibrating - not propagating over distances like lower mass WDM particles, so it should continue to gravitationally interact with other particles (theoretically forming cusps)... Thanks!

Heleri,

Thanks again for the very interesting new reference, http://arxiv.org/abs/1402.1764 https://www.researchgate.net/publication/260169505_Cold_dark_matter_heats_up. It is well-researched and a good source of cusp/core reference information. I think though, that the most critical issues may lie more with a more complete understanding of possible interactions among baryonic matter and dark matter than with understanding the nature of star formation and gas flows within galaxies.

The researchers propose (if I can paraphrase) that it is gravitational interactions between clumps of accreting gas (dust) that - when violently dispersed would cause individual, (strongly) gravitationally bound dark matter particles to be violently ejected from center of the galaxy. They also seem to suggest that kinetic energy of gas out-flows would (I didn't understand how) be transferred to dark matter.

IMO, there are several flaws in the reasoning employed bu this research. First, it's presumed that it is the accretion of baryonic mass that causes dark matter to condense into increased density central halo cusps. As I understand, the cusp predictions were the result of CDM-only halo simulations. This is important because the research concludes that the sudden removal of baryonig mass from the center of the galaxy would therefore cause dark matter to be expelled. However, IMO it is the baryonic mass that is considered to be under the gravitational influence of disperse dark matter - rather than dark matter being bound to baryonic matter.

Secondly, it's considered that individual dark matter particles would proximally bound to baryonic clumps and that large gas outflows would transfer kinematic energy to dark matter. While kinematic energy is transferred among particles of neutral hydrogen gas, for example, neutral hydrogen atoms do participate in EM interactions by virtue of their charged components - electrons and protons. I can't understand how collisionless dark matter particles could be 'energized' by baryons.

It seems that this research presumes that the predicted number of dwarf galaxies could be reduced by intragalactic conditions, but as I understand it is the number of field galaxies produced that has been over-predicted by LCDM and that many dwarf satellite galaxies have been captured by gravitational encounters... Galactic conditions could not produce the total number of field galaxies initially produced by LCDM.

I certainly can't express my concerns adequately here - the explanations I found in the proposal also seemed unclear to me. However, I must point out that there are strong motives to find a solution to LCDM issues. Previous attempts have included self-interacting dark matter - I'm sure there are others. It will be interesting to see how this proposal is received by the physics community!

Also see http://cosmicrays.wordpress.com/2014/02/21/cold-dark-matter-heats-up-a-review-of-a-review/

Article Cold dark matter heats up

Maybe I'm being stupid, but I didn't agree with the box in which this paper suggested the gravitational force decreases as dark matter particles move outwards. While I agree reducing the mass enclosed within the orbit of the DM particle causes it to fly outwards, I was under the impression that the force on a test particle in a region where the baryons have approximately constant density increases linearly with r. After all, the gravitational force must equal 0 at the centre of the galaxy and rise outwards, before falling to 0 again at very large radii. So there must be a region where the gravitational force increases outwards. I'm not sure how this woud affect the conclusions of the paper, but there was no mention of this region of incresing gravity outwards anywhere in the paper. I don't know how large this region might be, though, but it's quite certain to be above the scale we can observe at nowadays, in some of the dwarf galaxies at least.

http://arxiv.org/abs/1402.1764

I also must agree with James about the dark matter being the dominnt contribution to the mass, even at very small radii, in dwarf galaxies. So intuitively it does seem very strange that the DM can be so greatly affected by the baryons, considering that there can only be relatively weak gravitational interactions between them.

Naturally, in my opinion more important issues like the anisotropic satellite galaxies around the Milky Way and Andromeda can't really be solved with baryonic processes, at those lengthscales. And one would have thought the same applies to DM dominated dwarf galaxies. Yet more indication that some seriously unlikely fine tuning is required to get galaxies to work in LCDM.

The graphs showing the data can also be a bit misleading - there is much less scatter between properties of galaxies, if you put them on a Tully-Fisher Relation. What I want to know is, with all this effort, can they explain the existence of a Baryonic Tully-Fisher Relation with negligible intrinsic scatter based on highly complicated and stocahstic baryonic feedback processes of the sort they believe to be so important? And why is it preferrable to have a theory with lots of stochastic processes to achieve a tight correlation like the BTFR when we have a theory which predicted this 30 years ago? It is quite simple really: v_f = (GMa_0)^(1/4).

Coincidences are rare in science, and usually it progresses by treating patterns in data as not coincidences.

The illustration in Box A - "how gas affects dark matter through gravity" in http://arxiv.org/abs/1402.1764 is inviting - showing that a clump of dense, star forming gas with an isolated cold dark matter (CDM) particle orbiting it will, by 'exploding', cause the CDM particle to move to a larger orbit (when gas again cools and clumps together, and gravitationally captures the accelerated CDM particle.

This conception is fraught with difficulties. The detailed process being proposed is described in http://arxiv.org/abs/1106.0499. The most serious failing is that the model represents only the gravitational interaction between a single (massless, for computational convenience) CDM particle and a fluctuating gravitational potential composed of gas.

In reality, a gas gravitational potential and a CDM particle do not exist in isolation. Collisionless CDM particles are likely to gravitationally interact with not only accreting gas particles but many neighboring CDM particles as well! In fact, whether and how much any CDM particle would be accelerated by the sudden 'disappearance' of gas gravitational potential would depend of the density of CDM particles and their disperse gravitational potentials.

Moreover, at sufficient CDM particle densities it should be impossible for colliding gas (and dust) particles to bind together - potentially preventing star formation!

At any rate, even presuming the acceleration of a CDM particle (or two) caused by the sudden 'disappearance' of gas gravitational potential, inertial effects of the massive particle and gravitational interactions with neighboring CDM particles would work to reduce and disperse the CDM particle's acceleration.

While the researchers successfully "tested" their proposal using simulation models, IMO their gravitational presumptions are fundamentally flawed. Discounting the researcher's proposed gravitational feedback mechanism, the basic idea that stellar feedback can resolve CDM cusp/core problems has been considered and rejected as long ago as 2002, in one of the subject paper's references - http://arxiv.org/abs/astro-ph/0108108.

That's exciting, James! I am going to be supervised by the coauthor of the paper from 2002, Hongsheng Zhao. He considered an instantaneous removal of the entire baryonic disk. More realistic models of stellar feedback would give even smaller effects. There is little chance of such feedback resolving the cusp-core issue if it does not with an instantaneous removal of the entire disk, unless the effect of actual feedback is similar to repeatedly doing this, a dubious prospect.

Also, we see rapidly star forming galaxies at high redshift which don't just explode as they are forming a lot of stars. In fact, the shockwaves from the resulting supernovae can even lead to more stars forming. This whole idea of extremely strong feedback from stars does not seem to make much sense. The authors of the recent paper even admit shutting off cooling for some gas particles near a supernova, conceding that if they just let the simulation run with realistic physics, it would cool rapidly. If you look at real supernova remnants in our galaxy, it's obvious little energy has gone into kinetic energy of interstellar gas. With really strong feedback, the galaxy would basically be unable to form any stars as all the gas would be in a hot form. Obviously, outflows do happen, but it's dubious for them to be so significant.

More importantly, one must use common sense: DM dominates the gravitational mass, so how can playing around with the mass distribution of the baryons solve the problem? It must be an intrinsic property of the dark matter, or the dark matter doesn't exist. One of these must be the correct solution, otherwise we're asking for too much of the subdominant baryons, which don't all fly away every time a star forms.

@ James

L-CDM model allows a cold dark matter and a non vanishing value

of the cosmological constant. However when one is trying to introduce

a kev scale neutrino it is not CDM. Then the question is will such a

case survive microwave anisotropy measurements from Planck

as well as the CBMR polarization studies at BICEP2. As far as I

understand Planck data as well as BICEP2 data are is very good

agreement with L-CDM model, even though some other model

that we do not know (such as warm dark matter) can also be

a good fit.

Biswajoy,

As I understand from papers referenced in the question statement, higher mass sterile neutrinos are being considered (as WDM) to account for the identified phenomena attributed to CDM in the LCDM model - and potentially addressing the sub-universal scale observational discrepancies that the model exhibits. As I understand, sterile neutrinos would not produce the same effects as very low mass, high velocity 'ordinary' neutrinos in primordial processes responsible for producing anisotropy effects. Please review the articles referenced in the question posting for additional information. I do not propose these changes to cosmological dark matter specifications - a number of cosmologists do...

Please see the interesting new observation of "dim matter" - the intergalactic medium - http://www.caltech.edu/content/intergalactic-medium-unveiled-caltechs-cosmic-web-imager-directly-observes-dim-matter. It seems that this is the first clear evidence that ordinary baryonic matter may contribute much of the mass comprising the cosmic web. The CalTech page states:

"The Cosmic Web Imager was conceived and developed by Caltech professor of physics Christopher Martin. [...] "Not only does it comprise most of the normal matter in the universe, it is also the medium in which galaxies form and grow."

If previously hypothesized but undetected material has been excluded from cosmological estimates of 'ordinary matter', should it instead diminish estimates of cosmological dark matter - changing their established proportions?

I think grossly there are two things to worry.

1) Structure formation which has happened much later

in the formation of the universe

2) Primordial power spectrum, which is a much earlier

effect. L-CDM model describes the primordial power

spectrum very well.

However, as L-CDM model has cold dark matter, it

may give rise to more structure at small scales. We

must note that the plus point is primordial and

minus point is to do with structure formation which

has happened much later. The following picture

gives the agreements of L-CDM model with the

primordial power spectrum.

Dear Biswajoy,

There have been a few L-CDM anomalies identified even in the Planck CMB data - see http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_reveals_an_almost_perfect_Universe

"One of the most surprising findings is that the fluctuations in the CMB temperatures at large angular scales do not match those predicted by the standard model – their signals are not as strong as expected from the smaller scale structure revealed by Planck.

"Another is an asymmetry in the average temperatures on opposite hemispheres of the sky. This runs counter to the prediction made by the standard model that the Universe should be broadly similar in any direction we look.

"Furthermore, a cold spot extends over a patch of sky that is much larger than expected."

Moreover, to this pedestrian, it seems that L-CDM power spectrum analyses may be more an exercise in parametric curve-fitting than stringent testing of model results. For example, see http://physics.aps.org/articles/v1/31.

There are also some very fundamental observations of low mass, large separation stellar binary systems in our galactic neighborhood that do not comply with Keplerian expectations for inverse-square diminishment of rotational velocity as a function of separation distance. While they do fit with the general expectations of modified gravity theories, there seems to be no configuration of dark matter that could produce the observed rotational characteristics. See http://arxiv.org/abs/1401.7063.

Dark matter's only purpose, even in the primordial universe, is to increase the effects attributed to mass-energy to fit observations. Alternatively, the analytical determination of effects produced by mass-energy, especially under widely varying conditions, could be different than currently established evaluation methods presume. If the effects of mass-energy in the early universe were different than determined by the L-CDM model, results might still fit with observations despite the absence of any dark matter.

Again, our interpretation of early, large scale conditions may be skewed by averaging methods and other scale dependent factors - see http://arxiv.org/abs/1109.2314.

http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2013/03/planck_enhanced_anomalies/12583990-3-eng-GB/Planck_enhanced_anomalies.jpg