Small-scale challenges to the cold dark matter model

Small-scale challenges to the cold dark matter model
The cusp-core problem. An optical image of the galaxy F568-3 (small inset, from the Sloan Digital Sky Survey) is superposed on the the dark matter distribution from the “Via Lactea” cosmological simulation of a Milky Way-mass cold dark matter halo (Diemand et al. 2007). In the simulation image, intensity encodes the square of the dark matter density, which is proportional to annihilation rate and highlights low mass substructure. Credit: arXiv:1306.0913.
(—A collaborative of researchers from several U.S. universities has published a new paper that explains the major contradictions presented by the prevailing cold dark matter (CDM) cosmological model, and proposes approaches for reconciling cosmological observations with the CDM model's predictions. The paper, titled "Cold dark matter: Controversies on small scales," was published in the Proceedings of the National Academy of Sciences in December.

In the last decade, investigations of the CDM cosmological model have explained cosmic structure over large spans of redshift. The prevailing theory holds that 80 percent of the matter comprising the universe consists of CDM, with a smaller percentage of baryonic matter composing the visible and more easily observed structures such as stars and planets.

The CDM model successfully demonstrates how the smooth early state of the universe evolved into the lumpy distribution of matter observed in galaxies and galactic clusters today. By contrast with the older "hot " model, CDM theory holds that universal structure grows hierarchically, with small-scale gravitational structures forming successively larger structures over time.

Despite its success as a model of universal evolution over long periods, the predictions made by the CDM cosmological model diverge from observational data in several problematic ways.

The "cusp-core" problem

Cosmologists believe that galaxies are suspended within vast, essentially spherical halos of dark matter. Cosmological simulations suggest that that CDM halos should have a "cuspy" distribution, with density spikes of dark matter at the centers of galaxies. However, the observed rotational speeds of galaxies fail to indicate the predicted density of dark matter in galactic cores.

The authors note that the the biggest discrepancies between the CDM model's predictions and the observational data arise for fairly small galaxies, and that a majority of galaxy rotation curves have so-called "cored" dark matter profiles, rather than the "cuspy" profiles of the predictions.

They describe possible solutions to this contradiction derived from the physics of baryonic matter. Simulations incorporating episodic models of star formation and supernovae feedback into the overall CDM model yield results more closely aligned with the flat CDM distributions actually observed in galaxies. The authors note that in a hydrodynamic simulation with star formation and feedback, "over time, the central dark matter density drops and the cuspy profile is transformed to one with a nearly constant density core."

The "missing satellite" problem

Large galaxies are orbited by systems of smaller , each with its own . The authors explain, "Because CDM preserves primordial fluctuation down to very small scales, halos today are filled with enormous numbers of subhalos that collapse in the early times and preserve their identities after falling into larger systems."

The problem is that CDM simulations predict a higher number of these satellites around galaxies like the Milky Way than observations reveal. Prior to 2000, only nine dwarf galaxies were known at the 250 kiloparsec virial radius of the Milky Way halo; researchers had predicted on the order of five to 20 more dwarf galaxies above a certain velocity threshold at that radius. Where are these missing satellite galaxies?

The authors believe that the "missing satellite" problem can be easily resolved by incorporating baryonic physics. The velocity threshold at which at which the subhalo observations diverge from the predictions is close to the value at which heating of intergalactic gases by the UV photoionizing background should suppress gas accretion onto halos. This dynamic could explain why proposed subhalos remain dark.

The observation of these subhalos is difficult for many reasons, including the relative dimness of many dwarf galaxies, and consequently, there remain gaps in the understanding of dwarf galaxy populations.

The "too big to fail" problem

Galactic simulations predict subhalos with central masses which cosmologists expect to host classical dwarf galaxies like the Milky Way's. However, the mass at the centers of these simulated subhalos actually exceeds the observed masses in the most luminous galactic satellites. The authors concede that it is possible in principle that the observed dwarf galaxies reside within less massive host subhalos, but it is physically very unlikely to be the case. Thus, researchers have described this conflict as the "too big to fail" problem.

Here, the authors write, "The degree of discrepancy varies with the particular realization of halo substructure and with the mass of the main halo, but even for a halo mass at the low end of estimates for the Milky Way, the discrepancy appears too large to be a statistical fluke, and a similar conflict is found in the satellite system of the Andromeda galaxy."

Additionally, they note that satellites in low-mass subhalos may also be explained by baryonic effects for which simulations have not yet accounted, but the "too big to fail" problem arises within more massive systems with gravitational potential that is dominated by dark matter. As in the "cusp-core" problem, the simulations predict too much mass in the central regions of subhalos.

Exploring dark-matter solutions

If future researchers want to upend the current CDM model in favor of an alternative, the authors suggest that a potential solution may lie in assuming a "warm dark matter" model in which free-streaming velocities in the early universe are substantial enough "to erase primordial fluctuations on subgalactic scales." The good news: Collisionless collapse of warm dark matter (WDM) still leads to a cuspy halo profile in simulations, but the central concentration is actually lower than that of CDM models, more closely aligning with the observations of galaxy rotation curves. As a result, the mass function of halos and subhalos drops at low masses in the absence of small-scale perturbations that produce collapsed objects, and the subhalo mass function corresponds with observational dwarf satellite counts.

The bad news: WDM eliminates power on small scales, resulting in too few subhalos in the Milky Way to support the number of observed. The authors conclude that despite some remaining uncertainties in the numerical simulations and observational data, it appears that WDM cannot solve the cusp-core and missing satellite problems with regard to cosmological observations.

Future developments

Hope for the refinement of CDM to align it with observations may come from one or more future research directions:

• Improved simulations of models interacting with dark matter may solve the small-scale problems, or find that parameters chosen to match one set of observations fail when applied to another set.

• Researchers could make a direct test of the CDM prediction that vast numbers of low-mass subhalos are orbiting within the virial radius of large galaxies.

• Improved measurements of stellar velocities in satellite may better delineate the satellite problem.

• Underground detection experiments, next-generation observatories, or collider experiments could identify the dark matter particle within the next decade.

Explore further

Research pair suggests dark matter could create halos of light around galaxies

More information: David H. Weinberg, James S. Bullock, Fabio Governato, Rachel Kuzio de Naray, Annika H. G. Peter. "Cold dark matter: controversies on small scales." PNAS (2015) … /1308716112.abstract . On Arxiv: arXiv:1306.0913.

The cold dark matter (CDM) cosmological model has been remarkably successful in explaining cosmic structure over an enormous span of redshift, but it has faced persistent challenges from observations that probe the innermost regions of dark matter halos and the properties of the Milky Way's dwarf galaxy satellites. We review the current observational and theoretical status of these "small-scale controversies." Cosmological simulations that incorporate only gravity and collisionless CDM predict halos with abundant substructure and central densities that are too high to match constraints from galaxy dynamics. The solution could lie in baryonic physics: Recent numerical simulations and analytical models suggest that gravitational potential fluctuations tied to efficient supernova feedback can flatten the central cusps of halos in massive galaxies, and a combination of feedback and low star formation efficiency could explain why most of the dark matter subhalos orbiting the Milky Way do not host visible galaxies. However, it is not clear that this solution can work in the lowest mass galaxies, where discrepancies are observed. Alternatively, the small-scale conflicts could be evidence of more complex physics in the dark sector itself. For example, elastic scattering from strong dark matter self-interactions can alter predicted halo mass profiles, leading to good agreement with observations across a wide range of galaxy mass. Gravitational lensing and dynamical perturbations of tidal streams in the stellar halo provide evidence for an abundant population of low-mass subhalos in accord with CDM predictions. These observational approaches will get more powerful over the next few years.

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Feb 11, 2015
The "early state of the universe" is not "known" since we are not there. Saying that CDM describes the "early universe" is asinine since we cannot observe it in any real sense.

It describes what we assume must have been based on inference. There isn't anything wrong with that per se, but its inaccurate to say we "know what the early universe was like"

Feb 11, 2015
The "early state of the 1800s" is not "known" since we are not there. Saying that history books describe the "early 1800s" is asinine because we cannot observe it in any real sense.

It describes what we assume must have been based on inference. There isn't anything wrong with that per se, but it's inaccurate to say we "know what the early 1800s was like"

Feb 11, 2015
Both CDM and WDM models fail to match observations.

Dark matter was created when our observations failed to match our models. The inclusion of dark matter fails to match our observations either. Perhaps it is time to eliminate dark matter and build a model which works.

In the brave new world of modern science, fine tuning and extra variables in computer models can sidestep most fundamental problems, but personally, I don't hold with secondary ad hoc tinkering to prevent falsification of endangered theories.

Feb 11, 2015
'Was the universe born spinning?'

"The universe was born spinning and continues to do so around a preferred axis"

Our Universe spins around a preferred axis because it is a larger version of a galactic polar jet.

'Mysterious Cosmic 'Dark Flow' Tracked Deeper into Universe'

"The clusters appear to be moving along a line extending from our solar system toward Centaurus/Hydra, but the direction of this motion is less certain."

The clusters are headed along this path because our Universe is a larger version of a polar jet.

It's not the Big Bang; it's the Big Ongoing.

Dark energy is dark matter continuously emitted into the Universal jet.

Feb 11, 2015
But you're assuming people recorded history correctly. You're assuming that they observed things correctly. You're assuming the world wasn't created last tuesday with all of us created as if we had memories of the past.

My point being that the argument "we weren't there, so we can't know" is specious in the context of science. Every measurement is, at some level, an inference about the state of reality. Every measurement relies on a myriad assumptions about what is "true."

In the context of the early universe, we take no grand leaps of faith, nothing more than is present in any other field of science, when we discuss its state. We can observe distant, and thus ancient, galaxies. We can observe light that comes from the period of recombination when an opaque plasma became transparent gas. And when I say we can observe, I mean that our observations fit precisely what you would expect from first principles derived from other, unrelated observations, and so one can infer its "truth"

Feb 11, 2015
Another article on dark matter and a tweak to CDM that makes it match observation said "Tweak theories are weak theories"

Isn't the whole dark matter theory to account for the dark matter effect a tweak theory?

Feb 11, 2015
@BS All measurements are of the past. Your argument, as shavera says, is specious. You can't "be" in the past. Measurements are the results/outcomes of earlier states. If I walk outside and the grass is wet, I didn't have to be there to know it rained. I DO have to eliminate alternative possibilities: snow, lawn irrigation, dew, leaking airplanes, fire truck spray, ...

Feb 11, 2015
Has the possibility that dark matter is a Majorana particle been considered in this context? As the density of dark matter increases, so does the probability of collisions between dark matter particles. Could this impose an upper limit on the density of dark halos that would account for their lack of observed "cuspiness?" The excess central mass predicted by the models isn't actually missing after all; it's just been lost to self-annihilation and dissipated as radiation. ---s

Feb 11, 2015
There is a distinct lack of discussion of the biggest uncertainty in these simulations, baryonic physics. It's not right to motivate WDM with these issues because of this uncertainty, WDM is studied because it's an interesting concept. We know baryonic feedback can prevent cusps forming and we also observe some satellite galaxies to have cusps. It does however fit into the growing consensus that warm dark matter doesn't differ significantly from CDM within existing constraints. Some have suggested with baryonic physics WDM, CDM and SIDM may be inseparable. The only hope is satellites but either way it's not significant.

Feb 11, 2015
Thank you for publishing this article.
Without it, I would not have questioned the nature of dark matter further.
I have now constructed a framework which is extremely simple, one which conforms with observations, and one which greatly elevates the sigma of dark matter.
It exists guys.

Feb 12, 2015
You hear a lot about if the universe is shrinking or expanding. Imagine the universe in a wave form. ( A ripple of formation). It slows as it gets to the top and speeds up as it lowers to the bottom. The speed of light remains constant only to bend, being dependent on the gravitational pull of the massive suns. Depending on how close the massive suns are to each other and their gravitational strength, light can create an arc or a small S pattern.Picturing the galaxies on a wave of space time is like picturing objects on water with wave patterns, they really don't clump up but expand away from each other. Something to think about in one's spare time.

Feb 12, 2015
What a great article, very informative and clear.

Feb 12, 2015
Perhaps it is time to eliminate dark matter and build a model which works.

Perhaps. No one is stopping you or anyone else from doing so. If you can find why we are seeing the various effects attributed to dark matter better w/o dark matter being needed people will be happy to listen and see if it holds water. If not you need to take their criticisms on it and Better refine your theory.

Feb 12, 2015
The "early state of the universe" is not "known" since we are not there. Saying that CDM describes the "early universe" is asinine since we cannot observe it in any real sense.

We can observe early universe directly, dont you know that by looking at distant galaxies we are also looking into the past? And what we see strongly implies the existence of dark matter.

Feb 12, 2015
I suspect that the review is dated, since it is just recently that the large scale LCDM models predict realistic galaxies, by including galactic winds.

"The galaxies formed in the new EAGLE-simulation (Evolution and Assembly of GaLaxies and their Environments) are a much closer reflection of real galaxies thanks to the strong galactic winds, which blow away the gas supply needed for the formation of stars. ... Having developed a simulation which produces galaxies with characteristics similar to observed galaxies, astronomers can now study the evolution of individual galaxies in detail." [ http://www.ljmu.a...le/1896/ ]


Feb 12, 2015
"the major contradictions presented by the prevailing cold dark matter (CDM) cosmological model".

There isn't any contradictions within the model. There is tensions (or "challenges" as the abstract says) with observations, not from the humongous redshift span the model covers but only at the smallest scales DM dominates at, at about 10 galaxy radii, to scales where baryonic matter dominates (galaxy centers).

Those are completely different things. Contradictions are bad, tensions are opportunities to improve both model and observations.

Feb 12, 2015
"We can observe early universe directly, dont you know that by looking at distant galaxies we are also looking into the past?"

Furthermore, in the basis for science that measurement theory is, there is no difference in hypothesis testing between which times the observations were made. And we can see that "assume", "inference" et cetera are originally philosophical, later popular theological, deceits that are erroneously used to raise unreasonable doubt over science when it tests beyond reasonable doubt. (It became popular to attack evolution.)

'Assume' makes an ass out of u and me, and so does philosophy/magic thinking.

We even have witnesses from that time, the cosmic microwave background is older than the first galaxies and yet has an imprint of their formation. We can ask fossil relic witnesses same as we ask any other observations.

Feb 12, 2015
Here are some problems with Dark Matter theories:

-They don't know what it is.
-They don't know it's temperature.
-They don't know it's mass per entity.
-They can't even prove it's a particle, instead of some other entity.
-They can't prove it isn't the result of error in formula, observation, or interpretation.
-They can't explain why, if it is a particle, it never gets captured, as even a non-interacting particle can still lose angular momentum in a system and fall into the core of a planet, star, or b lack hole. If and when this happens, it would be indistinguishable from the outside to ordinary matter and would throw off their calculations even more.
-They use the wrong distribution/configuration of the mystery mass in the model simulations they do run, because the distribution they use is less efficient at producing the observed curve than is a plain old disk.
-The sphere of varying density distribution is hocus-pocus as other matter does not behave that way..

Feb 12, 2015
-Stars and solar systems are proof that extended spheres of matter are not stable over cosmic time scales, and collapse into systems of central objects and orbiting objects in disks, with a few strays above or below the disk. Every system reaches this configuration eventually. Even globular clusters tend toward this distribution as the center gains mass and stars are ejected.
-The Sun (and it's progenitor mass) has orbited the galactic center some 55-60 times in it's history, or some 23 times in it's history as a Star.
-This means the hocus-pocus Dark Matter moving around in alleged spherical orbits has passed through the Sun any number of times, and we are expected to believe it never gets captured and stored in the heart of massive objects.
-Stars' surface gravity actually has a higher escape velocity than the Galaxy itself in inter-stellar space. This means Dark Matter is actually more easily captured by individual Stars than the group gravity total.

Feb 12, 2015
Dark matter wont be easily captured because it does not interact. So it should form halos, and not disks or localized objects.

Feb 12, 2015
-They don't know it's temperature or mass

Constrained by observation to the point it doesn't have much of a difference.

-They can't prove it isn't the result of error in formula, observation, or interpretation.

No model can ever claim that.

They can't explain why it never gets captured, as even a non-interacting particle can still lose angular momentum

No. If it interacts though gravitation only there is no reason it would loose angular momentum.

They use the wrong distribution/configuration ...The sphere of varying density distribution is hocus-pocus

The NFW profile they use is specifically chosen because it is not hocus-pocus unlike a disk, it ubiquitously forms in dark matter simulations. A disk requires less mass but doesn't reproduce clustering or observations of elliptical. It's not the same so why would it behave like other matter?

Feb 12, 2015
Stars and solar systems are proof that extended spheres of matter

And yet ellipticals are even smaller than dark matter halos and haven't collapsed.

Stars' surface gravity actually has a higher escape velocity than the Galaxy itself in inter-stellar space.

A particle falling into the sun however will gain that escape velocity falling from infinity. It wouldn't be captured.

Feb 12, 2015
Well dark matter outside the galaxy is cooler than that dark matter that makes up the central cores galactic mass its,super hot radioactive mass with a massive super hot charged particle plasma field orbiting it producing all that central core light

Feb 15, 2015
Dark Matter is not a theory. Its a model. With all these simulations they are trying to refine their models to match what is actually observer. Once that is done and it works pretty much flawlessly wherever it is used, it can be called a law. Then they'll try to figure why it is so and build various hypothesis. The ones which make testable predictions will be loosely called "theories". Then one of them will make at least one accurate prediction that no other theory has made and that particular prediction would be tested and retested till there is confidence that it is accurate and is able to derive known laws of physics that are already known to be very accurate.
It is then it'll be a theory - which is a description of why certain things are so - like the theory of gravity.

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