Super Colliders in Space: Dark Matter not Colliding

What’s bigger and more powerful than the Large Hadron Collider at CERN? Why colliding galaxy clusters of course.

A cluster of galaxies consists of hundreds or even thousands of galaxies bound together by their mutual gravitation. Both dark matter and ordinary matter in and between galaxies is responsible for the gravitational field of a cluster. And typically there is about 5 times as much dark matter as ordinary matter. The main component of ordinary matter is hot intracluster gas; only a small percentage of the mass is locked up in stars.

One stunning example of dark matter detection is the Bullet Cluster. This is the canonical example found revealing dark matter separation from ordinary matter in a pair of clusters colliding and merging. The dark matter just passes right through, apparently unaffected by the collision. The hot gas (ordinary matter) is seen through its X-ray emission, since the gas is heated by collisions to of order 100 million degrees. The Chandra X-ray Observatory (satellite) provided these measurements.

Image courtesy of Chandra X-ray Observatory

Bullet Cluster. The blue color shows the distribution of dark matter, which passed through the collision without slowing down. The purple color shows the hot X-ray emitting gas. Image courtesy of Chandra X-ray Observatory

The distribution of matter overall in the Bullet Cluster or other clusters is traced by gravitational lensing effects; general relativity tells us that  background galaxies will have their images displaced, distorted, and magnified as their light passes through a cluster on its way to Earth. The magnitude of these effects can be used to “weigh” the dark matter. These measurements are made with the Hubble Space Telescope.

In the Bullet Cluster the dark matter is displaced from the ordinary matter. The interpretation is that the ordinary matter from the two clusters, principally in the form of hot gas, is slowed by frictional, collisional processes as the clusters interact and form a larger single cluster of galaxies. Another six or so examples of galaxy clusters showing the displacement between the dark matter and the ordinary matter in gas and stars have been found to date.

Now, a team of astrophysicists based in the U.K. and Switzerland have examined 30 additional galaxy clusters with data from both Chandra and Hubble, and with redshifts typically 0.2 to 0.6. In aggregate there are 72 collisions in the 30 systems, since some have more than two subclusters. The offsets between the gas and dark matter are quite substantial, and in aggregate indicate the existence of dark matter in these clusters with over 7 standard deviations of statistical significance (probability of the null hypothesis of no dark matter is 1 in 30 trillion).

They then look at the possible drag force on the dark matter due to dark matter particles colliding with other dark matter particles. There are already much more severe constraints on ordinary matter – dark matter interactions from Earth-based laboratory measurements. But the dark matter mutual collision cross section could potentially be large enough to result in a drag. They measure the relative positions of hot gas, galaxies, and dark matter for all of the 72 subclusters.

From paper "The non-gravitational interactions of dark matter in colliding galaxy clusters"

From paper “The non-gravitational interactions of dark matter in colliding galaxy clusters” D. Harvey et al. 2015

The gas should and does lag the most, relative to the direction of the galaxies in a collision. If there is a dark matter drag, then dark matter should lag behind the positions of the stars. They find no lag of the dark matter average position, which allows them to place a new, tighter constraint on the mutual interaction cross-section for dark matter.

Their constraint is σ(DM)/m < 0.47 cm^2/g at 95% confidence level, where σ (sigma) is the cross-section and m is the mass of a single dark matter particle. This limit is over twice as tight as that previously obtained from the Bullet Cluster. And some dark matter models predict a cross section per unit mass of 0.6 cm^2/g, so these models are potentially ruled out by these new measurements.

In summary, using Nature’s massive particle colliders, the authors have found further highly significant evidence for the existence of dark matter in clusters of galaxies, and they have placed useful constraints on the dark matter self-interaction cross-section. Dark matter continues to be highly elusive.


D. Harvey et al. 2015 “The non-gravitational interactions of dark matter in colliding galaxy clusters”

Discovery of several dwarf galaxies near the Magellanic Clouds

Dwarf galaxies are, as the name implies, small or even tiny galaxies with much lower mass and luminosity than large galaxies such as our own Milky Way galaxy or the Andromeda galaxy or Triangulum galaxy. The first two galaxies are the dominant members of our Local Group of galaxies, which has over 50 members. While the Milky Way and Andromeda have over 200 billion stars each, most all of the others are much smaller and intrinsically fainter, and thus are considered dwarf galaxies. Around half of these known dwarf galaxies are companions to our Milky Way, and the rest are companions of Andromeda.


Previously known dwarf satellite galaxies around our Milky Way galaxy are shown as blue dots and the 9 new candidates are shown as red dots. Image: Yao-Yuan Mao, Ralf Kaehler, Risa Wechsler (KIPAC/SLAC).

The Dark Energy Survey “powered up” in the second half of 2013. Using the Dark Energy  Camera at the Cerro Tololo Inter-American observatory in Chile, two teams of astronomers have now made a stunning discovery of 9 new dwarf objects in the vicinity of, and gravitationally bound to, our own Milky Way. Three of these are confirmed to be dwarf galaxies. The other six objects are either dwarf galaxies or globular clusters, and further observations will be required to determine how many of these are indeed dwarf galaxies.

These new dwarf galaxies and dwarf galaxy candidates were found in the vicinity of the Magellanic Clouds, in the Southern Hemisphere. Those are themselves the two best known of all dwarf galaxies, but are substantially brighter and larger than these new dwarf galaxy candidates. In fact it is possible, but not certain, that the newly discovered dwarf galaxies have interacted with one or both of the Magellanic Clouds in the past.

This discovery of 3 or more new dwarf galaxies near to our Milky Way, in the range of about 100,000 light-years to 1.2 million light-years away from us, has important implications for our understanding of dark matter and cosmology generally. We know from a wide range of observations, including the latest Planck satellite results, that dark matter is 5 times more common than ordinary matter in the universe.

Dark matter and ordinary matter are distributed differently. Think of dark matter as the scaffolding which controls the overall distribution of matter at large scale. Ordinary matter is thus controlled gravitationally by the dark matter background. But ordinary matter also clumps together at smaller scales because as it collapses (falls into a gravitational potential well) it heats up via frictional processes. Next it radiates away energy, leading to cooling, and thus further collapse. This is how we end up with galaxies and stellar formation.

Large galaxies will be dominated by ordinary matter toward their centers, but by dark matter in their outer regions and halos. Many dwarf galaxies appear to have few stars, as little as only a few thousand, reflecting quite modest amounts of ordinary matter. These galaxies are heavily dominated by dark matter, sometimes 99% or more.

There is a whole theory of galaxy formation based on the growth of dark matter-dominated density perturbations that collapse under their own gravity, even while the universe as a whole is expanding. Ordinary matter is pulled into the regions of high dark matter density, leading to galaxy formation. Low density regions do not collapse, but keep on expanding in,the “Hubble flow”.

Numerical simulations of the growth of these dark matter density perturbations and of galaxy formation suggest there should be large numbers of dwarf galaxies. As we continue to discover more dwarf galaxies in the vicinity of our Milky Way, through the Dark Energy Survey and other experiments, our confidence in our understanding of cosmology and of galactic formation and evolution will continue to grow.

References  – CNET article – Article at University of Cambridge astronomy web site – Article at Fermilab web site (home of the Dark Energy Survey) – Dark Energy Survey web site – S. Koposov, V. Belokurov, G. Torrealba, N. Wyn Evans, ”Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds”

Planck Mission Full Results Confirm Canonical Cosmology Model

Dark Matter, Dark Energy values refined

The Planck satellite, launched by the European Space Agency, made observations of the cosmic microwave background (CMB) for a little over 4 years, beginning in August, 2009 until October, 2013.

Preliminary results based on only the data obtained over the first year and a quarter of operation, and released in 2013, established high confidence in the canonical cosmological model. This ΛCDM (Lambda-Cold Dark Matter) model is of a topologically flat universe, initiated in an inflationary Big Bang some 13.8 billion years ago and dominated by dark energy (the Λ component), and secondarily by cold dark matter (CDM). Ordinary matter, of which stars, planets and human beings are composed, is the third most important component from a mass-energy standpoint. The amount of dark energy is over twice the mass-energy equivalent of all matter combined, and the dark matter is well in excess of the ordinary matter component.


This general model had been well-established by the Wilkinson Microwave Anisotropy Probe (WMAP), but the Planck results have provided much greater sensitivity and confidence in the results.

Now a series of 28 papers have been released by the Planck Consortium detailing results from the entire mission, with over three times as much data gathered. The first paper in the series, Planck 2015 Results I, provides an overview of these results. Papers XIII and XIV detail the cosmological parameters measured and the findings on dark energy, while several additional papers examine potential departures from a canonical cosmological model and constraints on inflationary models.

In particular they find that:

Ωb*h²  = .02226 to within 1%.

In this expression Ωb is the baryon (basically ordinary matter) mass-energy fraction (fraction of total-mass energy in ordinary matter) and h = H0/100. H0 is the Hubble constant which measures the expansion rate of the universe, and indirectly, its age. The best value for H0 is 67.8 kilometers/sec/Megaparsec  (millions of parsecs, where 1 parsec = 3.26 light-years). H0 has an uncertainty of about 1.3% (two standard deviations). In this case h = .678 and the expression above becomes:

Ωb = .048, with uncertainty around 3% of its value. Thus, just under 5% of the mass-energy density in the universe is in ordinary matter.

The cold matter density is measured to be:

Ωc*h²  = .1186 with uncertainty less than 2% and with the h value substituted we have Ωc = .258 with similar uncertainty.

Since the radiation density in the universe is known to be very low, the remainder of the mass-energy fraction is from dark energy,

Ωe = 1 – .048 – .258 = .694

So in approximate percentage terms the Planck 2015 results indicate 69% dark energy, 26% dark matter, and 5% ordinary matter as the mass-energy balance of the universe. These results are essentially the same as the ratios found from the preliminary results reported in 2013. It is to be emphasized that these are present-day values of the constituents. The components evolve differently as the universe expands. Dark energy is manifested with its current energy density in every new unit of volume as the universe continues to expand, while the average dark matter and ordinary matter densities decrease inversely as the volume grows. This implies that in the past, dark energy was less important, but it will dominate more and more as the universe continues to expand.

Why is dark energy produced as the universe expands? The simplest explanation is that it is the irreducible quantum energy of empty space, of the vacuum. Empty space – space with no particles whatsoever – still has fields (scalar fields, in particular) permeating it, and these fields have a minimum energy. It also has ‘virtual’ particles popping in and out of existence very briefly. This is the cosmological constant (Λ) model for the dark energy.

This is the ultimate free lunch in nature. The dark energy works as a negative gravity; it enters into the equations of general relativity as a negative pressure which causes space to expand. And as space expands, more dark energy is created! A wonderful self-reinforcing process is in place. Since the dark energy dominates over matter, the expansion of the universe is accelerating, and has been for the last 5 billion years or so. Why wonderful? Because it adds billions upon billions of years of life to our universe.

The Planck Consortium also find the universe is topologically flat to a very high degree, with an upper limit of 1/2 of 1% deviation from flatness at large scales. This is an impressive observational result.

One of the most interesting results is Planck’s ability to constrain inflationary models. While a massive inflation almost certainly happened during the first billionth of a trillionth of a trillionth of a second as the Universe began, as indicated by the very uniformity of the CMB signal, there are many possible models of the inflationary field’s energy potential.

We’ll take a look at this in a future blog entry.

New calculations support dark-matter discovery by DAMA

Originally posted on physics4me:

A controversial claim by the DAMA group that it has detected dark matter in an underground lab in Italy might turn out to be true after all, according to physicists in Europe and the US. The new research reconciles the claimed detection with apparently null results from other experiments, as well as indirect astrophysical evidence. It proposes that dark matter interacts with ordinary matter not via one of the four known fundamental forces but instead through a fifth force mediated by an axion-like particle.

Dark matter is an as-yet-unknown substance that does not emit electromagnetic radiation but which numerous observations suggest makes up at least 80% of the matter in the universe. DAMA, a collaboration of physicists from Italy and China, says it has directly observed dark matter in a sodium-iodide detector located beneath Gran Sasso mountain east of Rome. The basis for its claim is a seasonal variation in…

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2014 in review

The stats helper monkeys prepared a 2014 annual report for this blog.

Here’s an excerpt:

A New York City subway train holds 1,200 people. This blog was viewed about 7,900 times in 2014. If it were a NYC subway train, it would take about 7 trips to carry that many people.

Click here to see the complete report.

Milky Way Dark Matter Halo Loses ‘Weight’

640px-Milky_Way_ArchMilky Way Arch, CC BY 3.0Bruno Gilli/ESO

Mass estimates for our Milky Way vary widely, from less than 1 trillion, to as high as 4 trillion, times the mass of the Sun.

A recent paper by a group of astronomers in Australia argues for a mass that is very much at the low end of this range. Prajawal Kafle and collaborators present a kinematic analysis and build a model of the Milky Way that incorporates a disk, a bulge, and a dark matter halo. The analysis utilizes K giant and horizontal branch star catalogs.

The disk component – in which our Sun resides – contains stars and gas and active star formation from this gas. The spheroidal bulge contains the oldest stellar population of the galaxy, including globular clusters. The spherical halo, significantly larger and more massive than both the other components, is dominated by dark matter. It is chiefly responsible for the overall gravitational potential of the Milky Way, and is evidenced by the high rotational velocity of our galaxy in its outer regions.

The result of their analysis is that the dark matter halo “weighs in” at about 800 billion solar masses, the disk is about 100 billion solar masses, and the bulge is only about 10 billion solar masses. They also find a dark matter density in the solar neighborhood equivalent to about 1/3 of a proton per cubic centimeter, consistent with other estimates. (This number is important for calibrating Earth-bound direct detection experiments for dark matter.)

The relatively low mass they determine for the dark matter halo implies fewer satellite galaxies in close proximity to the Milky Way. We see only 3, the two Magellanic Clouds and the Sagittarius Dwarf Galaxy. In the past this has been seen as an issue for the favored Lambda – Cold Dark Matter (ΛCDM) Cosmology.

However their lower halo mass is actually consistent with the Milky Way gathering only 3 so-called sub-halos (satellite galaxies) and thus there may be no Missing Satellite Problem with ΛCDM. Some had suggested warm dark matter, rather than cold dark matter, may be necessary because of the putative missing satellite problem, but this may not be the case, with a lighter Milky Way dark matter halo.

Another team, Penarrubia and collaborators, has recently modeled the dynamics of the Local Group of galaxies. They are thus using a different methodology to determine the total mass of the Milky Way. They find a total mass for the Local Group of 2.3 trillion solar masses. The Local Group mass is almost entirely due to the Andromeda Galaxy and our Milky Way. They also determine a Milky Way to Amdromeda mass ratio of about 1/2. This implies a mass of about 0.8 trillion solar masses for our Milky Way, consistent with the Australian team’s result.

These two latest measurements of the Milky Way mass seem to indicate that the total mass of the Milky Way galaxy is less than 1 trillion solar masses. And these two results thus suggest that the ΛCDM cosmology is in fact consistent with the small number of satellite galaxies around our Milky Way. Another success for ΛCDM, it seems.

References: – P. Kafle et al. 2014, “On the shoulders of giants: Properties of the stellar halo and Milky Way mass distribution”

J. Penarrubia et al. 2014, Monthly Notices of the Royal Astronomical Society, 443, 2204, “A dynamical model of the local cosmic expansion”

Dark Matter: Made of Sterile Neutrinos?


Composite image of the Bullet Group showing galaxies, hot gas (shown in pink) and dark matter (indicated in blue). Credit: ESA / XMM-Newton / F. Gastaldello (INAF/IASF, Milano, Italy) / CFHTLS 

What’s more elusive than a neutrino? Why a sterile neutrino, of course. In the Standard Model of particle physics there are 3 types of “regular” neutrinos. The ghost-like neutrinos are electrically neutral particles with 1/2 integer spins and very small masses. Neutrinos are produced in weak interactions, for example when a neutron decays to a proton and an electron. The 3 types are paired with the electron and its heavier cousins, and are known as electron neutrinos, muon neutrinos, and tau neutrinos (νe, νμ, ντ).

A postulated extension to the Standard Model would allow a new type of neutrino, known as a sterile neutrino. “Sterile” refers to the fact that this hypothetical particle would not feel the standard weak interaction, but would couple to regular neutrino oscillations (neutrinos oscillate among the 3 types, and until this was realized there was consternation around the low number of solar neutrinos detected). Sterile neutrinos are more ghostly than regular neutrinos! The sterile neutrino would be a neutral particle, like other neutrino types, and would be a fermion, with spin 1/2. The number of types, and the respective masses, of sterile neutrinos (assuming they exist) is unknown. Since they are electrically neutral and do not feel the standard weak interaction they are very difficult to detect. But the fact that they are very hard to detect is just what makes them candidates for dark matter, since they still interact gravitationally due to their mass.

What about regular neutrinos as the source of dark matter? The problem is that their masses are too low, less than 1/3 of an eV (electron-Volt) total for the three types. They are thus “too hot” (speeds and velocity dispersions too high, being relativistic) to explain the observed properties of galaxy formation and clumping into groups and clusters. The dark matter should be “cold” or non-relativistic, or at least no more than “warm”, to correctly reproduce the pattern of galaxy groups, filaments, and clusters we observe in our Universe.

Constraints can be placed on the minimum mass for a sterile neutrino to be a good dark matter candidate. Observations of the cosmic microwave background and of hydrogen Lyman-alpha emission in quasar spectra have been used to set a lower bound of 2 keV for the sterile neutrino’s mass, if it is the predominant component of dark matter. A sterile neutrino with this mass or larger is expected to have a decay channel into a photon with half of the rest-mass energy and a regular (active) neutrino with half the energy.

A recent suggestion is that an X-ray emission feature seen at 3.56 keV (kilo-electron Volts) from galaxy clusters is a result of the decay of sterile neutrinos into photons with that energy plus active (regular) neutrinos with similar energy. This X-ray emission line has been seen in a data set from the XMM-Newton satellite that stacks results from 73 clusters of galaxies together. The line was detected in 2 different instruments with around 4 or 5 standard deviations significance, so the existence of the line itself is on a rather strong footing. However, it is necessary to prove that the line is not from an atomic transition from argon or some other element. The researchers argue that an argon line should be much, much weaker than the feature that is detected.

In addition, a second team of researchers, also using XMM-Newton data have claimed detection of lines at the same 3.56 keV energy in the Perseus cluster of galaxies as well as our neighbor, the Andromeda galaxy.

There are no expected atomic transition lines at this energy, so the dark matter decay possibility has been suggested by both teams. An argon line around 3.62 KeV is a possible influence on the signal, but is expected to be very much weaker. Confirmation of these XMM-Newton results are required from other experiments in order to gain more confidence in the reality of the 3.56 keV feature, regardless of its cause, and to eliminate with certainty the possibility of an atomic transition origin. Analysis of stacked galaxy cluster data is currently underway for two other X-ray satellite missions, Chandra and Suzaku. In addition, the astrophysics community eagerly awaits the upcoming Astro-H mission, a Japanese X-ray astronomy satellite planned for launch in 2015. It should be able to not only confirm the 3.56 keV X-ray line (if indeed real), but also detect it within our own Milky Way galaxy.

Thus the hypothesis is for dark matter composed primarily of sterile neutrinos of a little over 7.1 keV in mass (in E = mc^2 terms), and that the sterile neutrino has a decay channel to an X-ray photon and regular neutrino. Each decay product would have an energy of about 3.56 keV. Such a 7 keV sterile neutrino is plausible with respect to the known density of dark matter and various cosmological and particle physics constraints. If the dark matter is primarily due to this sterile neutrino, then it falls into the “warm” dark matter domain, intermediate between “cold” dark matter due to very heavy particles, or “hot” dark matter due to very light particles.

The abundance of dwarf satellite galaxies found in the Milky Way’s neighborhood is lower than predicted from cold dark matter models. Warm dark matter could solve this problem. As Dr. Abazajian puts in in his recent paper “Resonantly Produced 7 keV Sterile Neutrino Dark Matter Models and the Properties of Milky Way Satellites”

the parameters necessary in these models to produce the full dark matter density fulfill previously determined requirements to successfully match the Milky Way galaxy’s total satellite abundance, the satellites’ radial distribution, and their mass density profile..



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