Monthly Archives: March 2015

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.