Monthly Archives: July 2012

Dark Matter Bridge Discovered

A team of astronomers claims to have detected an enormous bridge or filament of dark matter, with a mass estimated to be of order 100 trillion solar masses, and connecting two clusters of galaxies. The two clusters, known as Abell 222 and Abell 223, are about 2.8 billion light-years away and separated from one another by 400 million light-years. Each cluster has around 150 galaxies; actually one of the pair is itself a double cluster.

Clusters of galaxies are gravitationally bound collections of hundreds to a thousand or more galaxies. Often a cluster will be found in the vicinity of other clusters to which it is also gravitationally bound. The universe as a whole is gravitationally unbound – the matter, including the dark matter – is insufficient to stop the continued expansion, which is driven to acceleration in fact, by dark energy.

Dark matter bridge

Figure: Subaru telescope optical photo with mass density shown in blue and statistical significance contours superimposed. In the filament area found near the center of the image, the contours indicate four standard deviations of significance in the detection of dark matter. The cluster Abell 222 is in the south, and Abell 223 is the double cluster in the north of the image. The distance between the two clusters is about 14 arc-minutes, or about ½ the apparent size of the Moon.

Dark matter was originally called “missing matter”, and was first posited by Fritz Zwicky ( in the 1930s because of his studies of the kinematics of galaxies and galaxy clusters. He measured the velocities of galaxies moving around inside a cluster and found they were significantly greater than expected from the amount of ordinary matter seen in the galaxies themselves. This implied there was more matter than seen in galaxies because the velocities of the galaxies would be determined by the total gravitational field in a cluster, and the questions have been where is, and what is, the “missing matter” inferred by the gravitational effects. X-ray emission has been detected from most clusters of galaxies, and this is due to an additional component of matter outside of galaxies, namely hot gas between galaxies. But it is still insufficient to explain the total mass of clusters as revealed by both the galaxy velocities and the temperature of the hot gas itself, since both are a reflection of the gravitational field in the cluster.

Dark matter is ubiquitous, found on all scales and is generally less clumped than ordinary matter, so it is not surprising that significant dark matter would be found between two associated galaxy clusters. In fact the researchers in this study point out that “It is a firm prediction of the concordance Cold Dark Matter cosmological model that galaxy clusters live at the intersection of large-scale structure filaments.”

The technique used to map the dark matter is gravitational lensing, which is a result of general relativity. The gravitational lensing effect is well established; it has been seen in many clusters of galaxies to date. In gravitational lensing, light is deflected away from a straight-line path by matter in its vicinity.

In this case the gravitational field of the dark matter filament and the galaxy clusters deflect light passing nearby. The image of a background galaxy located behind the cluster will be distorted as the light moves through or nearby the foreground cluster. The amount of distortion depends on the mass of the cluster (or dark matter bridge) and how near the line of sight passes to the cluster center.

There is also a well-detected bridge of ordinary matter in the form of hot X-ray emitting gas connecting the two clusters and in the same location as the newly discovered dark matter bridge.  The scientists used observations from the XMM-Newton satellite to map the X-ray emission from the two clusters Abell 222 and Abell 223 and the hot gas bridge connecting them. Because of the strong gravitational fields of galaxy clusters, the gas interior to galaxy clusters (but exterior to individual galaxies within the cluster) is heated to very high temperatures by frictional processes, resulting in thermal X-ray emission from the clusters.

The research team, led by Jörg Dietrich at the University of Michigan, then performed a gravitational lensing analysis, focusing on the location of the bridge as determined from the X-ray observations. The gravitational lensing work is based on optical observations obtained from the Subaru telescope (operated by the Japanese government, but located on the Big Island of Hawaii) to map the total matter density profile around and between the two clusters. This method detects the sum of dark matter and ordinary matter.

They analyzed the detailed orientations and shapes of over forty thousand background galaxies observable behind the two clusters and the bridge. This work allowed them to determine the contours of the dark matter distribution. They state a 98% confidence in the existence of a bridge or filament dominated by dark matter.

The amount of dark matter is shown to be much larger than that of ordinary matter, representing over 90% of the total in the filament region, so the gravitational lensing effects are primarily due to the dark matter. Less than 9% of the mass in the filament is in the form of hot gas (ordinary matter). The estimated total mass in the filament is about 1/3 of the mass of either of the galaxy clusters, each of which is also dominated by dark matter.

Observations of galaxy distributions show that galaxies are found in groups, clusters, and filaments connecting regions of galaxy concentration. Cosmological simulations of the evolution of the universe on supercomputers indicate that the distribution of dark matter should have a filamentary structure as well. So although the result is in many ways not surprising, it represents the first detection of such a structure to date.


References: – press release from the University of Michigan “Dark matter filaments detected for the first time”

J. Dietrich et al. 2012 “A filament of dark matter between two clusters of galaxies”


Why the Higgs Boson is not Dark Matter

The Higgs boson is considered a necessary part of the Standard Model of particle physics. In the Standard Model there are 3 main forces of nature: the electromagnetic force, the weak nuclear force, and the strong nuclear force. The Standard Model does not address gravity and we do not yet have a proven theory for the unification of gravity with the other 3 forces.

On July 4th CERN, the European particle physics lab near Geneva, announced that two experiments using the Large Hadron Collider accelerator, ATLAS and CMS, have both amassed strong statistical evidence (around 5 sigma) for a new particle. This new particle has a mass of about 126 GeV* and “smells” very much like it is the long sought after, and elusive, Higgs boson. The prediction of the Higgs dates from 1964. For comparison, the proton mass is about 0.94 GeV, so the Higgs is around 134 times more massive. Further work is necessary to determine all of its properties, but at this point it looks as if the new particle decays into other particles in the expected manner. It is these decay products that are actually detected.

This decades-long search has proceeded in fits and starts, principally at CERN in Europe and Fermilab in the U.S., with different accelerators and detectors. Over time the experiments were able to exclude possible masses for the Higgs, since the rate of creation of different decay products varies for different putative masses. By the end of 2011 it looked like there was a preliminary signal, not yet of sufficient statistical strength, but that the mass would have to be in the range of about 115 to 130 GeV.


The CMS detector at the Large Hadron Collider. Credit: Mark Thiessen/National Geographic Society/Corbis

One of my professors, Steven Weinberg, won the Nobel Prize in Physics years ago for his work on unifying the electromagnetic force and the weak force. While the Standard Model and the body of work in particle physics provides a theoretical underpinning for all of the particles which we observe, and their quantum properties, and describes a unification of the strong force (which holds together the quarks inside a proton or neutron) with these other two forces, it also requires an additional mechanism to explain why most particles have non-zero masses.

The Higgs mechanism is the favored explanation, and it predicts a particle as the mediator to provide masses to other particles. The Higgs mechanism is theorized as an all-pervasive Higgs field, which slows down particles as they move through it. As you swim through water you feel a drag that slows you down. A fish with a very hydrodynamic design will feel less drag. In the particle world, more massive ones slow down more than the lighter ones, since they interact more strongly with the Higgs field.

The particle corresponding to this mechanism is known as the Higgs boson. Particles can have quantum spin that is a multiple of ½ or an integer multiple. Bosons have integer multiple spins. Actually the spin of the Higgs boson is zero. All of the force mediator particles such as the photon (spin = 1), which mediates electromagnetism, are bosons.

The Large Hadron Collider is in some sense recreating the conditions of the very early universe by smashing particles together at 7000 GeV, or 7 TeV. The Higgs originally would have been created in Nature in the very early part of the Big Bang, around the first one-trillionth of a second. The appearance of the Higgs broke the unification, or symmetry, between the electromagnetic and weak forces that Steven Weinberg demonstrated are one at very high energies. And the Higgs gave mass to particles.

Without the Higgs mechanism, all particles would be massless, and thus travelling at the speed of light, and structure in the universe – stars, planets, galaxies, human beings, would not be possible. Even the existence of the proton itself requires that quarks have mass, although most of the proton mass comes from the energy of the gluons (strong force mediation particle) and ‘virtual’ quark-antiquark pairs found inside it.

The Higgs boson cannot be the explanation for dark matter for a very simple reason. Dark matter must be stable with a very long lifetime, persisting over the universe’s present age of 14 billion years. It mostly sits in space doing nothing except providing additional gravitational interaction with ordinary matter. The favored candidate for dark matter is the least massive supersymmetric particle; being the least massive, it would have nothing to decay into. Supersymmetry is a theoretical extension beyond the Standard Model. No supersymmetric particles are detected as of yet, but the theory has a lot of support and has the benefit of stabilizing the mass of the Higgs itself.

The Higgs boson, on the other hand, decays very rapidly. There are various decay channels, including into quarks, W/Z bosons, leptons or photons, producing these in pairs (two Zs, two top quarks etc.). Sometimes even four particles are produced from a single Higgs decay. It is these decay products that are actually detected in the Large Hadron Collider at CERN.

There are a few experiments that are claiming to have directly detected dark matter. The favored mass range from the COGENT and DAMA/LIBRA experiments is around 10 GeV for dark matter, much more than a proton, but less than 10% of the Higgs’ mass. Now that the Higgs appears to have been found, work will proceed on confirming and elucidating its properties. And the next great hunt for particle physics may be the direct detection of dark matter particles and the beginning of a determination if supersymmetry is real.

* GeV = Giga-electronVolt or 1 billion electron Volts. 1 TeV (Tera-electronVolt) = 1000 GeV

References: – Don Lincoln of Fermilab on how we search for the Higgs at particle accelerators – BBC documentary