Dreaming of Dark Matter

Sam Raredon February 25, 2010 0

“Deep into that darkness peering, long I stood there, wondering, fearing, doubting, dreaming dreams no mortal ever dared to dream before.” ~Edgar Allan Poe, “The Raven”

Left: The cosmic neighborhood of Segue 1 by Sloan Digital Sky Survey. Right: Segue 1, with its 24 known stars, by Maria Geha.

The stars have always fascinated us with their beauty. Yet a small group of celestial objects have recently sparked the imagina­tions of astronomers and physicists for a peculiar reason. By using advanced meth­ods of observation during the past decade, scientists have found hundreds of galaxies that appear to be spinning too fast for their gravitational field. The stars within them are traveling far faster than the escape velocity calculated from their luminous matter. The galaxies should literally fly apart.

This realization has led most physicists to incorporate the concept of “dark matter” into their theories of the universe. Based on astronomic evidence, dark matter would outweigh regular matter in the universe by a factor of about 4.5 to 1; the galaxies mentioned above are theorized to contain enough dark matter to allow them to spin as fast as they do. Dark matter particles are thought to be massive, uncharged particles which barely interact with regular matter except via gravity. This is why we cannot see dark matter and why we have never directly detected it.

Along with confirmation of the existence of Higg’s Boson, the particle thought to bestow mass, the issue of whether dark matter actually exists is one of the most pressing questions in the field. Around the world, numerous experiments are aiming to shed light on this (dark) matter. Four of these collaborations involve profes­sors within Yale’s Physics and Astronomy departments.

Segue 1 and the Seven Dwarves

During the period of rapid expansion following the Big Bang, ripples from quan­tum fluctuations in the super-dense matter of the universe inflated to form the galaxies we see today. This model of the formation of the universe is known as the Cold Dark Matter (CDM) model.

According to this model, most of the visible galaxies in the sky, including our own, are thought to be accumulations of dark matter that attracted conglomerations of luminous matter. Additionally, there should be hundreds of leftover, dark-matter dominated “dwarf galaxies” surrounding the Milky Way. However, as of 2005 only 11 dwarf galaxies had been discovered orbit­ing our galaxy. This was hard astronomical evidence that challenged the validity of the CDM model.

Astronomers have realized, however, that maybe they just were not looking hard enough. Marla Geha, Assistant Professor of Astronomy at Yale and a member of the National Research Council of Canada, along with Joshua Simon, Robert A. Mil­likan Postdoctoral Scholar at Caltech, recently revealed the discovery of 8 addi­tional dwarf galaxies in our vicinity – bring­ing the current tally to 24.

Using data from the Sloan Digital Sky Survey and the 10-meter Keck II telescope in Hawaii, Geha and Simon focused on local dwarf galaxy candidates. At first glance, many of them appear to be little more than tiny clusters of stars. Yet by ana­lyzing red-shift data from individual stars within each cluster, Geha realized that many of the stars were moving nearly ten times faster than they should have been. At such speeds, the stars would have had enough kinetic energy to completely escape the gravitational pull of the luminous matter around them.

The only reasonable conclusion, the researchers theorized, was that these gal­axies were dominated by dark matter. One galaxy in particular, Segue 1, appears to be the most faint and therefore the most dark-matter-dominated galaxy ever discov­ered. Although only 300 times as bright as our sun, the galaxy is a thousand times as massive as it appears, which suggests that it must be composed almost entirely of dark matter.

Although only 24 dwarf galaxies have been discovered, Geha believes that there are many more. The Sloan Digital Sky Survey covers only about one-fifth of the sky. If the other four-fifths each contain a similar number, the final count could be as high as four to five hundred, nearly exactly the number predicted by the CDM model. If this is shown to be the case, we will have moved one step further towards verifying the existence of dark matter.

Shining a Light Through a Wall

Nothing would more concretely demonstrate the existence of dark matter than the creation of a dark matter particle in the laboratory. This is exactly what Yale Physics Professor Oliver Baker is planning to accomplish with the Free Electron Laser, or FEL, at Jefferson Labs in Virginia.

FEL at Jefferson Laboratories.

The FEL is the most powerful tunable laser in the world, producing more photons per second than any other in use. The laser has been set up to direct a beam of photons through a particular high-energy magnetic field. In theory, this should cause some of the photons in the beam to convert to dark matter candidates called axions. The beam is then pointed directly at a block of aluminum several inches thick. Since dark matter is minimally interactive with regular matter, the axions should travel directly through the slab of metal.

Of course, the problem is then to detect the axions on the other side: since they are minimally-interactive, there is currently no way of directly detecting them. Therefore, the beam is then passed through another magnetic field which would convert any axions back into detectable photons.

The experiment is also designed to test for the lesser-known but more prevalent cousin of dark matter, dark energy. A physics model known as the Quintessence Scalar Field Model predicts a form of dark energy that manifests itself in the form of “chameleon particles.” These particles change their properties according to their environment, gaining mass when around other massive objects. The experiment is set up in a fashion which would allow such particles to form, but would then trap them in a vacuum tube between two windows of glass. Whenever the particle approached the glass, it would gain too much mass to pass through. By bouncing between the two windows, the particles would create an afterglow, which would be present minutes to hours after the laser is turned off.

Such observations would provide strong evidence for the existence of dark matter and energy, and these experiments are the first examples of using a high-intensity light source in particle physics experiments.

Building the Big Bang

Nowhere are hopes higher than in Geneva, Switzerland, where CERN’s Large Hadron Collider was turned on. The LHC, as it is known around the world, is the larg­est and most powerful particle accelerator ever built.

Many physicists are hoping that when the LHC is run at full power for the first time, the resulting collisions will open entirely new windows into unexplored regions of particle physics. Current theories concern­ing the formation of the universe postulate that in the moments immediately follow­ing the Big Bang, the rapidly expanding universe was filled with extremely massive particles, which then quickly decayed into less massive, more stable ones. While a significant portion decayed into luminous matter, the majority decayed into dark matter. Therefore, in order to create dark matter, one would have to create conditions which are similar to those present immedi­ately after the Big Bang.

The LHC is designed to achieve this. By smashing billions of protons together at an energy of 14 trillion electron volts, the LHC will create an astronomically large array of different particles, the distinctive paths of which will be traced by detectors able to record 600 million collisions per second.

In addition to revealing entirely new families of particles, data from the LHC has the potential to explain fundamental prop­erties of our universe, including why objects have mass and whether or not dark matter exists. Both Professor Baker and Professor Paul Tipton of the Yale Department of Physics are currently directing projects as part of the ATLAS experiment at the LHC that aim to investigate dark matter.

The ATLAS experiment detectors. Images courtesy of ATLAS image database: http://cdsweb.cern.ch/collection/ATLAS.

The many layers of detectors at ATLAS.

The problem is that a dark matter particle would be impossible to detect directly by any known means. There­fore, Tipton is looking for anomalies in the data which indicate the creation of an unseen particle. Although a dark matter particle is minimally interactive, it still obeys fundamental physical laws such as the conservation of momentum. Data indicating that energy and momentum have not been conserved would suggest that unde­tected particles have been created. If Tipton and his colleagues see this more frequently than expected, we would suddenly have strong evidence that dark matter exists.

Nevertheless, “the full solution will have to come from a two-pronged approach,” explains Tipton. “If we see dark matter particles at the LHC, we’re then going to have to observe them directly in the lab to try to confirm their mass and their inter­action strength and measure their other properties.”

The first event seen at ATLAS. It is a “splash event” – a first test.

A three-dimensional rendering of the same “splash” event.

A simulation of a collision at ATLAS producing supersymmetric particles. There is a clearly visible lack of energy conservation in the plane transverse to the beam – a clear indication that un-detected particles have been created.

Panning for Particles

Although one of the main goals of the LHC is to create dark matter particles, the detectors will not be able to prove that what they have recorded is actually dark matter. In order to truly confirm its existence, the particles must be observed in a lab under controlled conditions which allow their properties to be measured. Dan McKinsey, Yale Associate Professor of Physics, is doing just this.

Since dark matter particles are so mini­mally interactive, collisions that involve them are extremely rare. Additionally, any detector searching for an interaction between a dark matter particle and an ordi­nary matter particle would be completely corrupted by the amount of background radiation present in the universe. Therefore, any experiment detecting dark matter must be highly sen­sitive, deep underground, and heavily shielded from radiation. Many of McK­insey’s investigations are modeled from experi­ments crafted in the 1950’s to verify the existence of neutrinos, another mini­mally-interactive particle.

McKinsey’s primary research venture, entitled Project LUX, is a scaled up version of earlier neutrino detectors, located deep within the former Home­stake gold mine in South Dakota. LUX is essentially a closed tank of liquid xenon surrounded by light sensors. When a dark matter particle encounters the liquid xenon, the particle will hit a xenon molecule and will release energy that can then be recorded by the extremely sensitive detectors around the fluid.

The large size of LUX will be integral to its success; in addition to being beneath miles of solid rock, the outermost xenon will shield the innermost xenon from back­ground radiation. If a collision is seen deep within the detector, it is highly likely that it was caused by a dark matter particle. McK­insey is involved in a number of similar experiments in underground laboratories around the United States.

“There are a lot of theories that we’re going to see dark matter particles in our detector,” said McKinsey in a recent inter­view. “The real challenge is making sure that the background radiation is low enough that we’re sensitive to these dark matter candidates. Since these particles are lower energy that neutrinos, our equipment has to be vastly more sensitive.”

A schematic of Project LUX.

A detail of the Project LUX detector.

Finding the Hidden Valley

Tipton, Baker, Geha and McKinsey are each using diverse techniques to study various aspects of dark matter from the perspective of different fields. Yet they have one thing in common: all four scientists are extremely optimistic that the existence of dark matter will soon be confirmed. “It’s like coming over a hill and looking over a valley we’ve never seen before,” said Tipton, speaking of the LHC. “The valley might be populated with what we’ve predicted or it might be filled with lots of various features we’ve never before encountered. But to not find anything would be very, very unlikely. I think there are many things which point to something here.”

The impact of such a discovery cannot be underestimated. Both McKinsey and Baker went so far as to compare the dis­covery of dark matter to the Copernican Revolution. “All of the major scientific revolutions that have come about,” said Baker recently, “deal with maybe 5 percent of the universe. But the vast majority of the universe is made up of dark matter and dark energy – it would be naive to focus on only a small part. That’s what’s most exciting to me. If we could observe dark matter in a lab environment, if we could bring it down to our level, it would be a revolution in our understanding of the universe.”

About the Author
SAM BRICKMAN RAREDON is a junior in Timothy Dwight College. A Biomedical Engineering and History of Art double major, he is very interested in the intersections among the sciences.

Acknowledgements
The author would like to thank Professor Marla Geha, Professor Oliver Baker, Pro­fessor Daniel McKinsey, and Professor Paul Tipton for their time and patience in answering all of his questions. He wishes them great luck in their continued research.

Further Reading

  • “Astronomers Discover Most Dark Matter-Dominated Galaxy in Universe.” opa.yale.edu. http://opa.yale.edu/news/article.aspx?id=6037.

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