QUEST FOR DARK MATTER Dr. S. S. VERMA; Department of Physics, S.L.I.E.T., Longowal; Distt.-Sangrur (Punjab)-148 106
With the theoretical as well as experimental evidences for the existence of anti-matter particles like positrons, anti-protons etc., scientists have always been curious to know about the dark matter purported to make up a majority of the mass in the Universe. Dark matter, the mysterious substance that may account for nearly 25 percent of the universe, has so far evaded direct observation. Dark matter was initially proposed to explain how galaxies hold together; from what we know about how gravity works, much more matter is required to hold galaxies together than we can see. Many candidates for what dark matter actually is have been proposed, but most explanations have been refuted by experiments. Though dark matter is imagined to be everywhere, permeating the Universe and clumping around galaxies, what seems to align best with both theory and experiment so far is a class of particles that tend not to interact with the matter. No one knows what dark matter is, or if it even really exists. For now, it is just a placeholder, an x that must be plugged into various calculations in order to square astronomical observations with the rules of Newtonian physics. The name comes from Fritz Zwicky, a Swiss astronomer who in 1933 used two well-established methods to calculate the mass of the Coma cluster, a group of more than 1,000 galaxies. One calculation was extrapolated from the movement of eight galaxies in the cluster using Newton’s second law of motion, which says, in essence, that the bigger the galaxy, the faster it spins. The other estimated the cluster’s total mass by quantifying the amount of light given off by its stars. The results should have been equal, but instead the movement-based number was an order of magnitude greater. The Coma galaxies were spinning much faster than would be predicted by the amount of overall light emitted. For the Newtonian equation to add up, there had to be more mass. Zwicky dubbed this missing bulk “dark matter.” Zwicky’s work was largely ignored until 1970, when another astronomer, Vera Rubin, documented similar discrepancies in the Andromeda galaxy. Since then, researchers have found that the visible mass in hundreds of other galaxies is also too small to explain the rate of motion, at least within the context of our current understanding of physics. Astronomers have also discovered invisible “gravitational lenses” that cause light to bend around themselves—despite these lenses having no identifiable mass with which to distort the fabric of space-time and bend light in the first place. All of this suggested that more than 80 percent of the matter in the universe was simply invisible to us. Today, most (but by no means all) physicists agree that dark matter exists, and that it is probably made up of what they call WIMPs, or weakly interacting massive particles. “Massive” doesn’t 1
mean that the particles are large, but that they have mass and therefore both respond to and cause gravitational pull. Despite the differences between ordinary and dark matter, cosmologists believe the two have been linked since the beginning of the universe, with dark matter playing a key role in the coalescing of particles into stars, galaxies and other large-scale structures after the Big Bang. Ordinary matter, which makes up the stars, planets, gas and dust in our galaxy, emits or reflects light that can be observed using telescopes on Earth or in space. However, the effect of dark matter, according to several theories, can be observed only indirectly by the gravitational force exerted on the more visible portions of the galaxy around us. Though dark matter exerts a tangible force on the galaxy as a whole, individual weakly interacting massive particles (WIMPs) have proved far more difficult to detect. Because these particles interact only very weakly with normal matter, the small signal that might come from a WIMP detection above ground would be drowned out by the cosmic radiation that constantly bombards Earth's surface. Dark-matter theorists currently suspect that it concentrates in a spherical cloud encompassing most galaxies. As our own solar system rotates in the Milky Way, both it and the galaxy move within one of these clouds, and the particles in the cloud—the WIMPs—flow through the Earth and everything on it at a rate, physicists guess, of about 100,000 per square centimeter per second. Now a team of physicists and former miners has converted shipping warehouse into a new surface-level laboratory at the Sanford Underground Laboratory. Physicists assemble the LUX (Large Underground Xenon) detector. When in place inside the Homestake mine, the liquidxenon-filled capsule may detect three or four particles of dark matter a year. The LUX project is just one of the efforts worldwide to find direct evidence of dark matter. They've painted the walls and baseboards white and added yellow floor lines to steer visitors around giant nitrogen tanks, locker-size computers and plastic-shrouded machine parts. Soon they will gather many of these components into the lab’s clean room and combine them into LUX, the Large Underground Xenon dark-matter detector, which they will then lower halfway down the mine, where—if all goes well—it will eventually detect the presence of a few particles of dark matter. Next is the work of planting LUX 4,850 feet underground, sometime late this year. And then the wait begins, as physicists and the public collectively hope that the giant hole in the ground will yield the secret to understanding the universe. In another experimental setup, Gran Sasso National Laboratory in Italy, where the XENON experiment is housed deep beneath a mountain 70 miles west of Rome, represent the highestsensitivity search for dark matter yet, with background noise 100 times lower than competing efforts. The XENON researchers used a dark-matter detector known as XENON100 -- an instrumented vat filled with over 100 pounds of liquid xenon -- as a target for these WIMPs, which are thought to be streaming constantly through the solar system and Earth. And while the XENON100 experiment found no dark matter signal in 100 days of testing, the researchers' newly calculated upper limits on the mass of WIMPs and the probability of their interacting with other particles are the best in the world. XENON100 looks for a primary flash of light that occurs when a particle bounces off a xenon atom inside the detector and a secondary flash when an electron knocked free from a xenon atom by a collision is accelerated toward the top of the device by an electric field. With this configuration, a WIMP will generate a signal fundamentally different from that of cosmic radiation or emission from the equipment itself, making it possible to identify background readings that could be mistaken for a positive detection. Even though the 2
experiment did not detect a WIMP, the progress sets the stage for an ambitious next-generation project called XENON1T, which will use a much larger, one-ton liquid xenon instrument with highly specialized light-detectors developed at UCLA that make it 100 times more sensitive than XENON100. To eliminate the majority of background noise, the XENON100 experiment is buried beneath almost one mile of rock in the Gran Sasso lab, the largest underground facility of its kind in the world. While dark matter particles can travel easily through the vast expanse of stone and pass through the detector, only the most energetic particles from space are able to follow. Because the XENON100 experiment is shielded by large amounts of rock, as well as by several tons of copper, lead and water, the largest source of background detections is actually the radiation coming from the instrument itself. In an effort to address this issue researchers are working in collaboration with Hamamatsu Photonics (Japan) and have developed the Quartz Photon Intensifying Detector (QUPID), a new light-detector technology that emits no radiation. The XENON group hopes to incorporate this breakthrough technology into the future XENON1T experiment. Acknowledgement: The use of information retrieved through various references/sources of internet in this article is highly acknowledged.