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Hypothetical particles with real gravity
Scientists are still trying to determine exactly what dark matter is. Candidates include hypothetical particles such as the neutralino, the sterile neutrino, the axion or some other weakly interacting massive particle. Fortunately, researchers do not need to fully understand dark matter in order to simulate it. All they need to know is that dark matter interacts with other matter only through gravity and is cold, meaning the matter is made up of particles that were moving slowly when galaxies and clusters began to form. Using initial conditions provided by observations of the cosmic microwave background, Madau and his team were able to simulate dark matter through a computer application called PKDGRAV2, developed by a group of numerical astrophysicists at the University of Zurich, who ignored visible matter and focused entirely on the gravitational interaction among a billion

"The computer was basically just computing gravity," Madau explained. "We have to compute the gravitational force among 1 billion particles, and to do that is very tricky. We are following the orbits of these particles in a gravitational potential that is varying all the time. The code allows us to compute with very high precision the gravitational force due to the particles that are next to us and with increasingly less precision the gravitational force due to the particles that are very far away because the gravity becomes weaker and weaker with distance."

Dark matter is not evenly spread, although researchers speculate it was nearly homogeneously distributed immediately after the Big Bang. Over time, however, gravity pulled the matter together, first into tiny "clumps" having more or less the mass of Earth. Over billions of years these clumps were drawn together, a process that continued until they combined to form halos of dark matter massive enough to host galaxies.

One lingering question was whether the smaller clumps would remain identifiable or would smooth out within the larger galactic halos. The answer required a state-of-the-art supercomputer such as Jaguar, which at the time of the simulations in November 2007 was capable of nearly 120 trillion calculations a second. Because earlier simulations did not have the resolution to resolve any unevenness, the results appeared to show the dark matter smoothing out, especially in the galaxy's dense inner reaches. Madau's billion-cell simulation, however, provided enough resolution to verify that the earliest forms of dark matter do indeed survive and retain their identity, even in the very inner regions, where our solar system is located.

"What we find," he continued, "is the survival fraction is quite high."

Madau's team will be able to verify its simulation results using the National Aeronautics and Space Administration's Gamma-Ray Large Area Space Telescope. Launched on June 11, 2008, The telescope will scan the heavens to study some of the universe's most extreme and puzzling phenomena: gamma-ray bursts, neutron stars, supernovas and dark matter, just to name a few. While dark matter particles cannot themselves be detected (direct detection of dark matter is being pursued by large underground detectors), researchers believe that dark matter particles and antiparticles may be annihilated when they bump into each other, producing gamma rays that can be observed from space. The clumps of dark matter predicted by Madau's team should bring more particles together and thereby produce an increased level of gamma rays.

A second verification comes from an effect known as gravitational lensing, in which the gravity exerted by a galaxy along the line of sight bends the light traveling from faraway quasars in the background. If the dark matter halos of galaxies are as clumpy as this simulation suggests, the light from a distant quasar should be broken up, like a light shining through frosted glass.

"We already have some data there," Madau noted, "which seems to imply that the inner regions of galaxies are rather clumpy. The flux ratios of multiply imaged quasars are not as you would predict with a smooth intervening lens potential. Instead of a smooth lens, there is substructure that appears to be affecting the lensing process. Our simulation seems to produce the right amount of lumpiness."

Madau's simulations in less than two years have reshaped the discussion about how our universe is held together. As researchers have access to increasingly powerful supercomputers, their findings could enable them to join their predecessors Newton and Einstein in unlocking the door to some of humankind's most fundamental questions.—Leo Williams.

Relativity , theory, developed in the early 20th century, which originally attempted to account for certain anomalies in the concept of relative motion, but which in its ramifications has developed into one of the most important basic concepts in physical science. The theory of relativity, developed primarily by German American physicist Albert Einstein, is the basis for later demonstration by physicists of the essential unity of matter and energy, of space and time, and of the forces of gravity and acceleration.

CLASSICAL PHYSICS
Physical laws generally accepted by scientists before the development of the theory of relativity, now called classical laws, were based on the principles of mechanics enunciated late in the 17th century by the English mathematician and physicist Isaac Newton. Newtonian mechanics and relativistic mechanics differ in fundamental assumptions and mathematical development, but in most cases do not differ appreciably in net results; the behavior of a billiard ball when struck by another billiard ball, for example, may be predicted by mathematical calculations based on either type of mechanics and produce approximately identical results. Inasmuch as the classical mathematics is enormously simpler than the relativistic, the former is the preferred basis for such a calculation. In cases of high speeds, however, assuming that one of the billiard balls was moving at a speed approaching that of light, the two theories would predict entirely different types of behavior, and scientists today are quite certain that the relativistic predictions would be verified and the classical predictions would be proved incorrect.

In general, the difference between two predictions on the behavior of any moving object involves a factor discovered by the Dutch physicist Hendrik Antoon Lorentz, and the Irish physicist George Francis FitzGerald late in the 19th century. This factor is generally represented by the Greek letter b (beta) and is determined by the velocity of the object in accordance with the following equation: