Saturday, June 14, 2008

Dark matters: life, the universe and everything

On Wednesday NASA launched a rocket carrying its new $690 million long distance telescope it hopes will provide answers to some of the universe’s enduring mysteries. The Delta II rocket blasted off from Kennedy Space Center carrying aboard the Gamma Ray Large Area Space (GLAST) telescope. After a 75 minute flight, GLAST was deployed into a low Earth orbit and NASA expect it to transmit its first data in three weeks time. “After a 60-day checkout and initial calibration period, we'll begin science operations," said Steve Ritz, project scientist at Goddard Space Flight Center in Washington DC. "Glast soon will be telling scientists about many new objects to study, and this information will be available on the internet for the world to see."

GLAST is designed to study high energy sources of radiation in the universe. The project is a five to ten year operation designed to detect high energy gamma ray bursts, pinpoint their origin and shed light on the black holes where they mostly seem to reside. According to NASA, the new telescope will give astronomers a superior tool to “study how black holes, notorious for pulling matter in, can accelerate jets of gas outward at fantastic speeds.” The idea is to use a form of light invisible to the human eye to study and trace the origins of one of the most powerful forms of energy known - gamma ray bursts.

Gamma rays are energy-laden electromagnetic radiation produced by sub-atomic particles. They sit at the lowest end of the electromagnetic spectrum beyond ultraviolet and x-rays with a tiny wavelength of 0.00001 millimetres (ten to the power of minus five). They are found in the hottest areas of the universe and are produced by dramatic events such as supernova explosions, destruction of atoms, and the decay of radioactive material in space. A gamma ray burst release can travel across vast distances of the universe, and are absorbed by the Earth's atmosphere. Gamma ray bursts are spectacular events and can release more energy in ten seconds than the Sun will emit in its entire ten billion year life.

Scientists are particularly interested in gamma ray bursts because they throw light on some of the earliest events in the universe. They are so bright they can be detected as far back as the earliest five percent of the universe’s life time – over 13 billion years ago. One of the aims of GLAST is to investigate the link between gamma rays and dark matter. They hope to confirm that gamma ray bursts from the centre of our galaxy will reveal the presence of dark matter. Although known to science since 1933, dark matter remains one of science’s more hypothetical concepts. Yet it is believed to account for the majority of the mass of the observable universe.

The discovery of dark matter was made by Swiss astrophysicist Fritz Zwicky who worked at the California Institute of Technology in the 1930s. Zwicky was aware of Edwin Hubble’s discovery that the universe was expanding. But he was also aware that galaxies tend to cluster in complicated local movements. He measured the red shift from individual galaxies to see what was holding them together. What he found astonished him: a cluster of galaxies can remain bound together for billions of years, but only if it contains enough material to trap the individual members. But when Zwicky calculated the combined gravity of the known components – stars, gas and dust – it was nowhere near sufficient to form the cluster.

Zwicky concluded there must be an extra contribution to the gravitational pull. This unknown force outweighed the visible stuff hundreds of times over. He called this material “dark matter”. While Zwicky’s findings were initially ignored, a gradual weight of evidence emerged to show, as scientist Paul Davies says, “the luminous parts of galaxies represent just the tip of the iceberg, and that most of the matter in the universe is in fact dark”.

It is dark matter that keeps the galaxy in the familiar disk shape and it is dark matter that ensures the Sun stays firm on its 250 million year circuit of the Galaxy. Without it, the Milky Way would unravel like an exploding flywheel. Ever keen to find human significance in the universe, Scientists have come up with bizarre anthropomorphic names to describe the types of dark matter they think might be out there. There are two broad categories: MACHOs and WIMPs. MACHOs are “Massive Compact Halo Objects”. These are concentrations of mass residing in the galactic halo. They include dwarf stars and giant planets as well as smaller objects such as asteroids and comets. These objects are too dim to show up in telescopes but still exist in abundance. Yet MACHOs probably count for only a small percentage of dark matter. The rest are likely to be WIMPs.

WIMPs are Weakly Interactive Massive Particles. They are not so much dark as invisible, mostly passing through ordinary matter without betraying their existence. Because dark matter is concentrated at the centre of galaxies, scientists envisage a thick invisible soup of WIMPs through which stars swim as they perform their loop across the Milky Way. As WIMPs are weakly interacting, they only very rarely hit an ordinary atom; and as they are massive particles, possibly as heavy as a uranium atom they could account for the remaining dark matter in the galaxy.

Most importantly, dark matter played an essential role in shaping the universe. The smoothness of the early life of the universe was removed as regions over-dense in dark matter drew on surrounding material to amplify their denseness. Normal matter alone would have been too feeble to create galaxies, stars and planets and dark matter was needed to assist the clumping process. Peter Michelson, a Stanford astrophysicist and a lead investigator on the GLAST project, describes dark matter as “"mysterious, unseen substance that gravitationally holds the universe together”. He hopes the glimpses of gamma ray bursts will provide clues as to how dark matter formed the universe. "When you look at the night sky with your eyes, it is fairly quiescent and peaceful," Michelson said. "The gamma ray sky is not. It's a very different view of the universe. We're seeing exotic things like black holes and neutron stars and coalescing binary systems at the end of their life when they collapse into a black hole and there's an explosion."

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