domingo, 17 de fevereiro de 2008

Could the First Stars Have Been Powered by Dark Matter?

Written by Ian O'Neill
Time line of the Universe, early stars may have been powered by dark matter, and not fusion reactions (Credit: NASA/WMAP Science Team)
Early stars that began to form about 200 million years after the Big Bang were strange creatures. From observation, the earliest stars (formed from coalescing primordial gas clouds) were not dense enough to support fusion reactions in their cores. Something within the young suns was counteracting the collapsing gas clouds, preventing the core reactions from taking place. Yet, they still produced light, even in absence of nuclear processes. Could dark matter have had a part to play, fueling the stellar bodies and sparking early stars to life?

New research indicates that the energy generated by annihilating dark matter in the early universe may have powered the first stars. How? Well, the violent early universe will have had high concentrations of dark matter. Dark matter has the ability to annihilate when it comes into contact with other dark matter matter, it does not require anti-dark matter to annihilate. When "normal" matter collides with its anti-component (i.e. electron colliding with positron), annihilation occurs. Annihilation is a term often used to describe the energetic destruction of something. While this is true, the annihilation products from dark matter include huge amounts of energy to create neutrinos and "ordinary matter" such as protons, electrons and positrons. Dark matter annihilation energy therefore has the ability to condense and create the matter we see in the Universe today.
"Dark matter particles are their own anti. When they meet, one-third of the energy goes into neutrinos, which escape, one-third goes into photons and the last third goes into electrons and positrons." - Katherine Freese, Theoretical Physicist, University of Michigan.
Katherine Freese (University of Michigan), Douglas Spolyar (University of California, Santa Cruz) and Paolo Gondolo (University of Utah in Salt Lake City) believe the strange physics of the early "dark stars" may be attributed to dark matter. For a star to form from stellar gas cloud to a viable, burning star, it must cool first. This cooling allows the star to collapse so the gas is dense enough to kick-start nuclear reactions in the core. However, early stars appear to have some form of energy acting against the cooling and collapse of early stars, fusion shouldn't be possible, and yet the stars still shine.
The group believe that early stars may have passed through two stages of development. As the gas clouds collapse, the stars go through a "dark matter phase", generating energy and producing normal matter. As the phase progresses, dark matter will slowly be used up and converted into matter. As the star becomes sufficiently dense with matter, fusion processes take over, starting the "fusion phase". Fusion in turn generates heavier elements (such as metals, oxygen, carbon and nitrogen) during the lifetime of the star. When the early stars' fuel is used up, it will go supernova, exploding and distributing these heavy elements throughout space to form other stars. The "dark matter phase" appears only to have existed in the very first stars (a.k.a. "population three stars"); later stars are supported by fusion processes only.
However, this exciting new theory will have to wait until the James Webb Telescope goes into operation in 2013 before population three stars can be observed with any great accuracy. Light may then be shone on the processes powering the first "dark stars" of our early Universe.

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