Helium clue found in echo of the Big Bang
THE subtle signal of ancient helium has shown up for the first time in light left over from the big bang. The discovery will help astronomers work out how much of the stuff was made during the big bang and how much was made later by stars.
Helium is the second-most abundant element in the universe after hydrogen. The light emitted by old stars and clumps of hot pristine gas from the early universe suggest helium made up some 25 per cent of the ordinary matter created during the big bang.
The new data provides another measure. A trio of telescopes has found helium’s signature in the cosmic microwave background (CMB, pictured), radiation emitted some 380,000 years after the big bang. The patterns in this radiation are an important indicator of the processes at work at that time. Helium affects the pattern because it is heavier than hydrogen and so alters the way pressure waves must have travelled through the young cosmos. But helium’s effect on the CMB was on a scale too small to resolve until now.
By combining seven years of data from NASA’s Wilkinson Microwave Anisotropy Probe with observations by two telescopes at the South Pole, astronomers have confirmed its presence. “This is the first detection of pre-stellar helium,” says WMAP’s chief scientist, Charles Bennett.
These observations are in line with earlier measurements, although less accurate. “I think CMB measurements will surpass them eventually,” says team member David Spergel.
More accurate numbers could reveal how quickly the early universe expanded. Helium forms from the interaction between protons and neutrons. This is constrained by the number of available neutrons, which would have dropped during the time the brand new universe was expanding as they decayed into protons. So the amount of helium that formed places important limits on how quickly this expansion took place. That could help test theories that postulate extra dimensions or as-yet-unseen particles.
Better data should be available in the next few years. The European Space Agency’s Planck satellite, which launched last year, is poised to measure the amount of helium even more precisely.
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