We are all submerged in a sea of almost undetectable particles left over from the first few seconds of the big bang, according to the latest observations from a NASA satellite. The Wilkinson Microwave Anisotropy Probe (WMAP) has confirmed the theory that the universe is filled with a fluid of cold neutrinos that remain almost entirely aloof from ordinary matter.
Cosmologists think that in the hot, dense, young universe, neutrinos should have been created in high-energy particle collisions. About two seconds after the big bang, the cauldron of colliding particles would have cooled down so much that most would not have had enough energy to interact strongly with neutrinos. The neutrinos would then have "de-coupled" from other matter and radiation.
In theory, they should still be buzzing around, a soup of slippery particles that by today has been chilled to a temperature of only 1.9 ° Celsius above absolute zero.
Now WMAP has found evidence of this cosmic gazpacho. The spacecraft, launched in 2001, has been building up a picture of the cosmic microwave background radiation, which carries a detailed imprint of the state of the universe 380,000 years after the big bang. In particular, it reveals the pattern of density fluctuations in space, the "texture" of the early universe.
Travelling at nearly the speed of light, neutrinos should have discouraged matter from forming tight clumps, and so smoothed out the texture of the universe slightly.
The WMAP data clearly show this smoothing effect, implying that those fast-flowing neutrinos formed about 10% of all the energy in the 380,000-year-old universe. "This confirms the theory," says Eiichiro Komatsu of the University of Texas in Austin, US, lead author of a study about the result.
In 2005, another analysis also provided evidence for a cosmic neutrino background, but it relied on combining WMAP data from other sources, and making some assumptions about other cosmological parameters, says Komatsu. Now that WMAP has collected five years' worth of data, it is enough to show firm evidence of the neutrino background on its own.
The neutrinos are too weak to be detected individually. "These neutrinos cannot be detected on the ground; you need the CMB to do it," Komatsu told New Scientist.
Other neutrinos, for example those generated in the Sun's core, can be detected on Earth, often in large tanks of water buried deep underground, where an occasional neutrino is unlucky enough to hit an atomic nucleus. But cosmic background neutrinos have only a millionth of the energy of a typical solar neutrino, making them even more ethereal.
To stop a substantial fraction of solar neutrinos, you would already need a lead shield a light year thick, says Komatsu. How about cosmic background neutrinos? "I'd estimate you would need a block of lead that is thicker than the entire universe."
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