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The universe may be a lonelier place than previously thought. Of the estimated 100 billion galaxies in the observable universe, only one in 10 can support complex life like that on Earth, a pair of astrophysicists argues. Everywhere else, stellar explosions known as gamma ray bursts would regularly wipe out any life forms more elaborate than microbes. The detonations also kept the universe lifeless for billions of years after the big bang, the researchers say.
"It's kind of surprising that we can have life only in 10% of galaxies and only after 5 billion years," says Brian Thomas, a physicist at Washburn University in Topeka who was not involved in the work. But "my overall impression is that they are probably right" within the uncertainties in a key parameter in the analysis.
WHAT IS GAMMA RAY BURST
Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe.Bursts can last from ten milliseconds to several minutes. A typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime. But all observed GRBs have originated from outside the Milky Way galaxy.
Scientists have long mused over whether a gamma ray burst could harm Earth. The bursts were discovered in 1967 by satellites designed to spot nuclear weapons tests and now turn up at a rate of about one a day. They come in two types. Short gamma ray bursts last less than a second or two; they most likely occur when two neutron stars or black holes spiral into each other. Long gamma ray bursts last for tens of seconds and occur when massive stars burn out, collapse, and explode. They are rarer than the short ones but release roughly 100 times as much energy. A long burst can outshine the rest of the universe in gamma rays, which are highly energetic photons.