Gamma-ray burst, the most powerful kind of explosion known
in the universe, may have trigged a mass extinction on earth within the past
billion years, researchers say.
Gamma-ray bursts are flashes of gamma rays associated with extremely
energetic explosions that have been observed in distant universe. Bursts can
last for milliseconds to several minutes. The initial burst is usually followed
by a longer-lived afterglow emitted at longer wavelengths (X-ray, Ultraviolet,
optical, infrared, microwaves and radio). These deadly outbursts could help
explain Fermi paradox, the seeming contradiction between high chance of alien
life and lack of evidence for it, scientists added.
Most observed gamma-ray bursts are believed to consist of a
narrow beam of intense radiation released during a supernova or hyper nova as
rapidly rotating, high-mass star collapse to form neutron star, quark star or a
black hole.
If a gamma-ray burst exploded within the Milky Way, it could
wreak extraordinary havoc if it pointed directly to earth, even from a thousand
of light years away. Although gamma rays would not penetrate the atmosphere
well enough to burn the ground, they would chemically change the atmosphere,
depleting ozone layer that protects the planet from the damaging U.V light from
the sun that could trigger mass extinction.
It’s also possible that gamma-ray bursts may spew out cosmic
rays, which are high energy particles that may create similar effects to a
nuclear explosion for those on the side of the earth facing the explosion,
causing radiation sickness. To see how great a threat of gamma-ray burst might
pose to earth to earth, researchers investigated how likely it was that such
explosion could have inflicted damage on the planet to the past.
Gamma-ray bursts are traditionally divided into two groups- long
and short – depending whether they last more or less than 2 seconds. Long gamma
ray bursts are associated with the death of massive stars, while short
gamma-ray burst are most likely caused by the mergers of neutron stars.
The scientists discovered the chance that a long gamma-ray Burts
could trigger mass extinctions on earth was 50% in the past 500 million, 60% in
the past 1billion years, and more than 90% in the past 5billion years. For comparison
the solar system is about 4.6billion years old.
Short gamma-ray bursts happen about five times more often
than long ones. However, since these short bursts are weaker, the researchers found
that they had negligible life-threatening effects on earth.
These findings suggest that a nearby gamma-ray burst may
have caused one of the five greatest mass extinction on earth, such as
Ordovician extinction that occurred 440 million years ago. The Ordovician extinction
was the earliest of the so called big five extinction events, and was thought to
be the second largest.
The researchers also explored the danger gamma-ray burst
could pose to the universe as a whole. They suggest that because gamma-ray
bursts, life as it is known on earth may safely develop in only 10% of
galaxies. They also suggest that such life could only developed in the past
5biullion years. Before then, galaxies were smaller in size, and gamma-ray
bursts were therefore always close enough to cause mass extinction to any
potentially life-harboring planet.
This may be an explanation to the Fermin paradox or the Big
Silence, said Tsvi Piran, a physicist at the Hebrew University in Jerusalem. Why
we haven’t encountered any civilizations so far.
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