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2008-11-20 08:11:54
By Will Dunham Will Dunham Wed Nov 19, 4:25 pm ET
WASHINGTON (Reuters) A balloon-borne instrument soaring high over Antarctica
has found evidence of a possible large clump of mysterious so-called dark
matter relatively close to our solar system, scientists said on Wednesday.
It detected an unexpected amount of very high energy cosmic ray electrons
coming from an unknown source within about 3,000 light years of the solar
system. A light-year is 6 trillion miles (10 trillion km), the distance light
travels in a year.
One explanation is that the electrons may have been spawned as dark matter
particles collided with one another, triggering their mutual annihilation,
according to Louisiana State University physics professor John Wefel.
Scientists think perhaps 25 percent of the universe is made up of dark matter,
which responds to gravity the same way as does regular matter such as stars and
planets and the like.
While the stuff is thought to be strewn throughout the cosmos, it is invisible
and poorly understood. Scientists have struggled to find any solid evidence of
dark matter, and the new study could represent a major step forward in that
effort.
"This would be the first indirect detection of the annihilation signature of
predicted dark matter particles," Wefel, who helped lead the research published
in the journal Nature, said in a telephone interview.
Scientists think regular matter amounts to about 5 percent of the universe's
mass. The remaining roughly 70 percent is thought to be composed of dark
energy, a mysterious presence that may be making the universe expand at an
accelerated pace.
The scientists said it is possible that the electrons detected in this research
may have been produced by a more conventional source -- perhaps a spinning
neutron star called a pulsar that emits a beam of radiation, a medium-sized
black hole or the remnants of a bygone supernova.
Scientists think dark matter is distributed somewhat uniformly throughout the
universe, with clumps forming around concentrations of regular matter -- for
example, galaxies -- due to their gravitational pull.
The scientists think the electrons detected by the instrument may come from one
of these clumps located relatively close in astronomical terms to our solar
system.
"If our data is to be explained this way, there's got to be some sort of a
clump," Wefel said.
The electrons detected by the instrument seem to match theories about what
would be produced when dark matter particles collide and destroy each other.
"The annihilation of these exotic particles with each other would produce
normal particles such as electrons, positrons, protons and antiprotons that can
be observed by scientists," Eun-Suk Seo of the University of Maryland said in a
statement.
The NASA-funded instrument was carried to an altitude of about 24 miles above
Antarctica using a helium-filled balloon as big as the interior of a large
sports stadium.
The research was part of the Advanced Thin Ionization Calorimeter, or ATIC,
collaboration involving scientists in the United States, Germany, Russia and
China.