Scientists
trying to create a detailed inventory of all the matter and energy in the
cosmos run into a curious problem--the vast majority of it is missing.
"I
call it the dark side of the universe," said Michael Turner, a cosmologist
at the University of Chicago, referring to the great mysteries of dark matter
and dark energy.
In fact,
only 4 percent of the matter and energy in the universe has been found. The
other 96 percent remains elusive, but scientists are looking in the farthest
reaches of space and deepest depths of Earth to solve the two dark riddles.
Missing
matter
Einstein's
famous equation "E=mc^2" describes energy and matter (or mass) as one
and the same--maps of the cosmos refer to the energy-matter combination as
energy density, for short. The problem with detecting dark matter, thought to
make up 22 percent of the universe's mass/energy pie, is that light doesn't
interact with it.
But it does
exhibit the tug of gravity.
Initial
evidence for the mysterious matter was discovered 75 years ago when
astrophysicists noticed an anomaly in a jumble of galaxies: The galactic
cluster had hundreds of times more gravitational pull than it should have, far
outweighing its visible mass of stars.
"We
can predict the motions of the sun and planets very accurately, but when we
measure distant things we see anomalies," said Scott Dodelson, an
astrophysicist at Fermi National Accelerator Laboratory in Illinois. "Dark
matter is currently the best possible solution, even though we've never seen
any of it."
Another
hallmark of dark matter is gravitational lensing, similar to the effect of
light passing through a piece of polished glass. Massive objects like the sun
can bend light, but colossal clouds of dark matter create
"bubbles" in the cosmos that magnify, distort and duplicate the
light of galaxies or stars behind them.
Gravitational
lensing recently exposed evidence of the unseen mass in the Bullet cluster as
well as in
a ring around a cluster of colliding galaxies called ZwCl0024+1652.
Particle
hunt
In spite of
the ghostly evidence, pieces of dark matter have yet to be pinned down by
researchers. "Until we actually discover particles, we're not home
yet," Dodelson said.
Particle
physicists have detected neutrinos, which are extremely lightweight particles
that pour out of the sun and hardly interact into ordinary matter, but Turner
said they make up an extremely small fraction of dark matter in the universe.
"We
arrested one of the members of the gang, but not the leader of the gang,"
Turner said of neutrinos. He thinks the leader is actually a WIMP: a weakly
interactive, massive particle. Unfortunately, WIMPS are just a theory so
far.
The
thinking goes that WIMPs are very heavy, yet like neutrinos they rarely bump
into matter to produce a detectable signal. But the idea that WIMPS--such as
theoretical axion or neutralino particles--can bump into visible matter at all
gives scientists hope.
"This
is a story that may soon be at its end," Turner said, noting that the
Cryogenic Dark Matter Search in the Soudan mine of Minnesota and other
experiments below the ground should be sensitive enough to detect a WIMP.
The
anti-gravity
Perhaps the
biggest mystery of all is dark matter's big cousin, dark energy.
The
invisible force is thought to be a large-scale "anti-gravity,"
pushing apart galactic clusters and causing the unexplainable, accelerating
expansion of the universe. Turner thinks dark energy is the biggest mystery of
them all--and quite literally, since physicists predict that it makes up 74
percent of energy density in the universe.
"So
far, the greatest achievement with dark energy is giving it a name,"
Turner said of the elusive force. "We are really at the very beginning of
this puzzle."
Turner
described dark energy as "really weird stuff," best thought of as an
elastic, repulsive gravity that can't be broken down into particles. "We
know what it does, but we don't know what it is," Turner said.
While
astrophysicists look deep into space to gather more details about dark
energy's effects, Turner noted that theoretical physicists are focusing on
explaining how the force actually works. And at this point, he joked, any
physicist's explanation for dark energy is probably good enough to consider.
"We're
at this very early stage, at the crime scene of dark energy's existence, if you
will," Turner said. "It's a highly creative period, and now is the
time for ideas."
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