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Saturday, December 9, 2017
What is Dark Matter? 10 dece. 2017.
What is Dark Matter?
By Nola Taylor Redd, Space.com Contributor |
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Roughly 80 percent of the mass of the universe is made up of material
that scientists cannot directly observe. Known as dark matter, this
bizarre ingredient does not emit light or energy. So why do scientists
think it dominates?
Studies of other galaxies in the 1950s first indicated that the
universe contained more matter than seen by the naked eye. Support for
dark matter has grown, and although no solid direct evidence of dark
matter has been detected, there have been strong possibilities in recent years.
"Motions of the stars tell you how much matter there is," Pieter van Dokkum, a researcher at Yale University, said in a statement.
"They don't care what form the matter is, they just tell you that it's
there." Van Dokkum led a team that identified the galaxy Dragonfly 44, which is composed almost entirely of dark matter.
The familiar material of the universe, known as baryonic matter, is
composed of protons, neutrons and electrons. Dark matter may be made of
baryonic or non-baryonic matter. To hold the elements of the universe
together, dark matter must make up approximately 80 percent of its
matter. [Image Gallery: Dark Matter Across the Universe]
The missing matter could simply be more challenging to detect, made up
of regular, baryonic matter. Potential candidates include dim brown
dwarfs, white dwarfs and neutrino stars. Supermassive black holes
could also be part of the difference. But these hard-to-spot objects
would have to play a more dominant role than scientists have observed to
make up the missing mass, while other elements suggest that dark matter
is more exotic.
Most scientists think that dark matter is composed of non-baryonic matter. The lead candidate, WIMPS
(weakly interacting massive particles), have ten to a hundred times the
mass of a proton, but their weak interactions with "normal" matter make
them difficult to detect. Neutralinos, massive hypothetical particles
heavier and slower than neutrinos, are the foremost candidate, though
they have yet to be spotted.
Sterile neutrinos are another candidate. Neutrinos are particles that
don't make up regular matter. A river of neutrinos streams from the sun,
but because they rarely interact with normal matter, they pass through
the Earth and its inhabitants. There are three known types of neutrinos;
a fourth, the sterile neutrino, is proposed as a dark matter candidate.
The sterile neutrino would only interact with regular matter through
gravity.
"One of the outstanding questions is whether there is a pattern to the
fractions that go into each neutrino species," Tyce DeYoung, an
associate professor of physics and astronomy at Michigan State
University and a collaborator on the IceCube experiment, told Space.com.
The smaller neutral axion and the uncharched photinos are also potential placeholders for dark matter.
According to a statement
by the Gran Sasso National Laboratory in Itally (LNGS), "Several
astronomical measurements have corroborated the existence of dark
matter, leading to a world-wide effort to observe directly dark matter
particle interactions with ordinary matter in extremely sensitive
detectors, which would confirm its existence and shed light on its
properties. However, these interactions are so feeble that they have
escaped direct detection up to this point, forcing scientists to build
detectors that are more and more sensitive."
A third possibility exists — that the laws of gravity that have thus
far successfully described the motion of objects within the solar system
require revision.
Proving the unseen
If scientists can't see dark matter, how do they know it exists?
Scientists calculate the mass of large objects in space by studying
their motion. Astronomers examining spiral galaxies in the 1950s
expected to see material in the center moving faster than on the outer
edges. Instead, they found the stars in both locations traveled at the
same velocity, indicating the galaxies contained more mass than could be
seen. Studies of the gas within elliptical galaxies also indicated a
need for more mass than found in visible objects. Clusters of galaxies
would fly apart if the only mass they contained were visible to
conventional astronomical measurements.
These illustrations, taken from
computer simulations, show a swarm of dark matter clumps around our
Milky Way galaxy. Image released July 10, 2012.
Credit: J. Tumlinson (STScI)
Albert Einstein
showed that massive objects in the universe bend and distort light,
allowing them to be used as lenses. By studying how light is distorted
by galaxy clusters, astronomers have been able to create a map of dark matter in the universe.
All of these methods provide a strong indication that most of the matter in the universe is something yet unseen.
Experiments
Although dark matter is different from ordinary matter, there are a
number of experiments working to detect the unusual material.
The Alpha Magnetic Spectrometer (AMS), a sensitive particle detector on the International Space Station, has been operating since its instillation in 2011.
So far, AMS has tracked more than 100 billion cosmic ray hits in its
detectors, AMS lead scientist Samuel Ting, a Nobel laureate with the
Massachusetts Institute of Technology, told Space.com.
"We have measured an excess of positrons [the antimatter counterpart to
an electron], and this excess can come from dark matter. But at this
moment, we still need more data to make sure it is from dark matter and
not from some strange astrophysics sources," Ting said. "That will
require us to run a few more years."
Back on Earth, beneath a mountain in Italy, the LNGS's XENON1T is
hunting for signs of interactions after WIMPs collide with xenon atoms.
The lab recently released the first results of the experiment.
"A new phase in the race to detect dark matter with ultra-low
background massive detectors on Earth has just began with XENON1T,"
project spokesperson Elena Aprile, a professor at Columbia University,
said in a statement. "We are proud to be at the forefront of the race with this amazing detector, e first of its kind."
The Large Underground Xenon dark-matter experiment (LUX),
seated in a gold mine in South Dakota, has also been hunting for signs
of WIMP and xenon interactions. To date, the instrument has not revealed
the mysterious matter.
"Though a positive signal would have been welcome, nature was not so
kind!" Cham Ghag, a physicist at University College London and
collaborator on LUX, said in a statement. "Nonetheless, a null result is
significant as it changes the landscape of the field by constraining
models for what dark matter could be beyond anything that existed
previously." IceCube Neutrino Observatory,
an experiment buried under Antarctica's ice, is hunting for sterile
neutrinos. Sterile neutrinos only interact with regular matter through
gravity, making it a strong candidate for dark matter.
Other instruments hunt not for dark matter but for its effects. The European Space Agency's Planck spacecraft
has been building a map of the universe since it was launched in 2009.
By observing how the mass of the universe interacts, the spacecraft can
investigate both dark matter and its co-partner, dark energy.
In 2014, NASA's Fermi Gamma-ray Space Telescope made maps of the heart of the Milky Way in gamma-ray light, revealing an excess of gamma-ray emissions extending from its core.
"The signal we find cannot be explained by currently proposed
alternatives and is in close agreement with the predictions of very
simple dark matter models," lead author Dan Hooper, an astrophysicist at
Fermilab in Illinois, told Space.com.
The excess can be explained by annihilations of dark matter particles
with a mass between 31 and 40 billion electron volts, researchers said.
The result is not considered a smoking gun for dark matter, as
additional data from other observing projects and/or direct-detection
experiments would be required to validate the interpretation.
Dark matter versus dark energy
Although dark matter makes up most of the matter of the universe, it
only makes up about a quarter of the composition. The universe is
dominated by dark energy.
After the Big Bang, the universe began expanding outward. Scientists
once thought that it would eventually run out of the energy, slowing
down as gravity pulled the objects inside it together. But studies of
distant supernovae revealed that the universe today is expanding faster
than it was in the past, not slower, indicating that the expansion is
accelerating. This would only be possible if the universe contained
enough energy to overcome gravity — dark energy.
Nola Taylor Redd is a contributing writer for Space.com. She loves all
things space and astronomy-related, and enjoys the opportunity to learn
more. She has a Bachelor’s degree in English and Astrophysics from Agnes
Scott college and served as an intern at Sky & Telescope magazine.
In her free time, she homeschools her four children. Follow her on
Twitter at @NolaTRedd
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