- 4498 - DARK MATTER - what is it? - Dark matter is the mysterious stuff that fills the universe but no one has ever seen. Roughly 80% of the mass of the universe is made up of dark matter but what is it? Scientists have never seen it. We only assume it exists because, without it, the behavior of stars, planets and galaxies simply wouldn't make sense.
---------------------------------------- 4498 - DARK MATTER - what is it?
- Dark matter is completely invisible. It
emits no light or energy and thus cannot be detected by conventional sensors
and detectors. The key to its elusive nature must lie in its composition.
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- Visible matter, also called “baryonic
matter”, consists of baryons which are an overarching name for subatomic
particles such as protons, neutrons and electrons. Scientists only speculate
what dark matter is made of. It could be composed of baryons but it could also
be non-baryonic, that means consisting of different types of particles.
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- Most scientists think that dark matter is
composed of this non-baryonic matter. The lead candidate, “WIMPS” (weakly
interacting massive particles), are believed to have ten to a hundred times the
mass of a proton, but their weak interactions with "normal" matter
make them difficult to detect.
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- “Neutralinos”, massive hypothetical
particles heavier and slower than neutrinos, are the foremost candidate, though
they have yet to be spotted.
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- “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 Earth and its inhabitants.
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- 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.
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- One of the outstanding questions is whether
there is a pattern to the fractions that go into each neutrino species. The smaller “neutral axion” and the
“uncharged photinos”, both theoretical
particles, are also potential placeholders for dark matter.
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- There is also such a thing as “antimatter”,
which is not the same as dark matter. Antimatter consists of particles that are
essentially the same as visible matter particles but with opposite electrical
charges. These particles are called antiprotons and positrons (or
antielectrons).
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- When antiparticles meet particles, an
explosion ensues that leads to the two types of matter canceling each other
out. Because we live in a universe made of matter, it is obvious that there is
not that much antimatter around, otherwise, there would be nothing left. Unlike
dark matter, physicists can actually manufacture anti-matter in their
laboratories.
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- Does dark matter exist? We don't know! What we do know is that if we
look at a typical galaxy, take account of all the matter that we see (stars,
gas, dust) and use Newton's Laws of Gravity and motion (or, Einstein's General
Relativity), to try to describe the motions of that material, then we get the
wrong answer. The objects in galaxies (nearly all of them) are moving too fast.
There should not be enough gravity to keep them from flying out of the galaxy
that their in. The same thing is true about galaxies moving around in clusters.
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- There are two possible explanations:
-
------ 1. There is more stuff (matter) that we
don't see with our telescopes. We call this “dark matter”.
-
------- 2. Newton's laws and even GR are wrong on the
scale of galaxies and everything bigger. This idea is usually called “modified
gravity” (because we need to modify GR) or Modified Newtonian Dynamics (MOND).
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- Mostly, cosmologists believe that the
answer is that the behavior of galaxies is explained by dark matter. Why?
Partly. because it has been very hard to write down a successful theory of MOND
or modified gravity. And partly because it turned out that when we turned our
microwave telescopes to look at cosmic background radiation (CMB), the light
from the early universe, it turned out that, according to GR, the same amount
and type of dark matter was also required to explain the behavior of the sound
waves that traveled in the universe when it was less than 500,000 years old,
and whose imprints we are able to see. Modified gravity struggles to provide a
unified explanation across all these systems, galaxies, clusters of galaxies,
the universe.
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- If dark matter exists, it must have mass.
Massless dark matter would not behave in ways that solve the problems that dark
matter addresses. The two things we
know for sure about dark matter (assuming it exists), is that it exerts gravity
(has mass) and that it moves slowly (compared to the speed of light).
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- If we cannot see dark matter, how do we
know it exists? The answer is gravity, the force exerted by objects made of
matter that is proportional to their mass. Since the 1920s, astronomers have
hypothesized that the universe must contain more matter than we can see because
the gravitational forces that seem to be at play in the universe simply appear
stronger than the visible matter alone would account for.
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- Motions of the stars tell you how much
matter there is. Astronomers examining
spiral galaxies in the 1970s expected to see material in the center moving
faster than at 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.
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- Studies of 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 was the mass visible
to conventional astronomical measurements.
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- Different galaxies seem to contain different
amounts of dark matter. In 2016, a team found a galaxy called “Dragonfly 44”,
which seems to be composed almost entirely of dark matter. On the other hand,
since 2018 astronomers have found several galaxies that seem to lack dark
matter altogether.
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- The force of gravity doesn't only affect the
orbits of stars in galaxies but also the trajectory of light. Albert Einstein showed in the early 20th
century that massive objects in the universe bend and distort light due to the
force of their gravity. The phenomenon is called “gravitational lensing”. By
studying how light is distorted by galaxy clusters, astronomers have been able
to create a map of dark matter in the universe.
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- 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.
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- Despite all the evidence pointing towards
the existence of dark matter, there is also the possibility that no such thing
exists after all and that the laws of gravity describing the motion of objects
within the solar system require revision.
-
- Dark matter appears to be spread across the
universe in a net-like pattern, with galaxy clusters forming at the nodes where
fibers intersect. By verifying that gravity acts the same both inside and
outside our solar system, researchers provide additional evidence for the
existence of dark matter.
-
- (Things are even more complicated as in
addition to dark matter there also appears to be “dark energy”, an invisible
force responsible for the expansion of the universe that acts against gravity.)
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- Dark matter might be concentrated in black
holes, the powerful gates to nothing that due to the extreme force of their
gravity devour everything in their vicinity. As such, dark matter would have
been created in the Big Bang together with all other constituting elements of
the universe as we see it today.
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- Stellar remnants such as white dwarfs and
neutron stars are also thought to contain high amounts of dark matter, and so
are the “brown dwarfs”, failed stars that didn't accumulate enough material to
kick-start nuclear fusion in their cores.
-
- Since we can't see dark matter, can we
actually study it? There are two approaches to learning more about this
mysterious stuff. Astronomers study the distribution of dark matter in the
universe by looking at the clustering of material and the motion of objects in
the universe. Particle physicists, on the other hand, are on a quest to detect
the fundamental particles making up dark matter.
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- An experiment mounted on the International
Space Station called the “Alpha Magnetic Spectrometer” (AMS) detects antimatter
in cosmic rays. Since 2011, it has been hit by more than 100 billion cosmic
rays, providing fascinating insights into the composition of particles
traversing the universe.
-
- We have measured an excess of “positrons”
[the antimatter counterpart to an electron], and this excess can come from dark
matter.
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- 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 “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. But so
far, the instrument hasn't revealed the mysterious matter.
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- The IceCube Neutrino Observatory, an
experiment buried under the frozen surface of Antarctica, is hunting for the
hypothetical sterile neutrinos. Sterile neutrinos only interact with regular
matter through gravity, making it a strong candidate for dark matter.
-
- Experiments aiming to detect elusive dark
matter particles are also conducted in the powerful particle colliders at the
European Organization for Nuclear Research (CERN) in Switzerland.
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- Several telescopes orbiting Earth are
hunting for the effects of dark matter. The European Space Agency's Planck
spacecraft, retired in 2013, spent four years in the Lagrangian Point 2 (a
point in the orbit around the sun, where a spacecraft maintains a stable
position with respect to Earth), mapping the distribution of the cosmic
microwave background, a relic from the Big Bang, in the universe.
Irregularities in the distribution of this microwave background revealed
“clues” about the distribution of dark matter.
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- In 2014, NASA's Fermi Gamma-ray Space
Telescope made maps of the heart of our galaxy, the Milky Way, in gamma-ray
light, revealing an excess of gamma-ray emissions extending from its core.
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- The signal found cannot be explained by
currently proposed alternatives and is in close agreement with the predictions
of very simple dark matter models. The
excess can be explained by annihilations of dark matter particles with a mass
between 31 and 40 billion electron volts.
-
- The James Webb Space Telescope, launched
after 30 years of development on December 25, 2021, is also expected to
contribute to the hunt for the elusive substance. With its infrared eyes able
to see to the beginning of time, the telescope of the century won't be able to
see dark matter directly, but through observing the evolution of galaxies since
the earliest stages of the universe, it is expected to provide insights that
have not been possible before.
-
- ESA's Euclid mission launched on July 1,
2023, and is currently on the hunt for dark matter and dark energy. The mission
aims to map the geometry of matter in the universe, specifically the
distribution of galaxies to learn more about the elusive dark matter
-
- The “DAMA/LIBRA” experiment at the Gran
Sasso National Laboratory (LNGS) near L’Aquila, Italy, has been recording an
annual fluctuation of light flashes in its detector that appears to be a sign
of dark matter. But no one has been able to definitively replicate the
findings.
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- But beneath a mountain in Jeongseon, South
Korea, researchers are scaling up an experiment that could finally lay the
controversial dark-matter claim to rest. In June, researchers will finish
installing a revamped detector in a brand-new facility called “Yemilab”. If all
goes to plan, the upgraded “COSINE-100” experiment will be running by August,
2024.
-
- DAMA/LIBRA’s observations of the distinct
annual pattern is consistent with what physicists would expect with Earth’s
relative position in the galaxy throughout the year. As the Earth orbits the
Sun, the Sun orbits the black hole at the center of the Milky Way.
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- In June, the Earth hurtles through the
Milky Way in the same direction as the Sun, increasing its relative speed
through the haze of dark matter. But in December, the Earth travels with the
flow of dark matter as it moves in the opposite direction to the Sun. As
expected, the number of signals recorded by DAMA/LIBRA’s detector are highest
in June and lowest in December.
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- Yemilab also offers a better-shielded
environment for detecting elusive particles besides dark matter. The facility
will also hunt for “neutrinos”,
chargeless particles that barely have mass. The second phase of an
experiment called “AMoRE” will search for signs of two neutrons decaying into
protons and electrons without emitting a neutrino.
-
- This hypothesized process is called
“neutrinoless double β decay” and if observed, it will demonstrate that
neutrinos are their own antiparticle. This could offer clues about their mass
and explain why there is more matter than antimatter in the Universe.
-
- The upgraded neutrino detector will use
around 160 kilograms of crystals embedded with molybdenum-100, a naturally
occurring radioisotope. When “AMoRE-II” starts running at the end of this
year,2024, it will be 100 times more sensitive than the previous version of the
experiment.
-
- Whether the two experiments succeed or fail
at detecting the rare events they are looking for, they are nevertheless set to
raise more questions. If both will
deliver only null results, we should seriously start rethinking the Universe.
-
-
June 11, 2024 DARK
MATTER - what is it? 4498
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