- 3798 - DARK MATTER - mysteries yet to be discovered? Despite recent advances in astrophysics and astronomy, scientists still don't understand exactly how galaxies can exist. The most common explanation for this is a so-far undiscovered form of matter: “dark matter”.
--------------------- 3798 - DARK MATTER - mysteries yet to be discovered?
- Astronomers
understand how stars form, burn, and die, and they are improving their
understanding of how planets assemble themselves into planetary systems like
our own, but they don’t understand how galaxies can exist.
-
- Galaxies are
collections of stars held together by gravity. Like our solar system, they
rotate, with stars marching in paths, orbiting the galactic center. At any
fixed distance from the center of the galaxy, stars moving faster require
stronger gravity to hold them in that orbit.
-
- When astronomers
measure the orbital speed of stars in galaxies at a range of distances from the
center, they find that the stars are moving lunt of hydrogen gas in galaxies
and, while there’s a lot of it out there, there’s not enough to explain the
galaxy rotation mystery.
-
- Other explanations
that have been proposed include things like burned out stars, black holes, and
other objects that are known to exist within galaxies but don’t emit
light. However, astronomers searched for
such objects (called MACHOs, short for MAssive Compact Halo Objects) in the
1990s and, again, while they found examples of MACHOs, there weren’t enough to
explain the motion of stars in galaxies.
-
- Perhaps dark matter
exists as a kind of a “gas,” or as never-before-seen particles. These particles
are generically called “WIMPs,” short for “Weakly Interacting Massive
Particles.” WIMPs, if they exist, are basically stable subatomic particles,
with a mass somewhere in the range of the mass of a proton up to 10,000
protons, or even more.
-
- Like all dark
matter particle candidates, WIMPs interact gravitationally, but that “W” in the
name means that they also interact via the weak nuclear force. The weak nuclear
force is involved in some forms of radioactivity. much stronger than gravity,
but unlike gravity’s infinite range, the weak nuclear force only acts over tiny
distances, distances much smaller than a proton.
- If WIMPs exist,
they pervade galaxies, including our Milky Way, and even our own solar system.
Depending on the mass of the WIMPs, astronomers estimate that if you make a
fist, one dark matter particle could be found inside it.
-
- Scientists have
been looking for direct evidence for the existence of WIMPs for many decades.
They do this in several ways. For example, some WIMP theories suggest that
WIMPs can be made in particle accelerators, like the Large Hadron Collider in
Europe. Particle physicists look at their data, hoping to see the signature of
WIMP production. No evidence has been observed so far.
-
- Another way in
which researchers look for WIMPs is directly observing dark matter particles
that pass through the solar system. Scientists build very large detectors and
cool them to very cold temperatures so the atoms of the detectors are moving
slowly. They then put these detectors a
half-mile or more underground to shield them from radiation from space. Then
they wait, hoping that a dark matter particle will interact in their detector,
disturbing one of the nearly stationary atoms.
-
- But despite decades
of efforts, no WIMPs have been observed. Predictions in the 1980s suggested
researchers could expect to detect WIMPs at a particular rate. When no WIMPs
were detected, researchers built a series of detectors with much greater
sensitivity, all of which failed to find WIMPs.
-
- Current detectors
are 100 million times more sensitive than the ones of the 1980s, and no definitive
observation of WIMPs have occurred, including a very recent measurement by the
LZ experiment, which employs 10 tons of xenon to achieve unparalleled
sensitivity to WIMPs.
-
- After decades of
failing to detect dark matter, the scientific community is reexamining the
situation. Astronomers are certain that
galaxies rotate faster than can be accounted for using the known laws of motion
and gravity and the observed amount of matter. The dark matter hypothesis is a
solution for a matter deficit, but perhaps it’s not the answer. Maybe the
actual explanation is that the laws of motion and gravity need to be
reexamined.
-
- The name for such
an approach is called MOND, short for “MOdifications of Newtonian Dynamics.”
The laws of motion worked out by Isaac Newton back in the 1600s work just fine.
But for very small forces and very small accelerations (like in the outskirts
of galaxies), these laws needed to be adjusted. After making those adjustments,
he could correctly predict the rotation of galaxies.
-
- In order to test
the MOND theory, researchers needed to compare its predictions in other
situations, such as applying it to the motion of large clusters of galaxies
held together by their mutual gravitational attraction. MOND theory struggles
to make a prediction of this motion that agrees with theory, and it also
disagrees with other observations.
-
- If dark matter
exists, it is five times more prevalent than ordinary atomic matter. If we need
to revisit our laws of motion and gravity, this will have significant
consequences for our modeling of the history of the universe.
-
- Scientists
identified a data signature for dark matter that can potentially be detected by
experiments. The effect they found is a daily "diurnal modulation" in
the scattering of particles. Dark
matter, a type of matter that is predicted to make up around 27 percent of the
known universe, has never been detected experimentally.
-
- Astronomers propose
a new type of effect that relates to the so-called “sub-GeV dark matter” which
is boosted by cosmic rays. Looking for this effect can potentially allow direct
detection of dark matter using nuclear recoil techniques.
-
- Astronomers are
looking for a prominent signature of accelerated dark matter particles that
come from the galaxy’s center, where dark matter and cosmic rays are at high
density. They found that these particles have a “diurnal modulation”, a
scattering pattern that is linked to the time of day.
-
- At periods when the
Galaxy Center faces the side of the planet that’s opposite the location of the
detector, the Earth shadows a large amount of these particles. At other times,
they come in as a signal with “higher recoil energy.”
-
- The conventional
diurnal effect is only for slow moving (nonrelativistic) Dark Matter particles
in our galaxy (so-called standard DM halo).
The effect is negligibly small either from direct experimental
constraints, or due to the detection threshold. For light DM particles the
DM-nucleus interaction is much less constrained, which leaves room for strong
diurnal modulation.
-
December 24, 2022 DARK MATTER - yet to be discovered? 3798
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