- 4579 - DARK MATTER MYSTERIES? - Perhaps our theories of gravity are simply incorrect, and that the fundamental force works differently on large scales than we expect. Often known as "modified Newtonian dynamics," or MOND models, these suggestions that there is no dark matter and the ultrafast speeds at which stars and galaxies are seen to rotate around one another is a consequence of gravity behaving in surprising ways.
------------------------------------------- 4579 -
DARK
MATTER MYSTERIES?
-
- In the 1930s, a Swiss astronomer named Fritz
Zwicky noticed that galaxies in a distant cluster were orbiting one another
much faster than they should have been given the amount of visible mass they
had. He proposed than an unseen substance, which he called “dark matter”, might
be tugging gravitationally on these galaxies.
-
- Since then, researchers have confirmed that
this mysterious material can be found throughout the universe, and that it is
six times more abundant than the normal matter that makes up ordinary things
like stars and people.
-
- Originally, some scientists conjectured
that the missing mass in the universe was made up of small faint stars and
black holes, though detailed observations have not turned up nearly enough such
objects to account for dark matter's influence.
-
- The current leading contender for dark
matter's mantle is a hypothetical particle called a “Weakly Interacting Massive
Particle”, or WIMP, which would behave sort of like a neutron except would be
between 10 and 100 times heavier than a proton.
-
- If dark matter is made from WIMPs, they
should be all around us, invisible and barely detectable. So why haven't we
found any yet? While they wouldn't interact with ordinary matter very much,
there is always some slight chance that a dark-matter particle could hit a
normal particle like a proton or electron as it travels through space.
-
- Researchers have built experiment after
experiment to study huge numbers of ordinary particles deep underground, where
they are shielded from interfering radiation that could mimic a
dark-matter-particle collision. The problem? After decades of searching, not
one of these detectors has made a credible discovery.
-
- Earlier this year, the Chinese PandaX
experiment reported the latest WIMP nondetection. It seems likely that
dark-matter particles are much smaller than WIMPs, or lack the properties that
would make them easy to detect.
-
- Ordinary matter is made up of everyday
particles like protons and electrons, as well as a whole zoo of more exotic
particles like neutrinos, muons and pions. So, some researchers have wondered
if dark matter, which makes up 85 percent of the matter in the universe, might
also be just as complicated.
-
- There is no good reason to assume that all
the dark matter in the universe is built out of one type of particle. Dark protons could combine with dark
electrons to form dark atoms, producing configurations as diverse and
interesting as those found in the visible world.
-
- Along with additional particles of dark
matter, there is the possibility that dark matter experiences forces analogous
to those felt by regular matter. Some researchers have searched for "dark
photons," which would be like the photons exchanged between normal
particles that give rise to the electromagnetic force, except they would be
felt only by dark matter particles.
-
- Physicists in Italy are gearing up to smash
a beam of electrons and their antiparticles, known as positrons, into a
diamond. If dark photons do exist, the
electron-positron pairs could annihilate and produce one of the strange
force-carrying particles, potentially opening a brand-new sector of the
universe.
-
- Other dark-matter particles are starting to
gain favor. One of the leading replacements is a hypothetical particle known as
an “axion”, which would be extremely light, perhaps as little as 10 raised to
the 31st power less massive than a proton.
-
- Axions are now being searched for in a few
experiments. Recent computer simulations have raised the possibility that these
axions could form star-like objects, which might produce detectable radiation
that would be quite similar to mysterious phenomena known as “fast-radio
bursts”.
-
- Astronomers discovered dark matter through
its gravitational interactions with ordinary matter, suggesting that this is
its main way of making its presence known in the universe. According to some
theories, dark-matter particles should be their own antiparticles, meaning that
two dark-matter particles would annihilate with one another when they meet.
-
- The “Alpha Magnetic Spectrometer” (AMS)
experiment on the International Space Station has been searching for the
telltale signs of this annihilation since 2011and has already detected hundreds
of thousands of events. Scientists still aren't sure if these are coming from
dark matter, and the signal has yet to help them pin down exactly what dark
matter is.
-
- Because it so massively outweighs ordinary
matter, dark matter is often said to be the controlling force that organizes
large structures such as galaxies and galactic clusters. So, it was strange
when, earlier this year, 2024, astronomers announced that they had found a
galaxy named “NGC 1052-DF2” that seemed to contain hardly any dark matter at
all. "Dark matter is apparently not a requirement for forming a galaxy”.
-
- A long-standing mystery in particle
physics are the puzzling results of a European experiment known as DAMA/LIBRA.
This detector located in an underground mine below the Gran Sasso mountain in
Italy has been searching for a periodic oscillation in dark matter particles.
This oscillation should arise as the Earth moves in its orbit around the sun
while flying through the galactic stream of dark matter surrounding our solar
system, sometimes called the “dark matter wind”. Since 1997, DAMA/LIBRA has
claimed to see exactly this signal, though no other experiment has seen
anything like this.
-
- A signal from the beginning of time has led
some physicists to suggest that dark matter might have an electrical charge.
Radiation with a wavelength of 21 centimeters was emitted by stars in the
universe's infancy, just 180 million years after the Big Bang. It was then
absorbed by cold hydrogen that was around at the same time.
-
- Neutrons are regular matter particles with
a limited lifetime. After around 14.5 minutes, a lone neutron unmoored from an
atom will decay into a proton, an electron and a neutrino. But two different
experimental setups give slightly different lifetimes for this decay, with the
discrepancy between them about 9 seconds.
-
- Physicists suggested that if 1 percent of
the time, some neutrons were decaying into dark-matter particles, it could
account for this anomaly. Astronomers
monitored neutrons for a signal that could be dark matter but were unable to
detect anything. They suggested that other decay scenarios might still be
possible. Given the difficulties that
scientists have faced trying to detect and explain dark matter, a reasonable
questioner might wonder if they're going about it all wrong.
-
- Dark matter is still an “unconfirmed
model". However, the latest
evidence also suggests that dark matter is “real”.
-
-
October 18, 2024 DARK
MA?TTER MYSTERIES? 4579
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--------------------- --- Sunday, October 20,
2024
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