- 4476 - DARK MATTER - experiments to learn what is it? Dark matter makes up over 80% of all matter in the universe, but 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 according to laws of gravity
--------------- 4476
- DARK MATTER
- experiments to learn what is
it?
-
- ( See
Review 4475 about the discovery of light
photons)
-
- We have not discovered what dark matter is
yet? Dark matter is the mysterious
stuff that fills the universe but no one has ever seen. We only see its effects of gravity.
-
- If dark matter is 'invisible,' how do we
know it exists? 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.
-
- Visible matter, also called “baryonic
matter”, consists of baryons which is 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.
-
- Most scientists think that dark matter is
composed of 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.
-
- 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 Earth and all 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. The smaller “neutral axion” and the
“uncharged photinos,” both theoretical particles, are also potential
placeholders for dark matter.
-
- 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).
-
- 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.
-
- 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, more correctly, 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.
-
- 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 General
Relativity 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).
-
- 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.
-
- 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).
-
- Since we don't know what dark matter is for
every possible candidate for the dark matter there is a different strategy to
search for it. People build giant detectors deep underground (to get away from
all the other particles streaming through the environment around us) and look
for signals of the dark matter hitting their detector after passing through the
Earth overhead.
-
- How do we know dark matter 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- A vast majority of the astronomical
community today accepts that dark matter exists. 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.
-
- 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
cosmos 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.)
-
- But where does dark matter come from? The
obvious answer is that we don't know. But there are a few theories. 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.
-
- Stellar remnants such as white dwarfs and
neutron stars are also thought to contain high amounts of dark matter, and so
are the so-called 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.
-
- 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. But at this moment, we still
need more data to make sure it is from dark matter and not from some strange
astrophysics sources. 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. A new phase in the
race to detect dark matter with ultra-low background massive detectors on Earth
has just begun with XENON1T.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
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 result by itself isn't enough to be considered a smoking
gun for dark matter. Additional data from other observing projects or
direct-detection experiments would be required to validate the interpretation.
-
- 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
-
-
May 20, 2024 DARK
MATTER - experiments to learn what is it? 4472
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