- 4310 - DARK MATTER and ANTIMATTER? - Astronomers can observe blobs of Dark Matter down to a scale of 30,000 light-years across. But, “dark matter” remains mysterious. While we don’t yet have a definite idea of what this cosmic “stuff” is made of, astronomers are learning more about its distribution throughout the Universe.
--------------- 4310 - DARK MATTER and ANTIMATTER?
- Since we can’t see
dark matter directly, observers need to use indirect methods to detect it. One
way is through “gravitational lensing”. Another is by looking for emissions
from hydrogen gas associated with small-scale dark matter structures in the
Universe.
-
- A massive
foreground galaxy is bending and distorting the light from a distant quasar
that lies some 11 billion light-years away. The result is four images of the
“quasar”.
-
- There are actually
variations in the distribution of dark matter along this line of sight between
us and the quasar. The gravitational lens magnified the fluctuations and
analysis of the data allowed astronomers to map the fluctuations down to a
scale of 30,000 light-years.
-
- Throughout the
universe, dark matter is associated with massive galaxies and galaxy clusters.
However, small-scale clumps and distributions aren’t as well understood. So,
astronomers want to find ways to map the smaller concentrations of it.
Gravitational lensing provides one way to do that.
-
- In the case of “MG
J0414+0534”, the positions and shapes of the lensed quasar images look strange.
They don’t fit the model of gravitational lensing predicted when you plug in
the numbers for the galaxy and its associated dark matter component.
-
- The fluctuations
indicate that there’s a gravitational lensing effect from the smaller
concentrations, in addition to that of the galaxy and its dark matter shell. In
this case, there are spatial fluctuations in the density of dark matter down to
a size of about 30,000 light-years.
-
- Such smaller
concentrations work with predictions made about cold dark matter (CDM).
Essentially, it says that dark matter clumps exist within galaxies, but also
can populate intergalactic space. The gravitational lensing effects due to the
clumps of dark matter found in this study are so small that it is extremely
difficult to detect them alone.
-
- Astronomers used a
giant radio telescope, the “Five-Hundred-Aperture Spherical Radio Telescope”
(FAST) in China to look at another interesting dark matter-related object that
lies near the galaxy M94. The system, which they call “Cloud-9, is a source of
21-centimeter radio emissions from cold neutral interstellar hydrogen.
-
- Cloud-9 appears to
be relatively starless. Astronomers
using FAST wondered if this 21-cm emission from the cloud could function as a
tracer of dark matter. Astronomers
describe Cloud-9 as being very similar to something called a
“REionization-limited HI Cloud” (RELHIC).
-
- RELHIC is a
starless dark matter halo. It’s filled with gas in equilibrium with the cosmic
ultraviolet background. That’s the “wash” of UV radiation produced by stars and
galaxies.
-
- Many low-mass
versions of dark matter halos remain dark or starless even after billions of
years of cosmic evolution. Not every
such halo contains a galaxy. The RELHICs are halos that have no stars, or, at
least none that have been detected so far.
-
- Hydrogen and
helium, the building blocks of stars, were attracted by the clumps of dark
matter and began to form stars. However, that applies to the distribution of
dark matter at the galaxy and larger scales. Accumulations of dark matter on
smaller scales, such as with Cloud-9, really aren’t well understood.
-
- Further
observations of Cloud-9 will definitely help determine the full extent of its
dark matter content. They could also shed some light on how galaxies form at
the small scale of smaller dark matter accumulations.
-
- Physicists have
shown that, like everything else experiencing gravity, antimatter falls
downwards when dropped. This outcome is
not surprising. A difference in the
gravitational behavior of matter and antimatter would have huge implications
for physics, but observing it directly had been a dream for decades.
-
- Because gravity is
much weaker than other forces such as electrostatic attraction or magnetism,
separating it from other effects in the laboratory is a delicate affair. Experiments will aim to test whether gravity
acts with the same strength on antimatter as it does on matter. Any tiny
discrepancies could help to solve one of the biggest problems in physics: “how the Universe came to be made almost
exclusively of matter, even though equal amounts of matter and antimatter
should have arisen from the Big Bang”.
-
- In the world of
antimatter, atomic nuclei are made of negatively charged antiprotons, orbited
by positively charged antielectrons, or positrons. According to the standard
model of particle physics the opposite charges should be pretty much the only
difference: particles and antiparticles should have nearly all the same
properties. Experiments have confirmed that positrons and antiprotons have the
same masses as their matter counterparts.
-
- According to
Einstein’s general theory of relativity, all objects of the same mass should
weigh the same. In other words, they
should experience exactly the same gravitational acceleration.
-
- What happens when
the neutral atom antihydrogen is dropped?
It’s almost impossible to do an experiment with a charged particle, so
antihydrogen is the perfect candidate.
-
- Antimatter
particles are routinely created in laboratories. Most particles produced by high-energy
particle collisions are made in pairs, one particle of matter and its antiparticle.
But it is hard to get antiparticles to combine into antiatoms because
antimatter particles are typically very short-lived. When an antiparticle meets
a particle, they both cease to exist and turn back into energy, in a process
called “annihilation”. In a world made primarily of matter, this makes it hard
for antimatter particles to find each other.
-
- CERN is currently
the only place in the world where antihydrogen can be made. It has an
accelerator that makes antiprotons from high-speed proton collisions, and a
‘decelerator’ called “ELENA” that slows them down enough to be held for further
manipulation.
-
- Several different
experiments feed off ELENA. “ALPHA-g” is one of them, and it combines
antiprotons with positrons it collects from a radioactive source. After making a thin gas of thousands of
antihydrogen atoms, researchers pushed it up a 3-meter-tall vertical shaft
surrounded by superconducting electromagnetic coils.
-
- These can create
a kind of magnetic ‘tin can’ to keep the antimatter from coming into contact
with matter and annihilating. Next, the researchers let some of the hotter
antiatoms escape, so that the gas in the can got colder, down to just 0.5 °C
above absolute zero and the remaining antiatoms were moving slowly.
-
- They then gradually
weakened the magnetic fields at the top and bottom of their trap and detected the antiatoms using two sensors
as they escaped and annihilated. When opening any gas container, the contents
tend to expand in all directions, but in this case the antiatoms’ low
velocities meant that gravity had an observable effect: most of them came out
of the bottom opening, and only one-quarter out of the top.
-
- To make sure that
this asymmetry was due to gravity, the researchers had to control the strength
of the magnetic fields to a precision of at least one part in 10,000. The results were consistent with the
antiatoms experiencing the same force of gravity as hydrogen atoms would.
-
- No one would have
expected antimatter to fall up, if nothing else, because antiprotons are made
of antiquarks, but these only constitute less than 1% of an antiproton’s mass:
the rest is the energy that keeps them together.
-
- A third CERN
experiment, called “AEgISz', will
attempt to measure the gravitational force on a beam of antihydrogen atoms in
the absence of any magnetic fields. 1%
precision to be made by first making positive antihydrogen ions (antihydrogen
with an extra positron), which will help to cool the gas down to a fraction of
a degree above absolute zero.
-
- Other efforts aim
to measure gravity acting on positronium, a short-lived particle made of one
electron and one positron orbiting each other. ALPHA-g itself plans to aim for
1% precision by letting antihydrogen atoms bump up and down and form a quantum
superposition with themselves.
-
-
January 8, 2023 DARK MATTER
and ANTIMATTER? 4310
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