- 4443 - DARK MATTER - what is it , really? - Dark matter is the mysterious stuff that fills the universe but no one has ever seen. “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.
------------------------- 4443 - DARK MATTER - what is it , really?
- 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, 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 other 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 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.
-
- The smaller “neutral axion” and the
uncharged “photinos” 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: There is more stuff (matter) that we don't
see with our telescopes. We call this “dark matter”. Or, 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.
-
- Some are looking for a form of dark matter
that is quite massive, between about 100grams and many tons, and would have
more easily visible effects. But because it is massive, it is very rare. (We
know how much total mass of dark matter there should be, so if the individual
dark matter particles are heavy, there are fewer of them.)
-
- But 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.
-
- 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.
-
- 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.)
-
- 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. A null
result is significant as it changes the landscape of the field by constraining
models for what dark matter could be beyond anything that existed previously.
-
- 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 excess can be explained by annihilation
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 this 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. Tiny black holes left over from the Big Bang
may be prime dark matter suspects
-
- Tiny black holes, created seconds after
the birth of the universe, may survive longer than expected, reigniting a
suspicion that primordial black holes could account for dark matter, the
universe's most mysterious stuff.
-
- Dark matter currently represents one of the
most pressing problems in physics. That is because, despite making up an
estimated 85% of the matter in the cosmos, dark matter remains effectively
invisible to our eyes because it doesn't interact with light.
-
- Because the particles that comprise atoms
that compose "everyday" stuff we can see, like stars, planets, and
our own bodies, clearly do interact with light, this has prompted the search
for dark matter particles outside the Standard Model of particle physics.
-
- Some astronomers posit that tiny black
holes born over 13.8 billion years ago, just after the Big Bang, are no larger than a proton, could cluster to
become suspects for dark matter without the need for new physics.
-
- Not only has a recent change in thinking
regarding how black holes "evaporate" prompted a reassessment of
primordial black holes' viability as dark matter suspects, but as the search
for a dark matter particle continues to mostly draw a blank, more researchers
could begin to look at the primordial black hole dark matter theory more
seriously.
-
- Primordial black holes' are a type of black
hole that is formed at the beginning of the universe. Within the first fraction of a second of the
universe.
-
- All the structures we observe in the
universe, from superclusters of galaxies to the galaxies within themselves, are
formed from slight overdensities in space present during the early universe. If
the early universe experienced much stronger density fluctuations than the
those which created these features, and these fluctuations collapsed at an
earlier time than galaxies indeed formed, then those overly dense patches could
have spurred primordial black holes.
-
- Depending on the time at which this
collapse may have happened as well as the scale of the collapse, these
primordial black holes would have very different masses. The primordial black
holes candidates would have masses ranging between a few tons and a thousand
tons, which is less than the mass of a planet and more in the category of a
small asteroid.
-
- Considering how the smallest black holes
scientists have discovered to date, known as “stellar-mass black holes”, have
masses equivalent to between 3 and 50 times that of the sun, which itself is
2.2 times 10 to the power of 27 (22 followed by 26 zeroes) tons. these
primordial black holes are incredibly tiny.
-
- Like their larger black hole counterparts
formed from either the collapse of massive stars or the merger of relatively
smaller black holes. Primordial black
holes would have a light-trapping outer boundary called an event horizon. The
diameter of this horizon is determined by the mass of the black hole, which
means the event horizon would be incredibly small in those cases. Smaller than the radius of a proton.
-
- Small, primordial black holes had previously
been ruled out as dark matter candidates because all black holes are thought to
"leak" a type of thermal radiation first theorized by Stephen Hawking
in 1974 and later named "Hawking radiation."
-
- The smaller a black hole, the more rapidly
it should leak Hawking radiation and, thus, the faster it should evaporate.
That means if primordial black holes ever existed, the smallest examples
shouldn't be around today, yet, dark matter clearly is.
-
- Theoretical physicists have suggested that
the evaporation process breaks down at a certain point, however. This means
primordial black holes of the masses the scientists considered could achieve a
semi-stable state. In order to decrease
its mass through the emission of Hawking radiation, the black hole has to
'rewrite' its information. This rewriting process takes time.
-
- It is called 'memory burden' because of
this memory that now has to be passed along to something else, and that just
kind of slows down the evaporation process overall. So it's a kind of
stabilization. And that "rescue
mechanism" means primordial black holes are back as potentially dark
matter candidates!
-
- Perhaps the most obvious connection is
dark matter's lack of interaction with light. Dark matter doesn't emit or
reflect light, and the event horizon that bounds all black holes represents the
point at which the escape velocity necessary to cross it exceeds the speed of
light. That means primordial black holes would "trap" all incident
light, resulting in an apparent lack of interactions.
-
- If they are light enough, somewhere around
a planetary mass, primordial black holes behave like particles of dark
matter. Dark matter is 'collision-less'
in standard models, so dark matter particles do not interact with each other to
such a degree that it affects the universe.
-
- If primordial black holes are lighter than
planetary masses, then, even on cosmic timescales, they would be so small
they'd very rarely collide. These primordial black holes could rather cluster
to create the gravitational effects we currently attribute to dark matter, such
as providing the gravitational influence that prevents rapidly spinning
galaxies from flying apart.
-
- Still, primordial black holes are going to
be incredibly difficult to confirm as dark matter, if they really do explain
the phenomenon. Again, their light-trapping nature means they are effectively
invisible.
-
- Plus, at such diminutive sizes, they don't
have the same immense gravitational effects as their stellar and supermassive
brethren. Even then, should a cluster of
primordial black holes be detected, there is no real way to tell the difference
between lots of little black holes and one large black hole. The search goes on!
-
-
April 25, 2023 DARK
MATTER - what is it , really? 4443
------------------------------------------------------------------------------------------
-------- Comments appreciated and Pass it on to
whomever is interested. ---
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for
copies or Index of all reviews
--- to:
------
jamesdetrick@comcast.net
------ “Jim Detrick” -----------
--------------------- --- Saturday, April 27,
2024
---------------------------------
No comments:
Post a Comment