- 2790 - DARK MATTER - to discover what it is? - There is a race to discover “dark matter“. Dark matter is that elusive substance that has mystified science since the 1930s, when astronomers first realized galaxies needed some kind of invisible gravitational glue to hold them together. No one knew what it was, so it was named “dark matter“.
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-------------------------- 2790 - DARK MATTER - to discover what it is?
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- The universe seems to hold more than five times as much “dark matter” as it does “normal” matter that we an see. Scientists know very little about this universe’s dominant material. Dark matter could be made of one kind of particle or many different particles, we don’t know?
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- Dark matter particles might be massively heavy or light. We think dark matter only interacts with other matter and itself via gravity, but dark matter could turn out to have interactions with any force of nature. We just don’t know.
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- Addressing all these possibilities, physicists have conjured up quite a few dark matter candidates:
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- The current candidates range from the grandest scales of the universe to the tiniest, from galaxies to subatomic particles. Many of the experiments involve supercooling materials such as liquid xenon to subfreezing temperatures, which makes it easier for the materials’ atoms to bump into stray dark matter particles and thus find the elusive galactic particles existence.
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- The odds-on favorite candidate is called a WIMP, the “weakly interacting massive particle“, but it has not been found despite intensive search efforts.
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- Another candidate thehe “massive compact halo object“, or MACHO has fallen out of contention.
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- Particle masses are measured in units called gigaelectron volts, or GeV. A proton weighs about 1 GeV. Electrons measure 0.0005 GeV and the heaviest known particle, the top quark, measures 172.9 GeV.
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- Developed over many decades, the “standard model f elementary particles” is a stunning scientific success. With pinpoint precision, it describes three of nature’s four forces, electromagnetic, and the strong and weak nuclear forces. But the model also has gaps, including not being able to describe the fourth force, gravity, and failing to explain dark matter and its particles.
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- A refinement of the standard model called “supersymmetry” smoothes over many of these flaws. It fills the gaps by proposing new, heavier partner particles for all known particles. Plug these new heavies into the mix, and their total mass strikingly matches the estimates for dark matter.
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- Cosmologists had already been kicking around the idea of WIMPs without knowing what they might be, and suddenly they had a good match. Assuming supersymmetry’s heavier partners were WIMPs resolved everything so perfectly, researchers dubbed it the “WIMP miracle.” Conveniently, these WIMPs would interact with normal matter, albeit very weakly as their name implies; such interactions should render them discoverable.
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- WIMPs also have failed to appear in other detection methods. Theories suggest the particles may occasionally destroy each other or decay, resulting in showers of gamma rays, but searches have found no convincing evidence detecting gamma rays.
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- Many physicists expected that the Large Hadron Collider would produce heavy, novel particles, including WIMPs. But a decade of operations with no heavy partners to show for it has instead made some physicists question the whole notion of supersymmetry.
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- Another particle called the “Axion” was proposed in 1977. Its mass is expected to be about 0.000,000,000,000,001 GeV. Physicists originally came up with this particle to help fix a problem with the strong nuclear force, one of nature’s four fundamental forces.
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- Although the individual axion particles have a ridiculously low mass, the universe forming Big Bang could have churned out axions in abundance. Enough axions could constitute all the dark matter in the cosmos.
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- To catch any fleeting axions, researchers at the “Axion Dark Matter eXperiment” at the University of Washington cool a cylinder to nearly absolute zero before it emits a strong magnetic field, which should transform the theoretical dark matter particles into radio waves.
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- The “Axion Dark Matter eXperiment (ADMX)” became the first device with the sensitivity necessary to detect axions. The experiment uses a 13-foot-long metal cylinder sunk into the floor, cooled to just above absolute zero to silence any signal-masking perturbations.
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- A magnet inside cranks out a powerful magnetic field that, according to theory, should convert any nearby axions into radio waves. To detect these infinitesimal signals which are a billionth of a billionth of a billionth of a watt each, ADMX has specially designed amplifiers. With these amplifiers it is the most sensitive radio receiver ever built.
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- Researchers are presently “tuning” ADMX through millions of frequencies representing possible axion masses. No detections so far.
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- Another candidate is the “Sterile Neutrino” having a mass roughly 1 GeV. It is a hypothesized new type, or flavor, of neutrino. Neutrinos are ubiquitous particles that come in three flavors and are all but oblivious to matter, passing clear through our bodies by the hundreds of trillions every second.
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- The idea for sterile neutrinos gained traction when an experiment in the 1990s recorded a strange excess of one flavor, called the “electron neutrino“, over the other two known as muon and tau neutrinos.
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- The particles should have appeared in roughly equal numbers. Around that same time, though, experiments revealed that neutrinos transform from one flavor to another spontaneously as they fly about the universe.
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- Theorists postulated that the flavor skew arose because some neutrinos were temporarily morphing into a fourth, sterile flavor before “returning” as electron neutrinos. When other observations ended up contradicting the idea, physicists summarily dismissed that lone result as an experimental fluke.
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- Yet in June 2018, a second experiment, MiniBooNE, found the same flavor excess based on a whopping 15 year’s worth of data. MiniBooNE experiment is housed at the Fermi National Accelerator Laboratory just outside Chicago.
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- MiniBooNE is hardly mini. It is a sensor-studded sphere measuring nearly 40 feet across, filled with over 800 tons of pure mineral oil. The instrument registers the flashes of light emitted on the rare occasions when neutrinos bump into the oil’s constituent atoms.
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- Assuming sterile neutrinos are proved to exist, they are still likely neither sufficient in mass nor number to constitute the bulk of dark matter. But just as normal neutrinos come in three flavors, multiple kinds of sterile neutrinos, with different masses, may exist.
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- And going a step further, neutrinos may not be the only kind of particle with a sterile counterpart. Their could be an entire “unstandard model,” full of particle types that invisibly interact with each other, all around us.
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- Dark photons, dark gluons, dark quarks and more could exist. All could be repositories of the extra stuff in the universe we perceive as dark matter.
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- Multiple new teams hope to further research out neutrinos’ weirdness with new projects. A fresh experiment at Los Alamos, called “Coherent CAPTAIN-Mills” uses chilled vats of argon to capture any telltale oscillations between flavors of neutrinos.
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- A new particle called the “Strongly interacting massive particles” (SIMPs) was proposed in 2014. Its mass expected to be about 0.1 GeV. WIMPs, axions and sterile neutrinos are all postulated as indivisible, elementary particles. SIMPs, on the other hand, are composite particles, made of other, smaller particles. The most common examples of composite particles, protons and neutrons, make up the normal matter around us.
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- The smaller bits making up protons, neutrons and SIMPs are called “quarks“, but in the SIMP’s case, they would be individually composed of a quark paired with a hypothetical “antiquary“, which primarily goes about its business in the dark sector beyond the standard model.
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- Still, the physics of composite particles is well understood, and that degree of familiarity could make SIMPs easier to detect and understand than the more exotic indivisible dark matter candidates.
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- SIMPs are strongly interacting with other SIMPs. That’s in contrast to WIMPs, which only weakly interact with each other and normal matter. As a result, instead of WIMPily flowing past their fellow particles, SIMPs would bounce off one another like billiard balls.
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- Dark matter behaving in this boisterous manner would help explain two key astronomical observations that buck against WIMPs. The first concerns some colliding galaxies. Astronomers inferred that a great amount of dark matter had detached from its host galaxies in a celestial smashup happening some 1.4 billion light-years away. This suggests the dark stuff pushes against itself and cannot readily flow together with the visible stars and gas as WIMPs should.
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- However, a second analysis using more accurate measurements now suggests perhaps the dark matter may not have separated from its galaxies after all. The second puzzling observation involves the screwy distribution of dark matter within smaller galaxies.
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- Computer simulations show that due to gravity, WIMPs should stick together, forming dense clumps of dark matter in the centers of galaxies; they should also coalesce into chunks out in space. Yet observations clash with those predictions.
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- Galactically, dark matter seems too evenly spread out, and astronomers have never found the chunks the WIMP model predicts. One more thing points to SIMPs instead. There should be enough of them to explain away all of the universe’s dark matter, unlike the more complicated theories other particles require.
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- In the late 1980s, scientists got their hopes up that MACHOs, normal matter that were simply dim and tough to detect, could answer the dark matter question. These objects would range from planets to failed stars to black holes. Unfortunately, well-supported Big Bang models struggle to produce anywhere near enough regular matter for the MACHOs needed.
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- Observations have consistently ruled out any vast populations of black holes, which should give themselves away when their gravity bends background starlight. An October 2018 study took out the last leg for MACHOs to stand on, putting serious constraints on the possibility of primordial black holes being the last plausible reservoir of significant unaccounted-for matter.
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- WIMPs, SIMPs . . . and GIMPs? The only force definitely felt by both matter and dark matter is gravity. Accordingly, some researchers have created gravity-only models dubbed GIMPs, “gravitationally interacting massive particles“.
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- This concept is not new to physics. It simply submits that black holes actually have all that missing dark matter bound up within them and act in essence like particles.
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- Alternatively, physicists have conjured GIMPs as elementary particles required by theories of our universe that include an extra fifth spatial dimension.
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- Perhaps the weirdest theory is “Planckian interacting dark matter” (PIDM). It consists of individual particles that each could weigh as much as 10 quadrillion protons. PIDM that spawned in the early universe should have left an indelible imprint on the Big Bang’s relic afterglow, called the cosmic microwave background, which researchers study for clues about the universe’s origins.
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- Next-generation instruments could be sensitive enough to answer many of these this dark matter questions. Stay tuned there is still more to learn.
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----------------------------- Other Reviews about Dark Matter:
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- 2768 - DARK MATTER - What is the Universe Made of? Since 1970 astronomers have believed Dark Matter existed because studying the orbits of galaxies and stars around galaxies could not be calculated based on the stars and matter they could see. Either Kepler’s and Newton’s formulas for the laws of gravity and motion were incorrect, or there was matter there that they could not find.
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- 2767 - DARK MATTER - confirmed by new measurements? - The first Fast Radio Burst detected came from a galaxy that is about 4 billion light-years away from Earth. Using dispersion measurements for these FRB’s, astronomers are able to make a rough calculation of how much dark matter the radio waves passed through before reaching earth.
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- 2718 - DARK MATTER - What is the Universe Made Of? ? We do not know? 95% of our knowledge is just “dark”. We think 75% is energy and 25% is matter. Remember matter and energy are the same thing, simply separated by the speed of light squared, c^2.
- 2631 - DARK MATTER - dark coffee would help? Astronomer’s observations have determined the average density of matter in our universe to very high precision. But this density turns out to be much greater than can be accounted for with “ordinary atoms“. Is there some other matter that we still don’t know about?
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- This Review 2631 lists 33 more Reviews about Dark Matter. A real mystery in astronomy and all of science.
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- August 13, 2020 2790
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