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------------- 2401 - DARK MATTER - absence of evidence is not evidence of absence
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- Science is examining the evidence for Dark Matter making up 80% of all the matter in the Universe. But, we can not find what it is.
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- Matter and Energy are two forms of the same thing according E = mc^2. ALL MATTER is 30% of the total and ALL ENERGY is 70% of the total. Of the 30% of all matter 25% is Dark and only 5% is “Ordinary matter” that we see as our Ordinary Universe. 95% is unknown Dark stuff with 30% holding things together and 70% ever expanding and separating all the matter in its expansion.
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- This Review searches for the evidence that tells us Dark matter really exists. Whenever a dark matter experiment comes up empty, it only tests the model-dependent assumptions, not the model-independent ones. Here’s why that doesn’t mean anything for the existence of dark matter.
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- When you collide any two particles together, you probe the internal structure of the particles colliding. If one of them isn’t fundamental, but is rather a composite particle, these experiments can reveal its internal structure.
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- The particles and antiparticles of the Standard Model have now all been directly detected. Including the last holdout, the Higgs Boson, that as discovered in the Large Hadron Collider earlier this decade. All of these particles can be created at LHC energies, and the masses of the particles lead to fundamental constants that are absolutely necessary to describe them fully.
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- These particles can be described by the physics of the “quantum field theories” underlying the Standard Model, but they do not describe everything. They do not describe dark matter.
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- We know the Universe consists of all the protons, neutrons and electrons that make up our bodies, our planet and all the matter we’re familiar with, as well as photons (light, radiation, etc.)
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- Protons and neutrons can be broken up into even more fundamental particles called quarks and gluons. These few fundamental particles in Standard Model of particles, make up all the known matter in the Universe. That only adds up to 5% of the total mass - energy that is out there.
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- The scientific theory for dark matter is that there is something other than these known particles contributing in a significant way to the total amounts of matter in the Universe. Why do we think this?
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- The motivation comes from looking at the Universe itself. Science has taught us a lot about what’s out there in the distant Universe, and much of it is completely undisputed. We know how stars work and we have an understanding of how gravity works.
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- If we look at galaxies, clusters of galaxies, and go all the way up to the largest-scale structures in the Universe, there are two things we can extrapolate very well.
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- How much mass there is in these structures at every level. We look at the motions of these objects, we look at the gravitational rules that govern orbiting bodies, whether something is bound or not, how it rotates, how structure forms, etc., and we get a number for how much matter there has to be in there.
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- How much mass is present in the stars contained within these structures. We know how stars work, so as long as we can measure the starlight coming from these objects, we can know how much mass is in all the stars.
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- The two bright, large galaxies at the center of the Coma Cluster and the slightly smaller, each exceed a million light years in size. But the galaxies on the outskirts, zipping around so rapidly, point to the existence of a large halo of dark matter throughout the entire cluster. The mass of the normal matter alone is insufficient to explain this bound structure.
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- These same measurements can be repeated with many different galaxy structures and the results are always the same. There is simply more mass surrounding these structures than we can identify. We can not see it therefore it is “dark“. It is matter because gravity affects matter.
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- There must be something more than just stars responsible for the vast majority of mass in the Universe. This is true for the stars within individual galaxies of all sizes all the way up to the largest clusters galaxies in the Universe, and beyond that, the entire cosmic web.
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- Another observation has to do with nucleosynthesis of the first elements. The predicted abundances of helium-4, deuterium, helium-3 and lithium-7 by Big Bang Nucleosynthesis puts the Universe at 75–76% hydrogen, 24–25% helium, a little bit of deuterium and helium-3, and a trace amount of lithium by mass.
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- After tritium and beryllium decay away, this is what we are left with, and this remains unchanged until stars form. Only about 1/6th of the Universe’s matter can be in the form of this normal baryonic, or atom-like matter.
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- When we extrapolate the laws of physics all the way back to the earliest times in the Universe, we find that there was not only a time so early when the Universe was hot enough that neutral atoms could not form, but there was a time where even nuclei could not form! When they finally can form without immediately being blasted apart, that phase is where the lightest nuclei of hydrogen and helium, originate from.
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- This formation of the first elements in the Universe after the Big Bang is due to Nucleosynthesis This theory tells us with very, very small errors how much total “normal matter” is there in the Universe. Although there is significantly more than what’s around in stars, it’s only about 16% of the total amount of matter we know is there from the gravitational effects. Not only stars, but normal matter in general, isn’t enough.
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- Another measurement involves fluctuations in the Cosmic Microwave Background. This background radiation was first measured accurately by COBE in the 1990s, then more accurately by WMAP in the 2000s and Planck in the 2010s.
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- This additional evidence for dark matter comes to us from another early signal in the Universe: when neutral atoms form and the Big Bang’s leftover glow can travel, at last, unimpeded through the Universe. It’s very close to a uniform background of radiation that’s just a few degrees above absolute zero. But when we look at the temperatures on degrees microkelvin scales, and on small angular (< 1°) scales, we see it’s not uniform at all.
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- The fluctuations in the cosmic microwave background tell us what fraction of the Universe is in the form of normal (protons+neutrons+electrons) matter, what fraction is in radiation, and what fraction is in non-normal, or dark matter, among other things. Again, they give us that same ratio: that dark matter is about 80% of all the matter in the Universe.
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- And finally, there’s the evidence found in the great cosmic web. When we look at the Universe on the largest scales, we know that gravitation is responsible, in the context of the Big Bang, for causing matter to clump and cluster together. Based on the initial fluctuations that begin as overdense and underdense regions, gravitation, and, the interplay of the different types of matter with one another and radiation, determine what we will see throughout our cosmic history.
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- We can not only see the ratio of normal-to-dark matter in the magnitude of the wiggles we can tell that the dark matter is cold, or moving below a certain speed even when the Universe is very young. These pieces of knowledge lead to outstanding, precise theoretical predictions.
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- According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit.
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- All together, they tell us that around every galaxy and cluster of galaxies, there should be an extremely large, diffuse halo of dark matter. This dark matter should have practically no interactions with normal matter; upper limits indicate that it would take light-years of solid lead for a dark matter particle to have a 50/50 shot of interacting just once.
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- However, there should be plenty of dark matter particles passing undetected through Earth, me and you every second. In addition, dark matter should also not collide or interact with itself, the way normal matter does. That makes direct detection difficult, to say the least. But thankfully, there are some indirect ways of detecting dark matter’s presence. The first is to study what’s called gravitational lensing.
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- When there are bright, massive galaxies in the background of a cluster, their light will get stretched, magnified and distorted due to the general relativistic effects known as gravitational lensing.
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- By looking at how the background light gets distorted by the presence of intervening mass (solely from the laws of General Relativity), we can reconstruct how much mass is in that object. Again, it tells us that there must be about six times as much matter as is present in all types of normal matter alone.
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- There’s got to be dark matter in there, in quantities that are consistent with all the other observations. But occasionally, the Universe gives us two clusters or groups of galaxies that collide with one another. When we examine these colliding clusters of galaxies, we learn something even more profound.
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- The dark matter really does pass right through one another, and accounts for the vast majority of the mass; the normal matter in the form of gas creates shocks , and only accounts for some 15% of the total mass in there.
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- In other words, about 80% of that mass is dark matter! By looking at colliding galaxy clusters and monitoring how both the observable matter and the total gravitational mass behaves, we can come up with an astrophysical, empirical proof for the existence of dark matter. There is no modification to the law of gravity that can explain why: two clusters, pre-collision, will have their mass and gas aligned, but post-collision, will have their mass and gas separated.
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- Still, despite all of this model-independent evidence, we’d still like to detect dark matter directly.
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- Unfortunately, we don’t know what’s beyond the Standard Model. We have never discovered a single particle that is not part of the Standard Model, and yet we know there must be more than what we’ve presently discovered out there.
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- As far as dark matter goes, we don’t know what dark matter’s particle (or particles) properties should be, should look like, or how to find it. We don’t even know if it’s all one thing, or if it’s made up of a variety of different particles.
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- All we can do is look for interactions down to a certain cross-section, but no lower. We can look for energy recoils down to a certain minimum energy, but no lower. We can look for photon or neutrino conversions, but all these mechanisms have limitations. At some point, background effects, natural radioactivity, cosmic neutrons, solar/cosmic neutrinos, etc., make it impossible to extract a signal below a certain threshold.
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- To date, the direct detection efforts having to do with dark matter have come up empty. There are no interaction signals we’ve observed that require dark matter to explain them, or that aren’t consistent with Standard Model-only particles in our Universe. Direct detection efforts can disfavor or constrain specific dark matter particles or scenarios, but does not affect the enormous suite of indirect, astrophysical evidence that leaves dark matter as the only viable explanation.
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- Many people are working tirelessly on alternatives, but unless they’re misrepresenting the facts about dark matter (and some do exactly that), they have an enormous suite of evidence they are required to explain.
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- When it comes to looking for the great cosmic unknowns, we might get lucky, and that’s why we try. But absence of evidence is not evidence of absence. When it comes to dark matter, don’t let yourself be fooled.
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- Other Reviews about Dark Matter:
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- 2214 - What is holding galaxies together? This Review lists 7 more Reviews.
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- 2082 - Dark matter throws us a curve Dark Matter.
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- 1934 - Can Dark matter make blackholes?
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- 1888 - What is the Universe made of?
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- 1823 - Dark Matter and the extinction of the dinosaurs?
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- 1636 - What does Dark Matter look like?
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- 1594 - Why is 96% of the Universe “dark” ? Part I of VI Reviews.
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- June 17, 2019
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--------------------- Monday, June 17, 2019 --------------------
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