- 2886 - DARK MATTER - mysteries? When something seems a little mysterious or we just don’t understand what is going on we like to describe it with the adjective ‘dark’. We do not understand 95% of the universe we live in. The 5% that is left is everything we know about and are still learning about.
--------------------------- 2886 - DARK MATTER - mysteries?
- We need to learn about dark energy and dark matter. Here is everything I have collected in a couple pages of heavy reading. It’s dark.
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- This is one of the reasons why the term ‘dark’ matter got coined. It was first proposed to explain the anomaly observed in the rotational velocities of galaxies. That is the observed rotational velocities of the gas and dust at the outer edges of a galaxy is rotating just as fast as the gas and dust near its center.
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- Compare that with our solar system. Mercury is rotating the fastest and the outer planets Uranus and Neptune are rotating with the lowest velocities. The math that Galileo came up with calculates these velocities exactly. That math does not work with stars orbiting in galaxies.
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- This was first noted in 1978 by Vera Rubin and W. Kent Ford who made precise measurements utilizing a new instrument that Ford himself had designed. At first, they thought their data could be erroneous, but then their results were corroborated by more observations of galactic rotational velocities. There was indeed an anomaly between what is expected and what was observed!
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- As direct measurements for the mass of a celestial body cannot be made. Astronomers instead observe the emitted light and look how it changes with time. An analysis of how the light changes with time reveals the dynamics of the system. That is the velocity, which, based on the laws of physics, allows for the determination of the mass. That was the math I was referring to.
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- For galaxies their rotational velocities are calculated through the measurement of their Doppler shifts in the light spectra. These calculated velocities are then plotted against their respective distance away from the galactic center, producing a rotation curve. ( I have some reviews on this math available if requested.)
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- To measure the Doppler shift, astronomers disperse the light using a spectrograph, which uses a prism, allowing the spectral lines representing the electron transitions between orbitals to be observed.
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- These spectral lines will be of a specific wavelength depending on the atomic transition of the electrons, but, as well the wavelength will be shifted, shortened or increased, due to the gas moving towards or away from the astronomer’s line of sight.
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- To determine the rotation curve of the Galaxy we observe the light emitted from the galaxy. However, the “visible light“ from stars may suffer from interstellar extinction where the light cannot penetrate the galactic dust clouds and are therefore not the best source of galactic light.
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- Instead, neutral hydrogen which exists in low density regions of the interstellar medium and emits light with a wavelength of 21 centimeters, the Hydrogen 21-cm Line, is used. In the case of neutral hydrogen, the 21-cm wavelength (1420 MHz) radiation comes from the transition between the two levels of the hydrogen ground state.
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- From the laws of physics we can assume that the velocity will change with distance, where in the case of a rigid or homogeneous system, like that assumed for the galactic nucleus, the velocity will be proportional to the distance. The velocity should increase with radius. Just like it does for our planetary system.
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- However, even with this more accurate observation using the 21cm-Hydrogen line, the resulting rotation curve is not what we expect
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- How can we explain this unexpected rotation curve? Why does it increase in velocity and then flatten as radial distance increases from the center? Based on what we know and would assume, the proportionality constant in the equation governing the Kepler’s formulas would need to NOT be constant with radius.
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- The mass would need to increase with respect to the radius, keeping the velocity approximately constant. As there is no observed extra mass it was thus proposed that the extra mass must be a different kind of mass that is undetectable that is ‘dark’.
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- This idea of dark matter had already been proposed back in 1933, through the study of galactic velocities in the Coma Cluster, concluded that the total mass required to hold the Cluster together is about 400 times larger than what is observed.
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- Today it is calculated that 85% of all matter is ‘dark’ matter that we can not observe.
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- Shortly after the idea of dark matter first gained momentum, several theories were put forward proposing possible sources e.g. ‘dark’ objects formed at the early epoch of the Universe ; dark remnants of Population III stars (the original stars formed after the big bang and thus composed entirely of primordial gas) such as white dwarfs, neutron stars or black holes ; and exotic elementary particles such as massive neutrinos.
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- You formulate a hypothesis, you develop it, and then you try and tease out what the observable, measurable consequences would be.
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- In order to understand how we might find dark matter we have to first understand the what else we know.
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- The particles and antiparticles of the “Standard Model of Particle Physics have now all been directly detected, with the last holdout, the Higgs Boson, falling at the Large Hadron Collider in 2010.
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- 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. These particles can be well described by the physics of the quantum field theories underlying the Standard Model, but , they do not describe dark matter.
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- We can start with the undisputed basics: 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 some photons (light, radiation, etc.). That is the 5% I referred to earlier.
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- Protons and neutrons can be broken up into even more fundamental particles, the quarks and gluons, and along with the other Standard Model particles, make up all the known “matter” in the Universe.
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- The big idea of dark matter is that there’s something other than these known particles contributing in a significant way to the total amounts of matter in the Universe. And, this additional matter creates additional gravity.
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- We know how stars work and we have an incredible understanding of how gravity works. 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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----------------------- First, 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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----------------------- Second, 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 there in stars.
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- These two numbers don’t match, and the mismatch between the values we obtain for them is spectacular in magnitude: they miss by a factor of approximately 50. There must be something more than just stars responsible for the vast majority of mass in the Universe. Thus dark matter.
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- 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 of galaxies“.
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- The predicted abundances of helium-4, deuterium, helium-3 and lithium-7 as predicted by Big Bang Nucleosynthesis. The Universe is 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’re left with, and this remains unchanged until stars form. Only about 17% 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 couldn’t form, but there was a time where even nuclei couldn’t form!
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- When they finally can form without immediately being blasted apart, that phase is where the lightest nuclei of all, including different isotopes of hydrogen and helium, originate from.
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- The formation of the first elements in the Universe after the Big Bang due to Nucleosynthesis, tells us with very, very small errors how much total “normal matter” is there in the Universe.
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- 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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- The fluctuations in the Cosmic Microwave Background were first measured accurately by COBE in the 1990s, then more accurately by WMAP in the 2000s and Planck in the 2010s. The fluctuations are only tens to hundreds of microkelvin in magnitude, but definitively point to the existence of both normal and dark matter in a 1:5 ratio, that is 20%
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- 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 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 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 matter, protons +neutrons +electrons, 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 83% of all the matter in the Universe.
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- The observations of baryon acoustic oscillations in the magnitude where they’re seen, on large scales, indicate that the Universe is made of mostly dark matter (83%), with only a small percentage of normal matter (16%).
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- And finally, there’s the incontrovertible 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.
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- 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’ll see throughout history.
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- We can tell that the dark matter is cold, thst is, 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 for these predictions, 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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- 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 collision 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, but, 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. That is how gravity bends the light rays.
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- By looking at how the background light gets distorted by the presence of intervening mass 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 (Standard Model-based) 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.
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- 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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- Studying four colliding galaxy clusters show us the separation between X-rays and gravitation, indicative of dark matter. On large scales, cold dark matter is necessary, and no alternative is proposed.
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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 86% 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 empirical proof for the existence of dark matter.
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- There is no modification to the law of gravity that can explain why:
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------------------- two clusters, pre-collision, will have their mass and gas aligned,
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------------------- but post-collision, will have their mass and gas separated.
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- Unfortunately, we don’t know what is beyond the Standard Model. We have never discovered a single particle that isn’t 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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- Many people are working on alternatives, but unless they’re misrepresenting the facts about dark matter, they have an enormous suite of evidence they’re required to explain. But absence of evidence is not evidence of absence.
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- The idea of massive neutrinos being dark matter was shortly put to bed when the mass of an electron neutrino was measured to be 30 eV. An argument based on the “Pauli exclusion principle“, which states that two or more identical fermions cannot simultaneously occupy the same quantum state, showed that individual galaxy halos could not be made of neutrinos with masses that small .
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- It was assumed that dark matter will not significantly interact with the electromagnetic spectrum and only interacts via gravity or any other force which is as weak as, or weaker than the weak nuclear force.
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- In the early universe the materialization of particles and anti-particles from radiant energy through pair production and the subsequent destruction through annihilation were in equilibrium. That is the production rates for both the particles and the photons were the same as their destruction rates such that no photon and/or particle was permanent, just continuously fluctuating in and out of existence.
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- As the universe cooled the energy was not sufficient for pair production and thus the number of particles and photons decreased until the particle interaction probability reached a critical low such that particle annihilation ceased, and the number density of particles stabilized.
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- For a specific particle, the number density that stabilization occurs depends on the particles mass. For a dark matter candidate, it would need to be sufficiently massive and slow moving, sub-relativistic, such that it could clump together and form the structure we observe today. This is the general view of dark matter and is referred to as the “Cold Dark Matter model“.
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- Supersymmetryis one of the candidate theories for quantum gravity and it focuses on the relationship between ordinary particles (fermions) and ‘force carrying’ entities (bosons). It predicts new elementary particles that fit the description of a weakly Interacting Massive Particle (WIMP)
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- The lightest of these stable supersymmetric particles is the “neutral no” which happens to have a calculated number density approximately equal to the known density of dark matter. The neutralino is the most likely candidate for a WIMP and with a mass is within the energy levels that can be detected at particle accelerators such as the Large Hadron Collider (LHC).
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- Any detection of the WIMP particle at the LHC would not be direct, instead it would be in the form of missing energy of the specific order. However, as yet no such detection has been made.
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- Direct detection of a WIMP would be the optimum confirmation of dark matter, however as they are weakly interacting it is an extremely low probability that they will interact, let alone that we would detect such a small energy range interaction.
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- There are many experiments dedicated to detecting the interaction of a WIMP with atomic matter. Depending on the material of the detector, silicon, germanium, sodium iodide etc., phonons – vibrations in the atomic lattice – and/or scintillation – luminescence from ionized electrons.
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- To reduce background events, these experiments operate deep underground and at extremely cold temperatures where they are shielded from cosmic rays and thermal excitations are minimized.
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- Although numerous experiments are actively searching e.g. Deep Underground Science and Engineering Laboratory (DUSEL), Large Underground Xenon experiment (LUX), Sudbury Neutrino Observatory Laboratories (SNOLAB) and the China Jinping Underground Laboratory (CJPL), to date no WIMP has been detected.
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- These underground experimental methods allow for another source of indirect detection through the detection of neutrinos. If we assume that WIMPS are the dark matter particle and exist in the halo of galaxies, then they would have been passing through our local surroundings and at some point, in the last several billion years, they would have been scattered by nuclei.
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- This loss in energy would have trapped the WIMPS in the gravitational well of the Sun and/or Earth until the number density sufficiently increased for annihilation to occur. Annihilation of WIMPs results in high-energy neutrinos, so based on this reasoning you would expect a stream of neutrinos to be emanating from the sun.
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- Neutrinos produced in nuclear reactions in the solar core have a much lower energy than the neutrinos produced through WIMP annihilation. These higher-energy neutrinos interact in the atmosphere of the Earth producing muons.
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- However, muons are also created through cosmic ray interaction with the Earth’s atmosphere so again, to reduce background events the detectors are placed deep underground.
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- Detectors such as the Antarctic Muon And Neutrino Detector Array (AMANDA), the South Pole Neutrino Observatory (IceCube) and the Astronomy with a Neutrino Telescope and Abyss environmental RESearch project (ANTARES) are all searching for a signal, but these methods have also been to no avail.
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- Annihilation of WIMPS also produce gamma rays, which has been another focus of searching for these elusive dark matter particles. This annihilation is expected to take place in galactic halos and could be detected through an excess of gamma rays. However, distinguishing between gamma rays due to annihilation and those from the various astrophysical sources has proven difficult.
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- For example, a recent study using data from the Large Area Telescope on NASA’s Fermi Gamma-ray Space Telescope have suggested that the excess source of gamma ray emission may instead be a well-known population of pulsars.
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- The myriad of experiments and searching techniques have so far revealed no positive results, so it looks like the most promising candidate WIMPS is finally facing defeat and a team of leading scientists attending a workshop on new ideas in dark matter have been encouraged to look elsewhere and widen their perspective.
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- One such alternative candidate for dark matter that may well make up a fraction of this missing mass are MACHOs (MAssive Compact Halo Objects). Unlike WIMPs , MACHOs are baryonic and come in the form of astronomical objects such as jupiter mass objects; brown dwarfs; black hole remnants of early generation stars, primordial black holes, neutron stars and white dwarfs.
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- If the galactic halo was filled with objects such as these, they would not be detected by emission or absorption of light. Detection of a MACHO could be made through the phenomenon known as microlensing, where the light from a distant star is magnified when a MACHO type object passes in front of it.
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- Non-baryonic matter has a much higher density by a factor of 5 than that of baryonic matter and gravity is too weak to grow the present structures from the smooth initial conditions observed in the cosmic microwave background (CMB).
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- An additional mass would speed up the process, but only if it didn’t interact with light in the same way ordinary “baryonic” matter does. It was concluded that the majority of dark matter is most likely non-baryonic and if MACHOs did exist then they would only be responsible for a small fraction of dark matter primarily in the halos of spiral galaxies.
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- “Axions” are another viable dark matter candidate. As opposed to WIMPs, that are hypothesized to have been created thermally in the early universe, axions are suggested to have been created non-thermally during a phase transition event.
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- Axions were first proposed in the 1970s to explain the strong CP problem that is ‘C” as in charge and “P” as in parity (spatial inversion). The problem is why quantum chromodynamics (QCD) does not seem to break CP symmetry when in principle it permits such a violation.
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- The fact that we live in a matter dominated universe indicates that the laws of physics are not the same for matter and antimatter and there must have been a violation of the fundamental symmetry of nature, “CP violation” .
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- Although this is the case for weak interactions , this is not the case for electromagnetic and strong interactions and is thus known as the “Strong CP problem“. Axions were thus postulated to account for this, where the axions potential would exactly cancel out a CP violating term introduced into the QCD calculations.
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- Since 2016 the Axion Dark Matter Experiment has been trying to tune a microwave antenna to the broadcast frequency of dark matter, to no avail. The Ultra Cold Neutron Source (UCN) is primarily being utilized to determine the electric dipole moment of a neutron but measurements over time could reveal a fluctuation of a consistent frequency which would be indicative of an interaction between the neutron and the hypothetical axion particle.
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- Axions are 10,000 trillion-trillion times less massive than an electron and could theoretically condense into a Bose-Einstein condensate and thus be the hypothesized superfluid dark matter responsible for galaxy rotation as oppose to the normal dark matter responsible for galaxy clusters.
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- However, this has since been ruled out on the basis that axions are weak and attractive and the superfluid dark matter particles are required to be repulsive.
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- Maybe there is no ‘dark’ matter particle after all and instead it’s the physics that we need to look at. This is the premise with a theory on modified Newtonian dynamics (MOND).
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- The central idea of MOND is scale invariant physics, that is, physics that does not change across scales? However, this is not the general consensus, that the laws of physics change at different scales, for example, you have quantum mechanics at the quantum scale and general relativity at large scales.
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- What if this was not the case and instead we live in a scale invariant Universe, a unified physics view where quantum gravity is the physics across all scales.
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- The underlying scale invariant framework of MOND, where gravity is an emergent property, utilizes quantum information theory, string theory and black hole physics to suggest that space-time and gravity emerge together from entangled quantum interactions.
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- This “emergent gravity” contains an additional force which can explain the observed phenomena in galaxies and clusters currently attributed to dark matter.
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- Maybe there is an additional force that opposes gravity. This force is only small at very low densities. On Earth the force would be too small to make any measurable difference, but on the galactic scale it is strong enough to hold the rotating galaxies together with no need for dark matter.
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- There are many more alternative theories of gravity that attempt to remove the need for dark matter and dark energy, but these models are still works in progress and cannot explain all the features of dark matter yet.
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- Possibly these models have limits to the scale in-variance. If we are assuming a unified approach with scale invariance, where any dilation or contraction of space would not change the physics then you would assume this to be valid across all scales.
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- Do you get the idea we have a lot more to learn. Stay in school. A lifelong endeavor.
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November 1, 2020 Dark Matter Mysteries 2886
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--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
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--------------------- --- Wednesday, November 4, 2020 ---------------------------
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