What Is Dark Universe? Is Dark Matter Made of Particles?

By the time I have finished my sentence up to a billions of billions Dark Particles may have streamed through your body like ghosts. The particle or particles of the dark sector make up the vast majority of the mass in the universe. So, to them, you are the ghostly one. Today we are going to discuss about this only. 

                                              We see the influence of dark matter, in the orbits of stars and galaxies in the way light bends around galaxies and clusters in the clumpiness of the cosmic background radiation and more. It becomes disturbingly clear that we cannot see 80% of the matter in the universe. Even more disturbing is that they doesn't even  seem to be a candidate  for dark matter in the known family of particles. We are faced with the eerie reality of the dark sector-perhaps there is an entire family of particles that exists in parallel to those we can see-a dark universe that overlaps our own, but so far is hidden from even our most ingenious experiments.  Today we are going to oepn the gateway to the dark sector and see what we can find. 

The Dark Sector

When we talk about 'dark sector'  we typically mean a particle or a family of particles that contribute to the dark matter. Now it's possible that dark matter is not particles, it could be black holes or failed stars or even weirder so called 'compact objects'. It might be even that what we perceive as dark matter is really a glitch in the laws we use to describe gravity. But those possibilities are for another time, today we are focusing on bizarre physics of the dark sector. So let's begin with what we do know. 

                       

                                                      Our best understanding of the sub-atomic world is Standard Model- which describes the behaviour of the known family of particles with incredible success. The visible universe is made of these particles, interacting with each-other through the Standard Model forces-the strong and weak nuclear forces and electromagnetism plus gravity. In general,  the behaviour of a particle is determined by the forces it interact with. We can think of those forcesas the language through which particles communicate. Any electrically charged particle experiences the electromagnetic force and can communicate with other charged particles by exchanging photons. But for a electrically neutral particle like neutrino, electromagnetism is a language it doesn't speak. Neutrino are that particles which are unaffected by that force, and so they are quite literally invisible to photons. A more technical way to think about this stuff is in terms of quantum field - where each particle and force is a vibration in its own field.  These fields fill the universe, overlapping each other, and if a particle field is connected to or coupled with a force, then it can speak the language of that force. The force of gravity is a sort of lingua franca, a common language that every particle with mass can speak. But gravity is little different to other forces-it's not part of the Standard Model, and we do not know even if it has a quantum field. The main requirements for a Dark matter particle is that it doesn't speak electromagnetism. It doesn't produce light, hence, the dark particle part. But it also doesn't absorb light, otherwise we'd be able to detect it when it is blocked light from the more distant universe-in the same way we see the black lanes of dust that block the light from the centre of our galaxy. No, dark matter is both perfectly dark and perfectly transparent. So, it must be electrically neutral like the neutrino. Dark matter can't have charge but it also must've mass because the only thing we've ever actually seen dark matter do is to exert its gravitational influence. Hence dark matter speaks gravity. And we can learn an awful lot from how it exerts gravity. We can map where dark matter is found by how it affects the rotation of galaxies, and how it drives the orbits of galaxies I side galaxy clusters, and by the way it bends light around galaxies and clusters. These tell us something really important: the dark matter is far more spread out, more diffuse than almost all of the visible matter. And that tells us a lot about any prospective dark matter particle. For one thing, dark matter doesn't tend to interact with itself - at least not very much. If it did, then giant region of dark matter would lose energy in those collisions and collapse. They might into dark matter galaxies or dark matter stars or dark matter people. But no, dark matter seems to stay puffed up in gigantic halos surrounding the much more concentrated clumps of visible matter. Infact galaxies are really just shiny dusting of stars, sprinkled deep in the gravitational wells of massive reservoirs of dark matter. But the fact that dark matter particle forms those giant halos at all tells us something very important. It gives dark matter a temperature. More accurately, it tells us how far dark matter particles were able to travel in the early universe. This 'free streaming length' of dark matter is how far a dark matter particle could travel before interacting with something - typically another such particle. In the early universe, that distance influenced the size of the seed structure which galaxies would later formed from. Now, how that  structure did end up forming, It seems likely that dark matter was moving pretty slowly. We refer to such dark matter as 'cold'. So let's review, if dark matter is particle, it's electrically neutral and doesn't interact much with itself  and it's relatively slow moving, also insanely abundant. From a long time people thought the neutrino might be the dark matter, being electrically neutral and the most abundant known particle in the universe. But the neutrinos of the Standard Model move very fast and  they are hot & there is just enough in neutrinos to do the job because they are ridiculously light. There is nothing else in the  

Standard Model that works- which sounds annoying, but actually physisits get very excited, because discovering a dark matter particle may be our best for finding a bigger and deeper theory than the Standard Model. It would also be a no-brainer Nobel prize and many researchers have devoted their lives to hunting down this particle. One type searchers for new evidence out there in the universe or in our particle experiments here on earth for evidence of particles that do not fit the standard model. The other delves deep into the theory - in the speculative mathematics beyond the standard model for signs of new particles. Today we are going to focus on the theoretical prospects because we might as well as have some fun before those pesky observations ruin everything with their facts. Actually we don't have to go beyond the standard model to find out our first dark particle candidate. Completely independent of our quest for our dark matter, physisits have hypothesized  a new type of neutrino called sterile neutrino. In short, as ghostly as neutrinos are, sterile neutrinos are far ghostlier. They don't even interact by even weak force, which means nthey are almost impossible to detect. There are some exceedingly clever experiments to do so. If sterile neutrinos exist and are massive and slow-moving enough, they are a great dark matter candidate. Another dark matter candidate is axion. This is a weird little particle that popped up in the maths when the physisits were trying to solve another mystery of physics-the so called  CB Problem. Axions, if they exist, would may be incredibly light, maybe 1% or less the mass of the already-puny neutrino. So, to account for the dark matter they need to exist in prodigious numbers..... but according to pro-axion physisits that we'll may be the case. Explorations of the theoretical landscape has led physisits to multiple possibilities for dark matter particles. Supersymmetry is an extension of the Standard Model which proposes that all the regular particles both matter and force-carrying - have twins - counterparts on the opposite side of the table. Every matter particle or fermion has a supersymmetric force-carryier or boson. And every boson has its fermion twin. It is expected that these supersymmetric particles are much heavier than the standard model counterparts and that may explain why have not we seen in our particle accelerators- perhaps we have just not produced enough energy to make one yet. But they may have been produced in the insanely energetic early universe and the leftovers from that time could still be throwing their weight around so to speak. The simplest kind of dark matter we get from supersymmetry is called a 'neutralino.' It is sort of three in one particle where the electrically neutral superpartners of Z boson, photons and Higgs particle mix together. In some models, these are the lightest supersymmetric particles possibly 'LSPs.'  but they are still incredibly heavy. And while normally heavy things tend to decay to lighter things, if they can't decay into Standard Model particles then they'ld be stable and long-lived an almost perfect dark matter particle candidate. There are other dark matter candidates in different flavours of supersymmetry - all of them LSPs, for eg., the counterparts of the neutrino or gravitation. The expected mass of these particles is eerily close to the mass expected for a certain type of dark matter which some would say is a point in favor of supersymmetry. This seeming coincidence is sometimes called 'WIMP miracle'. But for that to make a sense I will explain what a WIMP miracle is. Supersymmetric dark matter particle like the neutralino are examples of a general dark matter particle type is matter particle type called the WIMP or weakly interacting massive particles. The idea of the WIMP was proposed independently of any actual WIMP candidates. It's description of what some physisits thought dark matter particles had to be like which is to say weakly interacting and massive. The massive part is obvious enough- it helps if you want to make up 60% of the mass in the universe and also slows down the particle  - helps make it 'cold'. We also covered weakly interacting - it helps dark matter halos stay puffed up. But it also turns out that the interaction strength of dark matter is extremely important - it may have governed how every interesting thing in our universe first formed. In the first fractions of a second after the Big Bang, particles and their antimatter counterparts would have been popping into existence constantly, borrowing energy from the crazy radiation of that time. And then when the particle bumps into its antiparticle, they both annihilate, releasing that energy again. As the universe cooled and energy droppled, that process ceased. We were left with a universe full of particle anti-particle pairs that would then just annihilate over time. But it's possible some particles may not have been able to find an anti-particle counterpart before the expanding universe pulled them too far apart. Things like electron and antielectron or positrons, interacts very strongly via the electromagnetic force which means they find each other very easily. The universe didn't expand fast enough to throw these particles apart, and so these particles almost annihilated. But a WIMP, with its extremely weak interaction, would more easily doudge it's anti-matter buddy and so countless may have survived to this day. So, it turns out you can do a calculation of what interaction strength such a  relic particle would need to have in order to survive in sufficient number to give us dark matter. And that interaction strength is about the same as the weak interaction.

Thank you. 

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