DARK MATTER AND DARK ENERGY

By Armaan Gupta, Independent Researcher and Undergraduate at BITS Pilani

Immediately after the Big Bang, universe expanded or ballooned rapidly in a split-second era called inflation. The universe continued its expansion for a couple of a billion years or so, creating the early universe- galaxies, clouds and many more things. But then the gravitational force (weakest of the four fundamental forces, namely gravitational, electromagnetic, strong nuclear, and weak nuclear) somehow managed to decrease the rate of rapidly expanding universe and began to slow it down. But as the universe got larger and normal matter got more diluted, something mysterious started happening and it again started the expansion of universe and till present, universe is expanding. Scientists called this mysterious force “dark energy”. Eleven billion years ago, dark energy made up less than 10 percent of the total content of the universe; today it makes up almost three-quarters.

DARK MATTER AND DARK ENERGY

DARK MATTER

Everything we see in our universe, ultimately is made from different combination of same fundamental particles. There are generally two types of particles: matter particles, also known as fermions. Fermions include all quarks and leptons, as well as any composite particle made of an odd number of these. They follow Pauli Exclusion Principle. They takes up space. And Force particles, also known as Bosons. They follow Bose–Einstein statistics. They can pile on top of each other.

Ordinary matter is made up of fermions, held together by bosons.

There are twelve types of known fermions. Some of them are electron, muon, tau, up quark, down quark etc.

Every type of force is associated with some bosons: For electromagnetism, there is photon, for gravitational force, there is gravitons, for higgs force, and there is higgs bosons, for weak interactions- W & Z bosons, for strong interactions- 8 gluons. Together these bosons and fermions comprise standard model of particle

physics. We can’t see all types of matter through our naked eyes. Some of them are invisible and transparent. To detect these, gravitational force is used. Since every matter cause gravity and gravity attracts objects towards it including light, we can infer the existence of matter even without actually seeing it. Using standard model of physics we find that there is much more matter in the universe.

Bosons & Fermions

That can be accounted for by ordinary matter such as, stars, gases, dusts, planets, galaxies. So, concept of dark matter is arisen here. Dark matter isn’t like the normal matter, it is comprised (maybe) entirely of different fundamental particles, somewhat heavier than fermions and bosons, but yet stable in nature.

Galaxies are surrounded by much larger envelop called dark matter.

DARK ENERGY

If we examine several of faint, fuzzy objects in universe called "galaxies” or standard candles. Called ‘nebulae’, we would see that light given off by the galaxies would be redder than it should have been. Using Doppler shift of light, when an object's light looked too red, that means it is moving away from Earth. Calculating the galaxies’ speeds from the redness of their light, we can see that they are all moving away from us incredibly quickly, for example ,the Sombrero galaxy moves away at 2.5 million miles per hour away from earth. Not considering ourselves too special to be situated at the center of the universe, the only explanation fits into this- our universe is expanding. The farther away a galaxy is, the faster it moves away from us.

So the concept of dark energy is arisen here, taking blame for accelerating the expansion of universe. Dark energy is smoothly distributed in space- it doesn’t fall into galaxies or clusters, with no distant gap in between. According to Einstein, it is possible for more space to come into existence. The version of Einstein's gravity theory, that contains a cosmological constant, says that: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy of space would appear. Dark energy is a natural property of empty

space: The more the universe expands, the stronger dark energy becomes. As a result, this form of energy would cause the Universe to expand faster and faster. However, the existence of Einstein’s cosmological constant is highly debated. Dark energy has constant or slowly changing density. It can only be detected by gravity.

Behavior of universe over time

Other theories state that dark energy is a temporary phenomenon like inflation, and that matter’s gravitational pull will one day take over(as it was dominant in very early universe) and temper the universe’s growth spurt. While some physicists suggest that dark energy will cause runaway expansion, perhaps to the point that in several billion years it will pull apart galaxies, stars, planets, and even atoms in a doomsday scenario called the Big Rip. We aren’t sure of any one of the existing theory and the research is still going on. Based on various experiments, it can be seen that universe consists of - ~68% dark energy, ~27% dark matter, ~5% normal matter. Dark matter’s influence shows up even in individual galaxies, while dark energy acts only on the scale of the entire universe. We can say that dark matter is holding our universe together and stabilizing it whereas, dark energy is making our universe expand at amazing speed and destabilizing it. Since, dark energy is much more than dark matter in our universe, the war is won by dark energy and thus, our universe expands.

Existence of dark matter can also be proofed by using the famous Galaxy rotation curve. It is the rotation curve of a disc galaxy (also called a velocity curve) is the ratio of the rotational velocity of visible stars or gas in that galaxy to their radial

distance from that galaxy's center, which can be showed graphically by plotting the orbital speed (in km/s) of the stars or gas in the galaxy on the ordinate against the distance from the center of the galaxy on the abscissa. If we observe the curve carefully, we can see that rotational velocity of stars and gas is constant as far from the galactic center as can be measured. Stars are observed to revolve around the center of these galaxies at a constant speed over a large range of distances from the center of any galaxy. But if we make the mass distribution of disc galaxies similar to that of the observed distribution of mass in galaxies, the rotation curves velocities should decline at large distances in the same way as that of any other system with concentrated mass in the center (assuming that most of the mass of any galaxy lies at its center.). But it isn’t so in the galaxy rotation curve. The discrepancy can be accounted for by a large amount of dark matter that permeates the galaxy and extends into the galaxy's halo.

Galaxy rotation curve

Gravitational lensing: Many a times, we’ll find a cluster of galaxies in front of another galaxy. When this happens, the background galaxy can be lensed by the mass of the foreground cluster, the same way an optical lens can bend light. As a result, galaxy appears as distorted arcs and sometimes there are multiple images of the same background galaxy. They also allow us to measure how much mass is in the foreground cluster. Let us make it simple: suppose light leaves a young star

forming blue galaxy near the edge of the visible universe and it comes toward us. Some of the light passes through large clusters of galaxies and surrounding dark matter, which are directly in the line of sight between earth and the distant galaxy. The dark matters’ gravity acts like a lens, bending the incoming light. Most of this light is scattered, but some of it is focused and directed towards the earth. Observers see multiple, distorted images of the background galaxy.

There has been some speculation about what happened to the missing mass- couldn’t it be the energy that surrounds us simply, assuming that the mass and energy are inter- convertible? Unfortunately isn’t true. The floating energy around us is certainly taken into account while considering dark matter in first place and in fact it makes the dominant contribution to the energy density of the universe at early times. Since universe is about 13.7 billion years old, density of most of the concentrated energy would have dropped to a very low level as energy density drops faster than that of ordinary matter. It simply constitutes the 2.7 degree K cosmic background radiation. Furthermore, because it consists totally of photons, i.e. it moves at the speed of light it cannot be clustered in the gravitational potential of a galaxy, which cannot trap particle traveling faster than about 300 km/sec. For that we indeed need particles with non-zero inertial mass.

If we consider String theory, which implies up to 11 dimensions, there is possibility that dark matter may be the gravitons leaking from other dimensions into our dimension. This idea actually comes out of non-perturbative string theory, called M-theory. In M-Theory, strings are not the only ingredient. Rather, multi-dimensional strings, called branes, are fundamental along with ordinary one dimensional strings.

Multiple (and distorted) images of the same galaxy is formed as seen from earth

M-Theory also explains string tension, the variable that determines string oscillations and hence particle values, can be explained in terms of how tightly the branes hold the ends of open strings, the strings that make up everything except gravity. Gravity, in string theory, is represented by a closed loop string. Since a brane cannot hold onto a closed loop, since it has no ends, it will occasionally float off of the brane. So it does make the sense that dark matter may actually be gravity leaking from other dimensions and it may also explain that why gravity is the weakest force of the four fundamental forces in nature, considering that it leaks into other dimensions.

If gravity leaks into other dimensions, then newton’s inverse square law should break at small distances (sub-atomic level), so the gravity will be stronger there.

Vacuum Energy:

Vacuum energy is an underlying background energy that exists in space throughout the entire Universe. Vacuum energy may originate from virtual particles supposedly particle pairs that blink into existence and then annihilate in

a timespan too short to observe. These particle pairs are present everywhere in every vacuum throughout the universe. Since universe is so immense that a large part of it can be stated as vacuum. If calculated, the total amount of vacuum energy comes out to be 10^120 times too big. Virtual particles follow the perturbation theory of quantum field theory where interactions between ordinary particles are described in terms of exchanges of virtual particles. Any process involving virtual particles admits a schematic representation known as a Feynman diagram, in which virtual particles are represented by internal lines. There is a very thin line between real particles and virtual particles. Virtual particles need not carry the same mass as the corresponding real particle, but they always conserve energy and momentum.

Vacuum Energy

The longer any virtual particle exists, the closer its characteristics come to those of ordinary particles. Electromagnetic repulsion or attraction between two particles can be thought of as due to the exchange of many virtual photons between the charges. Real particles are better understood to be excitations of the underlying quantum fields. Virtual particles are also excitations of the underlying

fields although for a temporary period. Virtual particles are created in pairs; one virtual particle is made of particle (like an electron), and the other is antiparticle (like a positron.).

There is one more occurrence of virtual particles. According to Stephen Hawking, it's possible for virtual particles to be created just outside the surface of a black hole, with one of the two particles falling into the black hole, and the other escaping. This process takes matter from the black hole, and allows it to leave in the form of the escaping particles. If one of the pair is pulled into the black hole then the other particle becomes "real" and energy or mass is essentially radiated into space from the black hole It appears that smaller black holes radiate more than large black holes, thus becoming more and smaller and radiating even more matter and eventually exhausting all of its mass and die. The emitted matter is also called hawking radiation.

DARK MATTER CANDIDATES:

The two main categories of objects that scientists consider as possibilities for dark matter include MACHOs (MAssive Compact Halo Objects), and WIMPs (Weakly Interacting Massive Particles), neutrinos and axions.

Neutrinos were first "invented" by physicists in the early 20th century to help make particle physics interactions work properly. They were later discovered and they, when investigated, were found to be massless, except one. Neutrinos are weakly interacting particles which are assumed to be created during the Big Bang. They are present everywhere in galaxies, constantly travelling through the entire galaxies, even though the earth also. They largely go undetected as they can only be sensed through very small electro-weak interactions. For neutrinos to be able to account for dark matter, their mass have to be increased by several electron volts, considering that the neutrinos discovered in particle accelerators, experiments around the world were much lighter.so, neutrinos are unlikely candidate for dark matter as of now.

It is also possible that some as yet undiscovered weakly interacting particle were created during the big bang and today remain in large enough abundance to make the dark matter. The masses typically required for these particles to be considered as dark matter are in the range 1 GeV–1 TeV, and these hypothetical dark matter particles are called WIMPs. Since, exact nature of particles making up

dark matter isn’t known, there are hundreds of elementary particles that fall into this class of dark matter particles, including super-symmetric particles such as neutralinos, photinos, higgsinos or sneutrinos, and new heavy neutrinos.

Role of gravity among Ordinary matter, dark matter, dark energy

Weakly interacting massive particle (WIMP): They are electromagnetically neutral subatomic particle that is hypothesized to make up most dark matter and therefore some 27% of the universe. These particles are assumed to be heavy and slow moving because if the dark matter particles were light and fast moving, they would not have come together in the density fluctuations from which galaxies and clusters of galaxies formed. Since, light isn’t reflected by these, indicating that they are electromagnetically neutral. WIMPs are assumed to be “nonbaryonic,” or something other than baryons because the amount of baryons in the universe has been determined by measuring the abundance of elements heavier than

hydrogen that were created in the first few minutes after the big bang. The precise nature of these particles is not currently known, and they are not predicted by the standard model of particle physics, but some extensions of standard model of physics, such as theory of super-symmetry predict certain hypothetical elementary particles—such as axions, neutralinos, and Kaluza-Klein particles—that may be the undetected WIMPs. WIMPs interacts only through the weak nuclear force and gravity, so they are very hard to detect. Roughly saying, there are two types of dark matter present in our universe, Cold dark matter-which produce galaxy distributions that are roughly similar to that which is observed, and Hot dark matter- which would smear out the large-scale structure of galaxies and thus is not considered a viable cosmological model.

Before considering anything, we have to choose whether the in question dark matter were created thermally, candidates being WIMPs or non-thermally, candidates being axions. Now, assuming that the WIMP fits the model of a relic dark matter particle from the early Universe. For sufficiently high temperature, such as existed in early universe, when all particles were in state of thermal equilibrium, considering dark matter were abundant then, the dark matter particle and its antiparticle would have been both forming from and annihilating into lighter particles, but as our universe expanded and became cooler, average thermal energy of these lighter particles decreased and eventually became insufficient to form a dark matter particle-antiparticle pair.

Expansion of universe over time

The annihilation of the dark matter particle-antiparticle pairs, which had already formed from lighter particles, however, would have continued, and the number density of dark matter particles would have begun to decrease exponentially. Then after some time, number density would become so low that the dark matter particle and antiparticle interaction would cease, and the number of dark matter particles would remain constant as the Universe continued to expand. Particles with a larger interaction cross section would continue to annihilate for a longer period of time, and thus would have a smaller number density when the annihilation interaction ceases.

WIMP

Based on the current estimated abundance of dark matter in the Universe, if the dark matter particle is such a relic particle, the interaction cross section governing the particle-antiparticle annihilation can be no larger than the cross section for the weak interaction. If this model is correct, the dark matter particle would have the properties of the WIMP. Since, thermally created dark matter has weak-scale interactions also means that it may be within reach of accelerator experiments such as LEP and LHC at CERN, and the proton collider experiments at Fermilab.

Supersymmetry: Supersymmetry is a conjectured symmetry of space and time. The theory states that every type of particle has one or more superpartners — other types of particles that share many of the same properties, but differ in a crucial way. If a particle is a fermion, its super-partner is a boson. If a particle is a boson, its super-partner is a fermion. Bosonic ordinary particles have fermonic superpartners with the same name except with the suffix ‘no’ added, while fermonic ordinary particles have Bosonic (scalar) Superpartner names with

prefix‘s’ added.

Supersymmetry

Some supersymmetric particles have the same quantum numbers as each other and therefore can mix together producing particles that are not exact partners of any standard model particle. Supersymmetry postulates that every particle we observe has a massive shadow particle partner.

If we want Supersymmetry theory to explain dark matter, we have to exclude hot dark matter in this discussion for the reasons mentioned above. Considering only cold dark matter, One source would be a superpartner such as a neutralino, and in this case it would need to have a mass in the range that would be accessible to the LHC — and again assuming that all of its superpartners have masses in the same range, the explanation would be that, Supersymmetry does explain cold dark matter. But we come to this conclusion only after assuming that all super partners of neutralinos have masses in the same range, which may not be true. So the debate still continues.

It is thought that, the lightest supersymmetric particle (LSP) is stable, and since supersymmetric particles have electroweak-strength interactions, they can be

considered as dark matter candidate. Considering that neutralinos are LSP, then there are many possible supersymmetric models and many free parameters in the models, so precise predictions of supersymmetric relic abundance and supersymmetric particle detectability are not possible. Typically experiments attempt to probe a range of model parameters. So far no concrete evidence of any supersymmetric partner exists. If even one supersymmetric partner is found, the theory predicts that they all must exist.

If we could detect WIMPs, then our life would have been a lot easier. We could detect WIMPs coming from our Galactic halo as they move past and through the Earth. Then we could measure the local density of dark matter and say for sure that the dark matter is non-baryonic cold dark matter. We can do direct detection of the WIMP particles in the laboratory. As we roughly know the speed that is more or less near to 220 km/s and the density, we can say that for a WIMP of mass of order 10–100 GeV, roughly 100 000 dark matter particles per second pass through every square centimeter of the Earth. However, even if WIMPs exist, they are very weakly interacting particles, so it is very rare that any one of them will interact with earthly matter at all; most of them pass right through the Earth undisturbed. In addition, if a WIMP does elastically scatter off a nucleus or two, the deposited energy is usually in the keV to 100 keV range, too small to be noticed by any laboratory equipment except the highly sensitive ones. The difficulty lies in the fact that WIMP events are rare and that there are many backgrounds that deposit similar amounts of energy on much more frequent time-scales. So we have to operate these experiments deep underground where Ionizing cosmic rays are less frequent, and typically operate their detectors at extremely cold temperatures to keep thermal excitations low. Also many types of shielding, as well as redundant detection methods, are now becoming standard.

The method is to detect the small energy deposited when a WIMP scatters off a nucleus in some well instrumented piece of material. When a WIMP scatters off a nucleus in a crystal, the nucleus recoils, causing dislocation in the crystal structure, vibrations of the crystal lattice and also ionization. These signals can be detected. For example, we can use kilogram size germanium and silicon crystals or sodium iodide crystals and detect the ionization and phonon signals or the ionized electrons, study their vibrations and detect WIMPs, if any. We have to be careful about radioactive signals present in surroundings as even tiny amounts of

radioactivity in the detector or shielding can swamp the expected signal. With effort, a background rate of under one event per kilogram of detector per day can be achieved.

Massive Compact Halo Object (MACHO):

An important difference between WIMPs and MACHOs is that WIMPs are non-baryonic and MACHOS are typically formed from baryonic material.

Microlensing: Microlensing occurs due to gravitational lensing effect. It can be used to detect objects ranging from the mass of a planet to the mass of a star, regardless of the light they emit. Earlier, scientists couldn’t find any object which was too faint, or didn’t emit any light but now, due to Microlensing nearly any object which emits electromagnetic radiation, no matter how faint can be detected. Microlensing is caused by the same physical effect as strong lensing and weak lensing, but it is studied using different observational techniques. In strong and weak lensing, the lens used is quite massive such as a galaxy or galaxy cluster and the displacement of light by the lens can be resolved with a high resolution telescope such as the Hubble Space Telescope.

Effect of MACHO

But with Microlensing, the lens used is a planet or a star, (very less mass as compared to galaxy) so, the displacement of light isn’t observed easily, but the apparent brightening of the source may still be detected. In such a situation, the lens will pass by the source in a reasonable amount of time, seconds to years instead of millions of years. As the alignment changes, the source's apparent

brightness changes, and this can be monitored to detect and study the event. Thus, unlike with strong and weak gravitational lenses, a Microlensing event is a transient phenomenon from a human timescale perspective.

Microlensing

The idea of Microlensing rests on Einstein’s observation that, if a massive object lies directly on the line of sight to a much more distant star, the light from the star will be lensed and form a ring around the lens. The ring is called the ‘Einstein ring’ and it sets the scale for all the Microlensing searches.

Dark matter may be Baryonic which are composed of baryons, i.e. protons and neutrons (even electrons, though they are made up of leptons) and combinations of these, such as non-emitting ordinary atoms. Baryonic matter include Clouds of cold gas, Planets, Comets and asteroids, Stars, Neutron stars, Black holes. While the matter from which black holes form is mainly baryonic matter, once swallowed by the black hole, this distinction is lost. For example, a theoretical black hole constructed purely out of photons (which are bosons and clearly not baryons) is indistinguishable from one made from normal baryonic matter. This is

often referred to as the ‘black holes have no hair’ theorem which simply states that black holes do not have properties such as baryonic or non-baryonic. A small proportion of dark matter may be baryonic dark matter such as massive compact halo objects, that are composed of ordinary matter but which emit little or no electromagnetic radiation. Study of nucleosynthesis in the Big Bang produces an upper bound on the amount of baryonic matter in the universe, which indicates that the vast majority of dark matter in the universe cannot be baryons, and thus does not form atoms. It also cannot interact with ordinary matter via electromagnetic forces; in particular, dark matter particles do not carry any electric charge.

Baryonic dark matter candidates

Nonbaryonic dark matter are hypothetical particles such as axions, or supersymmetric particles and neutrinos, but neutrinos can only form a small fraction of the dark matter, as they are limited by large-scale structure and high-redshift galaxies. In contrast to baryonic dark matter, nonbaryonic dark matter does not form any of the elements in the early universe and so its presence is revealed only via its gravitational attraction. Additionally, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions

with themselves, possibly producing observable products such as gamma rays and neutrinos.

We can classify Nonbaryonic dark matter in terms of the mass of the particle(s) that it is assumed to makeup of, and/or the typical velocity dispersion of those particles since, more massive particles move more slowly. There are possibly three types of prominent nonbaryonic dark matter, called Cold dark matter (CDM), Warm dark matter (WDM), and Hot dark matter (HDM); even some combination of these is also possible. Out of these, only cold dark matter is most widely and the corresponding particle is most commonly assumed to be a weakly interacting massive particle (WIMP). Hot dark matter may include (massive) neutrinos, but observations imply that only a small fraction of dark matter can be hot.

Non-Baryonic dark matter candidates

Cold dark matter leads to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario; hot dark matter has been ruled out by observations of high-redshift galaxies such as the Hubble Ultra-Deep Field.

Using the Microlensing, we can be sure whether, the dark matter in our universe is baryonic or not. The reason is that the Microlensing searches are probably sensitive to any objects in the range 10^–8Mo<m<10^3Mo., just the range in which such objects are theoretically allowed to exist. Objects made purely of H and He with masses less than ~(10^–9 to 10^–7Mo.) are expected to evaporate owing to the microwave background in less than present age of the universe, while dark matter objects with masses greater than ~10^3Mo would have disrupted known globular clusters. So unlike the searches for WIMP dark matter, which if they detect nothing will remain inconclusive, if the MACHO searches find nothing, we would at least know what the dark matter is not.

In order to interpret Microlensing events, the efficiency with which an experiment can detect Microlensing is needed. Bad weather, bad seeing, crowded stars, etc. cause Microlensing events to be missed by the experiment. The MACHO collaboration finds an efficiency of around 30% for Microlensing events that last 50 days. For events lasting less than a few days and events lasting longer than 6 months the efficiency is very low.

Some time ago, a modified law of gravity correctly predicted, in advance of the observations, the velocity dispersion -- the average speed of stars within a galaxy relative to each other -- in 10 dwarf satellite galaxies of the Milky Way's giant neighbor Andromeda. The relatively large velocity dispersions observed in these types of dwarf galaxies is usually attributed to dark matter. Yet predictions made using the alternative hypothesis Modified Newtonian Dynamics (MOND) succeeded in anticipating the observations. The researchers tested MOND on quasi-spherical, very low-surface brightness galaxies that are satellites of Andromeda. In the cosmic scale, they are among the smallest galaxies, containing only a few hundred thousand stars. But with conventional gravity, they are inferred to contain huge amounts of dark matter. We also need to understand that gravity is many orders of magnitude weaker than the other three fundamental forces of nature. One explanation is that gravity leaks into extra dimensions, leaving us to feel a mere fraction of its attractive force. Most theories predict that these dimensions would be no larger than a millimeter or so. But some say extra dimensions could be non-compact, meaning that odd gravitational effects would occur at astronomical distances. In the past, such long-range effects have also been put forward to explain the odd trajectories of NASA’s disused

Pioneer 10 and 11 probes, which appear to be being affected by a small additional force as they leave the Solar System.

So, we come to conclusion that dark matter and dark energy do exist, we know how much they account for total contents of our universe, we also know that how they affect some phenomena in universe, but we don’t know what exactly they are, what they are made up of and how they are going to behave. We know, where to look for them and some evolving techniques such as Microlensing are a great tool to understand and play with dark matter and dark energy.

Dark Matter vs. Dark Energy

Dark Matter vs. Dark Energy

Some may wonder, whether scientists had to wait for a billions of years or so, to get a complete overview of our universe and the movements of stars. It isn’t true.as technically saying we look into past while looking at stars and universe. The further we look into deep space, the further we see the past, covering time over billions of years. So we may look into different places into deep space and get to know about the happening in the past from where the considered electromagnetic radiation are coming. Quite simple, isn’t it?

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