What is Dark Matter?
Dark Matter is a new type of matter that was theorized in the early 1900. Its two principal characteristics are that it’s non luminous, which means that it doesn’t emit or reflect light and so cannot be directly observed like stars or galaxies, and that it interacts only very weakly with normal matter, which means that it’s very hard to detect.
According to the cosmological standard model, called the ΛCDM model (Figure 1), which describes the entire universe, the universe is made of three different components: Normal Matter, which encompasses all the particles that we know and that have been discovered, Dark Matter, which is a theorized non luminous matter which would clump in halos around galaxies but hasn’t been detected yet and Dark Energy, which is an unknown form of energy which is responsible for the expansion of the universe.
As it turns out, normal matter (all that we know) makes up for only ~5% of the mass of the universe, which means that the vast majority of the universe is made of something else, something that is still unknown. Out of this remaining 95%, ~25% is thought to be made of dark matter while the last ~70% make up the mysterious dark energy.
How do we Know Dark Matter Exists?
Proof of the existence of dark matter comes from many different astronomical and astrophysical observations, where observed effects and phenomena can only be explained if taking into account the presence of dark matter.
The first conclusive evidence for the existence of dark matter was garnered in the 1970s by analyzing the rotation curves of spiral galaxies. The rotation curve of a galaxy represents the dependence between the velocity of stars and objects in a galaxy with respect to their distance from the center of said galaxy. By following Newtonian dynamics and because most of the luminous mass is concentrated in the galactic bulge, one would expect the orbital velocity of stars, which is a measure of how fast they orbit around the galaxy, to decrease with increasing distance from the galactic center.
However, results obtained from studying stars at different distances from the galactic center demonstrated that this was not the case. Instead, the orbital velocity of stars stays constant with increasing distance from the galactic center, implying that most of the mass is not contained within the galactic bulge. This discrepancy can be solved by adding a uniform massive halo around the galaxy, as shown on figure 2. Because this halo has never been directly observed but is inferred from such observations, it cannot be made of normal matter but must be made of a new kind of non-luminous matter, dark matter.
Another source of evidence comes from gravitational lensing; a bending of the light emitted by a source object when passing through a ‘lens’, the gravitational field created by a massive object located between the source and the observer. Because the strength of the gravitational field is determined by the mass of the ‘lens’ object, observing such a lens gives information on the mass distribution around such an object.
Gravitational rings (figure 4) are a clear manifestation of this phenomenon. For example, the left panel of figure 4 shows the foreground galaxy (the lens) in the center with the bent light (rings) coming from the background galaxy (the source). The right panel is the residual light coming from the source after subtracting the light from the lens. This residual ring from the source clearly extends well beyond the luminous mass from the foreground galaxy, which implies that a lot more non luminous mass is present around that galaxy, confirming that the mass of galaxies is dominated by dark matter.
Perhaps the most compelling evidence for the existence of the dark matter is the Bullet Cluster (figure 5), a system made of two clusters of galaxies which have collided in the past, where visible matter and dark matter are spatially located at different points.A cluster of galaxy is made of cold point-like objects such as stars and galaxies, and of hot spread-out gas called plasma, both symmetrically distributed around the center. In addition, the plasma carries most of the mass of the cluster.
In the collision of two clusters, only the two plasma collide and are slowed by ram pressure. The stars and point-like object do not collide (left panel of figure 5), and so decouple from the plasma (right panel of figure 5). This creates a very peculiar system where gas and point-like objects are situated at different locations in the cluster.
Because most of the mass is contained within the hot plasma, gravitational contours of such a system are expected to match the plasma. However, when actually doing such a map, the gravitational contours (green lines on figure 5) match the point-like objects instead. This means that most of the mass of the cluster has not collided and has actually followed the stars and galaxies, once again indicating the clear presence of dark matter.
How do we know in what quantity dark matter is present in the universe?
Measurements of the dark matter density have been accomplished by studying anisotropies of the cosmic microwave background (CMB), the thermal radiation left over from the early universe, using satellites like WMAP and PLANCK.
The CMB is a stable radiation (photons) at an average temperature of 2.7K found throughout the universe. It comes from the time of photon decoupling, when the universe was roughly 380,000 years old. At this time, the universe went from opaque to transparent and photons, which were previously coupled with electrons and baryons, after a last scattering, become free and have been roaming the universe ever since. In addition at the time of photon decoupling, the universe is dominated by dark matter which is clustered in gravitational wells. At the time of last scattering, photons and baryons are more or less trapped in these gravitational wells and need more or less energy to get out of them. This gives rise to tiny difference in time at which photons underwent their last scattering.
These tiny differences in time are observed as tiny temperature fluctuations of the CMB (figure 6), which is otherwise a nearly perfect blackbody with temperature fluctuatuions smaller than 0.0001.
Because of the coupling between photon and baryons prior to photon decoupling and last scattering, studying these tiny fluctuations allows to accurately measure the baryon density in the universe and by extension, the dark matter density.
Can we detect Dark Matter?
In one simple word: YES!! The fact that dark matter is detectable is due to a phenomenon dubbed the WIMP miracle. WIMP (Weakly Interacting Massive Particle) is a term that coins a very generic candidate for dark matter, which happens to possess all the properties that dark matter should have to match all the astronomical observations. Coincidentally the interactions which happen to give the WIMP the correct density to be dark matter, also make it detectable , a happy ‘miracle’.
When going back to the early radiation dominated era, the universe was made of a plasma in thermal equilibrium, where particles (including WIMPs) are created and destroyed at the same rate. As the universe expanded, the plasma’s energy and so temperature decreased until it reached a point where WIMPs cannot ‘meet’ anymore and so cannot be produced or destroyed, leaving a constant relic WIMP density, which is still the same today (albeit taking into account the expansion of the universe). This time in the universe is referred to as “freeze out”.
In addition, this freeze-out moment depends strongly on the WIMP’s probability to undergo an interaction (called a cross section). The higher the probability to undergo an interaction the longer it will take for the freeze-out to happen.
It so happens that when matching the measurements of the dark matter density from the CMB, the resulting WIMP cross section necessary for the freeze-out happens to be at the scale of the weak interaction, which means that it is detectable in today’s experiments.
What is Dark Matter made of?
A likely candidate for dark matter comes from a theory called Supersymmetry (SUSY).
SUSY is an extension to the Standard Model of Particle Physics (the global theory governing the world of sub-atomic physics) which is known to be incomplete and does not encompass phenomenon such as dark matter.
In SUSY, the number of elementary particles from the Standard Model is doubled, by giving to each fermion a bosonic superpartner, and to each boson, a fermionic superpartner.
The appeal of SUSY is that it unifies strong, weak and electromagnetic forces, solves the hierarchy problem and provides a natural mechanism for electroweak symmetry breaking, all major shortcomings of the Standard Model. In addition, while SUSY was not specifically created to solve the dark matter issue, it does provide, under certain conditions, adequate dark matter candidates. Among these candidates, the most preferred is the lightest neutralino , a Majorana particle (meaning it’s its own anti-particle) expected to interact ever so slighlty with matter, and so making them directly detectable by experiments.
How do we detect Dark Matter?
There are three ways to detect dark matter, directly, indirectly and in colliders. These three detection techniques differ in what is actually detected to try and discover dark matter.
In direct detection techniques, what is actually detected is the light, charge or heat deposition coming from an actual interaction between a dark matter particle and a target material. Because of the rarity of such interactions, all direct dark matter experiments are located in underground laboratories which can provide shielding from external background radiation such as cosmic ray muons.
In indirect detection techniques, it is not the interaction of a dark matter particle with normal matter that is being detected but rather its annihilation products, whether they are under the form of neutrinos, gamma rays, electron positron pairs, antiprotons or antinuclei. These detectors are either space based or consist of wide arrays of telescopes on earth.
Finally in colliders, dark matter is in theory created (along with a plethora of other particles) by colliding very high energy protons or heavy ions together. Its presence can be ‘detected’ through careful analysis of the transverse momentum of the created products. As of now, the only collider capable of creating and detecting dark matter is the LHC.
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