Physicists have theorized that supersymmetric dark matter particles could be the solution to the Hubble Tension. One of these supersymmetric particles could be the hypothetical gravitino. The gravitino is the supersymmetric partner of the as of yet undiscovered graviton, the hypothesized messenger particle for gravity.
The Hubble Tension and Supersymmetry
A research paper recently published by a group of physicists (Gu et al.) have proposed a solution to the current Hubble Tension. In their model, Gu et al. states that the gravitino makes up a small fraction of the dark matter in the early universe and could explain the conflict found in the value of the Hubble constant found when analyzing type Ia supernova and when analyzing the speckles in the cosmic microwave background (CMB) map captured by the Planck spacecraft. Gu et al.
According to the paper published by Gu et al., gravitinos in the early universe behaved identically to neutrinos. The result is that this increases what cosmologists call effective neutrino density which in turn alters the value of the Hubble constant measured from the CMB. The data from WMAP, the Atacama Cosmology Telescope and the Hubble Space Telescope backs up this claim. The data shows a higher number of neutrinos in the universe than what has been predicted.
Gravitinos making up part of dark matter poses problems. For the model to work, Gu et al. has to deal with these problems. Luckily, most of these problems disappear if gravitinos are not too massive and come into existence in certain ways.
There are many ideas describing how symmetry breaking occurs between standard particles and their supersymmetric counterparts. Some of these models predict that the gravitino is the lightest supersymmetric particle. In such models, the gravitino has a mass between 1 electron volt (eV) to 1 GeV. This is a problem for Gu et al.’s hypothesis to explain the Hubble Tension.
If gravitinos had a mass above 1 KeV and the majority of them where created during the reheating phase after cosmic inflation, this extra mass would cause the mass-energy density of the universe to become too high. The result of this is the universe collapses in on it’s self before reaching it’s present age. This is know as over-closing the universe.
Balancing matter and energy
One way to overcome this problem is to propose that the reheat temperature after cosmic inflation was lower then expected. This means a smaller amount of particles created after the universe’s inflation phase. This would result in lowering the density matter-energy density of the universe and offset the effect of the gravitinos. The downside to this is that leptogensis which is believed to cause the imbalance in matter and anti-matter would not work in the why that conforms to observation.
To get around this problem, Gu et al. point one that supersymmetric models predict messenger particles that can decay. What this decay occurs, it increases the entropy in the universe reducing the effects of gravitinos on the universe if their mass is above 1 KeV.
The average distance in which gravitino can travel before interacting with other particles has to be considered as well. If the free-streaming length of the gravitino is very too long, large scale structures such as filaments and galaxy super clusters cannot found the way they did.
The bino and it’s decay
Gu et al. also proposed that a fraction of the gravitinos could be created later in formation of the universe. This happens in the decay of another supersymmetric particle known as the bino. The bino is the supersymmetric partner of the messanger particle of the weak hypercharge.
The bino is next lightest supersymmetric particle after the gravitino in some supersymmetric models. The advantage of generating gravitinos through bino decay is that they can have a larger mass without over-closing the universe. Gu et al. proposes that gravitinos created by this process is the solution to the Hubble Tension.
For this hypothesis to work, binos have to decay early before the majority of atomic nuclei have been formed. This is to prevent photons produced by bino decay from destroying atomic nuclei. Such a destruction of atomic nuclei will producing an effect on the abundances of elements that contradicts observations hence the constraints placed by Gu et al.
Testing the idea at the Large Hadron Collider
Gu et al. as well as other physicists propose that evidence for the gravitino could be found at the Large Handron Collider (LHC). They propose that we can find the evidence by looking at the Drell-Yan processes happening at the LHC.
Drell-Yan processes are when a quark and it’s anti-matter counterpart annihilate to create a photon or a Z boson. The photon or Z boson then splits to created a lepton and anti-lepton pair. According to Gu et al., some of these reactions create a slepton (sleptons are supersymmetric partners to leptons) and it’s anti-matter counterpart which then turn into a regular lepton and anti-lepton pair.
This lepton and anti-lepton pair will have a total energy is less than the one in the original quark and anti-quark pair. According to Gu et al., the gravitinos produced by the reaction will carry this missing energy.
The supersymmetric model used in this hypothesis is unique when compared to other supersymmtric models. In most of these supersymmetric models, the lightest supersymmetric particles are neutralinos such as the bino not the gravitino.
Therefore they are the most popular supersymmetric dark matter candidate. For this model to work, the energies at which supersymmetric breaking occurs has to be lower than want other models predict. That means that the gravitino ‘s mass must very light which does resolve problems with the model discussed previously.
Proving such a hypothesis right would be very interesting as not only will it prove supersymmetry and the Hubble Tension. It would also help resolve the discussion between theories that postulate gravitons such as string theory and theories such as loop quantum gravity. The logic behind this that the existence of gravitinos in my opinion strongly indicate the existence of gravitons.
Regarding the composition of dark matter, I thing dark matter is not made of one type of particle. Instead we will it to be a set of different particles including gravitinos, neutralinos and axios. It could also be that they are make of a set of particles that have never been predicted. This may help explain the mixed behavior of dark matter when observing large scale structures like the filaments made of galactic clusters and small scale structures such as the matter distribution within dwarf galaxies.