Hidden Dark Matter Research Articles

Hidden dark matter from Starobinsky inflation

The Starobinsky inflation model is one of the simplest inflation models that is consistent with the cosmic microwave background observations. In order to explain dark matter of the universe, we consider a minimal extension of the Starobinsky inflation model with introducing the dark sector which communicates with the visible sector only via the gravitational interaction. In Starobinsky inflation model, a sizable amount of dark-sector particle may be produced by the inflaton decay. Thus, a scalar, a fermion or a vector boson in the dark sector may become dark matter. We pay particular attention to the case with dark non-Abelian gauge interaction to make a dark glueball a dark matter candidate. In the minimal setup, we show that it is difficult to explain the observed dark matter abundance without conflicting observational constraints on the coldness and the self-interaction of dark matter. We propose scenarios in which the dark glueball, as well as other dark-sector particles, from the inflaton decay become viable dark matter candidates. We also discuss possibilities to test such scenarios.

An Alternative Model of Rotation Curve that Explains Anomalous Orbital Velocity, Mass Discrepancy and Structure of Some Galaxies

A new model of galaxy rotation based on the cyclostrophic model of vortices found in nature is developed. The model is tested using the SPARC dataset of 175 galaxies and a smaller dataset comprising of 60 galaxies. Analysis of the datasets showed that galactic rotation can be adequately described using the observed surface brightness of galaxies and the newly developed cyclostrophic velocity model. The use of the luminosity and the inverse mass-to-light ratio in lieu of the surface brightness, also yield a very good fit of the observed and computed galaxy rotation velocity. Evidently, galactic rotation greatly depends on the cyclostrophic balance of the pressure gradient and the centrifugal forces and the seismic-induced radial expansion occurring in various stars. This is the most probable origin of the action of a single force law that has been overlooked in previous studies. Therefore, the need for a super-massive black hole at the center of galaxies or hidden dark matter can be eliminated. Attractive gravitational force can occur even without a massive black hole at the center of galaxies. There appears to be a pressure gradient force between the center and the outer parts of galaxies that sustains attraction. The cyclostrophic model appears to be the physical basis of the Tully-Fisher relation. Furthermore, the missing mass problem associated with galactic rotation can be attributed to the orbital expansion of celestial objects perturbed by seismic-induced forces. In addition, massive tremors or starquakes may create a domino effect in perturbing nearby stars along the axis of the seismic-induced force and this could result in the formation of elliptical galaxies as the orbits of seismic-perturbed neighboring stars become larger.

Dark Matter Candidates from Particle Physics and Methods of Detection

The identity of dark matter is a question of central importance in both astrophysics and particle physics. In the past, the leading particle candidates were cold and collisionless, and typically predicted missing energy signals at particle colliders. However, recent progress has greatly expanded the list of well-motivated candidates and the possible signatures of dark matter. This review begins with a brief summary of the standard model of particle physics and its outstanding problems. I then discuss several dark matter candidates motivated by these problems, including weakly interacting massive particles (WIMPs), superWIMPs, light gravitinos, hidden dark matter, sterile neutrinos, and axions. For each of these, I critically examine the particle physics motivations and present their expected production mechanisms, basic properties, and implications for direct and indirect detection, particle colliders, and astrophysical observations. Upcoming experiments will discover or exclude many of these candidates, and progress may open up an era of unprecedented synergy between studies of the largest and smallest observable length scales.

Thermal relics in hidden sectors

Dark matter may be hidden, with no standard model gauge interactions. At the same time,in WIMPless models (WIMP: weakly interacting massive particles) with hiddenmatter masses proportional to hidden gauge couplings squared, the hidden darkmatter’s thermal relic density may naturally be in the right range, preservingthe key quantitative virtue of WIMPs. We consider this possibility in detail. Wefirst determine model-independent constraints on hidden sectors from big bangnucleosynthesis and the cosmic microwave background. Contrary to conventionalwisdom, large hidden sectors are easily accommodated. A flavour-free version ofthe standard model is allowed if the hidden sector is just 30% colder than theobservable sector after reheating. Alternatively, if the hidden sector contains aone-generation version of the standard model with characteristic mass scale below1 MeV, even identical reheating temperatures are allowed. We then analyse hiddensector freeze-out in detail for a concrete model, solving the Boltzmann equationnumerically and explaining the results from both observable and hidden sector points ofview. We find that WIMPless dark matter does indeed obtain the correct relicdensity for masses in the range . The upper bound results from the requirement of perturbativity, and the lower boundassumes that the observable and hidden sectors reheat to the same temperature, and israised to the MeV scale if the hidden sector is ten times colder. WIMPless dark mattertherefore generalizes the WIMP paradigm to the largest mass range possible for viablethermal relics and provides a unified framework for exploring dark matter signals acrossnine orders of magnitude in dark matter mass.

Heating in brane inflation and hidden dark matter

Towards the end of brane inflation, the brane pair annihilation produces massive closedstrings. The transfer of this energy to standard model (SM) open string modes depends onwhere the SM branes and the brane annihilation are located: in the bulk, in the samethroat, or in different throats. We find that, in all cases as long as the brane annihilationand the SM branes are not both in the bulk, the transfer of energy to start the hot bigbang epoch can be efficient enough to be compatible with big bang nucleosynthesis. Thesuppression of the abundance of the graviton and its Kaluza–Klein (KK) thermalrelics follows from the warped geometry in flux compactification. This worksout even in the scenarios where a long period of tunnelling is expected. In themulti-throat scenario, we find a dynamical mechanism of selecting a long throat asthe SM throat. We establish three new dark matter candidates: KK modes withspecific angular momentum in the SM throat, those in the brane annihilationthroat, and different matters generated by KK modes tunnelled to other throats.Since the latter two couple to the visible matter sector only through gravitonmediation, they behave as hidden dark matter. Hidden dark matter has novelimplications for the dark matter coincidence problem and the high energy cosmic rays.

Constructing physically valid fluid spheres in general relativity

We extend a method by Goldman to include conditions which are both necessary and sufficient for construction of physically acceptable relativistic fluid spheres. We thus give the conditions for having finite and non-negative pressure and finite and positive density inside the fluid sphere. We also give conditions for the pressure gradient and the density gradient to be negative and for the speed of sound to be less than the speed of light in vacuum. We further give the condition for the trace of the energy-momentum tensor to be positive and for the relativistic adiabatic index to be larger than 4/3. A model given by Goldman is examined, and we find that all these conditions are fulfilled. We further show that this model is stable. However, we also give a class ofnew exact solutions, and show that these models are physically valied. These models are also stable with respect to small radial disturbances. We calculated the total mass, the density and the physical radius for these fluid spheres. Some of these spheres turn out to be extremely compact. They can have a radius of only several centimeters, but typically contain the mass of a planet. We put these models forward as tentative exact solutions for spheres which could contain the hidden dark matter in the universe.