Axion as a cold dark matter candidate

Here we generally prove that the axion as a coherently oscillating scalar field acts as a cold dark matter in nearly all cosmologically relevant scales. The proof is made in the linear perturbation order. Compared with our previous proof based on solutions, here we compare the equations in the axion with the ones in the cold dark matter, thus expanding the valid range of the proof. Deviation from purely pressureless medium appears in very small scale where axion reveals a peculiar equation of state. Our analysis is made in the presence of the cosmological constant, and our conclusions are valid in the presence of other fluid and field components.     

Annihilating cold dark matter

Structure formation with cold dark matter (CDM) predicts halos with a central density cusp, which are observationally disfavored. If CDM particles have an annihilation cross section sigmav approximately 10(-29)(m/GeV) cm(2), then annihilations will soften the cusps. We discuss plausible scenarios for avoiding the early Universe annihilation catastrophe that could result from such a large cross section. The predicted scaling of core density with halo mass depends upon the velocity dependence of sigmav, and s-wave annihilation leads to a core density nearly independent of halo mass, which seems consistent with observations.     

Model for cold dark matter

We present a model for cold dark matter based upon continuum bound states of thee ⁺ e ^– system. These continuum bound states, referred to as photonium states to distinguish them from the well-known bound states of positronium, are shown to have cross sections and lifetime properties consistent with abundant production in the early universe and survival to the present time. Thus photonium can easily account for more than 90% of the total mass of the universe.The authors have benefitted from discussions with Charles J. Benesh. This work was supported in part by the US DOE grant no. DE-FG02-87ER40371, Division of High Energy and Nuclear Physics and by contract no. W-7405-ENG-82.     

Planck-mass-rotons cold dark matter hypothesis

Sakharov's conjecture that the vacuum is densely occupied with Planck-mass maximons is taken as a model to explain the missing mass as rotons of a superfluid made up from the Planck-mass maximons. Because rotons require a finite excitation energy, they not only can account for the missing mass but, in addition, can mimic a small, positive cosmological constant. According to Sakharov, the large vacuum energy of the Planck-mass maximons is compensated by ghost particles. In the proposed superfluid vacuum model, we assume that the compensation is done by a large, negative cosmological constant instead.     

Majorana neutrinos as the dark matters in the cold plus hot dark matter model

A simple model of the Majorana neutrino with the see-saw mechanism is studied, assuming that two light neutrinos are the hot dark matter each with a mass of 2.4 eV in the cold plus hot dark matter model of cosmology. We find that the heavy neutrino, which is the see-saw partner with the remaining one light neutrino, can be the cold dark matter, if the light neutrino is exactly massless. This cold dark matter neutrino is allowed to have a mass in the wide range from 5.9 × 10² eV to 2.2 × 10⁷ eV.     

Gamma ray line astronomy as a probe of cold dark matter

Some annihilation processes of cold dark matter particles in the galactic halo may result in monochromatic gamma rays with an astrophysically significant rate. This paper summarizes the calculation of these rates and discusses the expected gamma ray line flux in comparison with the diffuse cosmic background.     

Observational evidence for self-interacting cold dark matter

Cosmological models with cold dark matter composed of weakly interacting particles predict overly dense cores in the centers of galaxies and clusters and an overly large number of halos within the Local Group compared to actual observations. We propose that the conflict can be resolved if the cold dark matter particles are self-interacting with a large scattering cross section but negligible annihilation or dissipation. In this scenario, astronomical observations may enable us to study dark matter properties that are inaccessible in the laboratory.     

Pseudo-Majoron as dark matter

We consider the singlet Majoron model with softly broken lepton number. This model contains three right-handed neutrinos and a singlet scalar besides the standard model fields. The real part of the singlet scalar develops a vacuum expectation value to generate the lepton number violation for seesaw and leptogenesis. The imaginary part of the singlet scalar becomes a massive pseudo-Majoron to be a dark matter candidate with testability by colliders, direct detection experiments and neutrino observations.     

The inflaton as dark matter

Within the framework of an explicit dynamical model, in which we calculate the radiatively-corrected, tree-level potential that sets up inflation, we show that the inflaton can be a significant part of dark matter today. We exhibit potentials with both a maximum and a minimum. Using the calculated position of the potential minimum, and an estimate for fluctuations of the inflaton field in the early universe, we calculate a contribution to the matter energy density of in the present universe, from cold inflatons with mass of about . We show that the inflaton might decay in a specific way, and we calculate a possible lifetime that is several orders of magnitude greater than the present age of the universe. Inflaton decay is related to an interaction which, together with a spontaneous breakdown of CP invariance at a cosmological energy scale, can give rise to a neutrino-antineutrino asymmetry just prior to the time of electroweak symmetry breaking. Received: 26 November 1997 / Revised version: 8 December 1997 / Published online: 24 March 1998