Superheavy Dark Matter with Discrete Gauge Symmetries

We show that there are discrete gauge symmetries protect naturally heavy X particles from decaying into the ordinary light particles in the supersymmetric standard model. This makes the proposal very attractive that the superheavy X particles constitute a part of the dark matter in the present universe. It is more interesting that there are a class of discrete gauge symmetries which naturally accommodate a long-lived unstable X particle. We find that in some discrete Z_{10} models, for example, a superheavy X particle has lifetime \tau_X \simeq 10^{11}-10^{26} years for its mass M_X \simeq 10^{13}-10^{14} GeV. This long lifetime is guaranteed by the absence of lower dimensional operators (of light particles) couple to the X. We briefly discuss a possible explanation for the recently observed ultra-high-energy cosmic ray events by the decay of this unstable X particle.Comment: 9 pages, Late

Particle Production and Gravitino Abundance after Inflation

Thermal history after inflation is studied in a chaotic inflation model with supersymmetric couplings of the inflaton to matter fields. Time evolution equation is solved in a formalism that incorporates both the back reaction of particle production and the cosmological expansion. The effect of the parametric resonance gives rise to a rapid initial phase of the inflaton decay followed by a slow stage of the Born term decay. Thermalization takes place immediately after the first explosive stage for a medium strength of the coupling among created particles. As an application we calculate time evolution of the gravitino abundance that is produced by ordinary particles directly created from the inflaton decay, which typically results in much more enhanced yield than what a naive estimate based on the Born term would suggest.Comment: 23 pages + 13 figure

On the reheating stage after inflation

We point out that inflaton decay products acquire plasma masses during the reheating phase following inflation. The plasma masses may render inflaton decay kinematicaly forbidden, causing the temperature to remain frozen for a period at a plateau value. We show that the final reheating temperature may be uniquely determined by the inflaton mass, and may not depend on its coupling. Our findings have important implications for the thermal production of dangerous relics during reheating (e.g., gravitinos), for extracting bounds on particle physics models of inflation from Cosmic Microwave Background anisotropy data, for the production of massive dark matter candidates during reheating, and for models of baryogenesis or leptogensis where massive particles are produced during reheating.Comment: 8 pages, 2 figures. Submitted for publication in Phys. Rev.

Slepton and Neutralino/Chargino Coannihilations in MSSM

Within the low-energy effective Minimal Supersymmetric extension of Standard Model (effMSSM) we calculated the neutralino relic density taking into account slepton-neutralino and neutralino-chargino/neutralino coannihilation channels. We performed comparative study of these channels and obtained that both of them give sizable contributions to the reduction of the relic density. Due to these coannihilation processes some models (mostly with large neutralino masses) enter into the cosmologically interesting region for relic density, but other models leave this region. Nevertheless, in general, the predictions for direct and indirect dark matter detection rates are not strongly affected by these coannihilation channels in the effMSSM.Comment: 12 pages, 9 figures, revte

Squark-, Slepton- and Neutralino-Chargino coannihilation effects in the low-energy effective MSSM

Within the low-energy effective Minimal Supersymmetric extension of the Standard Model (effMSSM) we calculate the neutralino relic density taking into account slepton-neutralino, squark-neutralino and neutralino/chargino- neutralino coannihilation channels. By including squark (stop and sbottom) coannihilation channels we extend our comparative study to all allowed coannihilations and obtain the general result that all of them give sizable contributions to the reduction of the neutralino relic density. Due to these coannihilation processes some models (mostly with large neutralino masses) enter into the cosmologically interesting region for relic density, but other models leave this region. Nevertheless, in general, the predictions for direct and indirect dark matter detection rates are not strongly affected by these coannihilation channels in the effMSSM.Comment: 14 pages, 10 figures, corrected and to be published in Phys. Rev.

SuperWIMP Dark Matter Signals from the Early Universe

Cold dark matter may be made of superweakly-interacting massive particles, superWIMPs, that naturally inherit the desired relic density from late decays of metastable WIMPs. Well-motivated examples are weak-scale gravitinos in supergravity and Kaluza-Klein gravitons from extra dimensions. These particles are impossible to detect in all dark matter experiments. We find, however, that superWIMP dark matter may be discovered through cosmological signatures from the early universe. In particular, superWIMP dark matter has observable consequences for Big Bang nucleosynthesis and the cosmic microwave background (CMB), and may explain the observed underabundance of 7Li without upsetting the concordance between deuterium and CMB baryometers. We discuss implications for future probes of CMB black body distortions and collider searches for new particles. In the course of this study, we also present a model-independent analysis of entropy production from late-decaying particles in light of WMAP data.Comment: 19 pages, 5 figures, typos correcte

The Long-Term Future of Extragalactic Astronomy

If the current energy density of the universe is indeed dominated by a cosmological constant, then high-redshift sources will remain visible to us only until they reach some finite age in their rest-frame. The radiation emitted beyond that age will never reach us due to the acceleration of the cosmic expansion rate, and so we will never know what these sources look like as they become older. As a source image freezes on a particular time frame along its evolution, its luminosity distance and redshift continue to increase exponentially with observation time. The higher the current redshift of a source is, the younger it will appear as it fades out of sight. For the popular set of cosmological parameters, I show that a source at a redshift z=5-10 will only be visible up to an age of 4-6 billion years. Arguments relating the properties of high-redshift sources to present-day counterparts will remain indirect even if we continue to monitor these sources for an infinite amount of time. These sources will not be visible to us when they reach the current age of the universe.Comment: Phys. Rev. D, in press (2001

Dark energy as a mirage

Motivated by the observed cosmic matter distribution, we present the following conjecture: due to the formation of voids and opaque structures, the average matter density on the path of the light from the well-observed objects changes from Omega_M ~ 1 in the homogeneous early universe to Omega_M ~ 0 in the clumpy late universe, so that the average expansion rate increases along our line of sight from EdS expansion Ht ~ 2/3 at high redshifts to free expansion Ht ~ 1 at low redshifts. To calculate the modified observable distance-redshift relations, we introduce a generalized Dyer-Roeder method that allows for two crucial physical properties of the universe: inhomogeneities in the expansion rate and the growth of the nonlinear structures. By treating the transition redshift to the void-dominated era as a free parameter, we find a phenomenological fit to the observations from the CMB anisotropy, the position of the baryon oscillation peak, the magnitude-redshift relations of type Ia supernovae, the local Hubble flow and the nucleosynthesis, resulting in a concordant model of the universe with 90% dark matter, 10% baryons, no dark energy, 15 Gyr as the age of the universe and a natural value for the transition redshift z_0=0.35. Unlike a large local void, the model respects the cosmological principle, further offering an explanation for the late onset of the perceived acceleration as a consequence of the forming nonlinear structures. Additional tests, such as quantitative predictions for angular deviations due to an anisotropic void distribution and a theoretical derivation of the model, can vindicate or falsify the interpretation that light propagation in voids is responsible for the perceived acceleration.Comment: 33 pages, 2 figs; v2: minor clarifications, results unchanged; v3: matches the version published in General Relativity and Gravitatio

Radiative Decay of a Long-Lived Particle and Big-Bang Nucleosynthesis

The effects of radiatively decaying, long-lived particles on big-bang nucleosynthesis (BBN) are discussed. If high-energy photons are emitted after BBN, they may change the abundances of the light elements through photodissociation processes, which may result in a significant discrepancy between the BBN theory and observation. We calculate the abundances of the light elements, including the effects of photodissociation induced by a radiatively decaying particle, but neglecting the hadronic branching ratio. Using these calculated abundances, we derive a constraint on such particles by comparing our theoretical results with observations. Taking into account the recent controversies regarding the observations of the light-element abundances, we derive constraints for various combinations of the measurements. We also discuss several models which predict such radiatively decaying particles, and we derive constraints on such models.Comment: Published version in Phys. Rev. D. Typos in figure captions correcte

Many worlds in one

A generic prediction of inflation is that the thermalized region we inhabit is spatially infinite. Thus, it contains an infinite number of regions of the same size as our observable universe, which we shall denote as \O-regions. We argue that the number of possible histories which may take place inside of an \O-region, from the time of recombination up to the present time, is finite. Hence, there are an infinite number of \O-regions with identical histories up to the present, but which need not be identical in the future. Moreover, all histories which are not forbidden by conservation laws will occur in a finite fraction of all \O-regions. The ensemble of \O-regions is reminiscent of the ensemble of universes in the many-world picture of quantum mechanics. An important difference, however, is that other \O-regions are unquestionably real.Comment: 9 pages, 2 figures, comments and references adde