Gravitational fermion production in inflationary cosmology

We revisit the gravitational production of massive Dirac fermions in inflationary cosmology with a focus on clarifying the analytic computation of the particle number density in both the large and the small mass regimes. For the case in which the masses of the gravitationally produced fermions are small compared to the Hubble expansion rate at the end of inflation, we obtain a universal result for the number density that is nearly independent of the details of the inflationary model. The result is identical to the case of conformally coupled scalars up to an overall multiplicative factor of order unity for reasons other than just counting the fermionic degrees of freedom.     

Gravitational vacuum hypothesis and cosmology with variable particle number

We discuss a heurisitic model of gravitational vacuum as a set of virtual, radiating planckeons, particles with Planck size (L) and mass (/cL). Combined with Dirac's large number hypothesis, this gives the minimum universe scale factor valuea _min10^–13 cm, the strong interaction length (a = L just whena=a _min ). Taking this state as an initial one using standard quantum techniques, we consider particle creation by planckeons. Under some reasonable assumptions we obtain the present number of particles with nucleon mass close to observations,N 10⁸⁰. A criterion for gravitational stability of particles is formulated and some applications of the corresponding mass formula are considered. In particular, Fermi's weak interaction constant is expressed in terms ofa andL and a finite value for the neutrino mass is obtained.     

Gravitational effects in guth cosmology

Gravitational effects play a crucial role in cosmological phase transitions with Coleman-Weinberg type of symmetry breaking. For an SU(5) phase transition, it is argued that the false vacuumis destabilized at T ～ 10⁹ GeV.     

First law of thermodynamics and Friedmann-like equations in braneworld cosmology

We derive the Friedmann-like equations in braneworld cosmology by imposing the first law of thermodynamics and Bekenstein's area-entropy formula on the apparent horizon of a Friedmann–Robertson–Walker universe in both Randall–Sundrum II gravity and Dvali–Gabadadze–Porrati gravity models. Israel's boundary condition plays an important role in our calculations in both cases, besides the first law of thermodynamics and Bekenstein's area-entropy formula. The results indicate that thermodynamics on the brane world knows the behaviors of gravity.     

Gravitational particle production in massive chaotic inflation and the moduli problem

Particle production from vacuum fluctuations during inflation is briefly revisited. The moduli problem occurring with light particles produced at the end of inflation is addressed, namely, the fact that some results are in disagreement with nucleosynthesis constrains. A universal solution to this problem is found which leads to reasonable reheating temperatures in all cases. It invokes the assumption that, immediately after inflation, the moduli evolve like nonrelativistic matter. The assumption is justified in the context of massive chaotic inflation where, at the end of inflation, the Universe evolves as if it were matter dominated.     

Thermodynamics of gravitationally induced particle creation scenario in DGP braneworld

In this paper, we discuss the thermodynamical analysis for gravitationally induced particle creation scenario in the framework of DGP braneworld model. For this purpose, we consider apparent horizon as the boundary of the universe. We take three types of entropy such as Bakenstein entropy, logarithmic corrected entropy and power law corrected entropy with ordinary creation rate \(\Gamma \). We analyze the first law and generalized second law of thermodynamics analytically for these entropies which hold under some constraints. The behavior of total entropy in each case is also discussed which implies the validity of generalized second law of thermodynamics. Also, we check the thermodynamical equilibrium condition for two phases of creation rate, that is constant and variable \(\Gamma \) and found its vitality in all cases of entropy.     

Trans-Planckian particle creation in cosmology and ultrahigh energy cosmic rays

We consider observational constraints on the creation of particles induced by hypothetical trans-Planckian effects during the current stage of the Universe’s expansion. We show that compatibility with the diffuse γ-ray background measured by the EGRET experiment strongly restricts this creation. In particular, it rules out the possibility of detecting signatures of such short-distance effects in anisotropies of the cosmic microwave background radiation. On the other hand, the possibility that some part of ultrahigh-energy cosmic rays originates from new trans-Planckian physics remains open.     

The problems of singularity particle horizon and flatness in quantum cosmology

Classical relativistic cosmology is known to have the space-time singularity as an inevitable feature. The standard big bang models have very small particle horizons in the early stages which make it difficult to understand the observed homogeneity in the universe. The relatively narrow range of the observed matter density in the neighbourhood of closure density requires highly fine tuning of the early universe. In this paper it is argued that these three problems can be satisfactorily resolved in quantum cosmology. It is shown that it is extremely unlikely that the universe evolved to the present state from quantum states with singularity and particle horizon. Similarly, it is shown that of all possible states the Robertson-Walker model of flat spatial sections is the most likely state for the universe to evolve out of a quantum fluctuation. To demonstrate these results a suitable formalism for quantum cosmology is first developed.     

Gauge invariance in braneworld scenarios

Rubakov and Shaposhnikov (RSH), in a seminal paper, discussed the possibility that particles are confined in a potential well. This is considered as the first mention to the today?s idea that we live in a brane, i.e., the braneworld concept. In this work we show precisely that the proposed RSH model has a gauge invariant equivalent action and we discuss it in the light of braneworld structure. We analyzed the intrinsic features of both models trying to disclose new properties within RSH braneworld theory.     

Fluid cosmology with decay and production of particles

We investigate the influence of particle number nonconserving processes on the thermal history and the dynamics of the universe. There occurs a backreaction both on the temperature and on the scale factor if particles of one species decay into particles of another species, or if the overall particle number changes. This backreaction is determined by the ratios of the relevant decay and production rates to the Hubble expansion rate. It results in reheating phenomena and modifications of the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) expansion laws. A high production rate at the Planck scale tends to avoid the cosmological singularity.     

Static domain wall in braneworld gravity

Thermal radiation of electrically charged fermions from a rotating black hole with electric and magnetic charges in de Sitter space is considered. The tunneling probabilities for outgoing and incoming particles are obtained and the Hawking temperature is calculated. The relation for the classical action for the particles in the black hole’s background is also found.     

AdS braneworld with backreaction

We review the tachyon model derived from the dynamics of a 3-brane moving in the AdS₅ bulk. The bulk geometry is based on the Randall-Sundrum II model extended to include the radion. The effective tachyon Lagrangian is modified due to the back-reaction of the brane on the bulk geometry.