Narrow Results

In this paper, we explore the particle creation scenario in the context of $f({𝒢})$ gravity in flat-FLRW metric. For this purpose, from the perspective of thermodynamics and considering an adiabatic universe, we obtain the modified continuity equation in terms of the dynamic number of particles $N$. On the other hand, we obtain Friedmann’s equations in $f({𝒢})$ gravity and then write down the continuity equations of the components of matter and dark energy, taking into account the interaction between them. In what follows, by establishing a correspondence between the particle production scenario and $f({𝒢})$ gravity, we obtain the cosmological parameters in terms of $N$. After that, we find cosmological solutions using power-law cosmology and compare them with Hubble parameter data. Finally, we fit the current model with Hubble data and plot the best fit in terms of the redshift parameter.

In this study, we construct a theoretical framework based on the generalized Hubble parameter form which may arise within the particle creation, viscous and $f(R)$ gravity theory. The Hubble parameter is scrutinized for its compatibility with the observational data relevant to the late-time universe. By using Bayesian statistical techniques based on ${\chi }^2$ minimization method, we determine model parameters’ best fit values for the cosmic chronometer and supernovae Pantheon datasets. For the best fit values, the cosmographic and physical parameters are analyzed to understand the cosmic dynamics in model. We also analyze the model section criterion in comparison to the $\mathrm{\Lambda }$ cold dark matter model.

In this article we compute the density of Dirac particles created by a cosmological anisotropic Bianchi I universe in the presence of a constant electric field. We show that the particle distribution becomes thermal when one neglects the electric interaction.

The hypothesis that dark matter consists of superheavy particles with the mass close to the Grand Unification scale is investigated. These particles were created from vacuum by the gravitation of the expanding Universe and their decay led to the observable baryon charge. Some part of these particles with the lifetime larger than the time of breaking of the Grand Unification symmetry became metastable and survived up to the modern time as dark matter. However, in active galactic nuclei due to large energies of dark matter particles swallowed by the black hole and the possibility of the Penrose process for rotating black hole the opposite process can occur. Dark matter particles become interacting. Their decay on visible particles at the Grand Unification energies leads to the flow of ultra high energy cosmic rays observed by the Auger group. Numerical estimates of the effect leading to the observable numbers are given.

The problem of particle creation from vacuum in a flat Robertson–Walker spacetime is studied. Two sets of exact solutions for the Klein–Gordon equation are given when the scale factor is a²(η) = a+b tanh(λη)+c tanh² (λη). Then the canonical method based on Bogoliubov transformation is applied to calculate the pair creation probability and the density number of created particles. Some particular cosmological models such as radiation dominated universe and Milne universe are discussed. For both cases the vacuum to vacuum transition probability is calculated and the imaginary part of the effective action is extracted.

We discuss the dynamical effects of bulk viscosity and particle creation on the early evolution of the Friedmann–Robertson–Walker model in the framework of open thermodynamical systems. We consider bulk viscosity and particle creation as separate irreversible processes. Exact solutions of the Einstein field equations are obtained by using the "gamma-law" equation of state p = (γ-1)ρ, where the adiabatic parameter γ varies with scale factor of the metric. We consider the cosmological model to study the early phases of the evolution of the universe as it goes from an inflationary phase to a radiation-dominated era in the presence of bulk viscosity and particle creation. Analytical solutions are obtained for particle number density and entropy for all models. It is found that, by choosing appropriate functions for particle creation rate and bulk viscous coefficient, the models exhibit singular and non-singular beginnings.

In this paper, we study the massive spin-½ particle creation in de Sitter (dS) space where the related fields are written in (4+1)-dimensional bulk or the so-called ambient space approach. This approach mimics the flat space quantum field theory (QFT) and the field operators are defined globally on dS space. The main purpose of this study is defining the |in〉 and |out〉 modes for the proposed quantum field which has been written in terms of the dS plane wave in the 4+1 dimensions. We compute, via the Bogoliubov coefficients, the rate of particle creation in dS universe.

In this paper, we have studied the Klein’s paradox in the presence of both scalar and vector potential barriers. From the corresponding Dirac equation, we have calculated the transmission and reflection coefficients. It is shown that the presence of a scalar barrier the scalar potential widens the gap between positive and negative energies and so the forbidden region. Accordingly, the Klein’s paradox disappears when the scalar barrier exceeds a critical value. Considering the problem within the framework of quantum field theory, we have calculated the related pair creation probability, the mean number of created particles and the probability of a vacuum to remain a vacuum. Then it is shown that the scalar potential cuts down the Klein range and minimizes the creation of particles; the particle creation decreases as the scalar potential increases and ceases definitely when the scalar potential reaches the critical value.

By the use of coherent and squeezed thermal states formalism, we analyzed the phenomena of particle creation when coherently oscillating a homogeneous scalar field (Inflaton) in semi-classical gravity. We also obtained an estimated solution for the semi-classical Einstein equation in non-classical thermal states formalism perturbatively, which is similar to the power-law expansion of classical Einstein equation. This solution, apart from a particular condition, shows oscillatory behavior in nature. We also analyzed a coherently oscillating homogeneous scalar field, in a thermal vacuum, coherent thermal, squeezed thermal vacuum and coherent squeezed thermal states, suffering from particle creation, and created particles are showing oscillatory behavior. Particle production can be raised due to thermal and quantum effects. We also studied quantum fluctuations of a homogeneous scalar field in the above-mentioned non-classical thermal states.

In the course of study of the evolution of the universe, it is seen that perhaps the extra energy generated and particles created due to the accelerated expansion of the universe might be absorbed by the dark energy and dark matter which are already existing in this universe. It is found that the energy density of dark energy can be expressed as a function of the energy density of the remaining matter portion of the universe which shows that the different components of the universe are correlated. According to the forms of the different types of interaction occurring between dark energy and the other different contents of the universe it may be possible to utilize the dark energy in different ways as it may take different forms of energy. As an interesting phenomenon, it is also observed that the concept of negative time may exist in this universe, and it may revolutionize some of the original concepts of nature and the physical world.

The thermodynamical study of the universe allows particle production in modified $f(T)$ ($T$ is the torsion scalar) theory of gravity within a flat FLRW framework for line element. The torsion scalar $T$ plays the same role as the Ricci scalar $R$ in the modified theories of gravity. We derived the $f(T)$ gravity models by taking $f(T)$ as the sum of $T$ and an arbitrary function of $T$ with three different arbitrary function. We observe that the particle production describes the accelerated expansion of the universe without a cosmological constant or any unknown “quintessence” component. Also, we discussed the supplementary pressure, particle number density and particle production rate for three cases.

While two highly intensive laser beams collide, they create a region where the refractive index varies so quickly that photons are created. The variance of the refractive index is analog to the universe scale factor variance. Therefore, this laser system can be an analog to the expansion of the universe. We find that several hundreds of photons can be created under feasible conditions. This system can demonstrate the particle creation during inflation or other similar periods.

It is known that exponential particle creation can occur in situations which do not fall in the parametric resonance regime characterized by oscillations of the inflaton field about its minimum. Here we present a new analytical approach to exponential particle production which can occur when the inflaton is far from the minimum of its potential. Crucial for this effect is a term in the equation of motion which acts like a negative mass term, as occurs for tachyonic preheating and negative coupling particle production. Our techniques can be applied in models with a strong coupling between matter fields χ and the inflaton ϕ, or in some models in which the inflaton has a large amplitude of oscillation. On the other hand our analysis yields results which are quite model dependent. Although our analysis is general, in order to be clear we specialize, in this paper, to the model with interaction Lagrangian -g M_plϕχ².

The hypothesis that cold dark matter consists of primordial superheavy particles, the decay of short lifetime component of which led to the observable mass of matter while long living component survived up to modern times manifesting its presence in high energetic cosmic rays particles is investigated.

We investigate the Dynamical Casimir Effect (DCE) in the case of a scalar field enclosed in a box which undergoes a phase of strong acceleration (a kick) during its motion. Following a general-relativistic approach, we describe the acceleration field as a time-dependent space–time metric in the frame of a comoving, noninertial observer. Assuming a nonrelativistic motion of the box, we perturbatively solve the Klein–Gordon equation for the matter field, evaluating the β-Bogolubov coefficients, related to the particle creation. We show that, after the kick, a (small) number of created quanta is found inside the box. The resulting spectrum carries, in principle, information about the details of the box acceleration phase. The present approach can serve to shed light on the close relationship between DCE and Unruh–Hawking effect.

The hypothesis that dark matter consists of superheavy neutral particles with the mass of the Grand Unification scale is considered. The hypothesis that dark matter is converted into visible one in active galactic nuclei is investigated. If active galactic nuclei are rotating black holes then due to Penrose process superheavy particles can decay on nonstable particles with larger mass the decay of which on quarks and leptons leads to events in cosmic rays observed by the Auger group. Similar processes of decay of superheavy particles of dark matter into visible matter occurred in the early Universe. Numerical estimates of the processes in active galactic nuclei and in the early Universe are given.

States in quantum field theory (QFT) are represented by many-particle wave functions, such that a state describing n particles depends on n space–time positions. Since a general state is a superposition of states with different numbers of particles, the wave function lives in the configuration space identified with a product of an infinite number of four-dimensional Minkowski space–times. The squared absolute value of the wave function is interpreted as the probability density in the configuration space, from which the standard probabilistic predictions of QFT can be recovered. Such a formulation and probabilistic interpretation of QFT allows one to interpret the wave function as a pilot wave that describes deterministic particle trajectories, which automatically includes a deterministic and continuous description of particle creation and destruction. In particular, when the conditional wave function associated with a quantum measurement ceases to depend on one of the space–time coordinates, then the 4-velocity of the corresponding particle vanishes, describing a trajectory that stops at a particular point in space–time. In a more general situation a dependence on this space–time coordinate is negligibly small but not strictly zero, in which case the trajectory does not stop but the measuring apparatus still behaves as if this particle has been destroyed.

We discuss consistency of the concept of external background in QFT. Different restrictions on magnitude of magnetic and electric fields are analyzed. The back reaction due to strong electric field is calculated and restrictions on the magnitude and duration of such a field are obtained. The problem of consistency of Dirac equation with a superstrong Coulomb field is discussed.

The phenomenon of fermion production in a 5D FRW space–time with a warped extra dimension is studied by considering the canonical method based on Bogoliubov transformation connecting the "in" with the "out" states. For an exactly solvable model, two sets of exact solution are expressed in terms of Bessel functions. Studying the semiclassical Hamilton–Jacobi equation, the "in" and "out" states are identified. The pair creation probability and the mean number of created particles are calculated from the Bogoliubov coefficients. It is shown that the particle creation probability and the number density of created particles depend on the ratio of the scale factors associated with the ordinary space {x} and the extra dimension. It is also shown that the contraction of the extra dimension induces particle creation like the ordinary space expansion.

We study particle creation in a single pulse of an electric field in scalar quantum electrodynamics. We investigate the parameter condition for the case where the dynamical pair creation and Schwinger mechanism respectively dominate. Then, an asymptotic expansion for the particle distribution in terms of the time interval of the applied electric field is derived. We compare our result with particle creation in a constant electric field with a finite-time interval. These results coincide in an extremely strong field, however they differ in general field strength. We interpret the reason of this difference as a nonperturbative effect of high-frequency photons in external electric fields. Moreover, we find that the next-to-leading-order term in our asymptotic expansion coincides with the derivative expansion of the effective action.