Narrow Results

The inflationary models developed in presence of a background radiation can be a solution to the reheating problem faced by common cold (isentropic) inflationary scenario. A D-brane system comprising of k Neuvo–Schwarz (NS) 5-branes with a transverse circle and BPS D3-branes with world volume parallel to the NS 5-branes, placed at a point on the transverse circle diametrically to NS 5-brane has a point of unstable equilibrium and the D3-brane has a geometric tachyonic mode associated with displacement of the brane along the circle. Cold inflationary scenario has been studied in connection with this geometric tachyon [S. Panda, M. Sami and S. Tsujikawa, Phys. Rev. D73, 023515 (2006)] where it was found that one needs a background of minimum 10⁴ branes to realize a viable inflationary model. In this piece of work, we have tried to study a model of inflation driven by this geometric tachyon in presence of radiation. We have found that compared to the isentropic scenario, to satisfy the observational bounds, the number of background branes required in this case reduces drastically and a viable model can be obtained with even six to seven NS 5-branes in the background. In this context, we have also analyzed the non-gaussianity associated with the model and observed that the concerned parameter lies well within the observation limit.

Considering a constant dissipative coefficient $\mathrm{\Gamma }$, the pre-inflation dynamical behaviors of warm inflation in the loop quantum cosmology scenario are discussed. We consider three sets of initial conditions. The evolution of the background can always be divided into three phases, namely super-inflation, damping, and slow-roll inflation phases, with the duration of each phase depending on the initial conditions. As an example, we compare the background evolution between $\mathrm{\Gamma }=0.001m_{\mathrm{\text{Pl}}}$ and $\mathrm{\Gamma }=0$ under special initial conditions and find that there is no slow-roll inflation phase for $\mathrm{\Gamma }=0$ while the number of e-folds is about 60.209 for $\mathrm{\Gamma }=0.001m_{\mathrm{\text{Pl}}}$.

In this work, we consider a modern form of single-field inflationary models (known as power-law plateau inflation and preheating phase for a class of plateau inflationary model) and ASPIC models by considering the Dirac–Born–Infeld (DBI) non-canonical scalar field in the light of warm inflationary scenario. For these models, we assume constant sound speed $(c_s)$ and compute different slow-roll $(𝜖$, $\eta$ and $\kappa )$ as well as inflationary parameters (number of $e$-folds $(N)$, spectral index $(n_s)$, scalar spectral index $({\alpha }_s)$ and tensor-to-scalar ratio $(r)$) for specific anti-de Sitter warp factor of the DBI inflation. Next, we plot the trajectories of $n_s-N$, ${\alpha }_s-N$ and $r-N$ with different choices of $c_s$ and found the ranges $0.969_{-0.007}^{+0.007}$, $-0.0012_{-0.0002}^{+0.0002}$, $0.0009_{-0.0003}^{+0.0003}$ (for minimal inflation model), $0.962_{-0.006}^{+0.006}$, $-0.0009_{-0.0003}^{+0.0003}$, $-1.4_{-0.9}^{+0.9}$ (for modified minimal inflation model) and $0.98_{-0.02}^{+0.02}$, $-0.000157_{-0.000008}^{+0.000008}$, $0.00043_{-0.00012}^{+0.00012}$ (mixed large field inflation), respectively. Moreover, the ranges of these parametric plots for power-law plateau inflationary model are $0.961_{-0.005}^{+0.005}(n_s-N)$, $-0.009_{-0.005}^{+0.005}({\alpha }_s-N)$, $-1.15_{-0.85}^{+0.85}(r-N)$. Finally, we find that the results of all these parametric plots are consistent with 68% and 95% with current observational data except scalar-to-tensor ratio $r$ for modified minimal and power-law Plateau inflation models.

Considering the mechanism of dissipative slow-roll that has been used in warm inflation scenario, we show that dissipation may alter usual cosmological scenarios associated with SUSY-flat directions. We mainly consider SUSY-flat directions that have strong interactions with nonflat directions and may cause strong dissipation both in thermal and nonthermal backgrounds. An example is the Affleck–Dine mechanism in which dissipation may create significant (both qualitative and quantitative) discrepancies between the conventional scenario and the dissipative one. We also discuss several mechanisms of generating curvature perturbations in which the dissipative field, which is distinguished from the inflaton field, can be used as the source of cosmological perturbations. Considering the Morikawa–Sasaki dissipative coefficient, the damping caused by the dissipation may be significant for many MSSM flat directions even if the dissipation is far from thermal equilibrium.

In this work, we examined chromo-natural inflation in the context of warm inflation with constant and variable dissipation coefficients. We studied the cosmological perturbation theory in this model and obtained the scalar and tensor power spectra in each case. The sources of density fluctuations in this model are mainly the thermal fluctuations of the inflaton field like a general warm inflationary model. Finally, cosmological observables, namely, the spectral index and tensor-to-scalar ratio are calculated. It is found that the cosmological observables are consistent with observational Planck 2018 and BICEP bounds for sub-Planckian values of axion decay constant $f$ in case of temperature–inflaton-dependent dissipation coefficient. In case of the constant and temperature-dependent dissipation coefficient, the values of axion decay constant $f$ can be lowered below the GUT scale.

We study warm inflation in the framework of $f(\varphi )T$ gravity, where $\varphi$ is the inflaton and T is the trace of the energy–momentum tensor. The inflaton field is assumed to roll on the natural inflaton potential and the result is analyzed in light of Planck 2018 and BICEP/Keck 2021 data. We start by obtaining the field equations under slow-roll approximations. The scalar and tensor power spectra and their corresponding spectral index and tensor-to-scalar ratio with a temperature-dependent form of the dissipation coefficient during the inflationary era are then evaluated. It is found that the warm inflation model in $f(\varphi )T$ gravity is compatible with observational bands.

In this paper, we study warm-intermediate inflationary models in the context of the brane world models. We describe in great details the characteristics of these models in the high-energy limit. General conditions in the weak and strong dissipative regimes for these models are analyzed. The different parameters in both dissipative scenarios are constrained from the observational data.

In this paper, we study a warm intermediate inflationary model with a general form for the dissipative coefficient Γ(T, ϕ) = C_ϕT^m/ϕ^m-1 in the context of Loop Quantum Cosmology (LQC). We examine this model in the weak and strong dissipative regimes. In general, we discuss in great detail the characteristics of this model in the slow-roll approximation. Also, we assume that the modifications to perturbation equations result exclusively from Hubble rate. In this approach, we use recent astronomical observations from Planck and BICEP2 experiments to restrict the parameters in our model.

We review a recently proposed effective Tolman temperature and present its applications to various gravitational systems. In the Unruh state for the evaporating black holes, the free-fall energy density is found to be negative divergent at the horizon, which is in contrast to the conventional calculations performed in the Kruskal coordinates. We resolve this conflict by invoking that the Kruskal coordinates could be no longer proper coordinates at the horizon. In the Hartle–Hawking–Israel state, despite the negative finite proper energy density at the horizon, the Tolman temperature is divergent there due to the infinite blueshift of the Hawking temperature. However, a consistent Stefan–Boltzmann law with the Hawking radiation shows that the effective Tolman temperature is eventually finite everywhere and the equivalence principle is surprisingly restored at the horizon. Then, we also show that the firewall necessarily emerges out of the Unruh vacuum, so that the Tolman temperature in the evaporating black hole is naturally divergent due to the infinitely blueshifted negative ingoing flux crossing the horizon, whereas the outgoing Hawking radiation characterized by the effective Tolman temperature indeed originates from the quantum atmosphere, not just at the horizon. So, the firewall and the atmosphere for the Hawking radiation turn out to be compatible, once we discard the fact that the Hawking radiation in the Unruh state originates from the infinitely blueshifted outgoing excitations at the horizon. Finally, as a cosmological application, the initial radiation energy density in warm inflation scenarios has been assumed to be finite when inflation starts. We successfully find the origin of the nonvanishing initial radiation energy density in the warm inflation by using the effective Tolman temperature.

In this paper we investigate the observational signatures of Loop Quantum Cosmology (LQC) in the CMB data. First, we concentrate on the dynamics of LQC and we provide the basic cosmological functions. We then obtain the power spectrum of scalar and tensor perturbations in order to study the performance of LQC against the latest CMB data. We find that LQC provides a robust prediction for the main slow-roll parameters, like the scalar spectral index and the tensor-to-scalar fluctuation ratio, which are in excellent agreement within $1\sigma$ with the values recently measured by the Planck collaboration. This result indicates that LQC can be seen as an alternative scenario with respect to that of standard inflation.

This paper investigates the dynamics of warm logamediate inflation for flat isotropic and homogeneous universe in Einstein frame representation of $f(R)$ gravity. In this scenario, we study dissipative effects for weak and strong interactions of inflaton field via constant and generalized dissipative coefficient. In both interacting regimes, we find inflaton solution corresponding to scalar potential and radiation density of dissipating inflaton. Under slow-roll approximation, we formulate scalar and tensor power spectra, their spectral indices and tensor–scalar ratio for Starobinsky inflationary model and construct graphical analysis of these observational parameters. It is concluded that this model remains compatible with Planck 2015 constraints in weak and strong regimes for constant dissipative coefficient. For generalized dissipative coefficient, the inflationary model yields consistent results for $m=0,1$ and $-1$ in strong regime while condition of warm inflation is violated for $m=-1$ in weak regime.

We investigate the observational signatures of quantum cosmology in the Cosmic Microwave Background data provided by Planck collaboration. We apply the warm inflationary paradigm with a tachyon scalar field to the loop quantum cosmology. In this context, we first provide the basic cosmological functions in terms of the tachyon field. We then obtain the slow-roll parameters and the power spectrum of scalar and tensor fluctuations, respectively. Finally, we study the performance of various warm inflationary scenarios against the latest Planck data and we find a family of models which are in agreement with the observations.

The present work is an attempt to find inter relationship between the cold and warm inflationary scenarios. Here cold inflation is described by a single scalar field (inflaton) and nonequilibrium thermodynamical prescription is imposed. It is found that this quasi-de-Sitter phase of evolution is equivalent to warm inflation. The present work also addresses the question whether the dissipation is Hawking-type or not.

We have studied constant-roll inflation using the q-de Sitter scale factor. Both the cold and warm inflation have been considered, and the scalar potential and inflaton field have been determined. Also tensor to scalar ratio and the spectral index are obtained. In the cold inflation case, the perturbations have been investigated using Mukhanov–Sasaki equation, and the results determined for the spectral index and tensor to scalar ration are consistent with observations. In the warm inflation two cases of constant damping ($\mathrm{\Gamma }={\mathrm{\Gamma }}_0$) and the field dependent damping ($\mathrm{\Gamma }={\mathrm{\Gamma }}_{1}V(\varphi )$) have been studied. In both cases the power spectrum and the tensor to scalar ratio are determined as a function of the inflaton field. It has been shown that in both cases the spectral index is independent of the inflaton field. In the case of constant damping the obtained value for the spectral index is inconsistent with observations but for the case of field dependent damping the parameter $q$ can be chosen such that the value obtained for all the physical quantities including the spectral index be consistent with present cosmological observations.

In this paper, we study a constant-roll inflationary model in the context of brane-world cosmology caused by a quintessence scalar field for warm inflation with a constant dissipative parameter $Q=\mathrm{\Gamma }/3H$. We determine the analytical solution for the Friedmann equation coupled to the equation of motion of the scalar field. The evolution of the primordial scalar and tensor perturbations is also studied using the modified Langevin equation. To check the viability of the model, we use numerical approaches and plot some figures. Our results for the scalar spectral index and the tensor-to-scalar ratio show good consistency with observations.

We explore the swampland conjectures of quintessence and tachyon scalar field models in $f(T)$ and Einstein gravities, respectively. In $f(T)$ gravity, we consider cold and warm (by assuming generalized dissipative coefficient) inflationary scenarios and develop constraints on de-Sitter conjectures through Bekenstein entropy relation. In usual inflation, the de-Sitter conjecture constraint becomes constant (in terms of models constant parameters) which can be easily analyzed for its validity (i.e., $\frac{\left|V^{\prime }\right|}{V}\le O(1)$). However, in warm inflation, we analyze the de-Sitter conjecture constraint through $\frac{{\tau }^{\prime }V}{\tau V^{\prime }}$, i.e., if $\frac{{\tau }^{\prime }V}{\tau V^{\prime }}<1$ then $\frac{\left|V^{\prime }\right|}{V}\le O(1)$ and hence we can obtain the required result. For evaluating this condition, we choose three well-known potentials such as monomial chaotic, hilltop and generalized exponential. It is observed that the condition $\frac{{\tau }^{\prime }V}{\tau V^{\prime }}<1$ satisfied for all three potentials in both regimes of dissipative coefficient for specific choice of constants. We also obtain satisfactory results of de-Sitter conjecture for tachyon and polytropic (with quintessence scalar field) in general relativity. We investigate the consistency of assumed potentials through inflationary parameters $n_s$ and $r$ comparing with Planck’s observational data 2018 and found them compatible with the recent observations.

In this paper, $k$-inflation is analyzed in warm braneworld scenario. A general class of $k$-essence models with power-law kinetic term is investigated and weak and strong dissipation regimes are studied. Scalar perturbations and spectral index are derived. The results are discussed and applied to specific examples.

Nonisentropic inflation models attempt to fully account for the quantum effects that other field degrees of freedom, other than the inflaton field, might cause on the inflaton dynamics. In particular, it is known that these quantum effects can produce both quantum and thermal stochastic noise and dissipation terms in the inflaton’s equation of motion. Here we analyze how these quantum and thermal effects can alter the possibility for producing eternal inflation in the context of different potentials for the inflaton. It is shown how these quantum generated terms can reduce the probability of the universe entering in a regime of eternal inflation and the resulting implications are discussed.

It is shown how viscosities affect the dynamics of the perturbations at linear order for a two-fluid system composed by the inflaton and a radiation bath. These two-fluid systems are typical of nonisentropic models, like warm inflation. It is shown that viscosities effects must be considered to proper control the backreaction of the radiation fluctuations on the inflaton fluctuations and to give a proper value for the amplitude of the power spectrum.