Inflationary Cosmology Research Articles

Renormalization-group techniques for single-field inflation in primordial cosmology and quantum gravity

We study inflation as a ‘cosmic’ renormalization-group (RG) flow. The flow, which encodes the dependence on the background metric, is described by a running coupling α, which parametrizes the slow roll, a de Sitter free, analytic beta function and perturbation spectra that are RG invariant in the superhorizon limit. Using RG invariance as a guiding principle, we classify the main types of flows according to the properties of their spectra, without referring to their origins from specific actions or models. Novel features include spectra with essential singularities in α and violations of the relation r + 8n t = 0 to the leading order. Various classes of potentials studied in the literature can be described by means of the RG approach, even when the action includes a Weyl-squared term, while others are left out. In known cases, the classification helps identify the models that are ruled out by data. The RG approach is also able to generate spectra that cannot be derived from standard Lagrangian formulations.

Achieving the highest temperature during reheating with the Higgs condensate

We study the role of the Standard Model Higgs condensate, formed during cosmological inflation, in the epoch of reheating that follows. We focus on the scenario where the inflaton decays slowly and perturbatively, so that there is a long period between the end of inflation and the beginning of radiation domination. The Higgs condensate decays nonperturbatively during this period, and we show that it heats the primordial plasma to much higher temperatures than would result from the slowly decaying inflaton alone. We discuss the effect of this hot plasma on the thermalization of the inflaton's decay products, and study its phenomenological implications for the formation of cosmological relics like dark matter, with associated isocurvature fluctuations, and the restoration of the electroweak and Peccei-Quinn symmetries.

Warm constant-roll inflation in brane-world cosmology

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 [Formula: see text]. 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.

Testing an inflationary era in curvature–matter coupling gravity

In this paper, we investigate the cosmological inflation in the context of a minimal matter–geometry coupling, which is based on the [Formula: see text] gravity theory, where [Formula: see text] is a generic function of the curvature scalar [Formula: see text] and the trace [Formula: see text] of the energy–momentum tensor. Assuming that the slow-roll inflation conditions hold true in [Formula: see text] gravity, we obtain the various inflation-related observables such as the tensor-to-scalar ratio [Formula: see text], the scalar spectral index [Formula: see text], the running [Formula: see text] of the spectral index and the tensor spectral [Formula: see text] for two specific [Formula: see text] models from the Hubble slow-roll parameters. To observe the viability of these models, a numerical analysis of such parameters has been done. The results showed that, by using the various values of free parameters, it is possible to obtain a viable compatible with the observational data.

Primordial flat frame: A new view on inflation

Models of inflationary cosmology admit a choice of the metric for which the geometry of homogeneous isotropic solutions becomes flat Minkowski space in the infinite past. In this primordial flat frame all mass scales vanish in the infinite past and quantum scale symmetry is realized. The cosmological evolution is dominantly described by the slow increase of a scalar field which sets the scale of all masses. We construct the primordial flat frame for standard models of inflation as Starobinsky inflation or chaotic inflation. In particular, we discuss the evolution of inhomogeneous solutions in the neighborhood of the homogeneous isotropic background solution and their relation to the observable primordial fluctuation spectrum. If the propagators for the graviton and scalar field remain regular, our observed inhomogeneous Universe can be extrapolated back to the infinite past in physical time. In this case there is no physical big-bang singularity -- the latter reflects only a singular choice of ``field coordinates". Independently of the issue of singularity the primordial flat frame offers a new view on the physical properties of the inflationary universe, which can be characterized as a very slowly evolving almost empty vacuum state with approximate scale symmetry.

On stringy inflation potentials

We investigate cosmological inflation models from Randall–Sundrum brane physics. In particular, we examine certain cosmological parameters for two potentials, being a logarithmical one and open string tachyonic inflation (OSTI). In the presence of the brane tension, we give scalar field constraints providing inflationary models matching with Planck results. Then, we discuss the reheating phase of such models. Precisely, the reheating temperature depends on the potential forms. Among others, we find that OSTI provides small temperatures, supported by GUT scales.

Model independent prediction of the spectral index of primordial quantum fluctuations

One of the most important achievements of inflationary cosmology is to predict a departure from scale invariance of the power spectrum for cosmological scalar perturbations. This tilt is understood as a consequence of a quasi de Sitter classical equation of state describing the inflationary dark energy dominated era. Here, following previous work, we find a departure of scale invariance for the quantum Fisher information associated to de Sitter vacuum for scalar quantum spectator modes. This gives rise to a purely quantum cosmological tilt with a well defined dependence on energy scale. This quantum tilt is imprinted, in a scale dependent energy uncertainty for the spectator modes. The effective quasi de Sitter description of this model independent energy uncertainty uniquely sets the effective quasi de Sitter parameters (i.e., the small deviation from the cosmological constant equation of state) at all energy scales. In particular, in the slow-roll regime characterized by an almost constant ϵ, the quantum Fisher — model independent — prediction for the spectral index is n s = 0.9672. Moreover, the energy scale dependence of the quantum cosmological tilt implies the existence of a cosmological phase transition at energies higher than the CMB scale and of the order of 1 Mpc-1 where the tilt goes from red into blue. This strongly suggest the existence of a pre-inflationary phase where the effective scalaron contributes to the spectral index as normal relativistic matter and where the corresponding growth of the power spectrum can result in dark matter in the form of small mass primordial black holes. The source and features of the quantum cosmological tilt leading to these predictions are determined by the entanglement features of the de Sitter α- vacuum states.

Inflationary cosmology in [formula omitted] modified gravity

In this paper, we investigate the cosmological inflation scenario in the context of f(R,T) theory of gravity, where f is a generic function of the curvature scalar R and the trace T of the energy momentum tensor. Assuming that the slow roll inflation conditions hold true in f(R,T) gravity, we obtain from the Hubble slow roll parameters, the various inflation-related observables such as the tensor-to-scalar ratio r, the scalar spectral index ns, the running αs of the spectral index and the tensor spectral nT for two specific f(R,T) models. To see the viability of these models, a numerical analysis of such parameters has been done. The results show that by using the various values of free parameters it is possible to obtain a viable compatibility with the observational data.

Impact of Dynamical Collapse Models on Inflationary Cosmology.

Inflation solves several cosmological problems at the classical and quantum level, with a strong agreement between the theoretical predictions of well-motivated inflationary models and observations. In this Letter, we study the corrections induced by dynamical collapse models, which phenomenologically solve the quantum measurement problem, to the power spectrum of the comoving curvature perturbation during inflation and the radiation-dominated era. We find that the corrections are strongly negligible for the reference values of the collapse parameters.

Information-Measuring Complex to Detect High Frequency Gravitational Waves

The gravitational waves predicted by the general theory of relativity and detected by the Laser Interferometer Gravitational-wave Observatory (LIGO) have typical frequencies in the range of 30 ... 300 Hz. Current theories of gravity predict the existence of high-frequency gravitational waves with frequencies of 10 ... 100 MHz, including those of cosmological origin, induced by quantum fluctuations of the scalar field at the stage of cosmological inflation in the early Universe.Multi-beam optical resonators, in particular the Fabry-Perot interferometers, can be used to detect high-frequency gravitational waves. When using multi-beam optical resonators, it is possible to use the phenomenon of low-frequency optical resonance, which allows us to have a selective response to the gravitational wave effect. The gravitational-optical resonance in a multi-beam interferometer occurs if the condition is fulfilled that an integer number of half-waves of gravitational radiation is along the length of the resonator.The use of a multi-beam interferometer to detect high-frequency gravitational waves does not require the creation of a complex system for decoupling mirrors used for gravitational antennas operating in the low-frequency part of the spectrum. This is due to the fact that the frequency of mechanical vibrations of the interferometer mirrors is significantly less than the frequency of the gravitational wave.The paper considers possible optical schemes of a high-frequency gravitational antenna: based on the traditional Michelson interferometer, in the arms of which two Fabry-Perot interferometers are available, and on the basis of the Mach-Zehnder optical scheme, where Fabry-Perot interferometers can be made in the form of two perpendicular arms, with reflecting mirrors at the bend of the beam. The advantage of the second scheme is that three photo-detectors, one being main and two others being auxiliary, can be used, and there is a possibility to detect radiation transmitted by Fabry-Perot interferometers.To prove that detection of high-frequency gravitational waves is possible, a potential sensitivity of the high-frequency gravitational antenna has been estimated in the paper.

Re-interpretation of the Two-World Background of Special Relativity as Four-World Background II

Coexisting four universes in separate four-dimensional spacetimes constitute four-world background for the special theory of relativity (SR) in each universe, as developed in previous papers. The fact that the four universes exhibit perfect symmetry of state and perfect symmetry of natural laws is shown in this paper. The many universes concept involved is entitled compartment universes. Compartment universes are coexisting symmetrical universes in different fourdimensional spacetimes of identical extents. Material particles and bodies are symmetrically distributed in spacetimes and the same natural laws take on identical forms in compartment universes. These features differentiate the compartment universes concept from the multiverse of inflationary cosmology and the parallel branes of M-theory. The compartment universes concept opens new vista for many-world interpretations of the natural laws, as demonstrated for the special theory of relativity already, and it is a potential platform for the uniform formulation of the natural laws. Investigation of the possible existence of larger number of compartment universes than four and many-world interpretations of gravitation and other natural laws in the compartment universes picture are recommended.

Reheating in small-field inflation on the brane: the swampland criteria and observational constraints in light of the PLANCK 2018 results

We study cosmological inflation and its dynamics in the framework of the Randall–Sundrum II brane model. In particular, we analyze in detail four representative small-field inflationary potentials, namely Natural inflation, Hilltop inflation, Higgs-like inflation, and Exponential SUSY inflation, each characterized by two mass scales. We constrain the parameters for which a viable inflationary Universe emerges using the latest PLANCK results. Furthermore, we investigate whether or not those models in brane cosmology are consistent with the recently proposed Swampland Criteria, and give predictions for the duration of reheating as well as for the reheating temperature after inflation. Our results show that (i) the distance conjecture is satisfied, (ii) the de Sitter conjecture and its refined version may be avoided, and (iii) the allowed range for the five-dimensional Planck mass, M_5, is found to be 10^5,text {TeV}lesssim M_5lesssim 10^{12},text {TeV}. Our main findings indicate that non-thermal leptogenesis cannot work within the framework of RS-II brane cosmology, at least for the inflationary potentials considered here.

Nonlinearity analysis for cosmological inflation with minimal and non-minimal coupling of scalar field from horndeski theory for special cases: de sitter expansion and decaying scalar field

In this work, we consider De Sitter expansion and decaying scalar field in cosmological model with combination of minimal and non-minimal coupling of scalar field from Horndeski theory. As a continuity of the previous work, after the background solution and condition obtained, we continue the analysis to the nonlinearity aspect of perturbations via the spectral index of the perturbations. Spectral index of the perturbations and tensor to-scalar ratio of these special cases give sign of inflation model.

No-boundary wave function, Wheeler-DeWitt equation, and path integral analysis of the bouncing quantum cosmology

Bouncing models are alternatives to inflationary cosmology that replace the initial big bang singularity by a ``bouncing'' phase. A deeper understanding of the initial conditions of the universe, in these scenarios, requires knowledge of quantum aspects of bouncing models. In this work, we propose two classes of bouncing models that can be studied with great analytical ease and hence, provide a test bed for investigating more profound problems in quantum cosmology of bouncing universes. Our model's two key ingredients enable us to do straightforward analytical calculations: (i) a convenient parametrization of the minisuperspace of FRLW spacetimes and (ii) two distinct choices of the effective perfect fluids that source the background geometry of the bouncing universe. We study the quantum cosmology of these models using both the Wheeler-de Witt equations and the path integral approach. In particular, we found a bouncing model analogue of the no-boundary wave function and presented a Lorentzian path integral representation for the same. We also discuss the introduction of real scalar perturbations.

Curing inflationary degeneracies using reheating predictions and relic gravitational waves

It is well known that the inflationary scenario often displays different sets ofdegeneracies in its predictions for CMB observables. These degeneracies usually arise either because multiple inflationary modelspredict similar values for the scalar spectral index n S and the tensor-to-scalarratio r, or because within the same model, the values of {n S , r}are insensitive to some of the model parameters, making it difficult for CMB observations alone to constitute a unique probe of inflationary cosmology. We demonstrate that by taking into account constraints on the post-inflationary reheating parameters such as the duration of reheating N re, its temperature T re and especially its equationof state (EOS), w re, it is possible to break this degeneracy in certain classes of inflationary models where identical values of {n S , r} can correspond to different reheating w re. In particular, we show how reheating constraints can break inflationary degeneracies in the T-model and the E-model α-attractors. Non-canonical inflation is also studied. The relic gravitational wave (GW) spectrum provides us with another tool to break inflationary degeneracies.This is because the GW spectrumis sensitiveto the post-inflationary EOS of the universe. Indeed a stiff EOS during reheating (w re > 1/3) gives rise to a small scale blue tilt in the spectral indexn GW = d log ΩGW/d log k > 0, while a soft EOS (w re < 1/3) results in a red tilt.Relic GWs therefore provide us with valuable information about the post-inflationary epoch, and their spectrumcan be used to cure inflationary degeneracies in {n S , r}.

Scalaron\u2013Higgs inflation reloaded: Higgs-dependent scalaron mass and primordial black hole dark matter

We propose an extension of the scalaron-Higgs model by a non-minimal coupling of the Standard Model Higgs boson to the quadratic Ricci scalar resulting in a Higgs-dependent scalaron mass. The model predicts a successful stage of effective single-field Starobinsky inflation. It features a multi-field amplification mechanism leading to a peak in the inflationary power spectrum at small wavelengths which enhances the production of primordial black holes. The extended scalaron-Higgs model unifies inflationary cosmology with elementary particle physics and explains the origin of cold dark matter in terms of primordial black holes without assuming any new particles.

A semi-classical model of regular inflationary cosmology

Quantum-effective gravity must lead to a regularized cosmology and thereby by itself source and regulate the high energy inflaton fields required for an appropriate inflationary beginning of the Universe. Pursuing this premise, we involve in the generic code of extended theories of gravity in conceiving semi-classical imprinting of quantum effects into gravitation. Hence, we prescribe a general semi-classical procedure of constructing the regular, quantum-effective form of the spatially flat FLRW cosmology whereby two alternative classes of the admissible dynamics are distinguished. In this paper, we explore only the particular class holding a minimally inflationary de Sitter UV-phase. Curiously, the low energy phase of the newly modeled Universe recovers the conventional dynamics of loop quantum cosmology. However, unlike the loop quantum cosmology, the high energy phase of the newly conceived cosmology has no bounce, instead, has a regular whimpering emergence of the Universe replacing the Big Bang. Our analysis manifests that without needing any non-standard equation of state, ordinary matter admits an inflationary beginning of the Universe.

Dynamics of cosmological inflation and predictions for reheating in the light of 2018 PLANCK results

We study the dynamics of two concrete inflationary models, namely spontaneous symmetry breaking inflation and loop inflation. We constrain the parameters for which a viable inflationary Universe emerges using the latest PLANCK results of last year, and we give predictions for the duration of reheating as well as for the reheating temperature after inflation. Our numerical results show that baryogenesis via leptogenesis may be realized within the inflationary models considered in this work.

Cosmological inflation driven by a scalar torsion function

A viable model for inflation driven by a torsion function in a Friedmann background is presented. The scalar spectral index in the interval 0.92lesssim n_{s}lesssim 0.97 is obtained in order to satisfy the initial conditions for inflation. The post inflationary phase is also studied, and the analytical solutions obtained for scale factor and energy density generalizes that ones for a matter dominated universe, indicating just a small deviation from the standard model evolution. The same kind of torsion function used also describes satisfactorily the recent acceleration of the universe, which could indicate a possible unification of different phases, apart form specific constants

Inflationary cosmology- A new approach using Non-linear electrodynamics

We explore a new kind of NLED field as a source of gravity, which can accelerate the Universe during the inflationary era. We propose a new type of NLED lagrangian which is characterised by two parameters: α (dimensionless parameter) and β (dimensionful parameter). We investigate the classical stability and the causality aspects of this model of inflationary expansion by demanding that the speed of the sound wave and . Corresponding to , we find for α = 0.1(1.0). The equation of state parameter ω = −1/3 requires corresponding to . We find that the Universe is accelerating i.e. (which results in the deceleration parameter q < 0 (i.e. ω > −1/3)), provided . During inflation, the energy density is found to be maximum and is given by . The magnetic field necessary to trigger the inflation is found to be , where is the energy density of the Universe during the inflationary expansion. Our model also predicts the e-fold number that the magnetic field at the end of inflation is about B = 10−10 (10−4) Gauss corresponding to z = 0(1000) and this agrees quite well with the experimental prediction of the e-fold number. With α = 0.3(1.0) and , we find the scalar spectral index n s = 0.9649, consistent with the PLANCK 2018 CMB data. Further, with , we predicts the tensor-to-scalar ratio r = 0.1417(0.1449) and the tensorial spectral index .