Inflaton Potential Research Articles

Detuning primordial black hole dark matter with early matter domination and axion monodromy

We present a scenario that ameliorates the tuning problems present in models of primordial black hole (PBH) dark matter from inflation. Our setup employs the advantages of gravitational collapse in a long epoch of early matter domination with reheating temperature ≲ 106 GeV. Furthermore, we make use of a string-inspired class of models where the inflaton is identified with a non-compact axion field. In this framework, the presence of multiple local minima in the inflaton potential can be traced back to an approximate discrete shift symmetry. This scenario allows the formation of PBHs in the observationally viable range of masses (MPBH∼ 10−16M⊙–10−13M⊙) accounting for all dark matter, and in excellent agreement with the CMB . We find a significant reduction in the required tuning of the parameters of the inflationary potential, in contrast to the standard case of PBH formation during radiation domination. However, abundant formation of light PBHs during an early phase of matter domination can be more easily in conflict with evaporation bounds. We discuss how these can be avoided under mild assumptions on the collapsing energy density fraction.

A pure general relativistic non-singular bouncing origin for the Universe

In this article, we argue that the past of the Universe, extrapolated from standard physics and measured cosmological parameters, might be a non-singular bounce. We also show that, in this framework, quite stringent constraints can be put on the reheating temperature and number of inflationary e-folds, basically fixing T_{RH}sim T_{GUT} and Nsim 70. We draw some conclusions about the shape of the inflaton potential and raise the “naturalness” issue in this context. Finally, we argue that this could open a very specific window on the “pre big bounce" universe.

Primordial black holes from a tiny bump/dip in the inflaton potential

Scalar perturbations during inflation can be substantially amplified by tiny features in the inflaton potential. A bump-like feature behaves like a local speed-breaker and lowers the speed of the scalar field, thereby locally enhancing the scalar power spectrum. A bump-like feature emerges naturally if the base inflaton potential Vb(ϕ) contains a local correction term such as Vb(ϕ)[1+ε(ϕ)] at ϕ=ϕ0. The presence of such a localised correction term at ϕ0 leads to a large peak in the curvature power spectrum and to an enhanced probability of black hole formation. Remarkably this does not significantly affect the scalar spectral index nS and tensor to scalar ratio r on CMB scales. Consequently such models can produce higher mass primordial black holes (MPBH⩾ 1 M⊙) in contrast to models with `near inflection-point potentials' in which generating higher mass black holes severely affects nS and r. With a suitable choice of the base potential—such as the string theory based (KKLT) inflation or the α-attractor models—the amplification of primordial scalar power spectrum can be as large as 107 which leads to a significant contribution of primordial black holes (PBHs) to the dark matter density today, fPBH = Ω0, PBH/Ω0,DM ∼ O(1). Interestingly, our results remain valid if the bump is replaced by a dip. In this case the base inflaton potential Vb(ϕ) contains a negative local correction term such as Vb(ϕ)[1−ε(ϕ)] at ϕ=ϕ0 which leads to an enhanced probability of PBH formation. We conclude that primordial black holes in the mass range 10−17 M⊙ ⩽ MPBH ⩽ 100 M⊙ can easily form in single field inflation in the presence of small bump-like and dip-like features in the inflaton potential.

New K\xe4hler invariant Fayet\u2013Iliopoulos terms in supergravity and cosmological applications

Recently, a new type of constant Fayet–Iliopoulos (FI) terms was introduced in {mathcal {N}}=1 supergravity, which do not require the gauging of the R-symmetry. We revisit and generalise these constructions, building a new class of Kähler invariant FI terms parametrised by a function of the gravitino mass as functional of the chiral superfields, which is then used to describe new models of inflation. They are based on a no-scale supergravity model of the inflaton chiral multiplet, supplemented by an abelian vector multiplet with the new FI-term. We show that the inflaton potential is compatible with the CMB observational data, with a vacuum energy at the minimum that can be tuned to a tiny positive value. Finally, the axionic shift symmetry can be gauged by the U(1) which becomes massive. These models offer a mechanism for fixing the gravitino mass in no-scale supergravities, that corresponds to a flat direction of the scalar potential in the absence of the new FI-term; its origin in string theory is an interesting open problem.

Initial conditions for plateau inflation: a case study

We study initial conditions for inflation in scenarios where the inflaton potential has a plateau shape. Such models are those most favored by Planck data and can be obtained in a large number of model classes. As a representative example, we consider Higgs inflation with and without an R2 term in the context of Palatini gravity. We show that inflation with a large number of e-folds generically occurs in a large part of the parameter space without any fine-tuning of parameters even when the scale of inflation and the inflaton field value during inflation are much smaller than the Planck scale. We discuss consequences for detection of primordial gravitational waves and spectral tilt of curvature perturbations, as well as the recently proposed “Trans-Planckian Censorship” conjecture.

A walking dilaton inflation

We propose an inflationary scenario based on a many-flavor hidden QCD with eight flavors, which realizes the almost scale-invariant (walking) gauge dynamics. The theory predicts two types of composite (pseudo) Nambu-Goldstone bosons, the pions and the lightest scalar (dilaton) associated with the spontaneous chiral symmetry breaking and its simultaneous violation of the approximate scale invariance. The dilaton acts as an inflaton, where the inflaton potential is induced by the nonperturbative-scale anomaly linked with the underlying theory. The inflaton potential parameters are highly constrained by the walking nature, which are evaluated by straightforward nonperturbative analyses including lattice simulations. Due to the pseudo Nambu-Goldstone boson's natures and the intrinsic property for the chiral symmetry breaking in the walking gauge dynamics, the inflaton coupled to the pions naturally undergoes the small field inflation consistently with all the cosmological and astrophysical constraints presently placed by Planck 2018 data. When the theory is vector-likely coupled to the standard model in part in a way to realize a dynamical electroweak symmetry breaking, the reheating temperature is determined by the pion decays to electroweak gauge bosons. The proposed inflationary scenario would provide a dynamical origin for the small field inflation as well as the light pions as a smoking-gun to be probed by future experiments.

Minimal warm inflation

Slow-roll inflation is a successful paradigm. However we find that even a small coupling of the inflaton to other light fields can dramatically alter the dynamics and predictions of inflation. As an example, the inflaton can generically have an axion-like coupling to gauge bosons. Even relatively small couplings will automatically induce a thermal bath during inflation. The thermal friction from this bath can easily be stronger than Hubble friction, significantly altering the usual predictions of any particular inflaton potential. Thermal effects suppress the tensor-to-scalar ratio r significantly, and predict unique non-gaussianities. This axion-like coupling provides a minimal model of warm inflation which avoids the usual problem of thermal backreaction on the inflaton potential. As a specific example, we find that hybrid inflation with this axion-like coupling can easily fit the current cosmological data.

Chaotic inflation with four-form couplings

We consider chaotic inflation models with a pseudo-scalar field containing the general couplings to the four-form flux. The four-form mixing with the pseudo-scalar field induces a quadratic potential for inflaton while the coexisting four-form mixing with graviton generates a non-minimal gravity coupling for inflaton. The shift symmetry is respected by the derived inflaton couplings and it is broken spontaneously only by the fixed four-form flux. We discuss the success of inflationary predictions and robustness against higher order terms. Finally, the built-in reheating mechanism is also addressed.

Fuzzy Euclidean wormholes in the inflationary universe

In this paper, we investigate complex-valued Euclidean wormholes in the Starobinsky inflation. Due to the properties of the concave inflaton potential, the classicality condition at both ends of the wormhole can be satisfied, as long as the initial condition of the inflaton field is such that it is located sufficiently close to the hilltop. We compare the probabilities of classicalized wormholes with the Hartle-Hawking compact instantons and conclude that the Euclidean wormholes are probabilistically preferred than compact instantons, if the inflation lasts more than 50 e-foldings. Our result assumes that the Euclidean path integral is the correct effective description of quantum gravity. This opens a new window for various future investigations that can be either confirmed or refuted by future experiments.

Primordial black hole formation in inflationary α -attractor models

The formation of primordial black holes is studied using superconformal inflationary $\alpha$-attractors. An inflaton potential is constructed with a plateau-like region which brings about the onset of an ultra slow-roll region where the required enhancement in the curvature power spectrum takes place. This is accomplished by carefully fine-tuning the parameters in the potential. For the parameter sets, the amount of observable inflation is such that the curvature perturbation $\mathcal{P}_{\mathcal{R}}$ peaks at $\sim 10^{14}\text{Mpc}^{-1}$, producing PBHs in the mass window $10^{16}\text{g}\leq M \leq 10^{18}\text{g}$. The reheating period after inflation is taken into account to determine whether or not PBH formation takes place in such a phase. Finally, the spectrum of second order gravitational waves, that can arise due to the curvature perturbation enhancement, is calculated.

Exploring the inflation of F(R) gravity

The early-time expansion of the spacetime, namely, inflation, is introduced to solve some cosmological problems. [Formula: see text] gravity is a simple extension of the general relativity to induce the inflationary expansion. The precise observation of the Cosmic Microwave Background gives us the information to inspect the model of the inflation. [Formula: see text] gravity can be transformed to the Einstein–Hilbert term with a scalar field that plays the role of the inflaton. The inflaton potential is described as an explicit function of the inflaton field with a noncanonical kinetic term. In this paper, we obtain general formulae to derive the inflationary parameters including the models with a noncanonical kinetic term. The inflationary parameters are described as functions of the inflaton potential and its derivatives. We evaluate a well-known model, [Formula: see text], to confirm the validity of our formulae. Then, we apply the procedure to a model, [Formula: see text], in which it is not possible to represent the potential as an explicit function of the inflaton field with a canonical kinetic term.

Trans-Planckian Censorship and single-field inflaton potential

It was recently proposed that a field theory cannot be consistent with quantum gravity if it allows a mode shorter than the Planck length to exit the Hubble horizon. This is called the Trans-Planckian Censorship Conjecture (TCC). We discuss the implications of the TCC on the possible shape of the inflaton potential in single-field slow-roll inflation. We point out that (1) there is generically an initial condition in which the total e-folding number Ntotal is doubled or more compared to the e-folds necessary for the cosmic microwave background fluctuations, and (2) a sizable negative running of spectral index is generically expected to make Ntotal small. In concrete setups, we find a stringent constraint on the inflationary energy scale, Vinf1/4 < \U0001d4aa(10) TeV with r < \U0001d4aa(10−50), and the running parameter is bounded above as αs ≲ − 4 × 10−3.

Ricci subtraction for cosmological Coleman-Weinberg potentials

We reconsider the fine-tuning problem of scalar-driven inflation arising from the need to couple the inflaton to ordinary matter in order to make reheating efficient. Quantum fluctuations of this matter induce Coleman-Weinberg corrections to the inflaton potential, depending (for de Sitter background) in a complex way on the ratio of the inflaton to the Hubble parameter. These corrections are not Planck-suppressed and cannot be completely subtracted because they are not even local for a general geometry. A previous study showed that it is not satisfactory to subtract a local function of just the inflaton and the {\it initial} Hubble parameter. This paper examines the other allowed possibility of subtracting a local function of the inflaton and the Ricci scalar. The problem in this case is that the new, scalar degree of freedom induced by the subtraction causes inflation to end almost instantly.

Double-Running Inflaton Mass for a Flat Potential and Assisted Hilltop Inflation

We propose a scenario to flatten inflaton potential for a small-field slow-roll inflation from the origin. In the scenario, inflaton mass-square gets through a renormalization group running which depends on not only inflaton field but also an assisting field. Thanks to the assisting field participating in the running, initial condition for a slow-roll inflation can be set naturally, and inflaton potential can be flattened by the vacuum expectation value of the field. Applying this idea, we propose a scenario of inflation, dubbed here as \textit{assisted hilltop inflation}, which is free from the initial condition and the flatness problems of inflation.

Enlarging the space of viable inflation models: A slingshot mechanism

The viability of a given model for inflation is determined not only by the form of the inflaton potential, but also by the initial inflaton field configuration. In many models, field configurations which are otherwise well-motivated nevertheless fail to induce inflation, or fail to produce an inflationary epoch of duration sufficient to solve the horizon and flatness problems. In this paper, we propose a mechanism which enables inflation to occur even with such initial conditions. Our mechanism involves multiple scalar fields which experience a time-dependent mixing. This in turn leads to a "re-overdamping" phase as well as a parametric resonance which together "slingshot" the inflaton field from regions of parameter space that do not induce inflation to regions that do. Our mechanism is flexible, dynamical, and capable of yielding an inflationary epoch of sufficiently long duration. This slingshot mechanism can therefore be utilized in a variety of settings and thereby enlarge the space of potentially viable inflation models.

Constant-roll inflation driven by a scalar field with nonminimal derivative coupling

In this work, we study constant-roll inflation driven by a scalar field with nonminimal derivative coupling to gravity, via the Einstein tensor. This model contains a free parameter, [Formula: see text], which quantifies the nonminimal derivative coupling and a parameter [Formula: see text] which characterizes the constant-roll condition. In this scenario, using the Hamilton–Jacobi-like formalism, an ansatz for the Hubble parameter (as a function of the scalar field) and some restrictions on the model parameters, we found new exact solutions for the inflaton potential which include power-law, de Sitter, quadratic hilltop and natural inflation, among others. Additionally, a phase-space analysis was performed and it is shown that the exact solutions associated to natural inflation and a “cosh-type” potential, are attractors.

Warm pseudoscalar inflation

In this note, we will introduce an action for warm inflation model with direct coupling between the pseudoscalar field and mass-less $SU(2)$ gauge fields. The potential of inflaton is protected against the thermal corrections in a thermal bath of gauge fields even with strong direct interaction between inflaton and light fields. The dissipation parameter of this model is approximately constant in the high-dissipative regime. In this regime, the model is compatible with observational data and non-Gaussianity is in order of Hubble slow-roll parameter $\frac{\epsilon_{\phi}}{1+Q}$ even in $f<M_p$ limit.

(P)reheating after minimal plateau inflation and constraints from CMB

In this paper we have investigated the preheating phase for a class of plateau inflationary model considering the four-legs interaction term (1/2)g2ϕ2χ2 between the inflaton (ϕ) and reheating field (χ). We specifically focus on the effects of our model parameter ϕ* which controls inflationary dynamics. For ϕ* < Mp, the departure of the inflaton potential from the usual power-law behavior ϕn significantly modifies the microscopic behavior of the preheating dynamics. We analyze and compare the efficiency of production, thermalization behavior and the final equation of states of the system for different values of n = 2,4,6 considering two different values of ϕ*. Most importantly as we increase n, or decrease ϕ*, the preheating occurs very efficiently with the final equation of state to be that of the radiation, w = 1/3. However, for n = 2, the final equation of state turned out to be w ≃ 0.2. In the non-perturbative framework complete decay of inflaton could not be achieved with the four-legs interaction for any model under consideration. Therefore, in order to complete the reheating process, we perform the perturbative analysis for the second stage of the reheating phase. With the appropriate initial condition set by the non-perturbative dynamics, we solved a set of homogeneous Boltzmann equations for both the fields supplemented by the constraints coming from the subsequent entropy conservation. In so doing, we calculated the reheating temperature which was otherwise ill-defined right after the end of preheating. The temperature can be uniquely fixed for a given inflaton decay constant and the CMB temperature. We also compare our results with the conventional reheating constraint analysis and discuss the limits on the inflaton decay constant from the field theory perspective.

Numerical Study of Inflationary Preheating with Arbitrary Power-law Potential and a Realization of Curvaton Mechanism

Abstract During inflationary preheating, the energy stored in the inflaton field is rapidly converted into excitations of other entropy fields. This stage is characterized by exponential particle production due to parametric resonance and is notoriously difficult to analyze using analytic methods. We develop a detailed numerical simulation to investigate inflationary preheating when the potential of the inflaton is a power-law function with arbitrary power index. To achieve a successful graceful exit from a primordial inflationary phase to a smooth, oscillatory phase during preheating, we assume the inflaton potential reduces to a quadratic function in the infrared regime, which may be regarded as a mass term at low-energy scales. With this simplification, our numerical method may be applied to unconventional chaotic inflation models. To demonstrate its validity, we numerically analyze the preheating stage of axion-monodromy inflation, which is inspired by string theory. By performing perturbation analyses, our result shows that the power spectrum of primordial curvature perturbation can be dominated by fluctuations of entropy field rather than those of inflaton, which can be regarded as a particular realization of the curvaton mechanism through a preheating process.

Warm inflation in the light of swampland criteria

Warm Inflation seems to be the most befitting single-field slow-roll inflation scenario in the context of the recently proposed Swampland Criteria. We investigate the constraints these Swampland Criteria impose on Warm Inflation parameters and show that Warm Inflation is in accordance with both the current cosmological observations and the proposed Swampland Criteria in both weak and strong dissipative regimes depending on the value of the parameter $c$ which limits the slope of the inflaton potential according to the criteria.