Galileon Inflation Research Articles

Scalar induced gravity waves from ultra slow-roll galileon inflation

We consider the production of secondary gravity waves in Galileon inflation with an ultra-slow roll (USR) phase and show that the spectrum of scalar-induced gravitational waves (SIGWs) in this case is consistent with the recent NANOGrav 15-year data and with sensitivities of other ground and space-based missions, LISA, BBO, DECIGO, CE, ET, HLVK (consists of aLIGO, aVirgo, and KAGRA), and HLV(03). Thanks to the non-renormalization property of Galileon theory, the amplitude of the large fluctuation is controllable at the sharp transitions between SR and USR regions. We show that the behavior of the GW spectrum, when one-loop effects are included in the scalar power spectrum, is preserved under a shift of the sharp transition scale with peak amplitude ΩGWh2∼O(10−6), and hence it can cover a wide range of frequencies within O(10−9Hz−107Hz). An analysis of the allowed mass range for primordial black holes (PBHs) is also performed, where we find that mass values ranging from O(1M⊙−10−18M⊙) can be generated over the corresponding allowed range of low and high frequencies.

Primordial non-Gaussianity as a saviour for PBH overproduction in SIGWs generated by pulsar timing arrays for Galileon inflation

We investigate the explicit role of negative local non-Gaussianity, fNL, in suppressing the abundance of primordial black holes (PBHs) in the single-field model of Galileon inflation. PBH formation requires significantly enhancing the scalar power spectrum, which greatly affects their abundance. The associated frequencies in the nHz regime are also sensitive to the generation of scalar-induced gravitational waves (SIGWs) which may explain the current data from the pulsar timing arrays (PTAs). Our analysis using the threshold statistics on the compaction function demonstrates that Galileon theory not only avoids PBH overproduction using the curvature perturbation enhancements that give fNL∼O(−6), but also generates SIGWs that conform well with the PTA data.

Realisation of the ultra-slow roll phase in Galileon inflation andPBH overproduction

We demonstrate the explicit realisation of the ultra-slow roll phase in the framework of the effective field theory of single-field Galileon inflation. The pulsar timing array (PTA) collaboration hints at the scalar-induced gravity waves (SIGW) from the early universe as an explanation for the origin of the observed signal, which, however, leads to an enhancement in the amplitude of the scalar power spectrum giving rise to the overproduction of primordial black holes (PBHs). In the setup under consideration, we examine the generation of SIGW consistent with PTA (NANOGrav15 and EPTA) data, in addition to which we also consider the impact from QCD crossover at the nHz frequencies and address the PBH overproduction issue assuming linear approximations for the over-density without incorporating non-Gaussian effects from the comoving curvature perturbation. The framework is shown to give rise to SIGWs well consistent with the PTA signal with comfortable PBH abundance, 10-3 ≲ fPBH < 1, of near solar-mass black holes.

Primordial non-Gaussianity from ultra slow-roll Galileon inflation

We present a detailed study of the generation of large primordial non-Gaussianities during the slow-roll (SR) to ultra-slow roll (USR) transitions in the framework of Galileon inflation. We found out that due to having sharp transitions in the USR phase, which persist with a duration of Δ𝒩USR ∼ 2 e-folds, we are able to generate the non-Gaussianity amplitude of the order: |f NL| ∼ 𝒪(10-2) in the SRI, -5 < f NL < 5 in the USR, and -2 < f NL < 2 in the SRII phases. As a result, we are able to achieve a cumulative average value of |f NL| ∼ 𝒪(1). This implies that our results strictly satisfy Maldacena's no-go theorem in the squeezed limit only for SRI, while they strictly violate the same condition in both the USR and SRII phases.The non-renormalization theorem in the Galileon theory helps to support our results regarding the generation of large mass primordial black holes along with large non-Gaussianities, which we show to be dependent on the specific positions of the transition wave numbers fixed at low scales.

Galileon inflation evades the no-go for PBH formation in the single-field framework

We consider Galileon inflation in the Effective Field Theory (EFT) framework and examine the possibility for PBH formation during slow roll (SR) to ultra slow roll (USR) transitions. We show that loop corrections to the power spectrum, in this case, do not impose additional constraints on the masses of PBHs produced. We indicate that the remarkable non-renormalization property of Galileans due to generalized shift symmetry dubbed as Galilean symmetry is responsible for protecting PBH formation from quantum loop corrections.

Mechanism of primordial black holes production and secondary gravitational waves in α-attractor Galileon inflationary scenario

We study the process of the Primordial Black Holes (PBHs) production in the novel framework, namely α-attractor Galileon inflation (G-inflation) model. In our framework, we take the Galileon function as G(ϕ)=GI (ϕ)(1+GII (ϕ)), where the part GI (ϕ) is motivated from the α-attractor inflationary scenario in its original non-canonical frame, and it ensures for the model to be consistent with the Planck 2018 observations at the CMB scales. The part GII (ϕ) is invoked to enhance the curvature perturbations at some smaller scales which in turn gives rise to PBHs formation. By fine-tuning of the model parameters, we find three parameter sets which successfully produce a sufficiently large peak in the curvature power spectrum. We show that these parameter sets produce PBHs with masses \U0001d4aa(10)M ☉, \U0001d4aa(10-5)M ☉, and \U0001d4aa(10-13)M ☉ which can explain the LIGO events, the ultrashort-timescale microlensing events in OGLE data, and around 0.98% of the current Dark Matter (DM) content of the universe, respectively. Additionally, we study the secondary Gravitational Waves (GWs) in our setup and show that our model anticipates the peak of their present fractional energy density as ΩGW0 ∼ 10-8 for all the three parameter sets, but at different frequencies. These predictions can be located well inside the sensitivity region of some GWs detectors, and therefore the compatibility of our model can be assessed in light of the future data. We further estimate the tilts of the included GWs spectrum in the different ranges of frequency, and confirm that spectrum follows the power-law relation ΩGW0 ∼ fn in those frequency bands.

α-Attractors and reheating in a class of Galileon inflation

We study cosmological inflation in a Galileon inflationary model with the E-model potential to find possible [Formula: see text]-attractors. First, we calculate evolution of the perturbations in our setup. By adopting E-model potential, we show that values of the scalar spectral index and tensor-to-scalar ratio are universal. Also, we consider the reheating phase after inflation in this model. We compare the results with the Planck data and we find some constraints on the model’s parameter space. We find that in a Galileon inflation model, the reheating phase can last longer than that in the Standard Model. Also, in this model, the larger range of temperature is corresponding to observational viable values of [Formula: see text].

Galileon inflation field with three dark eras of the universe

In this paper, we discuss possible three early eras of the universe in the theoretical context of the Galilean cosmology. The first phase is a slow-roll inflation phase in which the friction term in the equations of motion dominates over the kinetic term. This is the initial vacuum state leading to quasi-de Sitter expansion. The second phase is where the cosmological perturbations are supposed to be generated. In this phase, the friction term is supposed to be negligible which leads to the appearance of the Galileon inflation field. The third phase is the reheating phase in the standard cosmology, where the oscillations of a canonical scalar field in the model lead to this phase. These three eras for the early universe are shown by a single master equation of the theory. However, we compute observational indices (the scalar spectral index parameter and the tensor-to-scalar ratio) for the era where the era that the perturbations are produced. Finally, we compare viability of the theoretical findings with the latest Planck observational data.

G-inflation: from the intermediate, logamediate and exponential models

The intermediate, logamediate and exponential inflationary models in the context of Galileon inflation or G-inflation are studied. By assuming a coupling of the form G(phi ,X)propto phi ^{nu },X^{n} in the action, we obtain different analytical solutions from the background cosmological perturbations assuming the slow-roll approximation. General conditions required for these models of G-inflation to be realizable are determined and discussed. In general, we analyze the condition of inflation and also we use recent astronomical and cosmological observations for constraining the parameters appearing in these G-inflationary models.

Realization of blue spectrum in generalized Galileon super-inflation models

In the spirit of Galileon inflation and by considering some sorts of noncanonical kinetic terms in the action, we realize a stage of super-inflation leading to a blue-tilted tensor perturbation. We show also that addition of Galileon-like term to the action leads to avoidance of ghost instabilities in this setup.

Consistency relation and non-Gaussianity in a Galileon inflation

We study a particular Galileon inflation in the light of Planck2015 observational data in order to constraint the model parameter space. We study the spectrum of the primordial modes of the density perturbations by expanding the action up to the second order in perturbations. Then we pursue by expanding the action up to the third order and find the three point correlation functions to find the amplitude of the non-Gaussianity of the primordial perturbations in this setup. We study the amplitude of the non-Gaussianity both in equilateral and orthogonal configurations and test the model with recent observational data. Our analysis shows that for some ranges of the non-minimal coupling parameter, the model is consistent with observation and it is also possible to have large non-Gaussianity which would be observable by future improvements in experiments. Moreover, we obtain the tilt of the tensor power spectrum and test the standard inflationary consistency relation (r = −8nT) against the latest bounds from the Planck2015 dataset. We find a slight deviation from the standard consistency relation in this setup. Nevertheless, such a deviation seems not to be sufficiently remarkable to be detected confidently.

DBI Galileon inflation in the light of Planck 2015

In this work we consider a DBI Galileon (DBIG) inflationary model and constrain its parameter space with the Planck 2015 and BICEP2/Keck array and Planck (BKP) joint analysis data by means of a potential independent analysis. We focus our attention on inflationary solutions characterized by a constant or varying sound speed as well as warp factor. We impose bounds on stringy aspects of the model, such as the warp factor (f) and the induced gravity parameter (m̃). We study the parameter space of the model and find that the tensor-to-scalar ratio can be as low as r ≃ 6 × 10−4 and inflation happens to be at GUT scale. In addition, we obtain the tilt of the tensor power spectrum and test the standard inflationary consistency relation (r = −8nt) against the latest bounds from the combined results of BKP+Laser Interferometer Gravitational-Waves Observatory (LIGO), and find that DBIG inflation predicts a red spectral index for the tensor power spectrum.

Primordial non-Gaussian features from DBI Galileon inflation

We have studied primordial non-Gaussian features from a model of potential driven single field DBI Galileon inflation. We have computed the bispectrum from the three point correlation function considering all possible cross correlation between scalar and tensor modes from the proposed setup. Further, we have computed the trispectrum from four point correlation function considering the contribution from contact interaction, scalar and graviton exchange diagrams in the in-in picture. Finally we have obtained the non-Gaussian consistency conditions from the four point correlator, which results in partial violation of the Suyama-Yamaguchi four-point consistency relation. This further leads to the conclusion that sufficient primordial non-Gaussianities can be obtained from DBI Galileon inflation.

Galileogenesis: A new cosmophenomenological zip code for reheating through R-parity violating coupling

In this paper we introduce an idea of leptogenesis scenario in higher derivative gravity induced DBI Galileon framework aka Galileogenesis in presence of one-loop R-parity violating couplings in the background of a low energy effective supergravity setup derived from higher dimensional string theory framework. We have studied extensively the detailed feature of reheating constraints and the cosmophenomenological consequences of thermal gravitino dark matter in light of PLANCK and PDG data. Finally, we have also established a direct cosmophenomenological connection among dark matter relic abundance, reheating temperature and tensor-to-scalar ratio in the context of DBI Galileon inflation.

On the trispectrum of Galileon inflation

We present a detailed study of the trispectrum of the curvature perturbation generated within a stable, well defined and predictive theory which comprises an inflationary phase. In this model the usual shift symmetry is enhanced up to the so-called Galileon symmetry. The appeal of this type of theories rests on being unitary and stable under quantum corrections. Furthermore, in the specific model under consideration here, these properties have been shown to approximately hold in realistic scenarios which account for curved spacetime and the coupling with gravity.In the literature, the analysis of the bispectrum of the curvature perturbation for this theory revealed non-Gaussian features which are shared by a number of inflationary models, including stable ones.It is therefore both timely and useful to investigate further and turn to observables such as the trispectrum. We find that, in a number of specific momenta configurations, the trispectrum shape-functions present strikingly different features as compared to, for example, the entire class of the so-called P(X,ϕ) inflationary models.

Multifield extension ofGinflation

We propose a multifield extension of (generalized) G inflation, based on covariant multi-Galileons and their generalization preserving second-order field equations. We compute the quadratic action for cosmological perturbations. By comparing the formulas for cosmological perturbations, it is highlighted that multifield Dirac-Born-Infield Galileon inflation is not included in the multifield version of generalized G inflation. Our result indicates that the generalized covariant multi-Galileon theory is not the most general multi-scalar-tensor theory with second-order field equations.

DBI Galileon in the effective field theory of inflation: orthogonal non-Gaussianities and constraints from the Trispectrum

Very few explicit inflationary scenarios are known to generate a large bispectrum of orthogonal shape. Dirac-Born-Infeld Galileon inflation, in which an induced gravity term is added to the DBI action, is one such model. We formulate it in the language of the effective field theory of inflation by identifying the unitary gauge operators that govern the behavior of its cosmological fluctuations. We show how to recover rather easily from this its power spectrum and bispectrum, which we calculated previously using standard cosmological perturbation theory. We push our calculations up to the determination of the fourth-order action and of the trispectrum, in which shapes absent in k-inflation arise due to the presence of higher-order derivative operators. We finally discuss the combined constraints set on this model by current observational bounds on the bispectrum and trispectrum.

Planck constraints on single-field inflation

We place observational constraints on slow-variation single-field inflationary models by carrying out the cosmological Monte Carlo simulation with the recent data of Planck combined with the WMAP large-angle polarization, baryon acoustic oscillations, and ACT/SPT temperature data. Our analysis covers a wide variety of models with second-order equations of motion-- including potential-driven slow-roll inflation, non-minimally coupled models, running kinetic couplings, Brans-Dicke theories, potential-driven Galileon inflation, field-derivative couplings to the Einstein tensor, and k-inflation. In the presence of running kinetic exponential couplings, covariant Galileon terms, and field-derivative couplings, the tensor-to-scalar ratio of the self-coupling potential gets smaller relative to that in standard slow-roll inflation, but the models lie outside the 68 % CL observational contour. We also show that k-inflation models can be tightly constrained by adding the bounds from the scalar non-Gaussianities. The small-field inflationary models with asymptotic flat Einstein-frame potentials in the regime phi >> M_{pl} generally fit the data very well. These include the models such as Kahler-moduli inflation, non-minimally coupled Higgs inflation, and inflation in Brans-Dicke theories in the presence of the potential V(phi)=3M^2 (phi-M_{pl})^2/4 with the Brans-Dicke parameter omega_{BD}<O(1) (which covers the Starobinsky's model f(R)=R+R^2/(6M^2) as a special case).

DBI Galileon inflation in background SUGRA

We introduce a model of potential driven DBI Galileon inflation in background N=1, D=4 SUGRA. Starting from D4–D4¯ brane–antibrane in the bulk N=2, D=5 SUGRA including quadratic Gauss–Bonnet corrections, we derive an effective N=1, D=4 SUGRA by dimensional reduction, that results in a Coleman–Weinberg type Galileon potential. We employ this potential in modeling inflation and in subsequent study of primordial quantum fluctuations for scalar and tensor modes. Further, we estimate the major observable parameters in both de Sitter (DS) and beyond de Sitter (BDS) limits and confront them with recent observational data from WMAP7 by using the publicly available code CAMB.

Potential-driven Galileon inflation

For the models of inflation driven by the potential energy of an inflaton field ϕ, the covariant Galileon Lagrangian (∂ϕ)2□ϕ generally works to slow down the evolution of the field.On the other hand, if the Galileon self-interaction is dominant relative to the standard kinetic term, we show that there is no oscillatory regime of inflaton after the end of inflation.This is typically accompanied by the appearance of the negative propagation speed squared cs2 of a scalar mode, which leads to the instability of small-scale perturbations.For chaotic inflation and natural inflation we clarify the parameter space in which inflaton oscillates coherently during reheating.Using the WMAP constraints of the scalar spectral index and the tensor-to-scalar ratio as well, we find that the self coupling λof the potential V(ϕ) = λϕ4/4 is constrained to be very much smaller than 1 and that the symmetry breaking scale f of natural inflationcannot be less than the reduced Planck mass Mpl.We also show that, in the presence of other covariant Galileon Lagrangians,there are some cases in which inflaton oscillates coherently even forthe self coupling λ of the order of 0.1, but still the instability associated with negative cs2 is generally present.