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- Conformal GUT inflation, proton lifetime and non-thermal leptogenesisPublication . Kumar, K. Sravan; Moniz, PauloIn this paper, we generalize Coleman-Weinberg (CW) inflation in grand unified theories (GUTs) such as $\text{SU}(5)$ and $\text{SO}(10)$ by means of considering two complex singlet fields with conformal invariance. In this framework, inflation emerges from a spontaneously broken conformal symmetry. The GUT symmetry implies a potential with a CW form, as a consequence of radiative corrections. The conformal symmetry flattens the above VEV branch of the CW potential to a Starobinsky plateau. As a result, we obtain $n_{s}\sim 1-\frac{2}{N}$ and $r\sim \frac{12}{N^2}$ for $N\sim 50-60$ e-foldings. Furthermore, this framework allow us to estimate the proton lifetime as $\tau_{p}\lesssim 10^{40}$ years, whose decay is mediated by the superheavy gauge bosons. Moreover, we implement a type I seesaw mechanism by weakly coupling the complex singlet, which carries two units of lepton number, to the three generations of singlet right handed neutrinos (RHNs). The spontaneous symmetry breaking of global lepton number amounts to the generation of neutrino masses. We also consider non-thermal leptogenesis in which the inflaton dominantly decays into heavy RHNs that sources the observed baryon asymmetry. We constrain the couplings of the inflaton field to the RHNs, which gives the reheating temperature as $10^{6}\text{ GeV}\lesssim T_{R}<10^{9}$ GeV.
- Non-slow-roll dynamics in α-attractorsPublication . Kumar, K. Sravan; Marto, João; Moniz, Paulo; Das, SuratnaIn this paper we consider the α−attractor model and study inflation under a non-slow-roll dynamics. More precisely, we follow the approach recently proposed by Gong and Sasaki [1] by means of assuming N=N(phi). Within this framework we obtain a family of functions describing the local shape of the potential during inflation. We study a specific model and find an inflationary scenario predicting an attractor at ns≈0.967 and r≈5.5×10^(−4). We further show that considering a non-slow-roll dynamics, the α−attractor model can be broaden to a wider class of models that remain compatible with value of r<0.1. We further explore the model parameter space with respect to large and small field inflation and conclude that the inflaton dynamics is connected to the α− parameter, which is also related to the Kähler manifold curvature in the supergravity (SUGRA) embedding of this model. We also comment on the stabilization of the inflaton's trajectory.
- Gravitational waves in α-attractorsPublication . Kumar, K. Sravan; Marto, João; Moniz, Paulo; Das, SuratnaWe study inflation in the alpha-attractor model under a non-slow-roll dynamics with an ansatz proposed by Gong & Sasaki of assuming N=N(phi). Under this approach, we construct a class of local shapes of inflaton potential that are different from the T-models. We find this type of inflationary scenario predicts an attractor at n_s~0.967 and r~0.00055. In our approach, the non-slow-roll inflaton dynamics are related to the $\alpha-$parameter which is the curvature of Kähler geometry in the SUGRA embedding of this model.
- K-essence model from the mechanical approach point of view: coupled scalar field and the late cosmic accelerationPublication . Bouhmadi Lopez, Mariam; Kumar, K. Sravan; Marto, João; Morais, João; Zhuk, AlexanderIn this paper, we consider the Universe at the late stage of its evolution and deep inside the cell of uniformity. At these scales, we can consider the Universe to be filled with dust-like matter in the form of discretely distributed galaxies, a K-essence scalar field, playing the role of dark energy, and radiation as matter sources. We investigate such a Universe in the mechanical approach. This means that the peculiar velocities of the inhomogeneities (in the form of galaxies) as well as the fluctuations of the other perfect fluids are non-relativistic. Such fluids are designated as coupled because they are concentrated around the inhomogeneities. In the present paper, we investigate the conditions under which the K-essence scalar field with the most general form for its action can become coupled. We investigate at the background level three particular examples of the K-essence models: (i) the pure kinetic K-essence field, (ii) a K-essence with a constant speed of sound and (iii) the K-essence model with the Lagrangian bX+cX2−V(phi). We demonstrate that if the K-essence is coupled, all these K-essence models take the form of multicomponent perfect fluids where one of the component is the cosmological constant. Therefore, they can provide the late-time cosmic acceleration and be simultaneously compatible with the mechanical approach.
- Inflation candidates: from string theory to particles physicsPublication . Korumilli, Sravan Kumar; Moniz, Paulo Rodrigues Lima VargasCosmic inflation is the cornerstone of modern cosmology. In particular, following the Planck mission reports presented in 2015 regarding cosmic microwave background (CMB), there is an increasing interest in searching for inflaton candidates within fundamental theories and to ultimately test them with future CMB data. This thesis presents inflationary models using a methodology that can be described as venturing top-down or bottom-up along energy scales. In the top-down motivation, we study inflationary scenarios in string theory and supergravity (SUGRA), namely with (multiple) 3-forms, Dirac-Born-Infeld Galileon model, a string field theory setup and N = 1 SUGRA attractor models. In the bottom-up motivation, we construct a grand unified theory based inflationary model with an additional conformal symmetry and study not only inflation but also provide predictions related to particle physics. Our research work includes various classes of inflation driven by scalar fields under a canonical, non-canonical and induced gravity frameworks. All these models are consistent with Planck data, supported by key primordial cosmological parameters such as the scalar spectral index ns, the tensor to scalar ratio r, together with the primordial non-Gaussianities. Future probes aiming to detect primordial gravitational waves and CMB non-Gaussianities can further help to distinguish between them.
- Inflation in a two 3-form fields scenarioPublication . Kumar, K. Sravan; Marto, João; Nunes, Nelson J.; Moniz, PauloA setting constituted by N 3-form fields, without any direct interaction between them, minimally coupled to gravity, is introduced in this paper as a framework to study the early evolution of the universe. We focus particularly on the two 3-forms case. An inflationary scenario is found, emerging from the coupling to gravity. More concretely, the fields coupled in this manner exhibit a complex interaction, mediated by the time derivative of the Hubble parameter. Our investigation is supported by means of a suitable choice of potentials, employing numerical methods and analytical approximations. In more detail, the oscillations on the small field limit become correlated, and one field is intertwined with the other. In this type of solution, a varying sound speed is present, together with the generation of isocurvature perturbations. The mentioned features allow to consider an interesting model, to test against observation. It is subsequently shown how our results are consistent with current CMB data (viz.Planck and BICEP2).
- DBI Galileon inflation in the light of Planck 2015Publication . Kumar, K. Sravan; Sánchez, Juan C. Bueno; Escamilla-Rivera, Celia; Marto, João; Moniz, PauloIn 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 (tilde m). We study the parameter space of the model and find that the tensor-to-scalar ratio can be as low as r sime 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.
- Interacting 3-form dark energy models: distinguishing interactions and avoiding the Little Sibling of the Big RipPublication . Morais, João; Bouhmadi Lopez, Mariam; Kumar, K. Sravan; Marto, João; Tavakoli, YaserIn this paper we consider 3-form dark energy (DE) models with interactions in the dark sector. We aim to distinguish the phenomenological interactions that are defined through the dark matter (DM) and the DE energy densities. We do our analysis mainly in two stages. In the first stage, we identify the non-interacting 3-form DE model which generically leads to an abrupt late-time cosmological event which is known as the little sibling of the Big Rip (LSBR). We classify the interactions which can possibly avoid this late-time abrupt event. We also study the parameter space of the model that is consistent with the interaction between DM and DE energy densities at present as indicated by recent studies based on BAO and SDSS data. In the later stage, we observationally distinguish those interactions using the statefinder hierarchy parameters {S3(1),S4 (1)},{S3 (1),S5 (1)} . We also compute the growth factor parameter ε(z) for the various interactions we consider herein and use the composite null diagnostic (CND) {S3(1), ε(z)} as a tool to characterise those interactions by measuring their departures from the concordance model. In addition, we make a preliminary analysis of our model in light of the recently released data by SDSS~III on the measurement of the linear growth rate of structure.
- Non-Gaussianity in multiple three-form field inflationPublication . Kumar, K. Sravan; Mulryne, David J.; Nunes, Nelson J.; Marto, João; Moniz, PauloIn this work, we present a method for implementing the δN formalism to study the primordial non-Gaussianity produced in multiple three-form field inflation. Using a dual description relating three-form fields to noncanonical scalar fields, and employing existing results, we produce expressions for the bispectrum of the curvature perturbation in terms of three-form quantities. We study the bispectrum generated in a two three-form field inflationary scenario for a particular potential that for suitable values of the parameters was found in earlier work to give values of the spectral index and ratio of tensor to scalar perturbations compatible with current bounds. We calculate the reduced bispectrum for this model, finding an amplitude in equilateral and orthogonal configurations of δ(1) and in the squeezed limit of δ(10-3). We confirm, therefore, that this three-form inflationary scenario is compatible with present observational constraints.
- Effective models of inflation from a non-local frameworkPublication . Koshelev, Alexey; Kumar, K. Sravan; Moniz, PauloThe dilaton is a possible inflaton candidate following recent CMB data allowing a non-minimal coupling to the Ricci curvature scalar in the early Universe. In this paper, we introduce an approach that has seldom been used in the literature, namely dilaton inflation with non-local features. More concretely, employing non-local features expressed in J. High Energy Phys. 04 (2007) 029, we study quadratic variations around a de Sitter geometry of an effective action with a non-local dilaton. The non-locality refers to an infinite derivative kinetic term involving the operator $\mathcal{F}\left(\Box\right)$. Algebraic roots of the characteristic equation $\mathcal{F}(z)=0$ play a crucial role in determining the properties of the theory. We subsequently study the cases when $\mathcal{F}\left(\Box\right)$ has one real root and one complex root, from which we retrieve two concrete effective models of inflation. In the first case we retrieve a class of single field inflations with universal prediction of $n_{s}\sim0.967$ with any value of the tensor to scalar ratio $r<0.1$ intrinsically controlled by the root of the characteristic equation. The second case involves a new class of two field conformally invariant models with a peculiar quadratic cross-product of scalar fields. In this latter case, we obtain Starobinsky like inflation through a spontaneously broken conformal invariance. Furthermore, an uplifted minimum of the potential, which accounts for the vacuum energy after inflation is produced naturally through this mechanism intrinsically within our approach.