Dirac-Born-Infeld Inflation Research Articles

Anisotropic power-law inflation for models of non-canonical scalar fields non-minimally coupled to a two-form field

In this paper, we investigate the validity of the so-called cosmic no-hair conjecture in the framework of anisotropic inflation models of non-canonical scalar fields non-minimally coupled to a two-form field. In particular, we focus on two typical k-inflation and Dirac–Born–Infeld inflation models, in which we find a set of exact anisotropic power-law inflationary solutions. Interestingly, these solutions are shown to be stable and attractive during an inflationary phase using the dynamical system analysis. The obtained results indicate that the non-minimal coupling between the scalar and two-form fields acts as a non-trivial source of generating stable spatial anisotropies during the inflationary phase and therefore violates the prediction of the cosmic no-hair conjecture, even when the scalar field is of non-canonical forms. In connection with the Planck 2018 data, tensor-to-scalar ratios of these anisotropic solutions are investigated. As a result, it appears that the tensor-to-scalar ratio of the anisotropic power-law inflationary solution of k-inflation model turns out to be more highly consistent with the Planck 2018 data than that of Dirac-Born-Infeld model.

The backreaction effect of sound speed resonance in DBI inflation

We examine the backreaction effect of the enhanced small-scale scalar perturbations from the sound speed resonance (SSR) mechanism for primordial black hole formation in Dirac-Born-Infeld (DBI) inflation Within the perturbative regime, the backreaction effect of perturbations on the background dynamics can be described by an effective action after integrating out the perturbation sector. Starting with the effective field theory of a specific DBI inflation model that incorporates SSR, we obtain the one-loop effective action by integrating out the scalar perturbations at the quadratic level. Using the effective Friedmann equations derived from this one-loop effective action, we solve the Hubble parameter with backreaction and the effective perturbation dynamics on this background as well. Our numerical findings reveal that, for a viable parameter space, the backreaction effect results in a relative correction to the Hubble parameter of approximately 10−7, whereas the relative correction to the slow-roll parameter can vary between −0.3 and 0.1, before gradually converging to 10−7. Furthermore, our results show that the backreaction effect on SSR sound speed causes a slight reduction in the resonant peak of the curvature power spectrum, and the subsequent PBH formation predicted by the SSR mechanism remains almost unchanged.

Observational viability of the intermediate DBI inflation in the presence of a minimal length

We consider an intermediate Dirac-Born-Infeld (DBI) inflationary model in the presence of a minimal measurable length in the theory. We show that the presence of a minimal measurable length modifies the definitions of the scalar and tensor spectral indices and also other inflation observables. This is due to modification of the momentum and corresponding wave number of the perturbations in the presence of a minimal length. By using the deformed definition of the scalar and tensor spectral indices, we perform numerical analysis on the intermediate DBI inflation model to find some constraints on the deformation parameter. In this regard, we compare our numerical results with both Planck2018 TT, TE, EE +lowE +lensing +BAO+ BK14 and Planck2018 TT, TE, EE +lowE+lensing+BK14 +BAO+LIGO and Virgo2016 data at the 68% CL and 95% CL. Our numerical study shows that the intermediate DBI inflation model in the presence of a minimal measurable length is observationally viable if the upper bound on the deformation parameter to be considered of the order of 1048 at 68% CL and 1049 at 95% CL. This is consistent with the results of other approaches to constrain such a quantity.

Dynamics of constant sound speed inspires warm inflation

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 [Formula: see text] and compute different slow-roll [Formula: see text], [Formula: see text] and [Formula: see text] as well as inflationary parameters (number of [Formula: see text]-folds [Formula: see text], spectral index [Formula: see text], scalar spectral index [Formula: see text] and tensor-to-scalar ratio [Formula: see text]) for specific anti-de Sitter warp factor of the DBI inflation. Next, we plot the trajectories of [Formula: see text], [Formula: see text] and [Formula: see text] with different choices of [Formula: see text] and found the ranges [Formula: see text], [Formula: see text], [Formula: see text] (for minimal inflation model), [Formula: see text], [Formula: see text], [Formula: see text] (for modified minimal inflation model) and [Formula: see text], [Formula: see text], [Formula: see text] (mixed large field inflation), respectively. Moreover, the ranges of these parametric plots for power-law plateau inflationary model are [Formula: see text], [Formula: see text], [Formula: see text]. 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 [Formula: see text] for modified minimal and power-law Plateau inflation models.

Dirac–Born–Infeld inflation under constant-roll conditions

In this paper, we aim to generalize the constant-roll inflation in the noncanonical set-up, in the context of scalar Dirac–Born–Infeld (DBI) theory. Assuming the simplest case, when the speed of sound of scalar fluctuations is constant, we obtain exact inflationary solutions under constant-roll conditions. We finally provide an observational bound to constrain the DBI constant-roll parameter by using the values of scalar spectral index and speed of sound provided by Planck and WMAP data.

DBI inflation and warped black holes

We study a possible amplification mechanism for the curvature perturbations generated during inflation by a Dirac-Born-Infeld (DBI) inflaton in the presence of a sharp feature in the warp factor. The large growth resulting in the scalar power spectrum is a consequence of the decreasing speed of sound. We obtain analytical approximate expressions for the relevant dynamical quantities both in the slow roll phase and during the transient phase which leads to the amplification. We finally compare our approximations to the exact numerical evolution and give, in such a context, the general rules for building a viable inflationary model leading to, during the subsequent radiation dominated era, an abundant primordial black holes formation.

A symmetric DBI action theory and its applications to inflationary cosmology

The Dirac-Born-Infeld (DBI) field theory in string theory is important and can provide the field of the universe’s inflation. At the same time, it provides a causal mechanism for generating the original density perturbation, thereby providing the necessary density perturbation for existing the dense and sparse matter distributions of the universe. We deduce a symmetric DBI action, introduce it into inflationary cosmology to calculate various inflation parameters, further calculate the scalar perturbation spectrum and the tensor-scalar ratio, which are compared with Planck + WMAP9 + BAO data, the power spectrum predicted by the new general DBI inflation theory satisfies the CMB Experiment constraints, i.e., is consistent with the current theories and experimental observations. Consequently, the theory of this paper conforms to current experiments and is supplying the current theories, and also a new way of explaining the inflation of the universe.

Dirac-Born-Infeld warm inflation realization in the strong dissipation regime

We consider warm inflation with a Dirac-Born-Infeld (DBI) kinetic term in which both the nonequilibrium dissipative particle production and the sound speed parameter slow the motion of the inflaton field. We find that a low sound speed parameter removes, or at least strongly suppresses, the growing function appearing in the scalar of curvature power spectrum of warm inflation, which appears due to the temperature dependence in the dissipation coefficient. As a consequence of that, a low sound speed helps to push warm inflation into the strong dissipation regime, which is an attractive regime from a model building and phenomenological perspective. In turn, the strong dissipation regime of warm inflation softens the microscopic theoretical constraints on cold DBI inflation. The present findings, along with the recent results from swampland criteria, give a strong hint that warm inflation may consistently be embedded into string theory.

Dirac-Born-Infeld realization of sound speed resonance mechanism for primordial black holes

We present a concrete realization of the sound speed resonance (SSR) mechanism for primordial black hole (PBH) formation within a specific model of Dirac-Born-Infeld (DBI) inflation. We perform a perturbative approach to phenomenologically construct such a viable DBI inflation model that involves the nonoscillating stage and the oscillating stage, with a type of specific forms of the warp factor and the potential. We show that the continuous but nonsmooth conjunction of sound speed between two stages does not yield manifest effects on the phenomenology of SSR, and thus, our model gives rise to the same PBH mass spectrum as the original predictions of SSR. Additionally, we also demonstrate that the violation of adiabaticity of the Mukhanov-Sasaki equation does not affect the comoving curvature perturbation after Hubble crossing in the nonresonant region. Making use of observational data, we derive various cosmological constraints on the parameter space. Our analyses show that the predicted tensor-to-scalar ratio is typically small, while the amplitude of primordial non-Gaussianity can meet with cosmic microwave background bounds, and additionally, the consistency relation for single-field slow-roll inflation is softly violated in our case due to the small sound speed variations.

The multi-feature universe: Large parameter space cosmology and the swampland

Under the belief that the universe should be multi-feature and informative, we employ a model-by-model comparison method to explore the possibly largest upper bound on the swampland constant c. Considering the interacting quintessence dark energy as the comparison model, we constrain the large parameter space interacting dark energy model, a 12-parameter extension to the ΛCDM cosmology, in light of current observations. We obtain the largest 2σ (3σ) bound so far, c≲1.62 (1.94), which would allow the existences of a number of string theory models of dark energy such as 11-dimensional supergravity with double-exponential potential, O(16)×O(16) heterotic string and some Type II string compactifications. For inflationary models with concave potential, we find the 2σ (3σ) bound c≲0.13 (0.14), which is still in strong tension with the string-based expectation c∼O(1). However, combining Planck primordial non-Gaussianity with inflation constraints, it is interesting that the Dirac–Born–Infeld inflation with concave potential gives the 2σ (3σ) bound c≲0.53 (0.58), which is now in a modest tension with the swampland conjecture.

Mimetic DBI Inflation in Confrontation with Planck2018 Data

Abstract We study mimetic gravity in the presence of a Dirac–Born–Infeld (DBI)-like term, which is a non-canonical setup of the scalar field’s derivatives. We consider two general cases with varying and constant sound speeds and construct the potentials for both the DBI and mimetic DBI (MDBI) models. By considering the power-law scale factor as a = a 0 t n , we search for the observational viability of these models. We show that the MDBI model in some ranges of the parameter space is free from ghost and gradient instabilities. By studying the behavior of r–n s and α s –n s in confrontation with Planck2018 data, we find some constraints on the model’s parameters. We show that, for the case with varying sound speed, power-law DBI inflation is not consistent with Planck2018 TT, TE, EE+lowE+lensing data, but the MDBI inflation is consistent with the same data at 95% confidence level, in some ranges of the model’s parameter space such as 40 ≤ n ≤ 55; the model is also free from instabilities in these ranges of parameters. For a constant sound speed, we study both DBI and MDBI models numerically by adopting some sample values of c s , and find n ∼ 102 for the DBI model and n ∼ 10 for the MDBI model. We also compare the results with Planck2018 TT, TE, EE+lowE+lensing+BK14+BAO data and see that the DBI and MDBI models with varying sound speed are ruled out by these joint data. However, these models with constant sound speed are consistent with the same data, with n ∼ 102 for the DBI model and n ∼ 10 for the MDBI model. In this case, we find some tighter constraints on the corresponding sound speed.

Planck limits on non-canonical generalizations of large-field inflation models

In this paper, we consider two case examples of Dirac-Born-Infeld (DBI) generalizations of canonical large-field inflation models, characterized by a reduced sound speed, cS < 1. The reduced speed of sound lowers the tensor-scalar ratio, improving the fit of the models to the data, but increases the equilateral-mode non-Gaussianity, fequil.NL, which the latest results from the Planck satellite constrain by a new upper bound. We examine constraints on these models in light of the most recent Planck and BICEP/Keck results, and find that they have a greatly decreased window of viability. The upper bound on fequil.NL corresponds to a lower bound on the sound speed and a corresponding lower bound on the tensor-scalar ratio of r ∼ 0.01, so that near-future Cosmic Microwave Background observations may be capable of ruling out entire classes of DBI inflation models. The result is, however, not universal: infrared-type DBI inflation models, where the speed of sound increases with time, are not subject to the bound.

Dirac-Born-Infeld inflation model with kinetic coupling to Einstein gravity

In this paper we study a new class of inflation models which generalize the Dirac-Born-Infeld (DBI) action with the addition of a nonminimal kinetic coupling (NKC) term. We dubbed this model as the {\it new DBI inflation model}. The NKC term does not bring new dynamical degree of freedom, so the equations of motion remain of second order. However, with such a coupling, the action is no longer linear with respect to the Einstein curvature term ($R$ or $G^{\mu\nu}$), which leads to a correction term of $k^4$ in the perturbations. The new DBI inflation model can be viewed as theories beyond Horndeski. Without violating nearly scale-invariance, such a correction may lead to new effects on the inflationary spectra that could be tested by future observations.

Orthogonal non-Gaussianity in DBI galileon: prospect for Planck polarization and post-Planck experiments

In this paper, we study cosmic microwave background (CMB) constraints on primordial non-Gaussianity in Dirac–Born–Infeld (DBI) galileon models in which an induced gravity term is added to the DBI action. In this model, the non-Gaussianity of orthogonal shape can be generated. We provide a relation between theoretical parameters and orthogonal/equilateral nonlinear parameters using the Fisher matrix approach for the CMB bispectrum. In doing so, we include the effect of the CMB transfer functions and experimental noise properties by employing the recently developed second order non-Gaussianity code. The relation is also shown in the language of effective theory so that it can be applied to general single-field models. Using the bispectrum Fisher matrix and the central values for equilateral and orthogonal non-Gaussianities found by the Planck temperature survey, we provide forecasts on the theoretical parameters of the DBI galileon model. We consider the upcoming Planck polarization data and the proposed post-Planck experiments Cosmic Origins Explore (COrE) and Polarized Radiation Imaging and Spectroscopy Mission (PRISM). We find that Planck polarization measurements may provide a hint for a non-canonical sound speed at the 68% confidence level. COrE and PRISM will not only confirm a non-canonical sound speed but also exclude the conventional DBI inflation model at more than the 95% and 99% confidence level respectively, assuming that the central values will not change. This indicates that improving constraints on non-Gaussianity further by future CMB experiments is invaluable to constrain the physics of the early universe.

Relations between canonical and non-canonical inflation

We look for potential observational degeneracies between canonical and non-canonical models of inflation of a single field ϕ. Non-canonical inflationary models are characterized by higher than linear powers of the standard kinetic term X in the effective Lagrangian p(X,ϕ) and arise for instance in the context of the Dirac-Born-Infeld (DBI) action in string theory. An on-shell transformation is introduced that transforms non-canonical inflationary theories to theories with a canonical kinetic term. The 2-point function observables of the original non-canonical theory and its canonical transform are found to match in the case of DBI inflation.

DBI inflation with a nonminimally coupled Gauss-Bonnet term

We study the inflation with a Gauss-Bonnet term which is nonminimally coupled to a Dirac-Born-Infeld (DBI) field. We study the spectrum of primordial perturbations in this setup in detail. The non-Gaussianity of perturbations is considered and the amplitude of the non-Gaussianity is studied in both the equilateral and orthogonal configurations. By adopting various functions of the DBI field, the inflaton potential, and the Gauss-Bonnet coupling term, we test the model with recent observational data and find some constraints on the Gauss-Bonnet coupling parameter.

DBI inflation inN=1supergravity

It was recently demonstrated that, when coupled to N=1 supergravity, the Dirac-Born-Infeld (DBI) action constructed from a single chiral superfield has the property that when the higher-derivative terms become important, the potential becomes negative. Thus, DBI inflation cannot occur in its most interesting, relativistic regime. In this paper, it is shown how to overcome this problem by coupling the model to one or more additional chiral supermultiplets. In this way, one obtains effective single real scalar field DBI models with arbitrary positive potentials, as well as multiple real scalar field DBI inflation models with hybrid potentials.

Supersymmetric DBI inflation

We discuss a supersymmetric version of DBI (Dirac–Born–Infeld) inflation, which is a typical inflation model in string cosmology. The supersymmetric DBI action together with a superpotential always leads to correction terms associated with the potential into the kinetic term, which drastically change the dynamics of DBI inflation. We find two significant features of supersymmetric DBI inflation. The first one is that ultra-relativistic motion is prohibited to cause inflation, which leads to order of unity sound velocity squared and hence small non-Gaussianities of primordial curvature perturbations. The second one is that the relation between the tensor-to-scalar ratio and the field variation is modified. Then, significant tensor-to-scalar ratio r≳0.01 is possible because the variation of the canonically normalized inflaton can be beyond the reduced Planck scale. These new features are in sharp contrast with those of the standard non-supersymmetric DBI inflation and hence have a lot of interest implications on upcoming observations of cosmic microwave background (CMB) anisotropies by the Planck satellite as well as direct detection experiments of gravitational waves like DECIGO and BBO.

Non-Gaussianities in multifield DBI inflation with a waterfall phase transition

We study multifield Dirac-Born-Infeld (DBI) inflation models with a waterfall phase transition. This transition happens for a D3 brane moving in the warped conifold if there is an instability along angular directions. The transition converts the angular perturbations into the curvature perturbation. Thanks to this conversion, multifield models can evade the stringent constraints that strongly disfavor single field ultraviolet (UV) DBI inflation models in string theory. We explicitly demonstrate that our model satisfies current observational constraints on the spectral index and equilateral non-Gaussianity as well as the bound on the tensor to scalar ratio imposed in string theory models. In addition, we show that large local type non-Gaussianity is generated together with equilateral non-Gaussianity in this model.

Inflationary predictions in scalar-tensor DBI inflation

The scalar-tensor Dirac-Born-Infeld (DBI) inflation scenario provides a simple mechanism to reduce the large values of the boost factor associated with single field models with DBI action, whilst still being able to drive 60 efolds of inflation. Using a slow-roll approach, we obtain an analytical expression for the spectral index of the perturbations and, moreover, determine numerically the regions of the parameter space of the model capable of giving rise to a power spectrum with amplitude and spectral index within the observed bounds. We find that regions that exhibit significant DBI effects throughout the inflationary period can be discarded by virtue of a blue-tilted spectral index, however, there are a number of viable cases — associated with a more red-tilted spectral index — for which the boost factor is initially suppressed by the effect of the coupling between the fields, but increases later to moderate values.