Ghost Inflation Research Articles

Scalar-induced gravitational waves from ghost inflation and parity violation

Scalar-induced gravitational waves from ghost inflation and parity violation

Inflation with 0 ≤ c s ≤ 1

We investigate a novel single field inflationary scenario which allowsa transition between a slow-roll k-inflation with c s of order unityand a ghost inflation with c s ≃ 0, where c s is the soundspeed for the curvature perturbations. We unify the two phasessmoothly by appropriately taking into account a higher derivativescordatura term, which is always there from the effective field theorypoint of view but which becomes important only in the c s ≃ 0regime. The model achieves the whole range of 0 ≤ c s ≤ 1avoiding strong coupling and gradient instability, and allows us toaccess the c s ≃ 0 regime in a self-consistent manner. We alsodiscuss implications to the formation of primordial black holes.

Mimetic inflation

We study inflationary solution in an extension of mimetic gravity with the higher derivative interactions coupled to gravity. Because of the higher derivative interactions the setup is free from the ghost and gradient instabilities while it hosts a number of novel properties. The dispersion relation of scalar perturbations develop quartic momentum correction similar to the setup of ghost inflation. Furthermore, the tilt of tensor perturbations can take either signs with a modified consistency relation between the tilt and the amplitude of tensor perturbations. Despite the presence of higher derivative interactions coupled to gravity the tensor perturbations propagate with the speed equal to the speed of light as required by the LIGO observations. Furthermore, the higher derivative interactions induce non-trivial interactions in cubic Hamiltonian, generating non-Gaussianities in various shapes such as the equilateral, orthogonal and squeezed configurations with observable amplitudes.

Black hole formation from a general quadratic action for inflationary primordial fluctuations

The most up to date femto- and micro-lensing constraints indicate that primordial black holes of ∼ 10−16 M⊙ and ∼ 10−12 M⊙, respectively, may constitute a large fraction of the dark matter. We describe analytically and numerically the dynamics by which inflationary fluctuations featuring a time-varying propagation speed or an effective Planck mass can lead to abundant primordial black hole production. As an example, we provide an ad hoc DBI-like model. A very large primordial spectrum originating from a small speed of sound typically leads to strong coupling within the vanilla effective theory of inflationary perturbations. However, we point out that ghost inflation may be able to circumvent this problem. We consider as well black hole formation in solid inflation, for which, in addition to an analogous difficulty, we stress the importance of the reheating process. In addition, we review the basic formalism for the collapse of large radiation density fluctuations, emphasizing the relevance of an adequate choice of gauge invariant variables.

Generic analysis of kinetically driven inflation

We perform a model-independent analysis of kinetically driven inflation (KDI) which (partially) includes Generalized G-inflation and Ghost inflation. We evaluate the background evolution splitting into the inflationary attractor and the perturbation around it. We also consider the quantum fluctuation of scalar mode with a usual scaling and derive the spectral index ignoring the contribution from the second order products of slow-roll parameters. Using these formalisms, we find that within our generic framework, the models of KDI which possess the shift symmetry of scalar field cannot create the quantum fluctuation consistent to the observation. Breaking the shift symmetry, we obtain a few essential conditions for viable models of KDI associated with the graceful exit.

Ghost inflation and de Sitter entropy

In the setup of ghost condensation model the generalized second law of black hole thermodynamics can be respected under a radiatively stable assumption that couplings between the field responsible for ghost condensate and matter fields such as those in the Standard Model are suppressed by the Planck scale. Since not only black holes but also cosmology are expected to play important roles towards our better understanding of gravity, we consider a cosmological setup to test the theory of ghost condensation. In particular we shall show that the de Sitter entropy bound proposed by Arkani-Hamed, et al. is satisfied if ghost inflation happened in the early epoch of our universe and if there remains a tiny positive cosmological constant in the future infinity. We then propose a notion of cosmological Page time after inflation.

The CMB bispectrum

We use a separable mode expansion estimator with WMAP7 data to estimate the bispectrum for all the primary families of non-Gaussian models, including non-scaling feature (periodic) models,the flat (trans-Planckian) model, DBI and ghost inflation, as well as previously constrainedsimple cases. We review the late-time mode expansion estimator methodology which can be applied to any non-separable primordial and CMB bispectrum model, and we demonstrate how the method can be used to reconstruct the CMB bispectrum from an observational map. We extend the previous validation of the general estimatorusing local map simulations. We apply the estimator to the coadded WMAP 7-year V and W channel maps, reconstructing the WMAP bispectrum using l < 500 multipoles and n = 50 orthonormal 3D eigenmodes;both the mode expansion parameters and the reconstructed 3D WMAP bispectrum are plotted. We constrain all popular nearly scale-invariant models, ensuring that the theoretical bispectrum is well-described by a convergent mode expansion. Constraints from the local model fNL = 20.31±27.64 and the equilateral model fNL = 10.19±127.38(FNL = 1.90±23.79) are consistent with previously published results. (Here, we use a nonlinearity parameter FNL normalised to the local case, to allow more direct comparison between different models.) Notable new constraints from our method include those for the constant model FNL = 7.82±24.57, the flat model FNL = 7.31±26.22, and warm inflation FNL = 2.10±25.83. We investigate feature models,which break scale invariance, surveying a wide parameter range for both the scale and phase (scanning for feature models with an effective period l* > 150). We find no significant evidence of non-Gaussianity for all cases well-described by the given eigenmodes. In the overall non-Gaussian analysis, we find one anomalous mode n = 33 with a 3.39σ amplitude which could give rise to an oscillatory model signal with l* ⩽ 150. We propose a measure F̄NL for the total integrated bispectrum and find that the measured value is consistent with the null hypothesis that CMB anisotropies obey Gaussian statistics. We argue that this general bispectrum survey with the WMAP data represents the best test of Gaussianity to date and we discuss future prospects with higher precision and resolution, notably from the Planck satellite.

Trispectrum estimation in various models of equilateral type non-Gaussianity

We calculate the shape correlations between trispectra in various equilateral non-Gaussian models, including DBI inflation, ghost inflation and Lifshitz scalars, using the full trispectrum as well as the reduced trispectum. We find that most theoretical models are distinguishable from the shapes of primordial trispectra except for several exceptions where it is difficult to discriminate between the models, such as single field DBI inflation and a Lifshitz scalar model. We introduce an estimator for the amplitude of the trispectrum, $g_{\rm NL} ^{equil}$ and relate it to model parameters in various models. Using constraints on $g_{\rm NL} ^{equil}$ from WMAP5, we give constraints on the model parameters.

The trispectrum in ghost inflation

We calculate the trispectrum in ghost inflation where both the contact diagram and scale-exchange diagram are taken into account. The shape of trispectrum is discussed carefully and we find that the local form is absent in ghost inflation. In general, for the non-local shape trispectrum there are not analogous parameters to τNLloc. and gNLloc. which can completely characterize the size of local form trispectrum.

Trispectrum from ghost inflation

Ghost inflation predicts almost scale-invariant primordial cosmological perturbations with relatively large non-Gaussianity. The bispectrum is known to have a large contribution at the wavenumbers forming an equilateral triangle and the corresponding nonlinear parameter fNLequil is typically of order O(102). In this paper we calculate trispectrum from ghost inflation and show that the corresponding nonlinear parameter τNL is typically of order O(104). We investigate the shape dependence of the trispectrum and see that it has some features different from DBI inflation. Therefore, our result may be useful as a template to distinguish ghost inflation from other models of inflation by future experiments.

Effective field theory for inflation

The methods of effective field theory are used to study generic theories of inflation with a single inflaton field. For scalar modes, the leading corrections to the $\mathcal{R}$ correlation function are found to be purely of the $k$-inflation type. For tensor modes the leading corrections to the correlation function arise from terms in the action that are quadratic in the curvature, including a parity-violating term that makes the propagation of these modes depend on their helicity. These methods are also briefly applied to nongeneric theories of inflation with an extra shift symmetry, as in so-called ghost inflation.

The effective field theory of inflation

We study the effective field theory of inflation, i.e. the most general theory describing the fluctuations around a quasi de Sitter background, in the case of single field models. The scalar mode can be eaten by the metric by going to unitary gauge. In this gauge, the most general theory is built with the lowest dimension operators invariant under spatial diffeomorphisms, like g^{00} and K_{mu nu}, the extrinsic curvature of constant time surfaces. This approach allows us to characterize all the possible high energy corrections to simple slow-roll inflation, whose sizes are constrained by experiments. Also, it describes in a common language all single field models, including those with a small speed of sound and Ghost Inflation, and it makes explicit the implications of having a quasi de Sitter background. The non-linear realization of time diffeomorphisms forces correlation among different observables, like a reduced speed of sound and an enhanced level of non-Gaussianity.

General single field inflation with large positive non-Gaussianity

Recent analysis of the WMAP three-year data suggests in the WMAP convention. It is necessary to make surewhether general single field inflation can produce a large positivefNL before turning to other scenarios. We give some examples to generate a large positivefNLequil in general single field inflation. Our models are different from ghost inflation. Due to theappearance of non-conventional kinetic terms, can be realized in single field inflation.

A measure of de Sitter entropy and eternal inflation

We show that in any model of non-eternal inflation satisfying the null energy condition, the area of the de Sitter horizon increases by at least one Planck unit in each inflationary e-folding. This observation gives an operational meaning to the finiteness of the entropy S_dS of an inflationary de Sitter space eventually exiting into an asymptotically flat region: the asymptotic observer is never able to measure more than e^(S_dS) independent inflationary modes. This suggests a limitation on the amount of de Sitter space outside the horizon that can be consistently described at the semiclassical level, fitting well with other examples of the breakdown of locality in quantum gravity, such as in black hole evaporation. The bound does not hold in models of inflation that violate the null energy condition, such as ghost inflation. This strengthens the case for the thermodynamical interpretation of the bound as conventional black hole thermodynamics also fails in these models, strongly suggesting that these theories are incompatible with basic gravitational principles.

Tilted ghost inflation

In a ghost inflationary scenario, we study the observational consequences of a tilt in the potential of the ghost condensate. We show how the presence of a tilt tends to make contact between the natural predictions of ghost inflation and the ones of slow roll inflation. In the case of positive tilt, we are able to build an inflationary model in which the Hubble constant $H$ is growing with time. We compute the amplitude and the tilt of the two-point function, as well as the three-point function, for both cases of positive and negative tilt. We find that a good fraction of the parameter space of the model is within experimental reach.

The shape of non-Gaussianities

We study the dependence on the configuration in momentum space of the primordialthree-point function of density perturbations in several different scenarios: standardslow-roll inflation, curvaton and variable decay models, ghost inflation, models withhigher derivative operators and the DBI model of inflation. We define a cosinebetween the distributions using a measure based on the ability of experiments todistinguish between them. We find that models fall into two broad categorieswith fairly orthogonal distributions: models where non-Gaussianity is created oncrossing the horizon during inflation and models in which the evolution beyondthe horizon dominates. In the first case the three-point function is largest forequilateral triangles, while in the second the dominant contribution to the signal comesfrom the influence of long wavelength modes on small wavelength ones. We showthat, because the distributions in these two cases are so different, translatingconstraints on parameters of one model to those of the other on the basis of thenormalization of the three-point function for equilateral triangles can be very misleading.