Xingang Chen

Observational signatures and non-Gaussianities of general single-field inflation

We perform a general study of primordial scalar non-Gaussianities in single field inflationary models. We consider models where the inflaton Lagrangian is an arbitrary function of the scalar field and its first derivative, and the sound speed is arbitrary. We find that under reasonable assumptions, the non-Gaussianity is completely determined by 5 parameters. In special limits of the parameter space, one finds distinctive shapes of the non-Gaussianity. In models with a small sound speed, several of these shapes would become potentially observable in the near future. Different limits of our formulae recover various previously known results.

Quasi-Single Field Inflation and Non-Gaussianities

In quasi-single field inflation models, massive isocurvature modes, that are coupled to the inflaton and have mass of order the Hubble parameter, can have nontrivial impacts on density perturbations, especially non-Gaussianities. We study a simple example of quasi-single field inflation in terms of turning inflaton trajectory. Large bispectra with a one-parameter family of novel shapes arise, lying between the well-known local and equilateral shape. The trispectra can also be very large and its magnitude tNL can be much larger than fNL2.

Generation and characterization of large non-Gaussianities in single field inflation

Inflation driven by a single, minimally coupled, slowly rolling field generically yields a negligible primordial non-Gaussianity. We discuss two distinct mechanisms by which a non-trivial potential can generate large non-Gaussianities. Firstly, if the inflaton traverses a feature in the potential, or if the inflationary phase is short enough so that initial transient contributions to the background dynamics have not been erased, modes near horizon crossing can acquire significant non-Gaussianities. Secondly, potentials with small-scale structure may induce significant non-Gaussianities while the relevant modes are deep inside the horizon. The first case includes the step potential we previously analyzed while the second resonance case is novel. We derive analytic approximations for the three-point terms generated by both mechanisms written as products of functions of the three individual momenta, permitting the use of efficient analysis algorithms. Finally, we present a significantly improved approach to regularizing and numerically evaluating the integrals that contribute to the three-point function.

CMBPol Mission Concept Study Probing Ination with CMB Polarization

We summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of ination. We focus on the prospects for using CMB measurementsWe summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of inflation. We focus on the prospects for using CMB measurements to differentiate various inflationary mechanisms. In particular, a detection of primordial B‐mode polarization would demonstrate that inflation occurred at a very high energy scale, and that the inflaton traversed a super‐Planckian distance in field space. We explain how such a detection or constraint would illuminate aspects of physics at the Planck scale. Moreover, CMB measurements can constrain the scale‐dependence and non‐Gaussianity of the primordial fluctuations and limit the possibility of a significant isocurvature contribution. Each such limit provides crucial information on the underlying inflationary dynamics. Finally, we quantify these considerations by presenting forecasts for the sensitivities of a future satellite experiment to the inflationary parameters.

Large non-Gaussianities with intermediate shapes from quasi-single-field inflation

We study the slow-roll inflation models, where the inflaton slow-rolls along a trajectory whose orthogonal directions are lifted by potentials with masses of order the Hubble parameter. In these models large non-Gaussianities can be generated through the transformation from the isocurvature modes to the curvature mode, once the inflaton trajectory turns. We find large bispectra with a one-parameter family of novel shapes, interpolating between the equilateral and local shape. According to the in-in formalism, the shapes of these non-Gaussianities are different from a simple projection from the isocurvature non-Gaussian correlation functions.

Inflation from warped space

A long period of inflation can be triggered when the inflaton is held up on the top of a steep potential by the infrared end of a warped space. We first study the field theory description of such a model. We then embed it in the flux stabilized string compactification. Some special effects in the throat reheating process by relativistic branes are discussed. We put all these ingredients into a multi-throat brane inflationary scenario. The resulting cosmic string tension and a multi-throat slow-roll model are also discussed.

Running non-Gaussianities in Dirac-Born-Infeld inflation

We study the non-Gaussianity in the simplest infrared model of the Dirac-Born-Infeld (DBI) inflation. We show that the non-Gaussianity in such a model is compatible with the current observational bound and is within the sensitivity of future experiments. We also discuss the scale dependence of the non-Gaussianity. In DBI inflation, such a feature can be used as a probe to the properties of the background geometry of the extra dimensions or internal space.

Multithroat brane inflation

We present a scenario where brane inflation arises more generically. We start with D3 and anti-D3-branes at the infrared ends of two different throats. This setup is a natural consequence of the assumption that in the beginning we have a multithroat string compactification with many wandering anti-D3-branes. A long period of inflation is triggered when D3-branes slowly exit the highly warped infrared region, under a potential generically arising from the moduli stabilization. In this scenario, the usual slow-roll conditions are not required, and a large warping is allowed to incorporate the Randall-Sundrum model.

Large primordial trispectra in general single field inflation

We compute the large scalar four-point correlation functions in general single field inflation models, where the inflaton Lagrangian is an arbitrary function of the inflaton and its first derivative. We find that the leading order trispectra have four different shapes determined by three parameters. We study features in these shapes that can be used to distinguish among themselves, and between them and the trispectra of the local form. For the purpose of data analyses, we give two simple representative forms for these equilateral trispectra. We also study the effects on the trispectra if the initial state of inflation deviates from the standard Bunch-Davies vacuum.

Heating in brane inflation and hidden dark matter

Towards the end of brane inflation, the brane pair annihilation produces massive closed strings. The transfer of this energy to standard model (SM) open string modes depends on where the SM branes and the brane annihilation are located: in the bulk, in the same throat, or in different throats. We find that, in all cases as long as the brane annihilation and the SM branes are not both in the bulk, the transfer of energy to start the hot big bang epoch can be efficient enough to be compatible with big bang nucleosynthesis. The suppression of the abundance of the graviton and its Kaluza–Klein (KK) thermal relics follows from the warped geometry in flux compactification. This works out even in the scenarios where a long period of tunnelling is expected. In the multi-throat scenario, we find a dynamical mechanism of selecting a long throat as the SM throat. We establish three new dark matter candidates: KK modes with specific angular momentum in the SM throat, those in the brane annihilation throat, and different matters generated by KK modes tunnelled to other throats. Since the latter two couple to the visible matter sector only through graviton mediation, they behave as hidden dark matter. Hidden dark matter has novel implications for the dark matter coincidence problem and the high energy cosmic rays.