We compute the 3-point correlation function for a general model of inflation driven by a single, minimally coupled scalar field. Our approach is based on the numerical evaluation of both the perturbation equations and the integrals which contribute to the 3-point function. Consequently, we can analyze models where the potential has a "feature", in the vicinity of which the slow roll parameters may take on large, transient values. This introduces both scale and shape dependent non-Gaussianities into the primordial perturbations. As an example of our methodology, we examine the "step" potentials which have been invoked to improve the fit to the glitch in the T T C ℓ for ℓ ∼ 30, present in both the one and three year WMAP data sets. We show that for the typical parameter values, the non-Gaussianities associated with the step are far larger than those in standard slow roll inflation, and may even be within reach of a next generation CMB experiment such as Planck. More generally, we use this example to explain that while adding features to potential can improve the fit to the 2-point function, these are generically associated with a greatly enhanced signal at the 3-point level. Moreover, this 3-point signal will have a very nontrivial shape and scale dependence, which is correlated with the form of the 2-point function, and may thus lead to a consistency check on the models of inflation with non-smooth potentials.

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 nontrivial 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 3-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 3-point function.

We consider the gravitational effects of a single, fixed-norm, Lorentz-violating timelike vector field. In a cosmological background, such a vector field acts to rescale the effective value of Newton's constant. The energy density of this vector field precisely tracks the energy density of the rest of the universe, but with the opposite sign, so that the universe experiences a slower rate of expansion for a given matter content. This vector field similarly rescales Newton's constant in the Newtonian limit, although by a different factor. We put constraints on the parameters of the theory using the predictions of primordial nucleosynthesis, demonstrating that the norm of the vector field should be less than the Planck scale by an order of magnitude or more.

In many models of inflation, the period of accelerated expansion ends with preheating, a highly non-thermal phase of evolution during which the inflaton pumps energy into a specific set of momentum modes of field(s) to which it is coupled. This necessarily induces large, transient density inhomogeneities which can source a significant spectrum of gravitational waves. In this paper, we consider the generic properties of gravitational waves produced during preheating, perform detailed calculations of the spectrum for several specific inflationary models, and identify problems that require further study. In particular, we argue that if these gravitational waves exist they will necessarily fall within the frequency range that is feasible for direct detection experimentsfrom laboratory through to solar system scales. We extract the gravitational wave spectrum from numerical simulations of preheating after λφ 4 and m 2 φ φ 2 inflation, and find that they lead to a gravitational wave amplitude of around Ωgwh 2 ∼ 10 −10 . This is considerably higher than the amplitude of the primordial gravitational waves produced during inflation. However, the typical wavelength of these gravitational waves is considerably shorter than LIGO scales, although in extreme cases they may be visible at scales accessible to the proposed BBO mission. We survey possible experimental approaches to detecting any gravitational wave background generated during preheating.

We consider gravitational wave production due to parametric resonance at the end of inflation, or "preheating". This leads to large inhomogeneities which source a stochastic background of gravitational waves at scales inside the comoving Hubble horizon at the end of inflation. We confirm that the present amplitude of these gravitational waves need not depend on the inflationary energy scale.We analyze an explicit model where the inflationary energy scale is ∼ 10 9 GeV, yielding a signal close to the sensitivity of Advanced LIGO and BBO. This signal highlights the possibility of a new observational "window" into inflationary physics, and provides significant motivation for searches for stochastic backgrounds of gravitational waves in the Hz to GHz range, with an amplitude on the order of Ωgw(k)h 2 ∼ 10 −11 . Finally, the strategy used in our numerical computations is applicable to the gravitational waves generated by many inhomogeneous processes in the early universe.A successful model of inflation must have a "graceful exit" that describes the transition from the accelerated phase to a thermalized universe [1]. A widely studied mechanism for achieving this is preheating (e.g. [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]). After inflation, the inflaton (or a related field) oscillates about the bottom of its potential, driving the resonant amplification of specific momentum modes of some coupled field(s). This renders the universe inhomogeneous, and the resulting spatial gradients source gravitational waves. For GUT inflation, the present peak frequency is between 1 MHz and 1 GHz [5,15]. It was conjectured that the characteristic frequency is inversely proportional to the inflationary scale, while the amplitude can be independent of this scale [15], leading to a signal potentially detectable by future iterations of LIGO and BBO. We confirm this conjecture by numerically computing the gravitational wave spectrum in a toy model of preheating following low scale inflation. While simple inflationary models typically involve GUT scale physics, many stringy models have a much lower inflationary scale, so this signal may eventually lead to new constraints on these models. The tools developed for this analysis will allow us to explore fully realistic preheating models in an expanding background. Furthermore, our computational strategy applies to any inhomogeneous phase in the universe, and may have applications beyond the present problem.Computational Strategy & Results: During parametric resonance, momentum modes of a field χ are pumped by an oscillating field φ. In simple models φ is the inflaton, but in hybrid models φ is the direction orthogonal to the inflationary trajectory which induces the "waterfall" transition [20,21]. In either case the lagrangian can be expressed asWe numerically simulate the nonlinear field evolution in a conformally rigid spacetime background. We can then compute the spatial parts of T µν at any given time. The tensor contribution to the metric perturbation and Einstein equations readwhere the ove...

We 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 de tection 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. 10Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip. Striking advances in observational cosmology over the past two decades have provided us with a consistent account of the form and composition of the universe. Now that key cosmological parameters have been determined to within a few percent, we anticipate a generation of experiments that move beyond adding precision to measurements of what the universe is made of, but instead help us learn why the universe has the form we observe. In particular, during the coming decade, observational cosmology will probe the detailed dynamics of the universe in the earliest instants after the Big Bang, and start to yield clues about the physical laws that governed that epoch. Future experiments will plausibly reveal the dynamics responsible both for the large-scale homogeneity and flatness of the universe, and for the primordial seeds of small-scale inhomogeneities, including our own galaxy.The leading theoretical paradigm for the initial moments of the Big Bang is inflation [1][2][3][4][5][6], a period of rapid accelerated expansion. Inflation sets the initial conditions for conventional Big Bang cosmology by driving the universe towards a homogeneous and spatially flat configuration, which accurately describes the average state of the universe. At the same time, quantum fluctuations in both matter fields and spacetime produce minute inhomogeneities [7][8][9][10][11][12]. The seeds that grow into the galaxies, clusters of galaxies and the temperature anisotropies in the cosmic microwave background (CMB) are thus planted during the first moments of the universe's existence. By measuring the anisotropies in the microwave background and the large scale distribution of galaxies in the sky, we can infer the spectrum of the primordial perturbations laid down during inflation, and thus probe the underlying physics of this era. Any successful inflationary model will deliver a universe that is, on average, spatially flat and homogeneous -and one homogeneous universe looks very much like ano...

We introduce a novel class of field theories where energy always flows along timelike geodesics, mimicking in that respect dust, yet which possess non-zero pressure. This theory comprises two scalar fields, one of which is a Lagrange multiplier enforcing a constraint between the other's field value and derivative. We show that this system possesses no wave-like modes but retains a single dynamical degree of freedom. Thus, the sound speed is always identically zero on all backgrounds. In particular, cosmological perturbations reproduce the standard behaviour for hydrodynamics in the limit of vanishing sound speed. Using all these properties we propose a model unifying Dark Matter and Dark Energy in a single degree of freedom. In a certain limit this model exactly reproduces the evolution history of ΛCDM, while deviations away from the standard expansion history produce a potentially measurable difference in the evolution of structure. I. DUSTY FLUID WITH PRESSURE?How can one obtain dust from a scalar field? One can imagine a canonical scalar-field where the kinetic term is constrained to be equal to the potential. We can implement this property by introducing a Lagrange multiplier, λ, in the Lagrangian,We then find that the pressure is identically vanishing on all solutions and energy follows geodesics. This model describes the usual dust without vorticity. How can we obtain "dust with pressure"? We can generalise the above by adding some function of the scalar field and its derivatives to the Lagrangian,The constraint remains in effect and standard scalar-field dynamics are not restored. In fact, we will show that fluid elements in all such theories also always flow along geodesics, mimicking in that respect standard dust, yet the fluid has non-vanishing pressure. With this simple idea we have separated the notion that the pressure of the fluid is tied to the motion of a fluid element as is the situation in the usual case, e.g. radiation or cold dark matter. A parcel of such fluid will flow along geodesics, yet a manometer will record a pressure changing with time.In this paper, we introduce this new class of scalar-field models, which we will call λϕ-fluids. These theories are described by an action containing two scalar fields, ϕ and * Electronic address: eugene.a.lim@gmail.com † Electronic address: ignacy.sawicki@nyu.edu ‡ Electronic address: alexander.vikman@nyu.edu λ, where the latter plays the role of a Lagrange multiplier and enforces a constraint relating the value of the scalar field ϕ to the norm of its derivative. This constraint forces the dynamics of the λϕ-fluid to be driven by a system of two first-order ordinary differential equations, one for the field ϕ, the other for the Lagrange multiplier. As a consequence, there are no propagating wave-like degrees of freedom and the sound speed for perturbations is exactly zero irrespective of the background solution. However, the initial-value problem still requires the specification of two functions on the initial time slice. Thus, effectively, a single d...

We study gravitationally bound static and spherically symmetric configurations of k-essence fields. In particular, we investigate whether these configurations can reproduce the properties of dark matter haloes. The classes of Lagrangians we consider lead to non-isotropic fluids with barotropic and polytropic equations of state. The latter include microscopic realizations of the often-considered Chaplygin gases, which we find can cluster into dark matter halo-like objects with flat rotation curves, while exhibiting a dark energy-like negative pressure on cosmological scales. We complement our studies with a series of formal general results about the stability and initial value formulation of non-canonical scalar field theories, and we also discuss a new class of de Sitter solutions with spacelike field gradients.