Nishant Agarwal

Universality in D-brane inflation

We study the six-field dynamics of D3-brane inflation for a general scalar potential on the conifold, finding simple, universal behavior. We numerically evolve the equations of motion for an ensemble of more than 7107 realizations, drawing the coefficients in the scalar potential from statistical distributions whose detailed properties have demonstrably small effects on our results. When prolonged inflation occurs, it has a characteristic form: the D3-brane initially moves rapidly in the angular directions, spirals down to an inflection point in the potential, and settles into single-field inflation. The probability of Ne e-folds of inflation is a power law, P(Ne)∝Ne−3, and we derive the same exponent from a simple analytical model. The success of inflation is relatively insensitive to the initial position: we find attractor behavior in the angular directions, and the D3-brane can begin far above the inflection point without overshooting. Initial radial or angular velocities, on the other hand, can have a significant effect on the duration of inflation. In favorable regions of the spaces of initial velocities and of Lagrangian parameters, models yielding 60 e-folds of expansion arise approximately once in 103 trials. Realizations that are effectively single-field and give rise to a primordial spectrum of fluctuations consistent with WMAP, for which at least 120 e-folds are required, arise approximately once in 105 trials. The emergence of robust predictions from a six-field potential with hundreds of terms invites an analytic approach to multifield inflation.

Effective field theory and non-Gaussianity from general inflationary states

A bstractWe study the effects of non-trivial initial quantum states for inflationary fluctuations within the context of the effective field theory for inflation constructed by Cheung et al. which allows us to discriminate between different initial states in a model-independent way. We develop a Green’s function/path integral based formulation that incorporates initial state effects and use it to address questions such as how state-dependent is the consistency relation for the bispectrum, how many e-folds beyond the minimum required to solve the cosmological fine tunings of the big bang are we allowed so that some information from the initial state survives until late times, among others. We find that the so-called consistency condition relating the local limit of the bispectrum and the slow-roll parameter is a state-dependent statement that can be avoided for physically consistent initial states either with or without initial non-Gaussianities.

Sloan Digital Sky Survey III photometric quasar clustering: Probing the initial conditions of the Universe

The Sloan Digital Sky Survey has surveyed 14,555 square degrees of the sky, and delivered over a trillion pixels of imaging data. We present the large-scale clustering of 1.6 million quasars between z = 0.5 and z = 2.5 that have been classified from this imaging, representing the highest density of quasars ever studied for clustering measurements. This data set spans ~11,000 square degrees and probes a volume of 80(Gpc/h)^3. In principle, such a large volume and medium density of tracers should facilitate high-precision cosmological constraints. We measure the angular clustering of photometrically classified quasars using an optimal quadratic estimator in four redshift slices with an accuracy of ~25% over a bin width of l ~10 - 15 on scales corresponding to matter-radiation equality and larger (l ~ 2 - 30). Observational systematics can strongly bias clustering measurements on large scales, which can mimic cosmologically relevant signals such as deviations from Gaussianity in the spectrum of primordial perturbations. We account for systematics by employing a new method recently proposed by Agarwal et al. (2014) to the clustering of photometrically classified quasars. We carefully apply our methodology to mitigate known observational systematics and further remove angular bins that are contaminated by unknown systematics. Combining quasar data with the photometric luminous red galaxy (LRG) sample of Ross et al. (2011) and Ho et al. (2012), and marginalizing over all bias and shot noise-like parameters, we obtain a constraint on local primordial non-Gaussianity of fNL = -113+/-154 (1\sigma error). [Abridged]

Constraining the initial conditions of the Universe using large scale structure

Primordial non-Gaussianity induces a scale-dependent bias in large scale structure (LSS) data, proportional to fNL/k2 for the exact local ansatz. Recent work has shown that models of inflation that predict a large squeezed limit bispectrum, such as multi-field models and single field inflation with a modified initial state, typically give rise to a generalized local ansatz, with the scale-dependent bias now proportional to NL/kα. We use photometric measurements of the angular power spectrum of luminous red galaxies and quasars in the Sloan Digital Sky Survey Data Release Eight (SDSS DR8) with the above parameterization to constrain the amplitude NL and scale-dependence α. We find that the marginalized upper limit on α is 2.0 at the 95% confidence level, consistent with the local ansatz. We also present Fisher forecasts for a survey of the same size as DR8 to assess the role of systematics in current photometric LSS data. Moreover, we present analytic results on the expected mass dependence of NL for different inflationary models, which can be an important observable for future surveys, if primordial non-Gaussianity is non-zero.

Characterizing unknown systematics in large scale structure surveys

Photometric large scale structure (LSS) surveys probe the largest volumes in the Universe, but are inevitably limited by systematic uncertainties. Imperfect photometric calibration leads to biases in our measurements of the density fields of LSS tracers such as galaxies and quasars, and as a result in cosmological parameter estimation. Earlier studies have proposed using cross-correlations between different redshift slices or cross-correlations between different surveys to reduce the effects of such systematics. In this paper we develop a method to characterize unknown systematics. We demonstrate that while we do not have sufficient information to correct for unknown systematics in the data, we can obtain an estimate of their magnitude. We define a parameter to estimate contamination from unknown systematics using cross-correlations between different redshift slices and propose discarding bins in the angular power spectrum that lie outside a certain contamination tolerance level. We show that this method improves estimates of the bias using simulated data and further apply it to photometric luminous red galaxies in the Sloan Digital Sky Survey as a case study.

Signatures of pseudoscalar photon mixing in CMB polarization

We study the effect of photon and ultralight pseudoscalar mixing on the propagation of electromagnetic radiation through the extragalactic medium. The medium is modeled as a large number of uncorrelated magnetic domains. We obtain an analytic expression for the different Stokes parameters in the limit of a small mixing angle. Stokes parameters are found to change linearly with the number of domains. We verify this result by direct numerical simulations. We use the formalism to estimate the effect of pseudoscalar-photon mixing on the cosmic microwave background (CMB) polarization. We impose limits on the model parameters consistent with CMB observations. We find that the currently allowed parameter range admits a CMB circular polarization up to order 10{sup -7}.

The dynamical viability of scalar-tensor gravity theories

We establish the dynamical attractor behavior in scalar–tensor theories of dark energy, providing a powerful framework to analyze classes of theories, predicting common evolutionary characteristics that can be compared against cosmological constraints. In the Jordan frame the theories are viewed as a coupling between a scalar field, Φ, and the Ricci scalar, R, F(Φ)R. The Jordan frame evolution is described in terms of dynamical variables m ≡ dln F/dln Φ and r ≡ −ΦF/f, where F(Φ) = df(Φ)/dΦ. The evolution can be alternatively viewed in the Einstein frame as a general coupling between scalar dark energy and matter, β. We present a complete, consistent picture of evolution in the Einstein and Jordan frames and consider the conditions on the form of the coupling F and β required to give the observed cold dark matter (CDM) dominated era that transitions into a late-time accelerative phase, including transitory accelerative eras that have not previously been investigated. We find five classes of evolutionary behavior of which four are qualitatively similar to those for f(R) theories (which have β = 1/2). The fifth class exists only for , i.e. not for f(R) theories. In models giving transitory late-time acceleration, we find a viable accelerative region of the (r, m) plane accessible to scalar–tensor theories with any coupling, β (at least in the range |β| ≤ 1/2, which we study in detail), and an additional region open only to theories with .

Cosmic Expansion in Extended Quasidilaton Massive Gravity

Quasidilaton massive gravity offers a physically well-defined gravitational theory with nonzero graviton mass. We present the full set of dynamical equations governing the expansion history of the ...

Screening bulk curvature in the presence of large brane tension

We study a flat brane solution in an effective 5D action for cascading gravity in six dimensions, and propose a mechanism to screen extrinsic curvature in the presence of a large tension on the brane. The screening mechanism leaves the bulk Riemann-flat, thus making it simpler to generalize large extra-dimension dark energy models to higher codimensions. By studying an action with cubic interactions for the brane-bending scalar mode, we find that the perturbed action suffers from ghostlike instabilities for positive tension. The solution can be made ghost-free for sufficiently small negative tension, though the connection to 6D cascading gravity is less clear in this case.

A complete 3D numerical study of the effects of pseudoscalar-photon mixing on quasar polarizations

We present the results of three-dimensional simulations of quasar polarizations in the presence of pseudoscalar–photon mixing in the intergalactic medium. The intergalactic magnetic field is assumed to be uncorrelated in wave vector space but correlated in real space. Such a field may be obtained if its origin is primordial. Furthermore we assume that the quasars, located at cosmological distances, have negligible initial polarization. In the presence of pseudoscalar–photon mixing we show, through a direct comparison with observations, that this may explain the observed large scale alignments in quasar polarizations within the framework of big bang cosmology. We find that the simulation results give a reasonably good fit to the observed data.