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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 523 records · Page 29
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DOE OSTI2012

The full squeezed CMB bispectrum from inflation

The small-scale CMB temperature we observe on the sky is modulated by perturbations that were super-horizon at recombination, giving differential focussing and lensing that generate a non-zero bispectrum even for single-field inflation where local physics is identical. Understanding this signal is important for primordial non-Gaussianity studies and also parameter constraints from the CMB lensing bispectrum signal. Because of cancellations individual effects can appear larger or smaller than they are in total, so a full analysis may be required to avoid biases. I relate angular scales on the sky to physical scales at recombination using the optical equations, and give full-sky results for the large-scale adiabatic temperature bispectrum from Ricci focussing (expansion of the ray bundle), Weyl lensing (convergence and shear), and temperature redshift modulations of small-scale power. The δN expansion of the beam is described by the constant temperature 3-curvature, and gives a nearly-observable version of the consistency relation prediction from single-field inflation. I give approximate arguments to quantify the likely importance of dynamical effects, and argue that they can be neglected for modulation scales l∼<100, which is sufficient for lensing studies and also allows robust tests of local primordial non-Gaussianity using only the large-scale modulation modes. For accurate numerical results early and late-time ISW effects must be accounted for, though I confirm that the late-time non-linear Rees-Sciama contribution is negligible compared to other more important complications. The total corresponds to f{sub NL} ∼ 7 for Planck-like temperature constraints and f{sub NL} ∼ 11 for cosmic-variance limited data to l{sub max} = 2000. Temperature lensing bispectrum estimates are affected at the 0.2σ level by Ricci focussing, and up to 0.5σ with polarization.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

On the unlikeliness of multi-field inflation: bounded random potentials and our vacuum

Based on random matrix theory, we compute the likelihood of saddles and minima in a class of random potentials that are softly bounded from above and below, as required for the validity of low energy effective theories. Imposing this bound leads to a random mass matrix with non-zero mean of its entries. If the dimensionality of field-space is large, inflation is rare, taking place near a saddle point (if at all), since saddles are more likely than minima or maxima for common values of the potential. Due to the boundedness of the potential, the latter become more ubiquitous for rare low/large values respectively. Based on the observation of a positive cosmological constant, we conclude that the dimensionality of field-space after (and most likely during) inflation has to be low if no anthropic arguments are invoked, since the alternative, encountering a metastable deSitter vacuum by chance, is extremely unlikely.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Refining inflation using non-canonical scalars

This paper revisits the Inflationary scenario within the framework of scalar field models possessing a non-canonical kinetic term. We obtain closed form solutions for all essential quantities associated with chaotic inflation including slow roll parameters, scalar and tensor power spectra, spectral indices, the tensor-to-scalar ratio, etc. We also examine the Hamilton-Jacobi equation and demonstrate the existence of an inflationary attractor. Our results highlight the fact that non-canonical scalars can significantly improve the viability of inflationary models. They accomplish this by decreasing the tensor-to-scalar ratio while simultaneously increasing the value of the scalar spectral index, thereby redeeming models which are incompatible with the cosmic microwave background (CMB) in their canonical version. For instance, the non-canonical version of the chaotic inflationary potential, V(φ) ∼ λφ{sup 4}, is found to agree with observations for values of λ as large as unity! The exponential potential can also provide a reasonable fit to CMB observations. A central result of this paper is that steep potentials (such as V∝φ{sup −n}) usually associated with dark energy, can drive inflation in the non-canonical setting. Interestingly, non-canonical scalars violate the consistency relation r = −8n{sub T}, which emerges as a smoking gun test for this class of models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Stochastic approach to flat direction during inflation

We revisit the time evolution of a flat and non-flat direction system during inflation. In order to take into account quantum noises in the analysis, we base on stochastic formalism and solve coupled Langevin equations numerically. We focus on a class of models in which tree-level Hubble-induced mass is not generated. Although the non-flat directions can block the growth of the flat direction's variance in principle, the blocking effects are suppressed by the effective masses of the non-flat directions. We find that the fate of the flat direction during inflation is determined by one-loop radiative corrections and non-renormalizable terms as usually considered, if we remove the zero-point fluctuation from the noise terms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Large trispectrum in two-field slow-roll inflation

We calculate the conditions required to produce a large local trispectrum during two-field slow-roll inflation. This is done by extending and simplifying the 'heat-map' approach developed by Byrnes et al. The conditions required to generate a large trispectrum are broadly the same as those that can produce a large bispectrum. We derive a simple relation between τ{sub NL} and f{sub NL} for models with separable potentials, and furthermore show that g{sub NL} and τ{sub NL} can be related in specific circumstances. Additionally, we interpret the heatmaps dynamically, showing how they can be used as qualitative tools to understand the evolution of non-Gaussianity during inflation. We also show how f{sub NL}, τ{sub NL} and g{sub NL} are sourced by generic shapes in the inflationary potential, namely ridges, valleys and inflection points.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Separable and non-separable multi-field inflation and large non-Gaussianity

In this paper we provide a general framework based on δN formalism to estimate the cosmological observables pertaining to the cosmic microwave background radiation for non-separable potentials, and for generic end of inflation boundary conditions. We provide analytical and numerical solutions to the relevant observables by decomposing the cosmological perturbations along the curvature and the isocurvature directions, instead of adiabatic and entropy directions. We then study under what conditions large bi-spectrum and tri-spectrum can be generated through phase transition which ends inflation. In an illustrative example, we show that large f{sub NL} ∼ O(80) and τ{sub NL} ∼ O(20000) can be obtained for the case of separable and non-separable inflationary potentials.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Reheating, multifield inflation and the fate of the primordial observables

We study the effects of perturbative reheating on the evolution of the curvature perturbation ζ, in two-field inflation models. We use numerical methods to explore the sensitivity of f{sub NL}, n{sub ζ} and r to the reheating process, and present simple qualitative arguments to explain our results. In general, if a large non-Gaussian signal exists at the start of reheating, it will remain non-zero at the end of reheating. Unless all isocurvature modes have completely decayed before the start of reheating, we find that the non-linearity parameter, f{sub NL}, can be sensitive to the reheating timescale, and that this dependence is most appreciable for 'runaway' inflationary potentials that only have a minimum in one direction. For potentials with a minimum in both directions, f{sub NL} can also be sensitive to reheating if a mild hierarchy exists between the decay rates of each field. Within the class of models studied, we find that the spectral index n{sub ζ}, is fairly insensitive to large changes in the field decay rates, indicating that n{sub ζ} is a more robust inflationary observable, unlike the non-linearity parameter f{sub NL}. Our results imply that the statistics of ζ, especially f{sub NL}, can only be reliably used to discriminate between models of two-field inflation if the physics of reheating are properly accounted for.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Observational constraints on K-inflation models

We extend the ModeCode software of Mortonson, Peiris and Easther [1] to enable numerical computation of perturbations in K-inflation models, where the scalar field no longer has a canonical kinetic term. Focussing on models where the kinetic and potential terms can be separated into a sum, we compute slow-roll predictions for various models and use these to verify the numerical code. A Markov chain Monte Carlo analysis is then used to impose constraints from WMAP7 data on the addition of a term quadratic in the kinetic energy to the Lagrangian of simple chaotic inflation models. For a quadratic potential, the data do not discriminate against addition of such a term, while for a quartic (λφ{sup 4}) potential inclusion of such a term is actually favoured. Overall, constraints on such a term from present data are found to be extremely weak.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Evading the pulsar constraints on the cosmic string tension in supergravity inflation

The cosmic string is a useful probe of the early Universe and may give us a clue to physics at high energy scales which particle accelerators cannot reach. Although the most promising tool to observe it is the cosmic microwave background (CMB), the constraint from gravitational waves is becoming so stringent that detecting its signatures in CMB may be impossible. In this paper, we construct a scenario that contains cosmic strings observable in the cosmic microwave background while evading the constraint imposed by the recent pulsar timing data. We argue that cosmic strings with relatively large tension are allowed by diluting loops contributing to the relevant frequency range of the gravitational wave background. We also present a particle physics model to realize such dilution in the context of chaotic inflation in supergravity, where the phase transition occurs during inflation due to the time-dependence of the Hubble induced mass.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A statistical approach to multifield inflation: many-field perturbations beyond slow roll

We study multifield contributions to the scalar power spectrum in an ensemble of six-field inflationary models obtained in string theory. We identify examples in which inflation occurs by chance, near an approximate inflection point, and we compute the primordial perturbations numerically, both exactly and using an array of truncated models. The scalar mass spectrum and the number of fluctuating fields are accurately described by a simple random matrix model. During the approach to the inflection point, bending trajectories and violations of slow roll are commonplace, and 'many-field' effects, in which three or more fields influence the perturbations, are often important. However, in a large fraction of models consistent with constraints on the tilt the signatures of multifield evolution occur on unobservably large scales. Our scenario is a concrete microphysical realization of quasi-single-field inflation, with scalar masses of order H, but the cubic and quartic couplings are typically too small to produce detectable non-Gaussianity. We argue that our results are characteristic of a broader class of models arising from multifield potentials that are natural in the Wilsonian sense.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Generation of large-scale magnetic fields from inflation in teleparallelism

We explore the generation of large-scale magnetic fields from inflation in teleparallelism, in which the gravitational theory is described by the torsion scalar instead of the scalar curvature in general relativity. In particular, we examine the case that the conformal invariance of the electromagnetic field during inflation is broken by a non-minimal gravitational coupling between the torsion scalar and the electromagnetic field. It is shown that for a power-law type coupling, the magnetic field on 1 Mpc scale with its strength of ∼ 10{sup −9} G at the present time can be generated.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Unitarity-violation in generalized Higgs inflation models

Unitarity-violation presents a challenge for non-minimally coupled models of inflation based on weak-scale particle physics. We examine the energy scale of tree-level unitarity-violation in scattering processes for generalized models with multiple scalar fields where the inflaton is either a singlet scalar or the Higgs. In the limit that the non-minimal couplings are all equal (e.g. in the case of Higgs or other complex inflaton), the scale of tree-level unitarity-violation matches the existing result. However if the inflaton is a singlet, and if it has a larger non-minimal coupling than other scalars in the model, then this hierarchy increases the scale of tree-level unitarity-violation. A sufficiently strong hierarchy pushes the scale of tree-level unitarity-violation above the Planck scale. We also discuss models which attempt to resolve the issue of unitarity-violation in Higgs Inflation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Khronon inflation

We study the possibility that the approximate time shift symmetry during inflation is promoted to the full invariance under time reparametrization t → t-tilde (t), or equivalently under field redefinition of the inflaton φ → φ-tilde (φ). The symmetry allows only two operators at leading order in derivatives, so that all n-point functions of scalar perturbations are fixed in terms of the power spectrum normalization and the speed of sound. During inflation the decaying mode only decays as 1/a and this opens up the possibility to violate some of the consistency relations in the squeezed limit, although this violation is suppressed by the (small) breaking of the field reparametrization symmetry. In particular one can get terms in the 3-point function that are only suppressed by 1/k{sub L} in the squeezed limit k{sub L}→0 compared to the local shape.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

GUT scalar potentials for Higgs inflation

Motivated by the idea that there is new physics beyond the Standard Model (SM), we have investigated a number of models for Grand Unified Theories (GUTS) in four dimensions for the possibility that their Higgs fields might be responsible for inflation in the early universe. In addition to models having an intrinsic Planck mass parameter, we have entertained classically scale invariant models in which the Planck scale itself as well as the GUT scale is induced by spontaneous breaking of the gauge symmetry. We found that in non-supersymmetric SU(5) with the usual Higgs in the adjoint representation but with large non-minimal coupling to the curvature, there appear to be several possible flat directions that might lead to inflation. Interestingly, the one of lowest energy is the breaking into SU(3)SU(2)U(1) that is suggested by gauge coupling unification. Further, we show that this flat direction is stable against small fluctuations in other directions. We attempted to extend this to similar supersymmetric GUTS, both global and supergravity, but did not succeed in finding a phenomenologically acceptable model of this type, with a 'minimal' Kaehler potential augmented only by terms characterised by dimensionless coupling constants. As is often the case, such models suffered either from a negative vacuum energy or from tachyonic modes. We also considered a variant of an 'inverted hierarchy' model in which the GUT scale is set by dimensional transmutation, but were unable to find a phenomenologically acceptable model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Three-form inflation and non-Gaussianity

We calculate the perturbed action, at second and third order, for a massive three-form field minimally coupled to gravity, and use it to explore the observational predictions of three-form inflation. One intriguing result is that the value of the spectral index is nearly independent of the three-form potential, being fixed solely by the number of e-folds of inflation, with n{sub s} = 0.97 for the canonical number of 60. Considering the bispectrum, we employ standard techniques to give explicit results for two models, one of which produces a large non-Gaussianity. Finally, we confirm our results by employing a duality relating the three-form theory to a non-canonical scalar field theory and explicitly re-computing results in this dual picture.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Slow-walking inflation

We propose a new model of slow-roll inflation in string cosmology, based on warped throat supergravity solutions displaying 'walking' dynamics, i.e. the coupling constant of the dual gauge theory slowly varies over a range of energy scales. The features of the throat geometry are sourced by a rich field content, given by the dilaton and RR and NS fluxes. By considering the motion of a D3-brane probe in this geometry, we are able to analytically calculate the brane potential in a physically interesting regime. This potential has an inflection point: in its proximity we realize a model of inflation lasting sixty e-foldings, and whose robust predictions are in agreement with current observations. We are also able to interpret some of the most interesting aspects of this scenario in terms of the properties of the QFT dual theory.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Primordial spikes from wrapped brane inflation

Cosmic inflation driven by branes wrapping the extra dimensions involves Kaluza-Klein (KK) degrees of freedom in addition to the zero-mode position of the brane which plays the role of the inflaton. As the wrapped brane passes by localized sources or features along its inflationary trajectory in the extra dimensional space, the KK modes along the wrapped direction are excited and start to oscillate during inflation. We show that the oscillating KK modes induce parametric resonance for the curvature perturbations, generating sharp signals in the perturbation spectrum. The effective four dimensional picture is a theory where the inflaton couples to the heavy KK modes. The Nambu-Goto action of the brane sources couplings between the inflaton kinetic terms and the KK modes, which trigger significant resonant amplification of the curvature perturbations. We find that the strong resonant effects are localized to narrow wave number ranges, producing spikes in the perturbation spectrum. Investigation of such resonant signals opens up the possibility of probing the extra dimensional space through cosmological observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Observational constraints on gauge field production in axion inflation

Models of axion inflation are particularly interesting since they provide a natural justification for the flatness of the potential over a super-Planckian distance, namely the approximate shift-symmetry of the inflaton. In addition, most of the observational consequences are directly related to this symmetry and hence are correlated. Large tensor modes can be accompanied by the observable effects of a the shift-symmetric coupling φF F-tilde to a gauge field. During inflation this coupling leads to a copious production of gauge quanta and consequently a very distinct modification of the primordial curvature perturbations. In this work we compare these predictions with observations. We find that the leading constraint on the model comes from the CMB power spectrum when considering both WMAP 7-year and ACT data. The bispectrum generated by the non-Gaussian inverse-decay of the gauge field leads to a comparable but slightly weaker constraint. There is also a constraint from μ-distortion using TRIS plus COBE/FIRAS data, but it is much weaker. Finally we comment on a generalization of the model to massive gauge fields. When the mass is generated by some light Higgs field, observably large local non-Gaussianity can be produced.

79 ASTRONOMY AND ASTROPHYSICS↗