Abstract: Solid solutions of Sr1-xLaxPbO3 (x= 0, 0.05, 0.10, 0.15 and 0.20) were synthesized with... more Abstract: Solid solutions of Sr1-xLaxPbO3 (x= 0, 0.05, 0.10, 0.15 and 0.20) were synthesized with the help of high pressure. Temperature (T) dependences of the resistivity r and differential thermoelectric power S were measured. All samples demonstrate an increase of r for low T and no signs of a bulk superconducting transition. The sign of S (T) is positive for x= 0.05 and 0.10, whereas S (T) changes sign from positive to negative for x= 0.15. Tunnel measurements were carried out using the break-junction technique. The sample SrPbO3 ...
Until recently our knowledge of the primordial curvature perturbation was relatively modest. Ever... more Until recently our knowledge of the primordial curvature perturbation was relatively modest. Ever since COBE delivered its map of data we know the scalar spectrum of primordial perturbations is approximately flat, with the power being only slightly stronger at larger scales. Most inflationary models predict an approximately scale-invariant spectrum, which therefore cannot be used as a distinctive signature. To distinguish between different inflationary microphysics we need to study higher point statistics of the primordial perturbation, which can encode non-gaussian data. In the first part of this thesis we study the bispectrum in all single-field models with a well-defined quantum field theory during a quasi-de Sitter inflationary phase. Any single-field models without ghost-like instabilities fall into this description: from canonical, to Dirac-Born-Infeld inflation and galileon inflation theories. We investigate the scale and shape- dependences of the bispectrum to next-order in ...
In de Rham-Gabadadze-Tolley (dRGT) massive gravity and bi-gravity, a non-minimal mat- ter couplin... more In de Rham-Gabadadze-Tolley (dRGT) massive gravity and bi-gravity, a non-minimal mat- ter coupling involving both metrics generically re-introduces the Boulware–Deser (BD) ghost. A non-minimal matter coupling via a simple, yet specific composite metric has been proposed, which eliminates the BD ghost below the strong coupling scale. Working explicitly in the metric formu- lation and for arbitrary spacetime dimensions, we show that this composite metric is the unique consistent non-minimal matter coupling below the strong coupling scale, which emerges out of two diagnostics, namely, absence of Ostrogradski ghosts in the decoupling limit and absence of the BD ghost from matter quantum loop corrections.
Recently, several works have investigated the coupling to matter in ghost-free massive (bi- & mul... more Recently, several works have investigated the coupling to matter in ghost-free massive (bi- & multi-)gravity and a new effective coupling to matter has been proposed. In this note we clarify some confusion on the existence and the implications of a ghost above the strong coupling scale. We confirm that the standard constraint which is otherwise typically present in this type of theories disappears on generic backgrounds as soon as this new coupling is considered. This implies the re-emergence of the Boulware–Deser ghost. Nevertheless the absence of ghost in the decoupling limit implies that the cut-off scale (if identified with the scale at which the ghost enters) is higher than the strong coupling scale. Therefore there is a valid interesting region of applicability for these couplings at scales below the cut-off.
We investigate the coupling to matter in ghost-free massive (bi-)gravity. When species in the mat... more We investigate the coupling to matter in ghost-free massive (bi-)gravity. When species in the matter sector couple covariantly to only one metric, we show that at one-loop these couplings do not spoil the special structure of the graviton potential. When the same species couples directly to both metrics we show that a ghost is present at the classical level and that loops destroy the special structure of the potential at an unacceptably low scale. We then propose a new `composite' effective metric built out of both metrics. When matter fields couple covariantly to this effective metric, the would be Boulware--Deser ghost is absent in different representative limits. At one-loop such couplings do not detune the special structure of the potential. We conjecture that matter can couple covariantly to that effective metric in all generality without introducing any Boulware-Deser ghost below a cut-off scale parametrically larger than the strong coupling scale. We also discuss alternative couplings to matter where the kinetic and potential terms of the matter field couple to different metrics. In both cases we discuss preliminary implications for cosmology.
We provide a formula for the scaling behaviour of the inflationary bispectrum in the 'squeezed' l... more We provide a formula for the scaling behaviour of the inflationary bispectrum in the 'squeezed' limit where one momentum becomes much smaller than the other two. This determines the scaling of the halo bias at low wavenumber and will be an important observable for the next generation of galaxy surveys. Our formula allows it to be predicted for the first time for a generic inflationary model with multiple light, canonically-normalized scalar fields.
We derive the 'separate universe' method for the inflationary bispectrum, beginning directly from... more We derive the 'separate universe' method for the inflationary bispectrum, beginning directly from a field-theory calculation. We work to tree-level in quantum effects but to all orders in the slow-roll expansion, with masses accommodated perturbatively. Our method provides a systematic basis to account for novel sources of time-dependence in inflationary correlation functions, and has immediate applications. First, we use our result to obtain the correct matching prescription between the 'quantum' and 'classical' parts of the separate universe computation. Second, we elaborate on the application of this method in situations where its validity is not clear. As a by-product of our calculation we give the leading slow-roll corrections to the three-point function of field fluctuations on spatially flat hypersurfaces in a canonical, multiple-field model.
A pressing problem in comparing inflationary models with observation is the accurate calculation ... more A pressing problem in comparing inflationary models with observation is the accurate calculation of correlation functions. One approach is to evolve them using ordinary differential equations ("transport equations"), analogous to the Schwinger-Dyson hierarchy of in-out quantum field theory. We extend this approach to the complete set of momentum space correlation functions. A formal solution can be obtained using raytracing techniques adapted from geometrical optics. We reformulate inflationary perturbation theory in this language, and show that raytracing reproduces the familiar "delta N" Taylor expansion. Our method produces ordinary differential equations which allow the Taylor coefficients to be computed efficiently. We use raytracing methods to express the gauge transformation between field fluctuations and the curvature perturbation, zeta, in geometrical terms. Using these results we give a compact expression for the nonlinear gauge-transform part of fNL in terms of the principal curvatures of uniform energy-density hypersurfaces in field space.
We investigate the validity of effective field theory methods and the decoupling of heavy fields ... more We investigate the validity of effective field theory methods and the decoupling of heavy fields during inflation. Considering models of inflation in which the inflaton is coupled to a heavy (super-Hubble) degree of freedom initially in its vacuum state, we find that violations of decoupling are absent unless there is a breakdown of the slow-roll conditions. Next we allow for a temporary departure from inflation resulting in a period of non-adiabaticity during which effective field theory methods are known to fail. We find that the locality of the event and energy conservation lead to a tight bound on the size of the effects of the heavy field. We discuss the implications for the power spectrum and non-gaussianity, and comment on the connection with recent studies of the dynamics of multi-field inflation models. Our results further motivate the use of effective field theory methods to characterize cosmic inflation, and focus the question of observability of additional degrees of freedom during inflation to near the Hubble scale or below - as anticipated from the Wilsonian notions of decoupling and naturalness.
If the expansion of the early Universe was not purely de Sitter, the statistical imprints of the ... more If the expansion of the early Universe was not purely de Sitter, the statistical imprints of the primordial density perturbation on the cosmic microwave background can be quite different from those following slow-roll inflation. In this paper we study the inflationary signatures of all single-field models not plagued by ghost-like instabilities. We assume small deviations from exact scale-invariance, as supported by current cosmological constraints, allow for a rapid change of the Hubble parameter and the phase speed of scalar fluctuations. We obtain the propagator of scalar fluctuations and compute the bispectrum, keeping next-order corrections proportional to the deviation of the spectral index from unity. These theories offer an explicit example where the shape and scale dependences of the bispectrum are highly non-trivial for reasonable breaking of slow-roll.
We re-examine the importance of slow-roll corrections during the evolution of cosmological pertur... more We re-examine the importance of slow-roll corrections during the evolution of cosmological perturbations in models of multi-field inflation. We find that in many instances the presence of light degrees of freedom leads to situations in which next to leading order slow-roll corrections become significant. Examples where we expect such corrections to be crucial include models in which modes exit the Hubble radius while the inflationary trajectory undergoes an abrupt turn in field space, or during a phase transition. We illustrate this with two examples -- hybrid inflation and double quadratic inflation. Utilizing both analytic estimates and full numerical results, we find that corrections can be as large as 20%. Our results have implications for many existing models in the literature, as these corrections must be included to obtain accurate observational predictions -- particularly given the level of accuracy expected from CMB experiments such as Planck.
Abstract: Solid solutions of Sr1-xLaxPbO3 (x= 0, 0.05, 0.10, 0.15 and 0.20) were synthesized with... more Abstract: Solid solutions of Sr1-xLaxPbO3 (x= 0, 0.05, 0.10, 0.15 and 0.20) were synthesized with the help of high pressure. Temperature (T) dependences of the resistivity r and differential thermoelectric power S were measured. All samples demonstrate an increase of r for low T and no signs of a bulk superconducting transition. The sign of S (T) is positive for x= 0.05 and 0.10, whereas S (T) changes sign from positive to negative for x= 0.15. Tunnel measurements were carried out using the break-junction technique. The sample SrPbO3 ...
Until recently our knowledge of the primordial curvature perturbation was relatively modest. Ever... more Until recently our knowledge of the primordial curvature perturbation was relatively modest. Ever since COBE delivered its map of data we know the scalar spectrum of primordial perturbations is approximately flat, with the power being only slightly stronger at larger scales. Most inflationary models predict an approximately scale-invariant spectrum, which therefore cannot be used as a distinctive signature. To distinguish between different inflationary microphysics we need to study higher point statistics of the primordial perturbation, which can encode non-gaussian data. In the first part of this thesis we study the bispectrum in all single-field models with a well-defined quantum field theory during a quasi-de Sitter inflationary phase. Any single-field models without ghost-like instabilities fall into this description: from canonical, to Dirac-Born-Infeld inflation and galileon inflation theories. We investigate the scale and shape- dependences of the bispectrum to next-order in ...
In de Rham-Gabadadze-Tolley (dRGT) massive gravity and bi-gravity, a non-minimal mat- ter couplin... more In de Rham-Gabadadze-Tolley (dRGT) massive gravity and bi-gravity, a non-minimal mat- ter coupling involving both metrics generically re-introduces the Boulware–Deser (BD) ghost. A non-minimal matter coupling via a simple, yet specific composite metric has been proposed, which eliminates the BD ghost below the strong coupling scale. Working explicitly in the metric formu- lation and for arbitrary spacetime dimensions, we show that this composite metric is the unique consistent non-minimal matter coupling below the strong coupling scale, which emerges out of two diagnostics, namely, absence of Ostrogradski ghosts in the decoupling limit and absence of the BD ghost from matter quantum loop corrections.
Recently, several works have investigated the coupling to matter in ghost-free massive (bi- & mul... more Recently, several works have investigated the coupling to matter in ghost-free massive (bi- & multi-)gravity and a new effective coupling to matter has been proposed. In this note we clarify some confusion on the existence and the implications of a ghost above the strong coupling scale. We confirm that the standard constraint which is otherwise typically present in this type of theories disappears on generic backgrounds as soon as this new coupling is considered. This implies the re-emergence of the Boulware–Deser ghost. Nevertheless the absence of ghost in the decoupling limit implies that the cut-off scale (if identified with the scale at which the ghost enters) is higher than the strong coupling scale. Therefore there is a valid interesting region of applicability for these couplings at scales below the cut-off.
We investigate the coupling to matter in ghost-free massive (bi-)gravity. When species in the mat... more We investigate the coupling to matter in ghost-free massive (bi-)gravity. When species in the matter sector couple covariantly to only one metric, we show that at one-loop these couplings do not spoil the special structure of the graviton potential. When the same species couples directly to both metrics we show that a ghost is present at the classical level and that loops destroy the special structure of the potential at an unacceptably low scale. We then propose a new `composite' effective metric built out of both metrics. When matter fields couple covariantly to this effective metric, the would be Boulware--Deser ghost is absent in different representative limits. At one-loop such couplings do not detune the special structure of the potential. We conjecture that matter can couple covariantly to that effective metric in all generality without introducing any Boulware-Deser ghost below a cut-off scale parametrically larger than the strong coupling scale. We also discuss alternative couplings to matter where the kinetic and potential terms of the matter field couple to different metrics. In both cases we discuss preliminary implications for cosmology.
We provide a formula for the scaling behaviour of the inflationary bispectrum in the 'squeezed' l... more We provide a formula for the scaling behaviour of the inflationary bispectrum in the 'squeezed' limit where one momentum becomes much smaller than the other two. This determines the scaling of the halo bias at low wavenumber and will be an important observable for the next generation of galaxy surveys. Our formula allows it to be predicted for the first time for a generic inflationary model with multiple light, canonically-normalized scalar fields.
We derive the 'separate universe' method for the inflationary bispectrum, beginning directly from... more We derive the 'separate universe' method for the inflationary bispectrum, beginning directly from a field-theory calculation. We work to tree-level in quantum effects but to all orders in the slow-roll expansion, with masses accommodated perturbatively. Our method provides a systematic basis to account for novel sources of time-dependence in inflationary correlation functions, and has immediate applications. First, we use our result to obtain the correct matching prescription between the 'quantum' and 'classical' parts of the separate universe computation. Second, we elaborate on the application of this method in situations where its validity is not clear. As a by-product of our calculation we give the leading slow-roll corrections to the three-point function of field fluctuations on spatially flat hypersurfaces in a canonical, multiple-field model.
A pressing problem in comparing inflationary models with observation is the accurate calculation ... more A pressing problem in comparing inflationary models with observation is the accurate calculation of correlation functions. One approach is to evolve them using ordinary differential equations ("transport equations"), analogous to the Schwinger-Dyson hierarchy of in-out quantum field theory. We extend this approach to the complete set of momentum space correlation functions. A formal solution can be obtained using raytracing techniques adapted from geometrical optics. We reformulate inflationary perturbation theory in this language, and show that raytracing reproduces the familiar "delta N" Taylor expansion. Our method produces ordinary differential equations which allow the Taylor coefficients to be computed efficiently. We use raytracing methods to express the gauge transformation between field fluctuations and the curvature perturbation, zeta, in geometrical terms. Using these results we give a compact expression for the nonlinear gauge-transform part of fNL in terms of the principal curvatures of uniform energy-density hypersurfaces in field space.
We investigate the validity of effective field theory methods and the decoupling of heavy fields ... more We investigate the validity of effective field theory methods and the decoupling of heavy fields during inflation. Considering models of inflation in which the inflaton is coupled to a heavy (super-Hubble) degree of freedom initially in its vacuum state, we find that violations of decoupling are absent unless there is a breakdown of the slow-roll conditions. Next we allow for a temporary departure from inflation resulting in a period of non-adiabaticity during which effective field theory methods are known to fail. We find that the locality of the event and energy conservation lead to a tight bound on the size of the effects of the heavy field. We discuss the implications for the power spectrum and non-gaussianity, and comment on the connection with recent studies of the dynamics of multi-field inflation models. Our results further motivate the use of effective field theory methods to characterize cosmic inflation, and focus the question of observability of additional degrees of freedom during inflation to near the Hubble scale or below - as anticipated from the Wilsonian notions of decoupling and naturalness.
If the expansion of the early Universe was not purely de Sitter, the statistical imprints of the ... more If the expansion of the early Universe was not purely de Sitter, the statistical imprints of the primordial density perturbation on the cosmic microwave background can be quite different from those following slow-roll inflation. In this paper we study the inflationary signatures of all single-field models not plagued by ghost-like instabilities. We assume small deviations from exact scale-invariance, as supported by current cosmological constraints, allow for a rapid change of the Hubble parameter and the phase speed of scalar fluctuations. We obtain the propagator of scalar fluctuations and compute the bispectrum, keeping next-order corrections proportional to the deviation of the spectral index from unity. These theories offer an explicit example where the shape and scale dependences of the bispectrum are highly non-trivial for reasonable breaking of slow-roll.
We re-examine the importance of slow-roll corrections during the evolution of cosmological pertur... more We re-examine the importance of slow-roll corrections during the evolution of cosmological perturbations in models of multi-field inflation. We find that in many instances the presence of light degrees of freedom leads to situations in which next to leading order slow-roll corrections become significant. Examples where we expect such corrections to be crucial include models in which modes exit the Hubble radius while the inflationary trajectory undergoes an abrupt turn in field space, or during a phase transition. We illustrate this with two examples -- hybrid inflation and double quadratic inflation. Utilizing both analytic estimates and full numerical results, we find that corrections can be as large as 20%. Our results have implications for many existing models in the literature, as these corrections must be included to obtain accurate observational predictions -- particularly given the level of accuracy expected from CMB experiments such as Planck.
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