27 research outputs found
PyTransport: A Python package for the calculation of inflationary correlation functions
21 pages, 5 figures21 pages, 5 figures21 pages, 5 figuresPyTransport constitutes a straightforward code written in C++ together with Python scripts which automatically edit, compile and run the C++ code as a Python module. It has been written for Unix-like systems (OS X and Linux). Primarily the module employs the transport approach to inflationary cosmology to calculate the tree-level power-spectrum and bispectrum of user specified models of multi-field inflation, accounting for all sub and super-horizon effects. The transport method we utilise means only coupled differential equations need to be solved, and the implementation presented here combines the speed of C++ with the functionality and convenience of Python. At present the code is restricted to canonical models. This document details the code and illustrates how to use it with a worked example
Multi-field inflation with large scalar fluctuations: non-Gaussianity and perturbativity
Recently multi-field inflation models that can produce large scalar fluctuations on small scales have drawn a lot of attention, primarily because they could lead to primordial black hole production and generation of large second-order gravitational waves. In this work, we focus on models where the scalar fields responsible for inflation live on a hyperbolic field space. In this case, geometrical destabilisation and non-geodesic motion are responsible for the peak in the scalar power spectrum. We present new results for scalar non-Gaussianity and discuss its dependence on the model's parameters. On scales around the peak, we typically find that the non-Gaussianity is large and close to local in form. We validate our results by employing two different numerical techniques, utilising the transport approach, based on full cosmological perturbation theory, and the δN formalism, based on the separate universe approximation. We discuss implications of our results for the perturbativity of the underlying theory, focusing in particular on versions of these models with potentially relevant phenomenology at interferometer scales
Generating the cosmic microwave background power asymmetry with g(NL)
We consider a higher order term in the expansion for the CMB power
asymmetry generated by a superhorizon isocurvature field fluctuation. The term
can generate the asymmetry without requiring a large value of . Instead
it produces a non-zero value of . A combination of constraints lead to
an allowed region in space. To produce the asymmetry with this
term without a large value of we find that the isocurvature field
needs to contribute less than the inflaton towards the power spectrum of the
curvature perturbation.Comment: 6 pages, 1 figure. Updated to match published version. Minor
typographical correction
Exploring the small mass limit of stationary black holes in theories with Gauss-Bonnet terms
In this work we examine the small mass limit of black holes (BHs), with and without spin, in theories where a scalar field is non-minimally coupled to a Gauss–Bonnet (GB) term. First, we provide an analytical example for a theory where a static closed-form solution with a small mass limit is known, and later use analytical and numerical techniques to explore this limit in standard scalar-GB theories with dilatonic, linear and quadratic-exponential couplings. In most cases studied here, we find an inner singularity that overlaps with the event horizon of the static BH as the small mass limit is reached. Moreover, since solutions in this limit possess a non-vanishing Hawking temperature, a naked singularity is expected to be reached through evaporation, raising questions concerning the consistency of these theories altogether. On the other hand, we provide for the first time in this context an example of a coupling where the small mass limit is never reached, thus preferred from the point of view of cosmic censorship. Finally, we consider BHs with spin and numerically investigate how this changes the picture, using these to place the tightest upper bounds to date on the coupling constant for the dilatonic and linear theories, with α‾<1 km
Black Holes in the Scalar-Tensor Formulation of 4D Einstein-Gauss-Bonnet Gravity: Uniqueness of Solutions, and a New Candidate for Dark Matter
In this work we study static black holes in the regularized 4D Einstein-Gauss-Bonnet theory of gravity; a shift-symmetric scalar-tensor theory that belongs to the Horndeski class. This theory features a simple black hole solution that can be written in closed form, and which we show is the unique static, spherically-symmetric and asymptotically-flat black hole vacuum solution of the theory. We further show that no asymptotically-flat, time-dependent, spherically-symmetric perturbations to this geometry are allowed, which suggests that it may be the only spherically-symmetric vacuum solution that this theory admits (a result analogous to Birkhoff's theorem). Finally, we consider the thermodynamic properties of these black holes, and find that their final state after evaporation is a remnant with a size determined by the coupling constant of the theory. We speculate that remnants of this kind from primordial black holes could act as dark matter, and we constrain the parameter space for their formation mass, as well as the coupling constant of the theory
Derivation of regularized field equations for the Einstein-Gauss-Bonnet theory in four dimensions
6 pages + 2 pages with reference
Observational constraints on the regularized 4D Einstein-Gauss-Bonnet theory of gravity
In this paper we study the observational constraints that can be imposed on
the coupling parameter, , of the regularized version of the
4-dimensional Einstein-Gauss-Bonnet theory of gravity. We use the scalar-tensor
field equations of this theory to perform a thorough investigation of its
slow-motion and weak-field limit, and apply our results to observations of a
wide array of physical systems that admit such a description. We find that the
LAGEOS satellites are the most constraining, requiring . This constraint suggests that the possibility of
large deviations from general relativity is small in all systems except the
very early universe (), or the immediate vicinity of
stellar-mass black holes (). We then consider
constraints that can be imposed on this theory from cosmology, black hole
systems, and table-top experiments. It is found that early universe inflation
prohibits all but the smallest negative values of , while
observations of binary black hole systems are likely to offer the tightest
constraints on positive values, leading to overall bounds .Comment: 17 page
The 4D Einstein-Gauss-Bonnet theory of gravity: a review
57 pages, 2 figures57 pages, 2 figure
Primordial curvature perturbation from lattice simulations
We study the contribution to the primordial curvature perturbation on
observational scales generated by the reheating field in massless preheating.
To do so we use lattice simulations and a recent extension to the
formalism. The work demonstrates the functionality of these techniques for
calculating the observational signatures of models in which non-perturbative
reheating involves a light scalar field.Comment: 7 pages, 3 figure