New London Institute of Cosmology

At a London Cosmology Discussion Meeting last week, representatives of the London cosmology groups discussed ways of increasing collaboration and cooperation between the groups and how to utilize the various skills in London in a more coherent way.

One of the main outcomes of this meeting was a consensus to form a ‘virtual’ institute to act as an umbrella organisation for the collaboration efforts. This London Institute of Cosmology would initially act as a clearing house for event information, especially seminars in the groups, information about research visitors to London, and provide a structure for possible funding applications.

I am happy to say that a preliminary website has now been constructed with information about the groups in London, their seminar programmes and the London Cosmology Discussion Meetings. The address for the site is which hopefully will rise up through the ranks of Google search results soon. There is much work to be done on the site and the institute but hopefully this is a good start and provides a visible link between the research activities of the London groups.


Pyinspire – Python script to access INSPIRE database

The new INSPIRE HEP database has been up and running for a while now, and is going from strength to strength (despite some recent wobbles concerning citation counts).

I recently needed to get the BibTeX entries for a few papers and instead of copy-pasting from the results web page each time I wondered whether a more programmatic solution existed. There used to be a few utilities for SPIRES which enabled you to get results using a script but I haven’t seen any that do this with INSPIRE (although there is a plugin for Jabref).

I decided to cook up a quick script called pyinspire that will send INSPIRE a query and scrape the resulting page for results. It is available now in the Python Package Index with the source code on Bitbucket. I’ve released it under a modified BSD license so feel free to fork. Installation is as easy as pip install pyinspire or easy_install pyinspire.

The functionality is very basic at the moment but does include output in BibTeX, LaTeX(EU) and LaTeX(US) modes, and standard text output including citation counts. See the Bitbucket page for more details.


Trispectrum during Inflation

After a lot of work, “Large trispectrum in two-field slow-roll inflation” was released on the arXiv yesterday as arXiv:1203.6844. In this article Joe Elliston, Laila Alabidi, David Mulryne, Reza Tavakol and I look at the generation of higher order statistics during inflation in the early universe.

In the early universe the curvature perturbations, which later are seen as temperature fluctuations in the Cosmic Microwave Background (CMB), are initially thought to be Gaussian, but can become skewed during inflation depending on the physics of their evolution. In the last few years a lot of work has been done to both find evidence of this non-Gaussianity, and to construct physical models in which it is generated in the early universe.

In the past most of the focus has been on the 3-point function or bispectrum, and discussion of non-gaussianity has boiled down to finding bounds on the parameter f_{\mathrm{NL}}. In terms of the CMB the bispectrum in essence considers whether the temperature of three points on the sky is correlated. The WMAP satellite has not seen any definitive evidence of a non-zero value for f_{\mathrm{NL}} but the Planck satellite should be able to detect a signal if it is moderately large. In this work we look beyond the bispectrum to the 4-point function or trispectrum.  For the trispectrum the correlation we attempt to measure is between four different points on the sky.

In this work we have tried to find models which generate a large value for the trispectrum during inflation. We have found some new expressions for the parameters f_{\mathrm{NL}}\tau_{\mathrm{NL}} and g_{\mathrm{NL}}. The last two of these parametrise two different contributions to the trispectrum. The bottom line is that it is quite difficult to find conditions where the trispectrum can be large, at least under the assumptions we made of sum- and product-separable potentials using the \delta N formalism.

In the course of searching for models which give large values to these parameters we plotted the coefficient functions which need to be large as heatmaps, following Byrnes et al (arXiv:0807.1101). In order to generate these heatmaps I relied on the combination of Python, Numpy and Matplotlib, which I have used before on Pyflation. The script I used to generate the heatmap figures in the paper is now available as a repository on Bitbucket.

PS Someone really needs to work on the Wikipedia Non-Gaussianity page!


Paper published in Physical Review D

My latest paper “Calculating nonadiabatic pressure perturbations during multifield inflation”, written with Adam Christopherson, has now been published in Physical Review D as Phys. Rev. D 85, 063507. The DOI is 10.1103/PhysRevD.85.063507 and if you want to get the (freely available) arXiv version the number is arXiv:1111.6919.

This paper investigates the isocurvature or nonadiabatic perturbations during inflationary expansion with more than one field. We performed numerical simulations using Pyflation version 0.2 which can handle multiple field inflationary models at first order in perturbation theory.

We found that, as expected, the amount of isocurvature varied considerably for the three models we examined. We compared our results to those found using a different definition for the isocurvature, which coincides with ours during slow roll evolution on large scales. As slow roll breaks down near the end of inflation the evolution of the two different forms of isocurvature can vary dramatically.

One reason to be interested in non-adiabatic pressure during the early universe is that it can be a source for the generation of vorticity, which could be observable in the polarization of the cosmic microwave background.



Two new papers on the arXiv

Last Tuesday was a bit hectic for me as I tried to coordinate the last minute changes needed to put two papers on the arXiv servers for the next day. The two articles which are now available are numbered 1111.6919 and 1111.6940:

  • Calculating Non-adiabatic Pressure Perturbations during Multi-field Inflation
    Ian Huston, Adam J. Christopherson
    Abstract: Isocurvature perturbations naturally occur in models of inflation consisting of more than one scalar field. In this paper we calculate the spectrum of isocurvature perturbations generated at the end of inflation for three different inflationary models consisting of two canonical scalar fields. The amount of non-adiabatic pressure present at the end of inflation can have observational consequences through the generation of vorticity and subsequently the sourcing of B-mode polarisation. We compare two different definitions of isocurvature perturbations and show how these quantities evolve in different ways during inflation. Our results are calculated using the open source Pyflation numerical package which is available to download.
  • Quantifying the behaviour of curvature perturbations during inflation
    Ellie Nalson, Adam J. Christopherson, Ian Huston, Karim A. Malik
    Abstract: How much does the curvature perturbation change after it leaves the horizon, and when should one evaluate the power spectrum? To answer these questions we study single field inflation models numerically, and compare the evolution of different curvature perturbations from horizon crossing to the end of inflation. In particular we calculate the number of efolds it takes for the curvature perturbation at a given wavenumber to settle down to within a given fraction of their value at the end of inflation. We find that e.g. in chaotic inflation, the amplitude of the comoving and the curvature perturbation on uniform density hypersurfaces differ by up to 180 % at horizon crossing assuming the same amplitude at the end of inflation, and that it takes approximately 3 efolds for the curvature perturbation to be within 1 % of its value at the end of inflation.

As part of the work on the first of these papers, my Pyflation code has been updated to version 0.2 and now allows multiple fields systems to be used at first order.

There seemed to be a glut of papers released last Wednesday, with the post-Thanksgiving queue being worked through and people aiming to apply for jobs in December. Apologies to those irked by the overwhelming astro-ph listing that day for my part in it!