A Bayesian study of the primordial power spectrum from a novel closed universe model

TL;DR

This paper constrains the primordial power spectrum using CMB and galaxy data, comparing a standard power-law model with a novel closed universe model based on the LD spectrum, which naturally explains low multipole power deficits.

Contribution

It introduces and tests a new closed universe model (LD spectrum) for the primordial power spectrum, showing its preference over standard models with current observational data.

Findings

LD model is preferred over power-law spectrum for CMB-only data.

LD spectrum naturally explains low multipole power suppression.

Bayesian analysis supports the LD model across datasets.

Abstract

We constrain the shape of the primordial power spectrum using recent measurements of the cosmic microwave background (CMB) from the Wilkinson Microwave Anisotropy Probe (WMAP) 7-year data and other high-resolution CMB experiments. We also include observations of the matter power spectrum from the luminous red galaxy (LRG) subset DR7 of the Sloan Digital Sky Survey (SDSS). We consider two different models of the primordial power spectrum. The first is the standard nearly scale-invariant spectrum in the form of a generalised power-law parameterised in terms of the spectral amplitude $A_{\rm s}$, the spectral index $n_{\rm s}$ and (possibly) the running parameter $n_{\rm run}$. The second spectrum is derived from the Lasenby and Doran (LD) model. The LD model is based on the restriction of the total conformal time available in a closed Universe and the predicted primordial power spectrum depends upon just two parameters. An important feature of the LD spectrum is that it naturally incorporates an exponential fall-off on large scales, which might provide a possible explanation for the lower-than-expected power observed at low multipoles in the CMB. In addition to parameter estimation, we compare both models using Bayesian model selection. We find there is a significant preference for the LD model over a simple power-law spectrum for a CMB-only dataset, and over models with an equal number of parameters for all the datasets considered.