  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.
