arXiv subjects

Fergus Simpson

Publications and source records attributed to Fergus Simpson.

40 records · Page 3Linked to original sources

arXiv2007

An Alternative Approach to Holographic Dark Energy

Here we consider a scenario in which dark energy is associated with the apparent area of a surface in the early universe. In order to resemble the cosmological constant at late times, this hypothetical reference scale should maintain an approximately constant physical size during an asymptotically de-Sitter expansion. This is found to arise when the particle horizon - anticipated to be significantly greater than the Hubble length - is approaching the antipode of a closed universe. Depending on the constant of proportionality, either the ensuing inflationary period prevents the particle horizon from vanishing, or it may lead to a sequence of "Big Rips".

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arXiv2006

The Redshift Sensitivities of Dark Energy Surveys

Great uncertainty surrounds dark energy, both in terms of its physics, and the choice of methods by which the problem should be addressed. Here we quantify the redshift sensitivities offered by different techniques. We focus on the three methods most adept at constraining w, namely supernovae, cosmic shear, and baryon oscillations. For each we provide insight into the family of w(z) models which are permitted for a particular constraint on either w=w0 or w=w0+wa(1-a). Our results are in the form of "weight functions", which describe the fitted model parameters as a weighted average over the true functional form. For example, we find the recent best-fit from the Supernovae Legacy Survey (w=-1.023) corresponds to the average value of w(z) over the range 0<z<0.4. Whilst there is a strong dependence on the choice of priors, each cosmological probe displays distinctive characteristics in their redshift sensitivities. In the case of proposed future surveys, a SNAP-like supernova survey probes a mean redshift of z ~ 0.3, with baryon oscillations and cosmic shear at z ~ 0.6. If we consider the evolution of w, sensitivities shift to slightly higher redshift. Finally, we find that the weight functions may be expressed as a weighted average of the popular "principal components".

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arXiv2005

Observing Baryon Oscillations with Cosmic Shear

A cosmic shear survey, spanning a significant proportion of the sky, should greatly improve constraints on a number of cosmological parameters. It also provides a unique opportunity to examine the matter power spectrum directly. However, the observed lensing signal corresponds to a weighted average of the power spectrum across a range of scales, and so the potential to resolve the baryon oscillations has been somewhat neglected. These features originated prior to recombination, induced by the acoustics of the photon-baryon fluid. Recent galaxy surveys have detected the imprints, and in the future such measurements may even be used to refine our understanding of dark energy. Without redshift information, cosmic shear is an ineffective probe of the baryon oscillations. However, by implementing a novel multipole-dependent selection of photometric redshift bins, sensitivity is improved by an order of magnitude, bringing the "wiggles" within reach of future surveys. As an illustration, we show that data from surveys scheduled within the next ten years will be able to distinguish a smoothed power spectrum at the two sigma level.

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arXiv2005

Illuminating Dark Energy with Cosmic Shear

One of the principal goals of modern cosmology is to constrain the properties of dark energy. At present, numerous surveys are aiming to determine the equation of state, which is assumed constant due to our poor understanding of its behaviour (and since higher order parameterisations lead to unwieldy errors). This raises the question - how does our "best-fit" equation of state relate to the true function, which may vary with redshift. Saini et al. have demonstrated that the value of w attained by a supernovae study is well described by a weighted integral over the true function. Adopting a similar approach, we calculate the corresponding "weight function" for a fiducial cosmic shear survey. We disentangle contributions from the angular diameter distance and the growth factor, finding that they partially cancel each other out. Consequentially, the sensitivity to w at high redshift is enhanced beyond that of supernovae studies, and is comparable, if not higher, than lensing of the CMB. This illustrates the comlementary nature of the different techniques. Furthermore, we find that results which would naively be interpreted as inconsistent could arise from a time-dependent equation of state.

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