Publications (19)72.43 Total impact
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ABSTRACT: The stacked density profile of cosmic voids in the galaxy distribution provides an important tool for the use of voids for precision cosmology. We study the density profiles of voids identified using the ZOBOV watershed transform algorithm in realistic mock luminous red galaxy (LRG) catalogues from the Jubilee simulation, as well as in void catalogues constructed from the SDSS LRG and Main Galaxy samples. We compare different methods for reconstructing density profiles scaled by the void radius and show that the most commonly used method based on counts in shells and simple averaging is statistically flawed as it underestimates the density in void interiors. We provide two alternative methods that do not suffer from this effect; one based on Voronoi tessellations is also easily able to account from artefacts due to finite survey boundaries and so is more suitable when comparing simulation data to observation. Using this method we show that voids in simulation are exactly selfsimilar, meaning that their average rescaled profile does not depend on the void size. Within the range of our simulation we also find no redshift dependence of the profile. Comparison of the profiles obtained from simulated and real voids shows an excellent match. The profiles of real voids also show a universal behaviour over a wide range of galaxy luminosities, number densities and redshifts. This points to a fundamental property of the voids found by the watershed algorithm, which can be exploited in future studies of voids.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We examine the integrated SachsWolfe (ISW) imprint of voids and superclusters on the cosmic microwave background. We first study results from the Jubilee Nbody simulation, which models the fullsky ISW signal from structures out to redshift z=1.4 and provides a mock luminous red galaxy (LRG) catalogue, to confirm that the expected signal in the concordance \Lambda CDM model is very small and likely to always be much smaller than the anisotropies arising at the last scattering surface. Any current detections of such an imprint cannot, therefore, be caused by an ISW effect in a \Lambda CDM universe. Using the simulation as a guide, we then look for the signal using a catalogue of voids and superclusters from the Sloan Digital Sky Survey. We find a result that is consistent with the \Lambda CDM model, i.e. a signal consistent with zero.05/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We examine the integrated SachsWolfe (ISW) imprint of voids and superclusters on the cosmic microwave background. We first study results from the Jubilee Nbody simulation, which models the fullsky ISW signal from structures out to redshift z=1.4 and provides a mock luminous red galaxy (LRG) catalogue, to confirm that the expected signal in the concordance \Lambda CDM model is very small and likely to always be much smaller than the anisotropies arising at the last scattering surface. Any current detections of such an imprint cannot, therefore, be caused by an ISW effect in a \Lambda CDM universe. Using the simulation as a guide, we then look for the signal using a catalogue of voids and superclusters from the Sloan Digital Sky Survey. We find a result that is consistent with the \Lambda CDM model, i.e. a signal consistent with zero.04/2014; 
Article: The Jubilee ISW project  I. Simulated ISW and weak lensing maps and initial power spectra results
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ABSTRACT: We present initial results from the Jubilee Integrated SachsWolfe (ISW) project, which models the expected Λ cold dark matter ISW effect in the Jubilee simulation. The simulation volume is (6 h1 Gpc)3, allowing power on very large scales to be incorporated into the calculation. Haloes are resolved down to a mass of 1.5 × 1012 h1 M☉, which allows us to derive a catalogue of mock Luminous Red Galaxies (LRGs) for crosscorrelation analysis with the ISW signal. We find the ISW effect observed on a projected sky to grow stronger at late times with the evolution of the ISW power spectrum matching expectations from linear theory. Maps of the gravitationallensing effect are calculated using the same potential as for the ISW. We calculate the redshift dependence of the ISWLRG crosscorrelation signal for a fullsky survey with no noise considerations. For ℓ < 30, the signal is strongest for lower redshift bins (z ̃ 0.20.5), whereas for ℓ > 30, the signal is best observed with surveys covering z ̃ 0.61.0.01/2014; 438(1).  [Show abstract] [Hide abstract]
ABSTRACT: Sutter et al. have responded to the criticisms we made of their cosmic void catalogue in our recent paper presenting an alternative catalogue (Nadathur & Hotchkiss 2013). Unfortunately, their response contains several statements which are incorrect, as we point out in this note.10/2013;  [Show abstract] [Hide abstract]
ABSTRACT: The study of the interesting cosmological properties of voids in the Universe depends on the efficient and robust identification of such voids in galaxy redshift surveys. Recently, Sutter et al. (2012) have published a public catalogue of voids in the Sloan Digital Sky Survey Data Release 7 main galaxy and luminous red galaxy samples, using the voidfinding algorithm ZOBOV, which is based on the watershed transform. We examine the properties of this catalogue and show that it suffers from several inconsistencies and errors, including the identification of some extremely overdense regions as voids. As a result, cosmological results obtained using this catalogue need to be reconsidered. We provide instead an alternative, selfconsistent, public catalogue of voids in the same galaxy data, obtained from using an improved version of the same watershed transform algorithm. We provide a more robust method of dealing with survey boundaries and masks, as well as with a radially varying selection function, which means that our method can be applied to any other survey. We discuss some basic properties of the voids thus discovered, and describe how further information may be obtained from the catalogue. In addition, we apply an inversion of the algorithm to the same data to obtain a corresponding catalogue of largescale overdense structures, or "superclusters".10/2013; 440(2).  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the hemispherical power asymmetry in the cosmic microwave background on small angular scales. We find an anomalously high asymmetry in the multipole range l=6012048, with a naive statistical significance of 6.5 sigma. However, we show that this extreme anomaly is simply a coincidence of three other effects, relativistic power modulation, edge effects from the mask applied, and interscale correlations. After correcting for all of these effects, the significance level drops to ~1 sigma, i.e., there is no anomalous intrinsic asymmetry in the small angular scales. Using this null result, we derive a constraint on a potential dipolar modulation amplitude, A(k)<0.0045 on the ~10 Mpcscale, at 95% C.L. This new constraint must be satisfied by any theoretical model attempting to explain the hemispherical asymmetry at large angular scales.Journal of Cosmology and Astroparticle Physics 07/2013; · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A detection of the stacked integrated SachsWolfe (ISW) signal in the CMB of rare superstructures identified in the SDSS Luminous Red Galaxy catalogue has been reported at very high statistical significance. The magnitude of the observed signal has previously been argued to be more than 3 standard deviations larger than the theoretical \Lambda CDM expectation. However, this calculation was made in the linear approximation, and relied on assumptions that may potentially have caused the \Lambda CDM expectation to be underestimated. Here we update the theoretical model calculation and compare it with an analysis of ISW maps obtained from Nbody simulations of a \Lambda CDM universe. The differences between model predictions and the map analyses are found to be small and cannot explain the discrepancy with observation, which remains at >3 s.d. significance. We discuss the cosmological significance of this anomaly and speculate on the potential of alternative models to explain it.Journal of Cosmology and Astroparticle Physics 12/2012; 2013(02). · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We consider methods with which to answer the question "is any observed galaxy cluster too unusual for LambdaCDM?" After emphasising that many previous attempts to answer this question will overestimate the confidence level at which LambdaCDM can be ruled out, we outline a consistent approach to these rare clusters, which allows the question to be answered. We define three statistical measures, each of which are sensitive to changes in cluster populations arising from different modifications to the cosmological model. We also use these properties to define the 'equivalent mass at redshift zero' for a cluster  the mass of an equally unusual cluster today. This quantity is independent of the observational survey in which the cluster was found, which makes it an ideal proxy for ranking the relative unusualness of clusters detected by different surveys. These methods are then used on a comprehensive sample of observed galaxy clusters and we confirm that all are less than 2sigma deviations from the LambdaCDM expectation. Whereas we have only applied our method to galaxy clusters, it is applicable to any isolated, collapsed, halo. As motivation for future surveys, we also calculate where in the mass redshift plane the rarest halo is most likely to be found, giving information as to which objects might be the most fruitful in the search for new physics.Journal of Cosmology and Astroparticle Physics 10/2012; 2013(07). · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A crucial diagnostic of the ΛCDM cosmological model is the integrated SachsWolfe (ISW) effect of largescale structure on the cosmic microwave background (CMB). The ISW imprint of superstructures of size ∼ 100 h−1Mpc at redshift z ∼ 0.5 has been detected with > 4σ significance, however it has been noted that the signal is much larger than expected. We revisit the calculation using linear theory predictions in ΛCDM cosmology for the number density of superstructures and their radial density profile, and take possible selection effects into account. While our expected signal is larger than previous estimates, it is still inconsistent by > 3σ with the observation. If the observed signal is indeed due to the ISW effect then huge, extremely underdense voids are far more common in the observed universe than predicted by ΛCDM.Journal of Cosmology and Astroparticle Physics 01/2012; 2012(06):042042. · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We critically investigate current statistical tests applied to high redshift clusters of galaxies in order to test the standard cosmological model and describe their range of validity. We carefully compare a sample of highredshift, massive, galaxy clusters with realistic Poisson sample simulations of the theoretical mass function, which include the effect of Eddington bias. We compare the observations and simulations using the following statistical tests: the distributions of ensemble and individual existence probabilities (in the > M, > z sense), the redshift distributions, and the 2d KolmogorovSmirnov test. Using seemingly rare clusters from Hoyle et al. (2011), and Jee et al. (2011) and assuming the same survey geometry as in Jee et al. (2011, which is less conservative than Hoyle et al. 2011), we find that the ( > M, > z) existence probabilities of all clusters are fully consistent with ΛCDM. However assuming the same survey geometry, we use the 2d KS test probability to show that the observed clusters are not consistent with being the least probable clusters from simulations at > 95% confidence, and are also not consistent with being a random selection of clusters, which may be caused by the nontrivial selection function and survey geometry. Tension can be removed if we examine only a Xray selected sub sample, with simulations performed assuming a modified survey geometry.Journal of Cosmology and Astroparticle Physics 01/2012; 2012(02):009009. · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We address the issue of cosmological backreaction from nonlinear structure formation by constructing an approximation for the time evolved metric of a dust dominated universe based on a gradient expansion. Our metric begins as a perturbation of a flat FriedmannRobertsonWalker state described by a nearly scale invariant, Gaussian, powerlaw distribution, and evolves in time until nonlinear structures have formed. After describing and attempting to control for certain complications in the implementation of this approach, this metric then forms a working model of the universe. We numerically calculate the evolution of the average scale factor in this model and hence the backreaction. We argue that, despite its limitations, this model is more realistic than previous models that have confronted the issue of backreaction. We find that the \emph{instantaneous} effects of backreaction in this model could be as large as $\sim10%$ of the background. This suggests that a proper understanding of the \emph{cumulative} effects of backreaction could be crucial for precision cosmology and any future exploration of the dark sector.Journal of Cosmology and Astroparticle Physics 12/2011; 2012(03). · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The detection of primordial gravitational waves, or tensor perturbations, would be regarded as compelling evidence for inflation. The canonical measure of this is the ratio of tensor to scalar perturbations, r. For singlefield slowroll models of inflation with small field excursions, the Lyth bound dictates that if the evolution of the slowroll parameter epsilon is monotonic, the tensortoscalar ratio must be below observationally detectable levels. We describe how nonmonotonic evolution of epsilon can evade the Lyth bound and generate observationally large r, even with small field excursions. This has consequences for the scalar power spectrum as it necessarily predicts an enhancement in the spectrum at very small scales and significant scaledependent running at CMB scales. This effect has not been appropriately accounted for in previous analyses. We describe a mechanism that will generically produce the required behaviour in epsilon and give an example of this mechanism arising in a wellmotivated smallfield model. This model can produce r\geq0.05 while satisfying all current observational constraints.Journal of Cosmology and Astroparticle Physics 10/2011; 2012(02). · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: I show that the most common method of quantifying the likelihood that an extreme galaxy cluster could exist is biased and can result in false claims of tension with LambdaCDM. This common method uses the probability that at least one cluster could exist above the mass and redshift of an observed cluster. I demonstrate the existence of the bias using sample cluster populations, describe its origin and explain how to remove it. I then suggest potentially more suitable and unbiased measures of the rareness of individual clusters. Each different measure will be most sensitive to different possible types of new physics. I show how to generalise these measures to quantify the total `rareness' of a set of clusters. It is seen that, when mass uncertainties are marginalised over, there is no tension between the standard LambdaCDM cosmological model and the existence of any observed set of clusters. As a case study, I apply these rareness measures to sample cluster populations generated using primordial density perturbations with a nonGaussian spectrum.Journal of Cosmology and Astroparticle Physics 05/2011; 7(07). · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: There are observations of at least 14 highredshift massive galaxy clusters, which have an extremely small probability with a purely Gaussian initial curvature perturbation. Here we revisit the estimation of the contribution of nonGaussianities to the cluster mass function and point out serious problems that have resulted from the application of the mass function out of the range of its validity. We remedy the situation and show that the values of fNL previously claimed to completely reconcile (i.e. at ~ 100% confidence) the existence of the clusters with ΛCDM are unphysically small. However, for WMAP cosmology and at 95% confidence, we arrive at the limit fNL411, which is similar to previous estimates. We also explore the possibility of a large gNL as the reason for the observed excess of the massive galaxy clusters. This scenario, gNL > 2 × 106, appears to be in more agreement with CMB and LSS limits for the nonGaussianity parameters and could also provide an explanation for the overabundance of large voids in the early universe.Journal of Cosmology and Astroparticle Physics 04/2011; 2011(04):017. · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Number counts of massive highredshift clusters provide a window to study primordial nonGaussianity. The current quality of data, however, forces the statistical analysis to probe a region of parameter space  the extreme tail of the mass function  which is neither accessible in any of the currently available theoretical prescriptions for calculating the mass function, nor calibrated in Nbody simulations. In this work we present a new analytical prescription for calculating a "resummed" nonGaussian halo mass function, which is constructed to remain stable in the extreme tail. We show that the prescription works well in the parameter regime that has been currently explored in simulations. We then use Fisher matrix techniques to compare our prescription with an extrapolated fit to Nbody simulations, which has recently been used to obtain constraints from data collected by the South Pole Telecope. We show that for the current data, both prescriptions would lead to statistically consistent constraints. As the data improve, however, there is a possibility of introducing a statistically significant bias in the constraints due to the choice of prescription, especially if nonGaussianity is scale dependent and becomes relatively large on cluster scales. It would then be necessary to test the accuracy of the prescriptions in Nbody simulations that can probe clusters with high masses and redshifts in the presence of large nonGaussianity.Physical review D: Particles and fields 04/2011; 84.  [Show abstract] [Hide abstract]
ABSTRACT: We consider observational constraints and finetuning issues in a renormalizable model of inflection point inflation, with two independent parameters. We derive constraints on the parameter space of this model arising from the WMAP 7year power spectrum. It has previously been shown that it is possible to successfully embed this potential in the MSSM. Unfortunately, to do this requires severe finetuning. We address this issue by introducing a hybrid field to dynamically uplift the potential with a subsequent smooth phase transition to end inflation at the necessary point. Large parameter regions exist where this drastically reduces the finetuning required without ruining the viability of the model. A side effect of this mechanism is that it increases the width of the slowroll region of the potential, thus also alleviating the problem of the finetuning of initial conditions. The MSSM embedding we study has been previously shown to be able to explain the smallness of the neutrino masses. The hybrid transition does not spoil this feature as there exist parameter regions where the finetuning parameter is as large as $10^{1}$ and the neutrino masses remain small.Journal of Cosmology and Astroparticle Physics 01/2011; 6(06). · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Multiple inflation is a model based on N=1 supergravity wherein there are sudden changes in the mass of the inflaton because it couples to 'flat direction' scalar fields which undergo symmetry breaking phase transitions as the universe cools. The resulting brief violations of slowroll evolution generate a nongaussian signal which we find to be oscillatory and yielding f_NL ~ 520. This is potentially detectable by e.g. Planck but would require new bispectrum estimators to do so. We also derive a modelindependent result relating the period of oscillations of a phase transition during inflation to the period of oscillations in the primordial curvature perturbation generated by the inflaton. Comment: 21 pages, 6 figures; Clarifying comments and references added; Accepted for publication in JCAPJournal of Cosmology and Astroparticle Physics 10/2009; · 6.04 Impact Factor 
Article: Fine tuning and the ratio of tensor to scalar density fluctuations from cosmological inflation
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ABSTRACT: The form of the inflationary potential is severely restricted if one requires that it be natural in the technical sense, i.e. terms of unrelated origin are not required to be correlated. We determine the constraints on observables that are implied in such natural inflationary models, in particular on $r$, the ratio of tensor to scalar perturbations. We find that the naturalness constraint does not require $r$ to be lare enough to be detectable by the forthcoming searches for Bmode polarisation in CMB maps. We show also that the value of $r$ is a sensitive discriminator between inflationary models. Comment: 8 pages LaTeX; clarifications and a reference added; to appear in JCAPJournal of Cosmology and Astroparticle Physics 04/2008; · 6.04 Impact Factor
Publication Stats
166  Citations  
72.43  Total Impact Points  
Top Journals
Institutions

2013–2014

University of Sussex
 Astronomy Centre
Brighton, England, United Kingdom


2012

Bielefeld University
 Faculty of Physics
Bielefeld, North RhineWestphalia, Germany


2011–2012

University of Helsinki
 Department of Physics
Helsinki, Southern Finland Province, Finland


2008

Universidad Nacional Autónoma de México
 Institute of Physical Science
Ciudad de México, The Federal District, Mexico
