Publications (27)99.06 Total impact

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ABSTRACT: We discuss the relation between the output of Newtonian Nbody simulations on scales that approach or exceed the particle horizon to the description of general relativity. At leading order, the Zeldovich approximation is correct on large scales, coinciding with the general relativistic result. At second order in the initial metric potential, the trajectories of particles deviate from the second order Newtonian result and hence the validity of second order Lagrangian perturbation theory initial conditions should be reassessed when used in very large simulations. We also advocate using the expression for the synchronous gauge density as a well behaved measure of density fluctuations on such scales.Classical and Quantum Gravity 12/2014; 31(23). DOI:10.1088/02649381/31/23/234004 · 3.10 Impact Factor 
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ABSTRACT: We examine the deviation of Cold Dark Matter particle trajectories from the Newtonian result as the size of the region under study becomes comparable to or exceeds the particle horizon. To first order in the gravitational potential, the general relativistic result coincides with the Zel'dovich approximation and hence the Newtonian prediction on all scales. At second order, General Relativity predicts corrections which overtake the corresponding second order Newtonian terms above a certain scale of the order of the Hubble radius. However, since second order corrections are very much suppressed on such scales, we conclude that simulations which exceed the particle horizon but use Newtonian equations to evolve the particles, reproduce the correct trajectories very well. At z=49, the dominant relativistic corrections to the power spectrum on scales close to the horizon are at most of the order of $10^{3}$, while the differences in the positions of real space features are affected at a level below $10^{6}$. Similar conclusions hold at z=0. In the process, we clarify the relation of Nbody results to relativistic considerations.Monthly Notices of the Royal Astronomical Society 07/2013; 446(1). DOI:10.1093/mnras/stu2070 · 5.23 Impact Factor 
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ABSTRACT: A perfect irrotational fluid with the equation of state of dust, Irrotational Dark Matter (IDM), is incapable of virializing and instead forms a cosmoskeleton of filaments with supermassive black holes at the joints. This stark difference from the standard cold dark matter (CDM) scenario arises because IDM must exhibit potential flow at all times, preventing shellcrossing from occurring. This scenario is applicable to general nonoscillating scalarfield theories with a small sound speed. Our model of combined IDM and CDM components thereby provides a solution to the problem of forming the observed billionsolarmass black holes at redshifts of six and higher. In particular, as a result of the reduced vortical flow, the growth of the black holes is expected to be more rapid at later times as compared to the standard scenario.Physical Review D 07/2013; 88(8):083520. DOI:10.1103/PhysRevD.88.083520 · 4.86 Impact Factor 
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ABSTRACT: We present a new analysis on how to distinguish between isotropic and anisotropic cosmological models based on tracking the angular displacements of a large number of distant quasars over an extended period of time, and then performing a multipolevector decomposition of the resulting displacement maps. We find that while the GAIA mission operating at its nominal specifications does not have sufficient angular resolution to resolve anisotropic universes from isotropic ones using this method within a reasonable timespan of ten years, a nextgeneration GAIAlike survey with a resolution ten times better should be equal to the task. Distinguishing between different anisotropic models is however more demanding. Keeping the observational timespan to ten years, we find that the angular resolution of the survey will need to be of order 0.1 microarcsecond in order for certain rotating anisotropic models to produce a detectable signature that is also unique to models of this class. However, should such a detection become possible, it would immediately allow us to rule out large local void models.Journal of Cosmology and Astroparticle Physics 07/2013; DOI:10.1088/14757516/2013/12/042 · 5.88 Impact Factor 
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ABSTRACT: There is an approximately 9% discrepancy, corresponding to 2.4sigma, between two independent constraints on the expansion rate of the universe: one indirectly arising from the cosmic microwave background and baryon acoustic oscillations, and one more directly obtained from local measurements of the relation between redshifts and distances to sources. We argue that by taking into account the local gravitational potential at the position of the observer this tension  strengthened by the recent Planck results  is partially relieved and the concordance of the standard model of cosmology increased. We estimate that measurements of the local Hubble constant are subject to a cosmic variance of about 2.4% (limiting the local sample to redshifts z>0.010) or 1.3% (limiting it to z>0.023), a more significant correction than that taken into account already. Nonetheless, we show that one would need a very rare fluctuation to fully explain the offset in the Hubble rates. If this tension is further strengthened, a cosmology beyond the standard model may prove necessary.Physical Review Letters 03/2013; 110(24):241305. DOI:10.1103/PhysRevLett.110.241305 · 7.73 Impact Factor 
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ABSTRACT: We argue that there is an intrinsic noise on measurements of the equation of state parameter w=p/rho from largescale structure around us. The presence of the large scale structure leads to an ambiguity in the definition of the background universe and thus there is a maximal precision with which we can determine the equation of state of dark energy. To study the uncertainty due to local structure, we model density perturbations stemming from a standard inflationary power spectrum by means of the exact LemaitreTolmanBondi solution of Einstein's equation, and show that the usual distribution of matter inhomogeneities in a LCDM cosmology causes a variation of w  as inferred from distance measures  of several percent.02/2013; 2(4). DOI:10.1016/j.dark.2014.01.001 
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ABSTRACT: We present a new program for placing constraints on radial inhomogeneity in a darkenergy dominated universe. We introduce a new measure to quantify violations of the Copernican principle. Any violation of this principle would interfere with our interpretation of any darkenergy evolution. In particular, we find that current observations place reasonably tight constraints on possible latetime violations of the Copernican principle: the allowed area in the parameter space of amplitude and scale of a spherical inhomogeneity around the observer has to be reduced by a factor of three so as to confirm the Copernican principle. Then, by marginalizing over possible radial inhomogeneity we provide the first constraints on the cosmological constant which are free of the homogeneity prior prevalent in cosmology.Monthly Notices of the Royal Astronomical Society 09/2012; 438(1). DOI:10.1093/mnrasl/slt140 · 5.23 Impact Factor 
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ABSTRACT: We describe inhomogeneities in a {\Lambda}CDM universe with a gradient series expansion and show that it describes the gravitational evolution far into the nonlinear regime and beyond the capacity of standard perturbation theory at any order. We compare the gradient expansion with exact inhomogeneous {\Lambda}LTB solutions (Lema\^itreTolmanBondi metric with the inclusion of a cosmological constant) describing growing structure in a {\Lambda}CDM universe and find that the expansion approximates the exact solution well, following the collapse of an overdensity all the way into a singularity.Physical review D: Particles and fields 08/2012; 86(4). DOI:10.1103/PhysRevD.86.043523 · 4.86 Impact Factor 
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ABSTRACT: If we live in the vicinity of the hypothesized Great Attractor, the age of the universe as inferred from the local expansion rate can be off by three per cent. We study the effect that living inside or near a massive overdensity has on cosmological parameters induced from observations of supernovae, the Hubble parameter and the Cosmic Microwave Background. We compare the results to those for an observer in a perfectly homogeneous LCDM universe. We find that for instance the inferred value for the global Hubble parameter changes by around three per cent if we happen to live inside a massive overdensity such as the hypothesized Great Attractor. Taking into account the effect of such structures on our perception of the universe makes cosmology perhaps less precise, but more accurate.Monthly Notices of the Royal Astronomical Society 03/2012; 424(1). DOI:10.1111/j.13652966.2012.21218.x · 5.23 Impact Factor 
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ABSTRACT: We find that if we live at the center of an inhomogeneity with total density contrast of roughly 0.1, dark energy is not a cosmological constant at 95% confidence level. Observational constraints on the equation of state of dark energy, w, depend strongly on the local matter density around the observer. We model the local inhomogeneity with an exact spherically symmetric solution which features a pressureless matter component and a darkenergy fluid with constant equation of state and negligible sound speed, that reaches a homogeneous solution at finite radius. We fit this model to observations of the local expansion rate, distant supernovae and the cosmic microwave background. We conclude that the possible uncertainty from largescale structure has to be taken into account if one wants to progress towards not just precision but also accurate cosmology.Monthly Notices of the Royal Astronomical Society 03/2012; 431(2). DOI:10.1093/mnras/stt309 · 5.23 Impact Factor 
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ABSTRACT: The Cold Spot could be an adiabatic perturbation on the surface of last scattering, in which case it is an overdensity with comoving radius of the order of 1 Gpc. We assess the effect that living in a similar structure, without knowing it, has on our perception of the equation of state of Dark Energy. We find that structures of dimensions such that they could cause the Cold Spot on the CMB, affect the perceived equation of state of Dark Energy possibly up to ten percent.Journal of Cosmology and Astroparticle Physics 06/2011; 2012(01). DOI:10.1088/14757516/2012/01/047 · 5.88 Impact Factor 
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ABSTRACT: We present semianalytical solutions to the background equations describing the Lema\^itreTolmanBondi (LTB) metric as well as the homogeneous Friedmann equations, in the presence of dust, curvature and a cosmological constant Lambda. For none of the presented solutions any numerical integration has to be performed. All presented solutions are given for expanding and collapsing phases, preserving continuity in time and radius. Hence, these solutions describe the complete space time of a collapsing spherical object in an expanding universe. In the appendix we present for completeness a solution of the Friedmann equations in the additional presence of radiation, only valid for the RobertsonWalker metric.General Relativity and Gravitation 04/2011; 44(10). DOI:10.1007/s1071401214059 · 1.73 Impact Factor 
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ABSTRACT: We present semianalytical solutions to the background equations describing the Lema\^itreTolmanBondi (LTB) metric as well as the homogeneous Friedmann equations, in the presence of dust, curvature and a cosmological constant Lambda. For none of the presented solutions any numerical integration has to be performed. All presented solutions are given for expanding and collapsing phases, preserving continuity in time and radius. Hence, these solutions describe the complete space time of a collapsing spherical object in an expanding universe. In the appendix we present for completeness a solution of the Friedmann equations in the additional presence of radiation, only valid for the RobertsonWalker metric. 
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ABSTRACT: In this paper, instead of invoking Dark Energy, we try and fit various cosmological observations with a large Gpc scale underdense region (Void) which is modeled by a LemaitreTolmanBondi metric that at large distances becomes a homogeneous FLRW metric. We improve on previous analyses by allowing for nonzero overall curvature, accurately computing the distance to the lastscattering surface and the observed scale of the Baryon Acoustic peaks, and investigating important effects that could arise from having nontrivial Void density profiles. We mainly focus on the WMAP 7yr data (TT and TE), Supernova data (SDSS SN), Hubble constant measurements (HST) and Baryon Acoustic Oscillation data (SDSS and LRG). We find that the inclusion of a nonzero overall curvature drastically improves the goodness of fit of the Void model, bringing it very close to that of a homogeneous universe containing Dark Energy, while by varying the profile one can increase the value of the local Hubble parameter which has been a challenge for these models. We also try to gauge how well our model can fit the largescalestructure data, but a comprehensive analysis will require the knowledge of perturbations on LTB metrics. The model is consistent with the CMB dipole if the observer is about 15 Mpc off the centre of the Void. Remarkably, such an offcenter position may be able to account for the recent anomalous measurements of a large bulk flow from kSZ data. Finally we provide several analytical approximations in different regimes for the LTB metric, and a numerical module for CosmoMC, thus allowing for a MCMC exploration of the full parameter space. Comment: 82 pages, 12 figures. Numerical module available at http://web.physik.rwthaachen.de/download/valkenburg/Journal of Cosmology and Astroparticle Physics 07/2010; DOI:10.1088/14757516/2010/11/030 · 5.88 Impact Factor 
06/2010; DOI:10.1063/1.3462615

Article: Swiss Cheese and a Cheesy CMB
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ABSTRACT: It has been argued that the SwissCheese cosmology can mimic Dark Energy, when it comes to the observed luminosity distanceredshift relation. Besides the fact that this effect tends to disappear on average over random directions, we show in this work that based on the ReesSciama effect on the cosmic microwave background (CMB), the SwissCheese model can be ruled out if all holes have a radius larger than about 35 Mpc. We also show that for smaller holes, the CMB is not observably affected, and that the small holes can still mimic Dark Energy, albeit in special directions, as opposed to previous conclusions in the literature. However, in this limit, the probability of looking in a special direction where the luminosity of supernovae is sufficiently supressed becomes very small, at least in the case of a lattice of spherical holes considered in this paper. Comment: 23 pages, 10 figures. Matches published versionJournal of Cosmology and Astroparticle Physics 02/2009; DOI:10.1088/14757516/2009/06/010 · 5.88 Impact Factor 
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ABSTRACT: We summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of inflation. We focus on the prospects for using CMB measurements to differentiate various inflationary mechanisms. In particular, a detection of primordial Bmode polarization would demonstrate that inflation occurred at a very high energy scale, and that the inflaton traversed a superPlanckian distance in field space. We explain how such a detection or constraint would illuminate aspects of physics at the Planck scale. Moreover, CMB measurements can constrain the scaledependence and nonGaussianity of the primordial fluctuations and limit the possibility of a significant isocurvature contribution. Each such limit provides crucial information on the underlying inflationary dynamics. Finally, we quantify these considerations by presenting forecasts for the sensitivities of a future satellite experiment to the inflationary parameters. Comment: 107 pages, 14 figures, 17 tables; Inflation Working Group contribution to the CMBPol Mission Concept Study; v2: typos fixed and references added 
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ABSTRACT: There has been much recent discussion, and some confusion, regarding the use of existing observational data to estimate the likelihood that nextgeneration cosmic microwave background (CMB) polarization experiments might detect a nonzero tensor signal, possibly associated with inflation. We examine this issue in detail here in two different ways: (1) first we explore the effect of choice of different parameter priors on the estimation of the tensortoscalar ratio r and other parameters describing inflation, and (2) we examine the Bayesian complexity in order to determine how effectively existing data can constrain inflationary parameters. We demonstrate that existing data are not strong enough to render full inflationary parameter estimates in a parametrization and priorindependent way and that the predicted tensor signal is particularly sensitive to different priors. For parametrizations where the Bayesian complexity is comparable to the number of free parameters we find that a flat prior on the scale of inflation (which is to be distinguished from a flat prior on the tensortoscalar ratio) leads us to infer a larger, and in fact slightly nonzero tensor contribution at 68% confidence level. However, no detection is claimed. Our results demonstrate that all that is statistically relevant at the current time is the (slightly enhanced) upper bound on r, and we stress that the data remain consistent with r = 0.Physical review D: Particles and fields 05/2008; 78(6). DOI:10.1103/PhysRevD.78.063521 · 4.86 Impact Factor 
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ABSTRACT: Temperature anisotropies in the cosmic microwave background (CMB) are affected by the late integrated SachsWolfe (lISW) effect caused by any time variation of the gravitational potential on linear scales. Dark energy is not the only source of lISW, since massive neutrinos induce a small decay of the potential on small scales during both matter and dark energy domination. In this work, we study the prospect of using the cross correlation between CMB and galaxydensity maps as a tool for constraining the neutrino mass. On the one hand massive neutrinos reduce the crosscorrelation spectrum because freestreaming slows down structure formation; on the other hand, they enhance it through their change in the effective linear growth. We show that in the observable range of scales and redshifts, the first effect dominates, but the second one is not negligible. We carry out an error forecast analysis by fitting some mock data inspired by the Planck satellite, Dark Energy Survey (DES) and Large Synoptic Survey Telescope (LSST). The inclusion of the cross correlation data from Planck and LSST increases the sensitivity to the neutrino mass mν by 38% (and to the dark energy equation of state w by 83%) with respect to Planck alone. The correlation between Planck and DES brings a far less significant improvement. This method is not potentially as good for detecting mν as the measurement of galaxy, cluster, or cosmic shear power spectra, but since it is independent and affected by different systematics, it remains potentially interesting if the total neutrino mass is of the order of 0.2 eV; if instead it is close to the lower bound from atmospheric oscillations, mν∼0.05 eV, we do not expect the ISWgalaxy correlation to be ever sensitive to mν.Physical review D: Particles and fields 03/2008; 77(6). DOI:10.1103/PhysRevD.77.063505 · 4.86 Impact Factor 
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ABSTRACT: We update constraints on the Hubble function H(phi) during inflation, using the most recent cosmic microwave background (CMB) and large scale structure (LSS) data. Our main focus is on a comparison between various commonly used methods of calculating the primordial power spectrum via analytical approximations and the results obtained by integrating the exact equations numerically. In each case, we impose naive, minimally restrictive priors on the duration of inflation. We find that the choice of priors has an impact on the results: the bounds on inflationary parameters can vary by up to a factor two. Nevertheless, it should be noted that within the region allowed by the minimal prior of the exact method, the accuracy of the approximations is sufficient for current data. We caution however that a careless minimal implementation of the approximative methods allows models for which the assumptions behind the analytical approximations fail, and recommend using the exact numerical method for a selfconsistent analysis of cosmological data. Comment: 16 pages, 3 figuresJournal of Cosmology and Astroparticle Physics 02/2008; DOI:10.1088/14757516/2008/04/016 · 5.88 Impact Factor
Publication Stats
451  Citations  
99.06  Total Impact Points  
Top Journals
Institutions

2011–2014

Leiden University
 InstituutLorenz for Theoretical Physics
Leyden, South Holland, Netherlands 
RWTH Aachen University
 Institut für Theoretische Teilchenphysik und Kosmologie (Theorie E)
Aachen, North RhineWestphalia, Germany


2010–2012

Heidelberg University
 Institute of Theoretical Physics
Heidelberg, BadenWuerttemberg, Germany


2008–2010

French National Centre for Scientific Research
Lutetia Parisorum, ÎledeFrance, France


2006

Universiteit Utrecht
 Institute for Theoretical Physics
Utrecht, Provincie Utrecht, Netherlands
