[Show abstract][Hide abstract] ABSTRACT: We provide a prescription for setting initial conditions for cosmological
N-body simulations, which simultaneously employ Lagrangian meshes (`particles')
and Eulerian grids (`fields'). Our description is based on coordinate systems
in arbitrary geometry, and can therefore be used in any metric theory of
gravity. We apply our prescription to a choice of Effective Field Theory of
Modified Gravity, and show how already in the linear regime, particle
trajectories are curved. For some viable models of modified gravity, the Dark
Matter trajectories are affected at the level of 5% at Mpc scales. Moreover, we
show initial conditions for a simulation where a scalar modification of gravity
is modelled in a Lagrangian particle-like description.
Journal of Cosmology and Astroparticle Physics 05/2015; 2015(09). DOI:10.1088/1475-7516/2015/09/054 · 5.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We discuss the relation between the output of Newtonian N-body 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.
[Show abstract][Hide abstract] 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. The dominant relativistic corrections to the power spectrum on scales close to the horizon are at most of the order
of ∼10−5 at z = 49 and ∼10−3 at z = 0. The differences in the positions of real-space features are affected at a level below 10−6 at both redshifts. Our analysis also clarifies the relation of N-body results to relativistic considerations.
Monthly Notices of the Royal Astronomical Society 07/2013; 446(1). DOI:10.1093/mnras/stu2070 · 5.11 Impact Factor
[Show abstract][Hide abstract] 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 shell-crossing from occurring. This scenario is applicable to general non-oscillating scalar-field 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 billion-solar-mass 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
[Show abstract][Hide abstract] 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 multipole-vector 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 next-generation GAIA-like 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 micro-arcsecond 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:10.1088/1475-7516/2013/12/042 · 5.81 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: We argue that there is an intrinsic noise on measurements of the equation of
state parameter w=p/rho from large-scale 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 Lemaitre-Tolman-Bondi
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.
Physics of the Dark Universe 02/2013; 2(4). DOI:10.1016/j.dark.2014.01.001 · 8.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a new program for placing constraints on radial inhomogeneity in a
dark-energy 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 dark-energy evolution. In particular,
we find that current observations place reasonably tight constraints on
possible late-time 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 · 5.11 Impact Factor
[Show abstract][Hide abstract] 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
non-linear regime and beyond the capacity of standard perturbation theory at
any order. We compare the gradient expansion with exact inhomogeneous
{\Lambda}LTB solutions (Lema\^itre-Tolman-Bondi 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 over-density all the way into a singularity.
[Show abstract][Hide abstract] 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.1365-2966.2012.21218.x · 5.11 Impact Factor
[Show abstract][Hide abstract] 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 dark-energy 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 large-scale 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 · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The International School for Advanced Studies (SISSA) was founded in 1978 and was the first institution in Italy to promote post-graduate courses leading to a Doctor Philosophiae (or PhD) degree. A centre of excellence among Italian and international universities, the school has around 65 teachers, 100 post docs and 245 PhD students, and is located in Trieste, in a campus of more than 10 hectares with wonderful views over the Gulf of Trieste.
SISSA hosts a very high-ranking, large and multidisciplinary scientific research output. The scientific papers produced by its researchers are published in high impact factor, well-known international journals, and in many cases in the world's most prestigious scientific journals such as Nature and Science. Over 900 students have so far started their careers in the field of mathematics, physics and neuroscience research at SISSA.
Visit www.sissa.it.
Journal of Cosmology and Astroparticle Physics 06/2011; 2012(01). DOI:10.1088/1475-7516/2012/01/047 · 5.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present semi-analytical solutions to the background equations describing
the Lema\^itre-Tolman-Bondi (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 Robertson-Walker metric.
General Relativity and Gravitation 04/2011; 44(10). DOI:10.1007/s10714-012-1405-9 · 1.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present semi-analytical solutions to the background equations describing the Lema\^itre-Tolman-Bondi (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 Robertson-Walker metric.
[Show abstract][Hide abstract] ABSTRACT: In this paper, instead of invoking Dark Energy, we try and fit various cosmological observations with a large Gpc scale under-dense region (Void) which is modeled by a Lemaitre-Tolman-Bondi 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 last-scattering 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 7-yr 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 large-scale-structure 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 off-center 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.rwth-aachen.de/download/valkenburg/
Journal of Cosmology and Astroparticle Physics 07/2010; 2010(11). DOI:10.1088/1475-7516/2010/11/030 · 5.81 Impact Factor
[Show abstract][Hide abstract] 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 B‐mode polarization would demonstrate that inflation occurred at a very high energy scale, and that the inflaton traversed a super‐Planckian 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 scale‐dependence and non‐Gaussianity 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.
CMB POLARIZATION WORKSHOP: THEORY AND FOREGROUNDS: CMBPol Mission Concept Study; 06/2009
[Show abstract][Hide abstract] ABSTRACT: It has been argued that the Swiss-Cheese cosmology can mimic Dark Energy, when it comes to the observed luminosity distance-redshift relation. Besides the fact that this effect tends to disappear on average over random directions, we show in this work that based on the Rees-Sciama effect on the cosmic microwave background (CMB), the Swiss-Cheese 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 version
Journal of Cosmology and Astroparticle Physics 02/2009; 6(06). DOI:10.1088/1475-7516/2009/06/010 · 5.81 Impact Factor
[Show abstract][Hide abstract] 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 B-mode polarization would demonstrate that inflation occurred at a very high energy scale, and that the inflaton traversed a super-Planckian 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 scale-dependence and non-Gaussianity 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
[Show abstract][Hide abstract] ABSTRACT: There has been much recent discussion, and some confusion, regarding the use of existing observational data to estimate the likelihood that next-generation 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 tensor-to-scalar 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 prior-independent 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 tensor-to-scalar 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.