Publications (40)47.77 Total impact
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Article: Accurate Weak Lensing of Standard Candles, Part 1: Flexible Cosmological Fits
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ABSTRACT: With the availability of thousands of type Ia supernovae in the near future the magnitude scatter induced by lensing will become a major issue as it affects parameter estimation. Current N-body simulations are too time consuming to be integrated in the likelihood analyses used for estimating the cosmological parameters. In this paper we show that in the weak lensing regime a statistical numerical approximation produces accurate results orders of magnitude faster. We write down simple fits to the second, third and fourth central moments of the lensing magnification probability distribution as a function of redshift, of the power spectrum normalization and of the present-day matter density. We also improve upon existing models of lensing variance and show that a shifted lognormal distribution fits well the numerical one. These fits can be easily employed in cosmological likelihood analyses. Moreover, our theoretical predictions make it possible to invert the problem and begin using supernovae lensing to constrain the cosmological parameters.04/2013; -
Article: How early is early dark energy?
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ABSTRACT: We investigate constraints on early dark energy (EDE) from the Cosmic Microwave Background (CMB) anisotropy, taking into account data from WMAP9 combined with latest small scale measurements from the South Pole Telescope (SPT). For a constant EDE fraction we propose a new parametrization with one less parameter but still enough to provide similar results to the ones previously studied in literature. The main emphasis of our analysis, however, compares a new set of different EDE parametrizations that reveal how CMB constraints depend on the redshift epoch at which Dark Energy was non negligible. We find that bounds on EDE get substantially weaker if dark energy starts to be non-negligible later, with early dark energy fraction Omega_e free to go up to about 5% at 2 sigma if the onset of EDE happens at z < 100. Tight bounds around 1-2% are obtained whenever dark energy is present at last scattering, even if its effects switch off afterwards. We show that the CMB mainly constrains the presence of Dark Energy at the time of its emission, while EDE-modifications of the subsequent growth of structure are less important.01/2013; -
Article: Internal Robustness: systematic search for systematic bias in SN Ia data
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ABSTRACT: A great deal of effort is currently being devoted to understanding, estimating and removing systematic errors in cosmological data. In the particular case of type Ia supernovae, systematics are starting to dominate the error budget. Here we propose a Bayesian tool for carrying out a systematic search for systematic contamination. This serves as an extension to the standard goodness-of-fit tests and allows not only to cross-check raw or processed data for the presence of systematics but also to pin-point the data that are most likely contaminated. We successfully test our tool with mock catalogues and conclude that the Union2.1 data do not possess a significant amount of systematics. Finally, we show that if one includes in Union2.1 the supernovae that originally failed the quality cuts, our tool signals the presence of systematics at over 3.8-sigma confidence level.arXiv. 09/2012; -
Article: Constraints on coupled dark energy using CMB data from WMAP and SPT
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ABSTRACT: We consider the case of a coupling in the dark cosmological sector, where a dark energy scalar field modifies the gravitational attraction between dark matter particles. We find that the strength of the coupling {\beta} is constrained using current Cosmic Microwave Background (CMB) data, including WMAP7 and SPT, to be less than 0.063 (0.11) at 68% (95%) confidence level. Further, we consider the additional effect of the CMB-lensing amplitude, curvature, effective number of relativistic species and massive neutrinos and show that the bound from current data on {\beta} is already strong enough to be rather stable with respect to any of these variables. The strongest effect is obtained when we allow for massive neutrinos, in which case the bound becomes slightly weaker, {\beta} < 0.084(0.14). A larger value of the effective number of relativistic degrees of freedom favors larger couplings between dark matter and dark energy as well as values of the spectral index closer to 1. Adding the present constraints on the Hubble constant, as well as from baryon acoustic oscillations and supernovae Ia, we find {\beta} < 0.050(0.074). In this case we also find an interesting likelihood peak for {\beta} = 0.041 (still compatible with 0 at 1{\sigma}). This peak comes mostly from a slight difference between the Hubble parameter HST result and the WMAP7+SPT best fit. Finally, we show that forecasts of Planck+SPT mock data can pin down the coupling to a precision of better than 1% and detect whether the marginal peak we find at small non zero coupling is a real effect.07/2012; -
Article: Detecting stable massive neutral particles through particle lensing
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ABSTRACT: Stable massive neutral particles emitted by astrophysical sources undergo deflection under the gravitational potential of our own galaxy. The deflection angle depends on the particle velocity and therefore non-relativistic particles will be deflected more than relativistic ones. If these particles can be detected through neutrino telescopes, cosmic ray detectors or directional dark matter detectors, their arrival directions would appear aligned on the sky along the source-lens direction. On top of this deflection, the arrival direction of non-relativistic particles is displaced with respect to the relativistic counterpart also due to the relative motion of the source with respect to the observer; this induces an alignment of detections along the sky projection of the source trajectory. The final alignment will be given by a combination of the directions induced by lensing and source proper motion. We derive the deflection-velocity relation for the Milky Way halo and suggest that searching for alignments on detection maps of particle telescopes could be a way to find new particles or new astrophysical phenomena.03/2012; -
Article: Oscillating non‐linear large‐scale structures in growing neutrino quintessence
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ABSTRACT: Growing neutrino quintessence describes a form of dynamical dark energy that could explain why dark energy dominates the universe only in recent cosmological times. This scenario predicts the formation of large-scale neutrino lumps which could allow for observational tests. We perform for the first time N-body simulations of the non-linear growth of structures for cold dark matter (CDM) and neutrino fluids in the context of growing neutrino cosmologies. Our analysis shows a pulsation increase and subsequent decrease in the neutrino density contrast. This could lead to interesting observational signatures, as an enhanced bulk flow in a situation where the DM density contrast differs only very mildly from the standard ΛCDM scenario. We also determine for the first time the statistical distribution of neutrino lumps as a function of mass at different redshifts. Such determination provides an essential ingredient for a realistic estimate of the observational signatures of growing neutrino cosmologies. Due to a breakdown of the non-relativistic Newtonian approximation, our results are limited to redshifts z≥ 1.Monthly Notices of the Royal Astronomical Society 11/2011; 418(1):214 - 229. · 4.90 Impact Factor -
Article: Testing coupled dark energy with next-generation large-scale observations
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ABSTRACT: Coupling dark energy to dark matter provides one of the simplest way to effectively modify gravity at large scales without strong constraints from local (i.e. solar system) observations. Models of coupled dark energy have been studied several times in the past and are already significantly constrained by cosmic microwave background experiments. In this paper we estimate the constraints that future large-scale observations will be able to put on the coupling and in general on all the parameters of the model. We combine cosmic microwave background, tomographic weak lensing, redshift distortions and power spectrum probes. We show that next-generation observations can improve the current constraint on the coupling to dark matter by two orders of magnitude; this constraint is complementary to the current solar-system bounds on a coupling to baryons.11/2011; -
Article: Oscillating nonlinear large scale structure in growing neutrino quintessence
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ABSTRACT: Growing Neutrino quintessence describes a form of dynamical dark energy that could explain why dark energy dominates the universe only in recent cosmological times. This scenario predicts the formation of large scale neutrino lumps which could allow for observational tests. We perform for the first time N-body simulations of the nonlinear growth of structures for cold dark matter and neutrino fluids in the context of Growing Neutrino cosmologies. Our analysis shows a pulsation - increase and subsequent decrease - of the neutrino density contrast. This could lead to interesting observational signatures, as an enhanced bulk flow in a situation where the dark matter density contrast only differs very mildly from the standard LCDM scenario. We also determine for the first time the statistical distribution of neutrino lumps as a function of mass at different redshifts. Such determination provides an essential ingredient for a realistic estimate of the observational signatures of Growing Neutrino cosmologies. Due to a breakdown of the non-relativistic Newtonian approximation our results are limited to redshifts z > 1.06/2011; -
Article: Real-time Cosmology
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ABSTRACT: In recent years the possibility of measuring the temporal change of radial and transverse position of sources in the sky in real time have become conceivable thanks to the thoroughly improved technique applied to new astrometric and spectroscopic experiments, leading to the research domain we call Real-time cosmology. We review for the first time great part of the work done in this field, analysing both the theoretical framework and some endeavor to foresee the observational strategies and their capability to constrain models. We firstly focus on real time measurements of the overall redshift drift and angular separation shift in distant source, able to trace background cosmic expansion and large scale anisotropy, respectively. We then examine the possibility of employing the same kind of observations to probe peculiar and proper acceleration in clustered systems and therefore the gravitational potential. The last two sections are devoted to the short time future change of the cosmic microwave background, as well as to the temporal shift of the temperature anisotropy power spectrum and maps. We conclude revisiting in this context the effort made to forecast the power of upcoming experiments like CODEX, GAIA and PLANCK in providing these new observational tools.11/2010; -
Article: Large-scale inhomogeneities may improve the cosmic concordance of supernovae.
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ABSTRACT: We reanalyze the supernova data from the Union Compilation including the weak-lensing effects caused by inhomogeneities. We compute the lensing probability distribution function for each background solution described by the parameters Ω(M), Ω(Λ), and w in the presence of inhomogeneities, approximately modeled with a single-mass population of halos. We then perform a likelihood analysis in the parameter space of Friedmann-Lemaître-Robertson-Walker models and compare our results with the standard approach. We find that the inclusion of lensing can move the best-fit model significantly towards the cosmic concordance of the flat Lambda-Cold Dark Matter model, improving the agreement with the constraints coming from the cosmic microwave background and baryon acoustic oscillations.Physical Review Letters 09/2010; 105(12):121302. · 7.37 Impact Factor -
Article: Neutrino lumps and the Cosmic Microwave Background
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ABSTRACT: The interaction between the cosmon and neutrinos may solve the "why now problem" for dark energy cosmologies. Within growing neutrino quintessence it leads to the formation of nonlinear neutrino lumps. For a test of such models by the integrated Sachs-Wolfe effect for the cosmic microwave background (CMB) we estimate the size and time evolution of the gravitational potential induced by these lumps. A population of lumps with size of 100 Mpc or more could lead to observable effects on the CMB anisotropies for low angular momenta. The linear approximation is found to be invalid for the relevant length scales. Quantitative estimates depend strongly on the details of the transition between the linear and nonlinear regimes. In particular, important backreaction effects arise from the nonlinearities of the cosmon interactions. At the present stage the uncertainties of the estimate make it difficult to constrain the parameter space of growing neutrino models. We explicitly discuss scenarios and models that are compatible with the CMB observations. Comment: 18 pages, 11 figures09/2010; -
Article: Measuring our peculiar velocity on the CMB with high-multipole off-diagonal correlations
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ABSTRACT: Our peculiar velocity with respect to the CMB rest frame is known to induce a large dipole in the CMB. However, the motion of an observer has also the effect of distorting the anisotropies at all scales, as shown by Challinor and Van Leeuwen (2002), due to aberration and Doppler effects. We propose to measure independently our local motion by using off-diagonal two-point correlation functions for high multipoles. We study the observability of the signal for temperature and polarization anisotropies. We point out that Planck can measure the velocity $\beta$ with an error of about 30% and the direction with an error of about 20 degrees. This method constitutes a cross-check, which can be useful to verify that our CMB dipole is due mainly to our velocity or to disentangle the velocity from other possible intrinsic sources. Although in this paper we focus on our peculiar velocity, a similar effect would result also from other intrinsic vectorial distortion of the CMB which would induce a dipolar lensing. Measuring the off-diagonal correlation terms is therefore a test for a preferred direction on the CMB sky.08/2010; -
Article: Distinguishing Between Void Models and Dark Energy with Cosmic Parallax and Redshift Drift
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ABSTRACT: Two recently proposed techniques, involving the measurement of the cosmic parallax and redshift drift, provide novel ways of directing probing (over a time-span of several years) the background metric of the universe and therefore shed light on the dark energy conundrum. The former makes use of upcoming high-precision astrometry measurements to either observe or put tight constraints on cosmological anisotropy for off-center observers, while the latter employs high-precision spectroscopy to give an independent test of the present acceleration of the universe. In this paper, we show that both methods can break the degeneracy between LTB void models and more traditional dark energy theories. Using the near-future observational missions Gaia and CODEX we show that this distinction might be made with high confidence levels in the course of a decade. Comment: Corrections made (regarding, e.g., Gaia measurements), one table and some references added. 17 pages, 10 figures, 3 tables, submitted to PRD09/2009; -
Article: Cosmic Parallax as a probe of late time anisotropic expansion
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ABSTRACT: Cosmic parallax is the change of angular separation between pair of sources at cosmological distances induced by an anisotropic expansion. An accurate astrometric experiment like Gaia could observe or put constraints on cosmic parallax. Examples of anisotropic cosmological models are Lemaitre-Tolman-Bondi void models for off-center observers (introduced to explain the observed acceleration without the need for dark energy) and Bianchi metrics. If dark energy has an anisotropic equation of state, as suggested recently, then a substantial anisotropy could arise at $z \lesssim 1$ and escape the stringent constraints from the cosmic microwave background. In this paper we show that such models could be constrained by the Gaia satellite or by an upgraded future mission. Comment: 9 pages, 7 figures, text extended, figures changed, accepted by PRD05/2009; -
Article: Possibility of detecting anisotropic expansion of the universe by very accurate astrometry measurements.
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ABSTRACT: Refined astrometry measurements allow us to detect large-scale deviations from isotropy through real-time observations of changes in the angular separation between sources at cosmic distances. This "cosmic parallax" effect is a powerful consistency test of the Friedmann-Robertson-Walker metric and may set independent constraints on cosmic anisotropy. We apply this novel general test to Lemaitre-Tolman-Bondi cosmologies with off-center observers and show that future satellite missions such as Gaia might achieve accuracies that would put limits on the off-center distance which are competitive with cosmic microwave background dipole constraints.Physical Review Letters 05/2009; 102(15):151302. · 7.37 Impact Factor -
Article: Cosmic Microwave Background as a Gravity Probe
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ABSTRACT: We show that cosmic microwave background (CMB) observations have a strong potentiality not only as a test of cosmic kinematics but also as a gravity probe. We compare the recent high-resolution CMB data to models with a direct coupling of dark energy to dark matter. This extra interaction violates the equivalence principle, acts as an additional scalar gravity on dark matter fluctuations, and imprints a characteristic signature on the CMB spectra. Defining the ratio β of the dark energy interaction to gravity, we that β < 0.16 (95% confidence level [c.l.]) from the current CMB data set, regardless of the potential. This implies that the effective equation of state between equivalence and tracking has been close to the pure matter equation of state within 1% and that scalar gravity is at least 40 times weaker than tensor gravity. Furthermore, we show that an ideal CMB experiment limited by cosmic variance only can put an upper bound β < 0.05 (95% c.l.), comparable to the best limits provided by local gravity experiments on the coupling to baryons.The Astrophysical Journal 12/2008; 583(2):L53. · 6.02 Impact Factor -
Article: Mapping the galactic gravitational potential with peculiar acceleration
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ABSTRACT: It has been suggested recently that the change in cosmological redshift (the Sandage test of expansion) could be observed in the next generation of large telescopes and ultra-stable spectrographs. In a recent paper we estimated the change of peculiar velocity, i.e. the peculiar acceleration, in nearby galaxies and clusters and shown it to be of the same order of magnitude as the typical cosmological signal. Mapping the acceleration field allows for a reconstruction of the galactic gravitational potential without assuming virialization. In this paper we focus on the peculiar acceleration in our own Galaxy, modeled as a Kuzmin disc and a dark matter spherical halo. We estimate the peculiar acceleration for all known Galactic globular clusters and find some cases with an expected velocity shift in excess of 20 cm/sec for observations fifteen years apart, well above the typical cosmological acceleration. We then compare the predicted signal for a MOND (modified Newtonian dynamics) model in which the spherical dark matter halo is absent. We find that the signal pattern is qualitatively different, showing that the peculiar acceleration field could be employed to test competing theories of gravity. However the difference seems too small to be detectable in the near future. Comment: 11 pages, 10 figures, 3 tables, minor changes, accepted for publication by MNRAS07/2008; -
Article: Peculiar acceleration
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ABSTRACT: It has been proposed recently to observe the change in cosmological redshift of distant galaxies or quasars with the next generation of large telescope and ultra-stable spectrographs (the so-called Sandage-Loeb test). Here we investigate the possibility of observing the change in peculiar velocity in nearby clusters and galaxies. This ``peculiar acceleration'' could help reconstructing the gravitational potential without assuming virialization. We show that the expected effect is of the same order of magnitude of the cosmological velocity shift. Finally, we discuss how to convert the theoretical predictions into quantities directly related to observations.09/2007; -
Article: Growing Matter
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ABSTRACT: We investigate quintessence cosmologies with a matter component consisting of particles with an increasing mass. While negligible in early cosmology, the appearance of a growing matter component has stopped the evolution of the cosmon field at a redshift around six. In turn, this has triggered the accelerated expansion of the Universe. We propose to associate growing matter with neutrinos. Then the presently observed dark energy density and its equation of state are determined by the neutrino mass.07/2007; -
Article: Are f(R) dark energy models cosmologically viable?
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ABSTRACT: All f(R) modified gravity theories are conformally identical to models of quintessence in which matter is coupled to dark energy with a strong coupling. This coupling induces a cosmological evolution radically different from standard cosmology. We find that, in all f(R) theories where a power of R is dominant at large or small R (which include most of those proposed so far in the literature), the scale factor during the matter phase grows as t(1/2) instead of the standard law t(2/3). This behavior is grossly inconsistent with cosmological observations (e.g., Wilkinson Microwave Anisotropy Probe), thereby ruling out these models even if they pass the supernovae test and can escape the local gravity constraints.Physical Review Letters 04/2007; 98(13):131302. · 7.37 Impact Factor
Top co-authors
Top Journals
Institutions
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2009–2012
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Heidelberg University
- Institute of Theoretical Physics
Heidelberg, Baden-Wuerttemberg, Germany -
University of Rome Tor Vergata
- Dipartimento di Fisica
Roma, Latium, Italy
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2011
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Ludwig-Maximilian-University of Munich
München, Bavaria, Germany
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