arXiv subjects

Enea Di Dio

Publications and source records attributed to Enea Di Dio.

35 records · Page 2Linked to original sources

arXiv2018

Optimal galaxy survey for detecting the dipole in the cross-correlation with 21 cm Intensity Mapping

We investigate the future perspectives of the detection of the relativistic dipole by cross-correlating the 21 cm emission in Intensity Mapping (IM) and galaxy surveys at low redshift. We model the neutral hydrogen (HI) and the galaxy population by means of the halo model to relate the parameters that affect the dipole signal such as the biases of the two tracers and the Poissonian noise. We investigate the behavior of the signal-to-noise as a function of the galaxy and magnification biases, for two fixed models of the neutral hydrogen. In both cases we found that the signal-to-noise does not grow by increasing the difference between the biases of the two tracers, due to the larger shot-noise yields by highly biased tracers. We also study and provide an optimal luminosity-threshold galaxy catalogue to enhance the signal-to-noise ratio of the relativistic dipole. Interestingly, we show that the maximum magnitude provided by the survey does not lead to the maximum signal-to-noise for detecting relativistic effects and we predict the optimal value for the limiting magnitude. Our work suggests that an optimal analysis could increase the signal-to-noise ratio up to a factor five compared to a standard one.

astro-ph.CO↗

arXiv2017

Lensing convergence in galaxy clustering in LambdaCDM and beyond

We study the impact of neglecting lensing magnification in galaxy clustering analyses for future galaxy surveys, considering the LambdaCDM model and two extensions: massive neutrinos and modifications of General Relativity. Our study focuses on the biases on the constraints and on the estimation of the cosmological parameters. We perform a comprehensive investigation of these two effects for the upcoming photometric and spectroscopic galaxy surveys Euclid and SKA for different redshift binning configurations. We also provide a fitting formula for the magnification bias of SKA. Our results show that the information present in the lensing contribution does improve the constraints on the modified gravity parameters whereas the lensing constraining power is negligible for the LambdaCDM parameters. For photometric surveys the estimation is biased for all the parameters if lensing is not taken into account. This effect is particularly significant for the modified gravity parameters. Conversely for spectroscopic surveys the bias is below one sigma for all the parameters. Our findings show the importance of including lensing in galaxy clustering analyses for testing General Relativity and to constrain the parameters which describe its modifications.

astro-ph.CO↗

arXiv2017

Impact of next-to-leading order contributions to CMB lensing

In this Letter we study the impact on cosmological parameter estimation, from present and future surveys, due to lensing corrections on CMB temperature and polarization anisotropies beyond leading order. In particular, we show how post-Born corrections, LSS effects and the correction due to the change in the polarization direction between the emission at the source and the detection at the observer, are non-negligible in the determination of the polarization spectra. They have to be taken into account for an accurate estimation of cosmological parameters sensitive to or even based on these spectra. We study in detail the impact of higher order lensing on the determination of the tensor-to-scalar ratio $r$ and on the estimation of the effective number of relativistic species $N_\text{eff}$. We find that neglecting higher order lensing terms can lead to misinterpreting these corrections as a primordial tensor-to-scalar ratio of about $\mathcal{O}(10^{-3})$. Furthermore, it leads to a shift of the parameter $N_\text{eff}$ by nearly two sigma considering the level of accuracy aimed by future S4 surveys.

astro-ph.CO↗

arXiv2017

Lensing smoothing of BAO wiggles

We study non-perturbatively the effect of the deflection angle on the BAO wiggles of the matter power spectrum in real space. We show that from redshift z~2 this introduces a dispersion of roughly 1 Mpc at BAO scale, which corresponds approximately to a 1% effect. The lensing effect induced by the deflection angle, which is completely geometrical and survey independent, smears out the BAO wiggles. The effect on the power spectrum amplitude at BAO scale is about 0.1% for z~2 and 0.2% for z~4. We compare the smoothing effects induced by the lensing potential and non-linear structure formation, showing that the two effects become comparable at z~4, while the lensing effect dominates for sources at higher redshifts. We note that this effect is not accounted through BAO reconstruction techniques.

astro-ph.CO↗

arXiv2017

The Alcock Paczynski test with Baryon Acoustic Oscillations: systematic effects for future surveys

We investigate the Alcock Paczynski (AP) test applied to the Baryon Acoustic Oscillation (BAO) feature in the galaxy correlation function. By using a general formalism that includes relativistic effects, we quantify the importance of the linear redshift space distortions and gravitational lensing corrections to the galaxy number density fluctuation. We show that redshift space distortions significantly affect the shape of the correlation function, both in radial and transverse directions, causing different values of galaxy bias to induce offsets up to 1% in the AP test. On the other hand, we find that the lensing correction around the BAO scale modifies the amplitude but not the shape of the correlation function and therefore does not introduce any systematic effect. Furthermore, we investigate in details how the AP test is sensitive to redshift binning: a window function in transverse direction suppresses correlations and shifts the peak position toward smaller angular scales. We determine the correction that should be applied in order to account for this effect, when performing the test with data from three future planned galaxy redshift surveys: Euclid, the Dark Energy Spectroscopic Instrument (DESI) and the Square Kilometer Array (SKA).

astro-ph.CO↗

arXiv2016

CMB-lensing beyond the Born approximation

We investigate the weak lensing corrections to the cosmic microwave background temperature anisotropies considering effects beyond the Born approximation. To this aim, we use the small deflection angle approximation, to connect the lensed and unlensed power spectra, via expressions for the deflection angles up to third order in the gravitational potential. While the small deflection angle approximation has the drawback to be reliable only for multipoles $\ell\lesssim 2500$, it allows us to consistently take into account the non-Gaussian nature of cosmological perturbation theory beyond the linear level. The contribution to the lensed temperature power spectrum coming from the non-Gaussian nature of the deflection angle at higher order is a new effect which has not been taken into account in the literature so far. It turns out to be the leading contribution among the post-Born lensing corrections. On the other hand, the effect is smaller than corrections coming from non-linearities in the matter power spectrum, and its imprint on CMB lensing is too small to be seen in present experiments.

astro-ph.CO↗

arXiv2016

Curvature constraints from Large Scale Structure

We modified the CLASS code in order to include relativistic galaxy number counts in spatially curved geometries; we present the formalism and study the effect of relativistic corrections on spatial curvature. The new version of the code is now publicly available. Using a Fisher matrix analysis, we investigate how measurements of the spatial curvature parameter $Ω_K$ with future galaxy surveys are affected by relativistic effects, which influence observations of the large scale galaxy distribution. These effects include contributions from cosmic magnification, Doppler terms and terms involving the gravitational potential. As an application, we consider angle and redshift dependent power spectra, which are especially well suited for model independent cosmological constraints. We compute our results for a representative deep, wide and spectroscopic survey, and our results show the impact of relativistic corrections on the spatial curvature parameter estimation. We show that constraints on the curvature parameter may be strongly biased if, in particular, cosmic magnification is not included in the analysis. Other relativistic effects turn out to be subdominant in the studied configuration. We analyze how the shift in the estimated best-fit value for the curvature and other cosmological parameters depends on the magnification bias parameter, and find that significant biases are to be expected if this term is not properly considered in the analysis.

astro-ph.CO↗

arXiv2016

The CLASSgal code for Relativistic Cosmological Large Scale Structure

We present some accurate and efficient computations of large scale structure observables, obtained with a modified version of the CLASS code which is made publicly available. This code includes all relativistic corrections and computes both the power spectrum Cl(z1,z2) and the corresponding correlation function xi(theta,z1,z2) in linear perturbation theory. For Gaussian initial perturbations, these quantities contain the full information encoded in the large scale matter distribution at the level of linear perturbation theory. We illustrate the usefulness of our code for cosmological parameter estimation through a few simple examples.

astro-ph.CO↗

arXiv2016

Cosmological Parameter Estimation with Large Scale Structure Observations

We estimate the sensitivity of future galaxy surveys to cosmological parameters, using the redshift dependent angular power spectra of galaxy number counts, $C_\ell(z_1,z_2)$, calculated with all relativistic corrections at first order in perturbation theory. We pay special attention to the redshift dependence of the non-linearity scale and present Fisher matrix forecasts for Euclid-like and DES-like galaxy surveys. We compare the standard $P(k)$ analysis with the new $C_\ell(z_1,z_2)$ method. We show that for surveys with photometric redshifts the new analysis performs significantly better than the $P(k)$ analysis. For spectroscopic redshifts, however, the large number of redshift bins which would be needed to fully profit from the redshift information, is severely limited by shot noise. We also identify surveys which can measure the lensing contribution and we study the monopole, $C_0(z_1,z_2)$.

astro-ph.CO↗

arXiv2016

Beyond $Λ$CDM: Problems, solutions, and the road ahead

Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, $Λ$CDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of $Λ$CDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.

astro-ph.CO↗

arXiv2016

Relativistic effects in Lyman-alpha forest

We present the calculation of the Lyman-alpha (Lyman-$α$) transmitted flux fluctuations with full relativistic corrections to the first order. Even though several studies exist on relativistic effects in galaxy clustering, this is the first study to extend the formalism to a different tracer of underlying matter at unique redshift range ($z=2-5$). Furthermore, we show a comprehensive application of our calculations of the Quasar-Lyman-$α$ cross-correlation function. Our results indicate that the signal of relativistic effects is sizeable at Baryonic Acoustic Oscillation (BAO) scale mainly due to the large difference in density bias factors of our tracers. We construct an observable, the anti-symmetric part of the cross-correlation function, that is dominated by the relativistic signal and offers a new way to measure the relativistic terms at relatively small scales. The analysis shows that relativistic effects are important when considering cross-correlations between tracers with very different biases, and should be included in the data analysis of the current and future surveys. Moreover, the idea presented in this paper is highly complementary to other techniques and observable trying to isolate the effect of the relativistic corrections and thus test the validity of the theory of gravity beyond the Newtonian regime.

astro-ph.CO↗

arXiv2015

The bispectrum of relativistic galaxy number counts

We discuss the dominant terms of the relativistic galaxy number counts to second order in cosmological perturbation theory on sub-Hubble scales and on intermediate to large redshifts. In particular, we determine their contribution to the bispectrum. In addition to the terms already known from Newtonian second order perturbation theory, we find that there are a series of additional `lensing-like' terms which contribute to the bispectrum. We derive analytical expressions for the full leading order bispectrum and we evaluate it numerically for different configurations, indicating how they can be measured with upcoming surveys. In particular, the new `lensing-like' terms are not negligible for wide redshift bins and even dominate the bispectrum at well separated redshifts. This offers us the possibility to measure them in future surveys.

astro-ph.CO↗

arXiv2015

Galaxy number counts to second order and their bispectrum

We determine the number counts to second order in cosmological perturbation theory in the Poisson gauge and allowing for anisotropic stress. The calculation is performed using an innovative approach based on the recently proposed "geodesic light-cone" gauge. This allows us to determine the number counts in a purely geometric way, without using Einstein's equation. The result is valid for general dark energy models and (most) modified gravity models. We then evaluate numerically the relevant contributions to the number counts bispectrum. In particular we consider the terms involving the density, redshift space distortion and lensing.

astro-ph.CO↗

arXiv2014

Distance-redshift relation in plane symmetric universes

Distance measurements are usually thought to probe the background metric of the universe, but in reality the presence of perturbations will lead to deviations from the result expected in an exactly homogeneous and isotropic universe. At least in principle the presence of perturbations could even explain the observed distance-redshift relation without the need for dark energy. In this paper we re-investigate a toy model where perturbations are plane symmetric, and for which exact solutions are known in the fluid limit. However, if perturbations are large, shell-crossing occurs and the fluid approximation breaks down. This prevents the study of the most interesting cases. Here we use a general-relativistic N-body simulation that does not suffer from this problem and which allows us to go beyond previous works. We show that even for very large plane-symmetric perturbations we are not able to mimic the observed distance-redshift relation. We also discuss how the synchronous comoving gauge breaks down when shell-crossing occurs, while metric perturbations in the longitudinal gauge remain small. For this reason the longitudinal (Newtonian) gauge appears superior for relativistic N-body simulations of large-scale structure formation.

astro-ph.CO↗

arXiv2013

A longitudinal gauge degree of freedom and the Pais Uhlenbeck field

We show that a longitudinal gauge degree of freedom for a vector field is equivalent to a Pais-Uhlenbeck scalar field. With the help of this equivalence, we can determine natural interactions of this field with scalars and fermions. Since the theory has a global U(1) symmetry, we have the usual conserved current of the charged fields, thanks to which the dynamics of the scalar field is not modified by the interactions. We use this fact to consistently quantize the theory even in the presence of interactions. We argue that such a degree of freedom can only be excited by gravitational effects like the inflationary era of the early universe and may play the role of dark energy in the form of an effective cosmological constant whose value is linked to the inflation scale.

hep-th↗

arXiv2012

Vector and Tensor Contributions to the Luminosity Distance

We compute the vector and tensor contributions to the luminosity distance fluctuations in first order perturbation theory and we expand them in spherical harmonics. This work presents the formalism with a first application to a stochastic background of primordial gravitational waves.

astro-ph.CO↗

arXiv2012

Back Reaction from Walls

We study the distance-redshift relation in a universe filled with 'walls' of pressure-less dust separated by under dense regions. We show that as long as the density contrast of the walls is small, or the diameter of the under dense regions is much smaller than the Hubble scale, the distance-redshift relation remains close to what is obtained in a Friedmann universe. However, when arbitrary density contrasts are allowed, every prescribed distance-redshift relation can be reproduced with such models.

astro-ph.CO↗