arXiv subjects

V. C. Busti

Publications and source records attributed to V. C. Busti.

18 recordsLinked to original sources

arXiv2018

X-ray surface brightness observations of galaxy clusters, cosmic opacity and the limits on the matter density parameter

In this work, we use two gas mass fraction samples of galaxy clusters obtained from their X-ray surface brightness observations jointly with recent $H(z)$ data in a flat $Λ$CDM framework to impose limits on cosmic opacity. It is assumed that the galaxy clusters are in hydrostatic equilibrium and their gas mass fraction measurement is constant with redshift. We show that the current limits on the matter density parameter obtained from X-ray gas mass fraction test are strongly dependent on the cosmic transparency assumption even for a flat scenario. Our results are consistent with a transparent universe within $1σ$ c.l. in full agreement with other analyses which used type Ia supernovae, gamma ray burst and $H(z)$ data.

astro-ph.CO↗

arXiv2017

Cosmological constraints on the gas depletion factor in galaxy clusters

The evolution of the X-ray emitting gas mass fraction ($f_{gas}$) in massive galaxy clusters can be used as an independent cosmological tool to probe the expansion history of the Universe. Its use, however, depends upon a crucial quantity, i.e., the depletion factor $γ$, which corresponds to the ratio by which $f_{gas}$ is depleted with respect to the universal baryonic mean. This quantity is not directly observed and hydrodynamical simulations performed in a specific cosmological model (e.g., a flat $Λ$CDM cosmology) have been used to calibrate it. In this work, we obtain for the first time self-consistent observational constraints on the gas depletion factor combining 40 X-ray emitting gas mass fraction measurements and luminosity distance measurements from type Ia supernovae. Using Gaussian Processes to reconstruct a possible redshift evolution of $γ$, we find no evidence for such evolution, which confirms the current results from hydrodynamical simulations. Moreover, our constraints on $γ$ can be seen as a data prior for cosmological analyses on different cosmological models. The current measurements are systematic limited, so future improvements will depend heavily on a better mass calibration of galaxy clusters and their measured density profiles.

astro-ph.CO↗

arXiv2017

cosmoabc: Likelihood-free inference via Population Monte Carlo Approximate Bayesian Computation

Approximate Bayesian Computation (ABC) enables parameter inference for complex physical systems in cases where the true likelihood function is unknown, unavailable, or computationally too expensive. It relies on the forward simulation of mock data and comparison between observed and synthetic catalogues. Here we present cosmoabc, a Python ABC sampler featuring a Population Monte Carlo (PMC) variation of the original ABC algorithm, which uses an adaptive importance sampling scheme. The code is very flexible and can be easily coupled to an external simulator, while allowing to incorporate arbitrary distance and prior functions. As an example of practical application, we coupled cosmoabc with the numcosmo library and demonstrate how it can be used to estimate posterior probability distributions over cosmological parameters based on measurements of galaxy clusters number counts without computing the likelihood function. cosmoabc is published under the GPLv3 license on PyPI and GitHub and documentation is available at http://goo.gl/SmB8EX

astro-ph.CO↗

arXiv2017

Probing the distance-duality relation with high-$z$ data

Measurements of strong gravitational lensing jointly with type Ia supernovae (SNe Ia) observations have been used to test the validity of the cosmic distance duality relation (CDDR), $D_L(z)/[(1+z)^2D_A(z)]=η=1$, where $D_L(z)$ and $D_A(z)$ are the luminosity and the angular diameter distances to a given redshift $z$, respectively. However, several lensing systems lie in the interval $1.4 \leq z \leq 3.6$ i.e., beyond the redshift range of current SNe Ia compilations ($z \approx 1.50$), which prevents this kind of test to be fully explored. In this paper, we circumvent this problem by testing the CDDR considering observations of strong gravitational lensing along with SNe Ia and { a subsample from} the latest gamma-ray burst distance modulus data, whose redshift range is $0.033 \leq z \leq 9.3$. { We parameterize their luminosity distances with a second degree polynomial function and search for possible deviations from the CDDR validity by using four different $η(z)$ functions: $η(z)=1+η_0z$, $η(z)=1+η_0z/(1+z)$, $η(z)=(1+z)^{η_0}$ and $η(z)=1+η_0\ln(1+z)$. Unlike previous tests done at redshifts lower than $1.50$, the likelihood for $η_0$ depends strongly on the $η(z)$ function considered, but we find no significant deviation from the CDDR validity ($η_0=0$). However, our analyses also point to the fact that caution is needed when one fits data in higher redshifts to test the CDDR as well as a better understanding of the mass distribution of lenses also is required for more accurate results.

astro-ph.CO↗

arXiv2017

Improved constraints on violations of the Einstein equivalence principle in the electromagnetic sector with complementary cosmic probes

Recent results have shown that a field non-minimally coupled to the electromagnetic Lagrangian can induce a violation of the Einstein equivalence principle. { This kind of coupling is present in a very wide class of gravitation theories.} In a cosmological context, this would break the validity of the cosmic distance duality relation as well as cause a time variation of the fine structure constant. Here, we improve constraints on this scenario by using four different observables: the luminosity distance of type Ia supernovae, the angular diameter distance of galaxy clusters, the gas mass fraction of galaxy clusters and the temperature of the cosmic microwave background at different redshifts. We consider four standard parametrizations adopted in the literature and show that, due to a high complementarity of the data, the errors are shrunk between 20\% and 40\% depending on the parametrization. We also show that our constraints are weakly affected by the geometry considered to describe the galaxy clusters. In short, no violation of the Einstein equivalence principle is detected up to redshifts $\sim$ 3.

gr-qc↗

arXiv2016

Galaxy clusters, type Ia supernovae and the fine structure constant

As is well known, measurements of the Sunyaev-Zeldovich effect can be combined with observations of the X-ray surface brightness of galaxy clusters to estimate the angular diameter distance to these structures. In this paper, we show that this technique depends on the fine structure constant, $α$. Therefore, if $α$ is a time-dependent quantity, e.g., $α=α_0 ϕ(z)$, where $ϕ$ is a function of redshift, we argue that current data do not provide the real angular diameter distance, $D_{\rm{A}}(z)$, to the cluster but instead $D_A^{data}(z) = ϕ(z)^2 D_{\rm{A}}(z)$. We use this result to derive constraints on a possible variation of $α$ for a class of dilaton runaway models considering a sample of 25 measurements of $D_A^{data}(z)$ in redshift range $0.023 < z < 0.784$ and estimates of $D_{\rm{A}}(z)$ from current type Ia supernovae observations. We find no significant indication of variation of $α$ with the present data.

astro-ph.CO↗

arXiv2016

Exploring the spectroscopic diversity of type Ia supernovae with DRACULA: a machine learning approach

The existence of multiple subclasses of type Ia supernovae (SNeIa) has been the subject of great debate in the last decade. One major challenge inevitably met when trying to infer the existence of one or more subclasses is the time consuming, and subjective, process of subclass definition. In this work, we show how machine learning tools facilitate identification of subtypes of SNeIa through the establishment of a hierarchical group structure in the continuous space of spectral diversity formed by these objects. Using Deep Learning, we were capable of performing such identification in a 4 dimensional feature space (+1 for time evolution), while the standard Principal Component Analysis barely achieves similar results using 15 principal components. This is evidence that the progenitor system and the explosion mechanism can be described by a small number of initial physical parameters. As a proof of concept, we show that our results are in close agreement with a previously suggested classification scheme and that our proposed method can grasp the main spectral features behind the definition of such subtypes. This allows the confirmation of the velocity of lines as a first order effect in the determination of SNIa subtypes, followed by 91bg-like events. Given the expected data deluge in the forthcoming years, our proposed approach is essential to allow a quick and statistically coherent identification of SNeIa subtypes (and outliers). All tools used in this work were made publicly available in the Python package Dimensionality Reduction And Clustering for Unsupervised Learning in Astronomy (DRACULA) and can be found within COINtoolbox (https://github.com/COINtoolbox/DRACULA).

astro-ph.SR↗

arXiv2016

Constraints on a possible variation of the fine structure constant from galaxy cluster data

We propose a new method to probe a possible time evolution of the fine structure constant $α$ from X-ray and Sunyaev-Zeldovich measurements of the gas mass fraction ($f_{gas}$) in galaxy clusters. Taking into account a direct relation between variations of $α$ and violations of the distance-duality relation, we discuss constraints on $α$ for a class of dilaton runaway models. Although not yet competitive with bounds from high-$z$ quasar absorption systems, our constraints, considering a sample of 29 measurements of $f_{gas}$, in the redshift interval $0.14 < z < 0.89$, provide an independent estimate of $α$ variation at low and intermediate redshifts. Furthermore, current and planned surveys will provide a larger amount of data and thus allow to improve the limits on $α$ variation obtained in the present analysis.

astro-ph.CO↗

arXiv2016

Probing the cosmic distance duality with strong gravitational lensing and supernovae Ia data

We propose and perform a new test of the cosmic distance-duality relation (CDDR), $D_L(z) / D_A(z) (1 + z)^{2} = 1$, where $D_A$ is the angular diameter distance and $D_L$ is the luminosity distance to a given source at redshift $z$, using strong gravitational lensing (SGL) and type Ia Supernovae (SNe Ia) data. We show that the ratio $D=D_{A_{12}}/D_{A_2}$ and $D^{*}=D_{L_{12}}/D_{L_{2}}$, where the subscripts 1 and 2 correspond, respectively, to redshifts $z_1$ and $z_2$, are linked by $D/D^*=(1+z_1)^2$ if the CDDR is valid. We allow departures from the CDDR by defining two funcions for $η(z_1)$, which equals unity when the CDDR is valid. We find that combination of SGL and SNe Ia data favours no violation of the CDDR at 1$σ$ confidence level ($η(z) \simeq 1$), in complete agreement with other tests and reinforcing the theoretical pillars of the CDDR.

astro-ph.CO↗

arXiv2015

Two new tests to the distance duality relation with galaxy clusters

The cosmic distance duality relation is a milestone of cosmology involving the luminosity and angular diameter distances. Any departure of the relation points to new physics or systematic errors in the observations, therefore tests of the relation are extremely important to build a consistent cosmological framework. Here, two new tests are proposed based on galaxy clusters observations (angular diameter distance and gas mass fraction) and $H(z)$ measurements. By applying Gaussian Processes, a non-parametric method, we are able to derive constraints on departures of the relation where no evidence of deviation is found in both methods, reinforcing the cosmological and astrophysical hypotheses adopted so far.

astro-ph.CO↗

arXiv2014

Robustness of $H_0$ determination at intermediate redshifts

The most recent Hubble constant ($H_0)$ estimates from local methods (z << 1), $H_0=73.8\pm 2.4$ km s$^{-1}$ Mpc$^{-1}$, and the one from high redshifts $H_0=67.3\pm 1.2$ km s$^{-1}$ Mpc$^{-1}$, are discrepant at $2.4 σ$ confidence level. Within this context, Lima & Cunha (LC) derived a new determination of $H_0$ using four cosmic probes at intermediate redshifts ($0.1<z<1.8$) based on the so-called flat $Λ$CDM model. They obtained $H_0=74.1\pm 2.2$ km s$^{-1}$ Mpc$^{-1}$, in full agreement with local measurements. In this Letter, we explore the robustness of the LC result searching for systematic errors and its dependence from the cosmological model used. We find that the $H_0$ value from this joint analysis is very weakly dependent on the underlying cosmological model, but the morphology adopted to infer the distance to galaxy clusters changes the result sizeably, being the main source of systematic errors. Therefore, a better understanding of the cluster morphology is paramount to transform this method into a powerful cross-check for $H_0$.

astro-ph.CO↗

arXiv2014

Probing cosmic opacity at high redshifts with gamma-ray bursts

Probing the evolution of the universe at high redshifts with standard candles is a powerful way to discriminate dark energy models, where an open question nowadays is whether this component is constant or evolves with time. One possible source of ambiguity in this kind of analyses comes from cosmic opacity, which can mimick a dark enery behaviour. However, most tests of cosmic opacity have been restricted to the redshift range $z<2$. In this work, by using luminosity distances of gamma-ray bursts (GRBs), {given the validity of the Amati relation}, and the latest $H(z)$ data we determine constraints on the cosmic opacity at high redshifts ($z>2$) for a flat $Λ$CDM model. A possible degenerescence of the results with the adopted cosmological model is also investigated by considering a flat XCDM model. The limits on cosmic opacity in the redshift range $0<z<2$ are updated with type Ia supernovae (SNe Ia) from the Union2.1 sample, where we added the most distant ($z=1.713$) spectroscopically confirmed SNe Ia. From the analyses performed, we find that both SNe Ia and GRBs samples are compatible with a transparent universe at $1σ$ level and the results are independent of the dark energy equation of state parameter $w$.

astro-ph.CO↗

arXiv2014

Studying light propagation in a locally homogeneous universe through an extended Dyer-Roeder approach

Light is affected by local inhomogeneities in its propagation, which may alter distances and so cosmological parameter estimation. In the era of precision cosmology, the presence of inhomogeneities may induce systematic errors if not properly accounted. In this vein, a new interpretation of the conventional Dyer-Roeder (DR) approach by allowing light received from distant sources to travel in regions denser than average is proposed. It is argued that the existence of a distribution of small and moderate cosmic voids (or "black regions") implies that its matter content was redistributed to the homogeneous and clustered matter components with the former becoming denser than the cosmic average in the absence of voids. Phenomenologically, this means that the DR smoothness parameter (denoted here by $α_E$) can be greater than unity, and, therefore, all previous analyses constraining it should be rediscussed with a free upper limit. Accordingly, by performing a statistical analysis involving 557 type Ia supernovae (SNe Ia) from Union2 compilation data in a flat $Λ$CDM model we obtain for the extended parameter, $α_E=1.26^{+0.68}_{-0.54}$ ($1σ$). The effects of $α_E$ are also analyzed for generic $Λ$CDM models and flat XCDM cosmologies. For both models, we find that a value of $α_E$ greater than unity is able to harmonize SNe Ia and cosmic microwave background observations thereby alleviating the well-known tension between low and high redshift data. Finally, a simple toy model based on the existence of cosmic voids is proposed in order to justify why $α_E$ can be greater than unity as required by supernovae data.

astro-ph.CO↗

arXiv2013

Supernovae as probes of cosmic parameters: estimating the bias from under-dense lines of sight

Correctly interpreting observations of sources such as type Ia supernovae (SNe Ia) require knowledge of the power spectrum of matter on AU scales - which is very hard to model accurately. Because under-dense regions account for much of the volume of the universe, light from a typical source probes a mean density significantly below the cosmic mean. The relative sparsity of sources implies that there could be a significant bias when inferring distances of SNe Ia, and consequently a bias in cosmological parameter estimation. While the weak lensing approximation should in principle give the correct prediction for this, linear perturbation theory predicts an effectively infinite variance in the convergence for ultra-narrow beams. We attempt to quantify the effect typically under-dense lines of sight might have in parameter estimation by considering three alternative methods for estimating distances, in addition to the usual weak lensing approximation. We find in each case this not only increases the errors in the inferred density parameters, but also introduces a bias in the posterior value.

astro-ph.CO↗

arXiv2012

Influence of Small-Scale Inhomogeneities on the Cosmological Consistency Tests

The current cosmological dark sector (dark matter plus dark energy) is challenging our comprehension about the physical processes taking place in the Universe. Recently, some authors tried to falsify the basic underlying assumptions of such dark matter-dark energy paradigm. In this Letter, we show that oversimplifications of the measurement process may produce false positives to any consistency test based on the globally homogeneous and isotropic LCDM model and its expansion history based on distance measurements. In particular, when local inhomogeneity effects due to clumped matter or voids are taken into account, an apparent violation of the basic assumptions ("Copernican Principle") seems to be present. Conversely, the amplitude of the deviations also probes the degree of reliability underlying the phenomenological Dyer-Roeder procedure by confronting its predictions with the accuracy of the weak lensing approach. Finally, a new method is devised to reconstruct the effects of the inhomogeneities in a LCDM model, and some suggestions of how to distinguish between clumpiness (or void) effects from different cosmologies are discussed.

astro-ph.CO↗

arXiv2012

Constraining the dark energy and smoothness parameter with type Ia Supernovae and Gamma-Ray Bursts

The existence of inhomogeneities in the observed Universe modifies the distance-redshift relations thereby affecting the results of cosmological tests in comparison to the ones derived assuming spatially uniform models. By modeling the inhomogeneities through a Zeldovich-Kantowski-Dyer-Roeder (ZKDR) approach which is phenomenologically characterized by a smoothness parameter $α$, we rediscuss the constraints on the cosmic parameters based on Supernovae type Ia and Gamma-Ray Bursts (GRBs) data. The present analysis is restricted to a flat $Λ$CDM model with the reasonable assumption that $Λ$ does not clump. A $χ^{2}$-analysis using 557 SNe Ia data from the Union2 Compilation Data (Amanullah {\it et al.} 2010) constrains the pair of parameters ($Ω_m, α$) to $Ω_m=0.27_{-0.03}^{+0.08}$($2σ$) and $α\geq 0.25$. A similar analysis based only on 59 Hymnium GRBs (Wei 2010) constrains the matter density parameter to be $Ω_m= 0.35^{+0.62}_{-0.24}$ ($2σ$) while all values for the smoothness parameter are allowed. By performing a joint analysis, it is found that $Ω_m = 0.27^{+0.06}_{-0.03}$ and $α\geq 0.52$. As a general result, although considering that current GRB data alone cannot constrain the smoothness $α$ parameter our analysis provides an interesting cosmological probe for dark energy even in the presence of inhomogeneities.

astro-ph.CO↗

arXiv2012

Constraining H0 from Lyman-alpha Forest and Baryon Acoustic Oscillations

A new method is proposed to measure the Hubble constant H0 through the mean transmitted flux observed from high redshift quasars. A semi-analytical model for the cosmological-independent volume density distribution function is adopted which allows one to obtain constraints over the cosmological parameters once a moderate knowlegde of the InterGalactic Medium (IGM) parameters is assumed. By assuming a flat LCDM cosmology, we show that such method alone cannot provide good constraints on the pair of free parameters (h, Omega_m). However, it is possible possible to break the degeneracy on the mass density parameter by applying a joint analysis involving the baryon acoustic oscillations (BAOs). Our analysis based on two different samples of Lyman-alpha forest restricts the parameters on the intervals 0.58 < h < 0.91 and 0.215 < Omega_m < 0.245 (1 sigma). Although the constraints are weaker comparatively to other estimates, we point out that with a bigger sample and a better knowledge of the IGM this method may present competitive results to measure the Hubble constant independently of the cosmic distance ladder.

astro-ph.CO↗

arXiv2011

Comment on "Constraining the smoothness parameter and dark energy using observational H(z) data"

In this Comment we discuss a recent analysis by Yu et al. [RAA 11, 125 (2011)] about constraints on the smoothness $α$ parameter and dark energy models using observational $H(z)$ data. It is argued here that their procedure is conceptually inconsistent with the basic assumptions underlying the adopted Dyer-Roeder approach. In order to properly quantify the influence of the $H(z)$ data on the smoothness $α$ parameter, a $χ^2$-test involving a sample of SNe Ia and $H(z)$ data in the context of a flat $Λ$CDM model is reanalyzed. This result is confronted with an earlier approach discussed by Santos et al. (2008) without $H(z)$ data. In the ($Ω_m, α$) plane, it is found that such parameters are now restricted on the intervals $0.66 \leq α\leq 1.0$ and $0.27 \leq Ω_m \leq 0.37$ within 95.4% confidence level (2$σ$), and, therefore, fully compatible with the homogeneous case. The basic conclusion is that a joint analysis involving $H(z)$ data can indirectly improve our knowledge about the influence of the inhomogeneities. However, this happens only because the $H(z)$ data provide tighter constraints on the matter density parameter $Ω_m$.

astro-ph.CO↗