arXiv subjects

Wilmar Cardona

Publications and source records attributed to Wilmar Cardona.

13 recordsLinked to original sources

arXiv2024

Lensing convergence and anisotropic dark energy in galaxy redshift surveys

Analyses of upcoming galaxy surveys will require careful modelling of relevant observables such as the power spectrum of galaxy counts in harmonic space $C_\ell(z,z')$. We investigate the impact of disregarding relevant relativistic effects by considering a model of dark energy including constant sound speed $c_{\rm eff}^2$, constant equation of state $w$, and anisotropic stress sourced by matter perturbations $π$. Cosmological constraints were computed using cosmic microwave background anisotropies, baryon acoustic oscillations, supernovae type Ia, and redshift space distortions. Our results are consistent with $w=-1$, $c_{\rm eff}^2=1$, and $π=0$. Then, a forecast for the performance of an Euclid-like galaxy survey was carried out also adding information from other probes. Here we show that, regardless of the galaxy survey configuration, neglecting the effect of lensing convergence will lead to substantial shifts in the galaxy bias $b_0$ and the neutrino mass $\sum m_ν$. Shifts in the dark energy sound speed and anisotropic stress also appear, but they depend on the survey configuration and hence lack robustness. While neglecting lensing convergence also leads to a Hubble constant $H_0$ moving downwards, the significance of the shift is not big enough to play a relevant part in the current $H_0$ tension.

astro-ph.CO↗

arXiv2024

Scrutinizing coupled vector dark energy in light of data

Since current challenges faced by $Λ$CDM might be hinting at new unravelled physics, here we investigate a plausible cosmological model where a vector field acts as source of dark energy. In particular, we examine whether an energy-momentum exchange between dark energy and dark matter could provide an explanation for current discrepancies in cosmological parameters. We carefully work out equations governing background and linear order perturbations and implement them in a Boltzmann code. We found that a negative coupling makes the dark energy equation of state less negative and closer to a cosmological constant during the matter dominated epoch than an uncoupled vector dark energy model. While the effect of the coupling is hardly noticeable through its effect on matter density perturbations, matter velocity perturbations and gravitational potentials are enhanced at late-times when dark energy dominates. Therefore, data of redshift space distortions help to narrow down these kinds of couplings in the dark sector. We computed cosmological constraints and found common parameters also present in $Λ$CDM are in good agreement with the Planck Collaboration baseline result. Our best fit for a negatively coupled vector field predicts a higher growth rate of matter perturbations at low redshift, thus exacerbating the disagreement with redshift space distortions data. While a positively coupled vector field can lead to power suppression of $P_{\rm{m}}(k,z=0)$ on small scales as well as a lower growth rate of matter perturbations than the standard model, it might compromise the goodness of fit to the CMB angular power spectrum on small scales. We conclude that our negatively coupled vector dark energy model does not solve current tensions (i.e., $H_0$ and $σ_8$). (abridged)

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arXiv2023

Using machine learning to compress the matter transfer function $T(k)$

The linear matter power spectrum $P(k,z)$ connects theory with large scale structure observations in cosmology. Its scale dependence is entirely encoded in the matter transfer function $T(k)$, which can be computed numerically by Boltzmann solvers, and can also be computed semi-analytically by using fitting functions such as the well-known Bardeen-Bond-Kaiser-Szalay (BBKS) and Eisenstein-Hu (EH) formulae. However, both the BBKS and EH formulae have some significant drawbacks. On the one hand, although BBKS is a simple expression, it is only accurate up to $10\%$, which is well above the $1\%$ precision goal of forthcoming surveys. On the other hand, while EH is as accurate as required by upcoming experiments, it is a rather long and complicated expression. Here, we use the Genetic Algorithms (GAs), a particular machine learning technique, to derive simple and accurate fitting formulae for the transfer function $T(k)$. When the effects of massive neutrinos are also considered, our expression slightly improves over the EH formula, while being notably shorter in comparison.

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arXiv2023

Holographic energy density, dark energy sound speed, and tensions in cosmological parameters: $H_0$ and $S_8$

Interesting discrepancies in cosmological parameters are challenging the success of the $Λ$CDM model. Direct measurements of the Hubble constant $H_0$ using Cepheid variables and supernovae turn out to be higher than inferred from the Cosmic Microwave Background (CMB). Weak galaxy lensing surveys consistently report values of the strength of matter clustering $σ_8$ lower than values derived from the CMB in the context of $Λ$CDM. In this paper we address these discrepancies in cosmological parameters by considering Dark Energy (DE) as a fluid with evolving equation of state $w_{\mathrm{de}}(z)$, constant sound speed squared $\hat{c}_{\mathrm{s}}^{2}$, and vanishing anisotropic stress $σ$. Our $w_{\mathrm{de}}(z)$ is derived from the Holographic Principle and can consecutively exhibit radiation-like, matter-like, and DE-like behaviour, thus affecting the sound horizon and the comoving angular diameter distance, hence $H_0$. Here we show DE sound speed plays a part in the matter clustering behaviour through its effect on the evolution of the gravitational potential. We compute cosmological constraints using several data set combinations including primary CMB, CMB lensing, redshift-space-distortions, local distance-ladder, supernovae, and baryon acoustic oscillations. In our analysis we marginalise over $\hat{c}_{\mathrm{s}}^{2}$ and find $\hat{c}_{\mathrm{s}}^{2}=1$ is excluded at $\gtrsim 3σ$. For our baseline result including the whole data set we found $H_0$ and $σ_8$ in good agreement (within $\approx 2σ$) with low redshift probes. Our constraint for the baryon energy density $ω_{\rm{b}}$ is however in $\approx 3σ$ tension with BBN constraints. We conclude evolving DE also having non-standard clustering properties [e.g., $\hat{c}_{\mathrm{s}}^{2}(z,k)$] might be relevant for the solution of current discrepancies in cosmological parameters.

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arXiv2022

Momentum transfer in the dark sector and lensing convergence in upcoming galaxy surveys

We investigated a cosmological model that allows a momentum transfer between dark matter and dark energy. The interaction in the dark sector mainly affects the behaviour of perturbations on small scales while the background evolution matches the $w$CDM solution. As a result of the momentum transfer, these kinds of models help alleviating the $σ_8$ discrepancy in the standard model, but do not resolve the so-called $H_0$ tension. We confirm that this is indeed the case by computing cosmological constraints. While our analysis tends to favour $σ_8$ values lower than in $Λ$CDM, we do not find evidence for a non-vanishing momentum transfer in the dark sector. Since upcoming galaxy surveys will deliver information on scales and red-shift relevant for testing models allowing momentum transfer in the dark sector, we also carried out forecasts using different survey configurations. We assessed the relevance of neglecting lensing convergence $κ$ when modelling the angular power spectrum of number counts fluctuations $C_\ell^{\rm ij}(z,z')$. We found that not including $κ$ in analyses leads to biased constraints ($\approx 1-5\,σ$) of cosmological parameters even when including information from other experiments. Incorrectly modelling $C_\ell^{\rm ij}(z,z')$ might lead to spurious detection of neutrino masses and exacerbate discrepancies in $H_0$ and $σ_8$.

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arXiv2022

An effective fluid description of scalar-vector-tensor theories under the sub-horizon and quasi-static approximations

We consider scalar-vector-tensor (SVT) theories with second-order equations of motion and tensor propagation speed equivalent to the speed of light. Under the sub-horizon and the quasi-static approximations we find analytical formulae for an effective dark energy fluid, i.e., sound speed, anisotropic stress as well as energy density and pressure. We took advantage of our general, analytical fluid description and showed that it is possible to design SVT cosmological models which are degenerate with $Λ$CDM at the background level while having gravity strength $G_{\rm eff}<G_{\rm N}$ at late-times as well as non-vanishing dark energy perturbations. We implemented SVT designer models in the widely used Boltzmann solver CLASS thus making it possible to test SVT models against astrophysical observations. Our effective fluid approach to SVT models reveals non trivial behaviour in the sound speed and the anisotropic stress well worth an investigation in light of current discrepancies in cosmological parameters such as $H_0$ and $σ_8$.

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arXiv2021

Cosmological constraints with the Effective Fluid approach for Modified Gravity

Cosmological constraints of Modified Gravity (MG) models are seldom carried out rigorously. First, even though general MG models evolve differently (i.e., background and perturbations) to the standard cosmological model, it is usual to assume a $Λ$CDM background. This treatment is not correct and in the era of precision cosmology could induce undesired biases in cosmological parameters. Second, neutrino mass is usually held fixed in the analyses which could obscure its relation to MG parameters. In a couple of previous papers we showed that by using the Effective Fluid Approach we can accurately compute observables in fairly general MG models. An appealing advantage of our approach is that it allows a pretty easy implementation of this kinds of models in Boltzmann solvers (i.e., less error--prone) while having a useful analytical description of the effective fluid to understand the underlying physics. This paper illustrates how an effective fluid approach can be used to carry out proper analyses of cosmological constraints in MG models. We investigated three MG models including the sum of neutrino masses as a varying parameter in our Markov Chain Monte Carlo analyses. Two models (i.e., Designer $f(R)$ [DES-fR] and Designer Horndeski [HDES]) have a background matching $Λ$CDM, while in a third model (i.e., Hu $\&$ Sawicki $f(R)$ model [HS]) the background differs from the standard model. In this way we estimate how relevant the background is when constraining MG parameters along with neutrinos' masses. We implement the models in the popular Boltzmann solver CLASS and use recent, available data (i.e., Planck 2018, CMB lensing, BAO, SNIa Pantheon compilation, $H_0$ from SHOES, and RSD Gold-18 compilation) to compute tight cosmological constraints in the MG parameters that account for deviation from the $Λ$CDM model. [abridged]

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arXiv2019

Analytic expressions for the background evolution of massive neutrinos and dark matter particles

We provide exact analytic expressions for the density, pressure, average number density and pseudo-pressure for massive neutrinos and generic dark matter particles, both fermions and bosons. We then focus on massive neutrinos and we compare our analytic expressions with the numerical implementation in the CLASS Boltzmann code. We find that our modifications including the exact analytic expressions are in agreement to better than $10^{-4}\%$ with the default CLASS implementation in the estimation of the CMB power spectrum; our modifications do not have an impact on the performance of the code. We also provide several specific limits of our expressions at the relativistic regime, but also at late times for the neutrino equation of state.

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arXiv2019

Designing Horndeski and the effective fluid approach

We present a family of designer Horndeski models, i.e. models that have a background exactly equal to that of the $Λ$CDM model but perturbations given by the Horndeski theory. Then, we extend the effective fluid approach to Horndeski theories, providing simple analytic formulae for the equivalent dark energy effective fluid pressure, density and velocity. We implement the dark energy effective fluid formulae in our code EFCLASS, a modified version of the widely used Boltzmann solver CLASS, and compare the solution of the perturbation equations with those of the code hi_CLASS which already includes Horndeski models. We find that our simple modifications to the vanilla code are accurate to the level of $\sim 0.1\%$ with respect to the more complicated hi_CLASS code. Furthermore, we study the kinetic braiding model both on and off the attractor and we find that even though the full case has a proper $Λ$CDM model limit for large $n$, it is not appropriately smooth, thus causing the quasistatic approximation to break down. Finally, we focus on our designer model (HDES), which has both a smooth $Λ$CDM limit and well-behaved perturbations, and we use it to perform Markov Chain Monte Carlo analyses to constrain its parameters with the latest cosmological data. We find that our HDES model can also alleviate the soft $2σ$ tension between the growth data and Planck 18 due to a degeneracy between $σ_8$ and one of its model parameters that indicates the deviation from the $Λ$CDM model.

astro-ph.CO↗

arXiv2019

Unraveling the effective fluid approach for $f(R)$ models in the subhorizon approximation

We provide explicit formulas for the effective fluid approach of $f(R)$ theories, such as the Hu & Sawicki and the designer models. Using the latter and simple modifications to the CLASS code, which we call EFCLASS, in conjunction with very accurate analytic approximations for the background evolution, we obtain competitive results in a much simpler and less error-prone approach. We also derive the initial conditions in matter domination and we find they differ from those already found in the literature for a constant $w$ model. A clear example is the designer model that behaves as $Λ$CDM in the background, but has nonetheless dark energy perturbations. We then use the aforementioned models to derive constraints from the latest cosmological data, including supernovae, BAO, CMB, $H(z)$ and growth-rate data, and find they are statistically consistent to the $Λ$CDM model. Finally, we show that the viscosity parameter $c_{vis}^2$ in realistic models is not constant as commonly assumed, but rather evolves significantly over several orders of magnitude, something which could affect forecasts of upcoming surveys.

astro-ph.CO↗

arXiv2017

Determining $H_0$ with Bayesian hyper-parameters

We re-analyse recent Cepheid data to estimate the Hubble parameter $H_0$ by using Bayesian hyper-parameters (HPs). We consider the two data sets from Riess et al 2011 and 2016 (labelled R11 and R16, with R11 containing less than half the data of R16) and include the available anchor distances (megamaser system NGC4258, detached eclipsing binary distances to LMC and M31, and MW Cepheids with parallaxes), use a weak metallicity prior and no period cut for Cepheids. We find that part of the R11 data is down-weighted by the HPs but that R16 is mostly consistent with expectations for a Gaussian distribution, meaning that there is no need to down-weight the R16 data set. For R16, we find a value of $H_0 = 73.75 \pm 2.11 \, \mathrm{km} \, \mathrm{s}^{-1} \, \mathrm{Mpc}^{-1}$ if we use HPs for all data points (including Cepheid stars, supernovae type Ia, and the available anchor distances), which is about 2.6 $σ$ larger than the Planck 2015 value of $H_0 = 67.81 \pm 0.92 \,\mathrm{km}\, \mathrm{s}^{-1} \, \mathrm{Mpc}^{-1}$ and about 3.1 $σ$ larger than the updated Planck 2016 value $66.93 \pm 0.62 \,\mathrm{km}\, \mathrm{s}^{-1} \, \mathrm{Mpc}^{-1}$. We test the effect of different assumptions, and find that the choice of anchor distances affects the final value significantly. If we exclude the Milky Way from the anchors, then the value of $H_0$ decreases. We find however no evident reason to exclude the MW data. The HP method used here avoids subjective rejection criteria for outliers and offers a way to test datasets for unknown systematics.

astro-ph.CO↗

arXiv2016

Lensing convergence and the neutrino mass scale in galaxy redshift surveys

We demonstrate the importance of including the lensing contribution in galaxy clustering analyses with large galaxy redshift surveys. It is well known that radial cross-correlations between different redshift bins of galaxy surveys are dominated by lensing. But we show here that also neglecting lensing in the autocorrelations within one bin severely biases cosmological parameter estimation with redshift surveys. It leads to significant shifts for several cosmological parameters, most notably the scalar spectral index and the neutrino mass scale. Especially the latter parameter is one of the main targets of future galaxy surveys.

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arXiv2014

The traces of anisotropic dark energy in light of Planck

We study a dark energy model with non-zero anisotropic stress, either linked to the dark energy density or to the dark matter density. We compute approximate solutions that allow to characterise the behaviour of the dark energy model and to assess the stability of the perturbations. We also determine the current limits on such an anisotropic stress from the cosmic microwave background data by the Planck satellite, and derive the corresponding constraints on the modified growth parameters like the growth index, the effective Newton's constant and the gravitational slip.

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