arXiv subjects

Adria Gomez-Valent

Publications and source records attributed to Adria Gomez-Valent.

At least 19 recordsLinked to original sources

arXiv2024

Phantom matter: a challenging solution to the cosmological tensions

The idea of composite dark energy (DE) is quite natural since on general grounds we expect that the vacuum energy density (associated with the cosmological term $\Lambda$) may appear in combination with other effective forms of DE, denoted X. Here we deal with model $w$XCDM, a simplified version of the old $\Lambda$XCDM model (Grande et al., 2006), and exploit the possibility that X behaves as `phantom matter' (PM), which appears in stringy versions of the running vacuum model (RVM). Unlike phantom DE, the PM fluid satisfies the strong energy condition like usual matter, hence bringing to bear positive pressure at the expense of negative energy. Bubbles of PM may appear in the manner of a transitory `phantom vacuum' tunneled into the late universe before it heads towards a new de Sitter era, thereby offering a crop field for the growing of structures earlier than expected. Using SNIa, cosmic chronometers, transversal BAO (BAO 2D), LSS data and the full CMB likelihood from Planck 2018, we find that the $H_0$ and growth tensions virtually disappear, provided that BAO 2D are the only source of BAO data used in the fit. In contrast, our preliminary analysis using exclusively anisotropic BAO (BAO 3D) indicates that the ability to ease the $H_0$ tension is significantly reduced as compared to the scenario with BAO 2D, despite the fact that the overall fit to the cosmological data is still better than in the $\Lambda$CDM. Finally, our approach with BAO 2D favors quintessence-like behavior of the DE below $z\simeq 1.5$ at $\gtrsim 3\sigma$ CL, which is compatible with the recent DESI measurements.

astro-ph.CO↗

arXiv2023

Running vacuum in the Universe: phenomenological status in light of the latest observations, and its impact on the $σ_8$ and $H_0$ tensions

A substantial body of phenomenological and theoretical work over the last few years strengthens the possibility that the vacuum energy density (VED) of the universe is dynamical, and in particular that it adopts the `running vacuum model' (RVM) form, in which the VED evolves mildly as $δρ_{\rm vac}(H)\sim ν_{\rm eff} m_{\rm Pl}^2{\cal O}\left(H^2\right)$, where $H$ is the Hubble rate and $ν_{\rm eff}$ is a (small) free parameter. This dynamical scenario is grounded on recent studies of quantum field theory (QFT) in curved spacetime and also on string theory. It turns out that what we call the `cosmological constant', $Λ$, is no longer a rigid parameter but the nearly sustained value of $8πG(H)ρ_{\rm vac}(H)$ around (any) given epoch $H(t)$, where $G(H)$ is the gravitational coupling, which can also be very mildly running (logarithmically). Of particular interest is the possibility suggested in past works that such a running may help to cure the cosmological tensions afflicting the $Λ$CDM. In the current study, we reanalyze it in full and we find it becomes further buttressed. Using the modern cosmological data, namely a compilation of the latest $SNIa+BAO+$H(z)$+LSS+CMB$ observations, we probe to which extent the RVM provides a quality fit better than the concordance $Λ$CDM model, paying particular emphasis on its impact on the $σ_8$ and $H_0$ tensions. We utilize the Einstein-Boltzmann system solver $CLASS$ and the Monte Carlo sampler $MontePython$ for the statistical analysis, as well as the statistical $DIC$ criterion to compare the running vacuum against the rigid vacuum ($ν_{\rm eff} = 0$). We show that with a tiny amount of vacuum dynamics ($|ν_{\rm eff}|\ll 1$) the global fit can improve significantly with respect to the $Λ$CDM and the mentioned tensions may subside to inconspicuous levels.

astro-ph.CO↗

arXiv2021

BD-$Λ$CDM and Running Vacuum Models: Theoretical background and current observational status

We present an analysis of the Brans-Dicke cosmological model with a cosmological constant and cold dark matter (BD-$Λ$CDM). We find that the BD-$Λ$CDM is favored by the overall cosmological data (SNIa+BAO+$H(z)$+LSS+CMB) when it is compared with the standard model of cosmology. The BD-$Λ$CDM model can be viewed from the GR perspective as a Running Vacuum Model (RVM) with a time evolving vacuum energy density. Due to this fact and also to its time evolving effective gravitational coupling, the model can alleviate the $σ_8$ and the $H_0$ tensions at a time. We also present the results for different types of RVM's when they are tested in the light of the cosmological data and we show that a mild dynamics for the vacuum energy density can help to smooth out the aforementioned tensions, thus improving the performance of $Λ$CDM model.

astro-ph.CO↗

arXiv2021

Running vacuum against the $H_0$ and $σ_8$ tensions

The cosmological term, $Λ$, was introduced $104$ years ago by Einstein in his gravitational field equations. Whether $Λ$ is a rigid quantity or a dynamical variable in cosmology has been a matter of debate for many years, especially after the introduction of the general notion of dark energy (DE). $Λ$ is associated to the vacuum energy density, $ρ_{\rm vac}$, and one may expect that it evolves slowly with the cosmological expansion. Herein we present a devoted study testing this possibility using the promising class of running vacuum models (RVM's). We use a large string $SNIa+BAO+H(z)+LSS+CMB$ of modern cosmological data, in which for the first time the CMB part involves the full Planck 2018 likelihood for these models. We test the dependence of the results on the threshold redshift $z_*$ at which the vacuum dynamics is activated in the recent past and find positive signals up to $\sim4.0σ$ for $z_*\simeq 1$. The RVM's prove very competitive against the standard $Λ$CDM model and give a handle for solving the $σ_8$ tension and alleviating the $H_0$ one.

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arXiv2020

Brans-Dicke cosmology with a $Λ$- term: a possible solution to $Λ$CDM tensions

We present a full-fledged analysis of Brans-Dicke cosmology with a cosmological constant and cold dark matter (BD-$Λ$CDM for short). We extend the scenarios where the current cosmological value of the BD-field is restricted by the local astrophysical domain to scenarios where that value is fixed only by the cosmological observations, which should be more natural in view of the possible existence of local screening mechanims. Our analysis includes both the background and perturbations equations in different gauges. We find that the BD-$Λ$CDM is favored by the overall cosmological data as compared to the concordance GR-$Λ$CDM model, namely data on distant supernovae, cosmic chronometers, local measurements of the Hubble parameter, baryonic acoustic oscillations, Large-Scale Structure formation and the cosmic microwave background under full Planck 2018 CMB likelihood. We also test the impact of Strong and Weak-Lensing data on our results, which can be significant. We find that the BD-$Λ$CDM can mimic effective quintessence with a significance of about $3-3.5σ$ c.l. (depending on the lensing datasets). The fact that the BD-$Λ$CDM behaves effectively as a Running Vacuum Model (RVM) when viewed from the GR perspective helps to alleviate some of the existing tensions with the data, such as the $σ_8$ excess predicted by GR-$Λ$CDM. On the other hand, the BD-$Λ$CDM model has a crucial bearing on the acute $H_0$-tension with the local measurements, which is rendered virtually harmless owing to the small increase of the effective value of the gravitational constant with the expansion. The simultaneous alleviation of the two tensions is a most remarkable feature of BD-gravity with a cosmological constant in the light of the current observations, and hence goes in support of BD-$Λ$CDM against GR-$Λ$CDM

astro-ph.CO↗

arXiv2019

Brans-Dicke gravity with a cosmological constant smoothes out $Λ$CDM tensions

We analyze Brans-Dicke gravity with a cosmological constant, $Λ$, and cold dark matter (BD-$Λ$CDM for short) in the light of the latest cosmological observations on distant supernovae, Hubble rate measurements at different redshifts, baryonic acoustic oscillations, large scale structure formation data, gravitational weak-lensing and the cosmic microwave background under full Planck 2015 CMB likelihood. Our analysis includes both the background and perturbations equations. We find that BD-$Λ$CDM is observationally favored as compared to the concordance $Λ$CDM model, which is traditionally defined within General Relativity (GR). In particular, some well-known persisting tensions of the $Λ$CDM with the data, such as the excess in the mass fluctuation amplitude $σ_8$ and specially the acute $H_0$-tension with the local measurements, essentially disappear in this context. Furthermore, viewed from the GR standpoint, BD-$Λ$CDM cosmology mimics quintessence at $\gtrsim3σ$ c.l. near our time.

astro-ph.CO↗

arXiv2019

Signs of Dynamical Dark Energy in Current Observations

Investigations on dark energy (DE) are currently inconclusive about its time evolution. Hints of this possibility do however glow now and then in the horizon. Herein we assess the current status of dynamical dark energy (DDE) in the light of a large body of updated $SNIa+H(z)+BAO+LSS+CMB$ observations, using the full Planck 2015 CMB likelihood. The performance of the $Λ$CDM model (with equation of state $w=-1$ for $Λ$) is confronted with that of the general XCDM and CPL parametrizations, as well as with the traditional $Φ$CDM model based on the scalar field potential $V\sim Φ^{-α}$. In particular, we gauge the impact of the bispectrum in the LSS and BAO parts, and show that the subset of $CMB+BAO+LSS$ observations may contain the bulk of the DDE signal. The departure from $w=-1$ is significant: roughly $2.6σ$ for XCDM and $2.9σ$ for $Φ$CDM. In both cases the full Bayesian evidence is found to be positive even for a prior range of the DDE parameters extending over several standard deviations from the mean when the bispectrum is taken into account. Positive signs follow as well from the preliminary results of Planck 2018 data using the compressed CMB likelihood. As a bonus we also find that the $σ_8$-tension becomes reduced with DDE.

astro-ph.CO↗

arXiv2018

Possible signals of vacuum dynamics in the Universe

We study a generic class of time-evolving vacuum models which can provide a better phenomenological account of the overall cosmological observations as compared to the $Λ$CDM. Among these models, the running vacuum model (RVM) appears to be the most motivated and favored one, at a confidence level of $\sim 3σ$. We further support these results by computing the Akaike and Bayesian information criteria. Our analysis also shows that we can extract fair signals of dynamical dark energy (DDE) by confronting the same set of data to the generic XCDM and CPL parametrizations. In all cases we confirm that the combined triad of modern observations on Baryonic Acoustic Oscillations, Large Scale Structure formation, and the Cosmic Microwave Background, provide the bulk of the signal sustaining a possible vacuum dynamics. In the absence of any of these three crucial data sources, the DDE signal can not be perceived at a significant confidence level. Its possible existence could be a cure for some of the tensions existing in the $Λ$CDM when confronted to observations.

astro-ph.CO↗

arXiv2018

Dynamical dark energy versus $Λ=$const. in light of observations

After about two decades of the first observational papers confirming the accelerated expansion of the universe, we are still facing the question whether the cause of it is a rigid cosmological constant $Λ$-term or a mildly evolving dynamical dark energy (DDE). While studies focusing mainly on CMB measurements do not perceive signs of physics beyond the $Λ$CDM, in this work we show that if we take a large string $SNIa+BAO+H(z)+LSS+CMB$ of modern cosmological observations, in which not only the CMB but also a rich sample of large scale structure formation data are included, one can extract $\sim 3.3σ$ signs of DDE using a simple XCDM parameterization. These signs can be enhanced up to near $3.8σ$ in the context of the running vacuum model (RVM), in which the vacuum energy density is in interaction with dark matter. Recently the RVM has been shown to provide an efficient and economical solution to the $σ_8$-tension, which is one of the intriguing phenomenological problems that has not been possible to solve within the $Λ$CDM so far. This fact contributes to strengthen the possibility that dynamical vacuum energy, or in general DDE, could be presently favored by the observations.

gr-qc↗

arXiv2018

Relaxing the $σ_8$-tension through running vacuum in the Universe

It has recently been shown that the class of running vacuum models (RVMs) has the capacity to fit the overall cosmological observations better than the concordance $Λ$CDM model, therefore supporting the possibility of dynamical dark energy (DE). Apart from the cosmic microwave background (CMB) anisotropies, the most crucial datasets involved are: i) baryonic acoustic oscillations (BAO), and ii) direct large scale structure (LSS) formation data. Analyses mainly focusing on CMB and with insufficient BAO+LSS input generally fail to capture the dynamical DE signature, whereas the few existing studies accounting for the wealth of known CMB+BAO+LSS data (see in particular Solà, Gómez-Valent \& de Cruz Pérez 2015, 2017; and Zhao et al. 2017) do converge to the remarkable conclusion that dynamical DE might well be encoded in the current cosmological observations at a $3-4σ$ c.l. A decisive factor is the persistent $σ_8$-tension between the $Λ$CDM and the data. Because the issue is obviously pressing, we devote this work to explain how and why running vacuum in the expanding universe successfully relaxes the existing $σ_8$-tension and describes the LSS formation data significantly better than the $Λ$CDM.

astro-ph.CO↗

arXiv2017

The $H_0$ tension in light of vacuum dynamics in the Universe

Despite the outstanding achievements of modern cosmology, the classical dispute on the precise value of $H_0$, which is the first ever parameter of modern cosmology and one of the prime parameters in the field, still goes on and on after over half a century of measurements. Recently the dispute came to the spotlight with renewed strength owing to the significant tension (at $>3σ$ c.l.) between the latest Planck determination obtained from the CMB anisotropies and the local (distance ladder) measurement from the Hubble Space Telescope (HST), based on Cepheids. In this work, we investigate the impact of the running vacuum model (RVM) and related models on such a controversy. For the RVM, the vacuum energy density $ρ_Λ$ carries a mild dependence on the cosmic expansion rate, i.e. $ρ_Λ(H)$, which allows to ameliorate the fit quality to the overall $SNIa+BAO+H(z)+LSS+CMB$ cosmological observations as compared to the concordance $Λ$CDM model. By letting the RVM to deviate from the vacuum option, the equation of state $w=-1$ continues to be favored by the overall fit. Vacuum dynamics also predicts the following: i) the CMB range of values for $H_0$ is more favored than the local ones, and ii) smaller values for $σ_8(0)$. As a result, a better account for the LSS structure formation data is achieved as compared to the $Λ$CDM, which is based on a rigid (i.e. non-dynamical) $Λ$ term.

astro-ph.CO↗

arXiv2017

Vacuum dynamics in the Universe versus a rigid $Λ=$const

In this year, in which we celebrate 100 years of the cosmological term, $Λ$, in Einstein's gravitational field equations, we are still facing the crucial question whether $Λ$ is truly a fundamental constant or a mildly evolving dynamical variable. After many theoretical attempts to understand the meaning of $Λ$, and in view of the enhanced accuracy of the cosmological observations, it seems now mandatory that this issue should be first settled empirically before further theoretical speculations on its ultimate nature. In this work, we summarize the situation of some of these studies. Devoted analyses made recently show that the $Λ=$const. hypothesis, despite being the simplest, may well not be the most favored one. The overall fit to the cosmological observables $SNIa+BAO+H(z)+LSS+CMB$ singles out the class RVM of the "running" vacuum models, in which $Λ=Λ(H)$ is an affine power-law function of the Hubble rate. It turns out that the performance of the RVM as compared to the "concordance" $Λ$CDM model (with $Λ=$const.) is much better. The evidence in support of the RVM may reach $\sim 4σ$ c.l., and is bolstered with Akaike and Bayesian criteria providing strong evidence in favor of the RVM option. We also address the implications of this framework on the tension between the CMB and local measurements of the current Hubble parameter.

astro-ph.CO↗

arXiv2017

Dynamical dark energy: scalar fields and running vacuum

Recent analyses in the literature suggest that the concordance $Λ$CDM model with rigid cosmological term, $Λ=$const., may not be the best description of the cosmic acceleration. The class of "running vacuum models", in which $Λ=Λ(H)$ evolves with the Hubble rate, has been shown to fit the string of $SNIa+BAO+H(z)+LSS+CMB$ data significantly better than the $Λ$CDM. Here we provide further evidence on the time-evolving nature of the dark energy (DE) by fitting the same cosmological data in terms of scalar fields. As a representative model we use the original Peebles & Ratra potential, $V\proptoΦ^{-α}$. We find clear signs of dynamical DE at $\sim 4σ$ c.l., thus reconfirming through a nontrivial scalar field approach the strong hints formerly found with other models and parametrizations.

astro-ph.CO↗

arXiv2017

First evidence of running cosmic vacuum: challenging the concordance model

Despite the fact that a rigid $Λ$-term is a fundamental building block of the concordance $Λ$CDM model, we show that a large class of cosmological scenarios with dynamical vacuum energy density $ρ_Λ$ and/or gravitational coupling $G$, together with a possible non-conservation of matter, are capable of seriously challenging the traditional phenomenological success of the $Λ$CDM. In this paper, we discuss these "running vacuum models" (RVM's), in which $ρ_Λ=ρ_Λ(H)$ consists of a nonvanishing constant term and a series of powers of the Hubble rate. Such generic structure is potentially linked to the quantum field theoretical description of the expanding Universe. By performing an overall fit to the cosmological observables $SNIa+BAO+H(z)+LSS+BBN+CMB$ (in which the WMAP9, Planck 2013 and Planck 2015 data are taken into account), we find that the class of RVM's appears significantly more favored than the $Λ$CDM, namely at an unprecedented level of $\gtrsim4.2σ$. Furthermore, the Akaike and Bayesian information criteria confirm that the dynamical RVM's are strongly preferred as compared to the conventional rigid $Λ$-picture of the cosmic evolution.

astro-ph.CO↗

arXiv2015

Background history and cosmic perturbations for a general system of self-conserved dynamical dark energy and matter

We determine the Hubble expansion and the general cosmic perturbations equations for a general system consisting of self-conserved matter and self-conserved dark energy (DE). While at the background level the two components are non-interacting, they do interact at the perturbations level. We show that the coupled system of matter and DE perturbations can be transformed into a single, third order, matter perturbation equation, which reduces to the (derivative of the) standard one in the case that the DE is just a cosmological constant. As a nontrivial application we analyze a class of dynamical models whose DE density $ρ_D$ consists of a constant term, $C_0$, and a series of powers of the Hubble rate. These models were previously analyzed from the point of view of dynamical vacuum models, but here we treat them as self-conserved DE models with a dynamical equation of state. We fit them to the wealth of expansion history and linear structure formation data and compare the obtained fit quality with that of the concordance $Λ$CDM model. Those with $C_0=0$ include the so-called "entropic-force" and "QCD-ghost" DE models, as well as the pure linear model $ρ_D\sim H$, all of which appear strongly disfavored. The models with $C_0\neq 0$, in contrast, emerge as promising dynamical DE candidates whose phenomenological performance is highly competitive with the rigid $Λ$-term inherent to the $Λ$CDM.

gr-qc↗

arXiv2015

Hints of dynamical vacuum energy in the expanding Universe

Recently there have been claims on model-independent evidence of dynamical dark energy. Herein we consider a fairly general class of cosmological models with a time-evolving cosmological term of the form $Λ(H)=C_0+C_H H^2+C_{\dot{H}} \dot{H}$, where $H$ is the Hubble rate. These models are well motivated from the theoretical point of view since they can be related to the general form of the effective action of quantum field theory in curved spacetime. Consistency with matter conservation can be achieved by letting the Newtonian coupling $G$ change very slowly with the expansion. We solve these dynamical vacuum models and fit them to the wealth of expansion history and linear structure formation data. The results of our analysis show a significantly better agreement as compared to the concordance $Λ$CDM model, thus supporting the possibility of a dynamical cosmic vacuum.

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arXiv2015

Vacuum models with a linear and a quadratic term in H: structure formation and number counts analysis

We focus on the class of cosmological models with a time-evolving vacuum energy density of the form $ρ_Λ=C_0+C_1 H+C_2 H^2$, where $H$ is the Hubble rate. Higher powers of $H$ could be important for the early inflationary epoch, but are irrelevant afterwards. We study these models at the background level and at the perturbations level, both at the linear and at the nonlinear regime. We find that those with $C_0=0$ are seriously hampered, as they are unable to fit simultaneously the current observational data on Hubble expansion and the linear growth rate of clustering. This is in contrast to the $C_0\neq 0$ models, including the concordance $Λ$CDM model. We also compute the redshift distribution of clusters predicted by all these models, in which the analysis of the nonlinear perturbations becomes crucial. The outcome is that the models with $C_0=0$ predict a number of counts with respect to the concordance model which is much larger, or much smaller, than the $Λ$CDM and the dynamical models with $C_0\neq 0$. The particular case $ρ_Λ\propto H$ (the pure lineal model), which in the past was repeatedly motivated by several authors from QCD arguments applied to cosmology, is also addressed and we assess in detail its phenomenological status. We conclude that the most favored models are those with $C_0\neq 0$, and we show how to discriminate them from the $Λ$CDM.

astro-ph.CO↗

arXiv2015

The $\barΛCDM$ cosmology: from inflation to dark energy through running $Λ$

Perhaps the deepest mystery of our accelerating Universe in expansion is the existence of a tiny and rigid cosmological constant, $Λ$. Its size is many orders of magnitude below the expected one in the standard model of particle physics. However, an expanding Universe is not expected to have a static vacuum energy density. We should rather observe a mildly dynamical behavior $δΛ(t)\sim R\sim H^2(t)$ with the expansion rate $H$. At the same time, it is natural to think that the huge value of the primeval vacuum energy (presumably connected to some grand unified theory) was responsible for the initial inflationary phase. In the traditional inflaton models such phase is inserted by hand in the early epoch of the cosmic evolution, and it is assumed to match the concordance $Λ$CDM regime during the radiation epoch. Here, instead, we consider a class of dynamical vacuum models which incorporate into a single vacuum structure $\barΛ(H)$ the rapid stage of inflation, followed by the radiation and cold matter epochs, until achieving our dark energy Universe. The early behavior of the model bares resemblance with Starobinsky's inflation and ptovides a solution to the large entropy problem. It is compatible with the latest cosmological data on Hubble expansion and structure formation, and presents distinctive observational features that can be tested in the near future.

gr-qc↗