arXiv subjects

Joan Solà Peracaula

Publications and source records attributed to Joan Solà Peracaula.

12 recordsLinked to original sources

arXiv2026

$Λ$XCDM: a running vacuum strategy for crossing the phantom divide

Composite dynamical dark energy (DDE) has recently been explored as an efficient way to help cure cosmological tensions through the so-called $w$XCDM model (Gomez-Valent & Solà Peracaula 2024; 2025), a toy-model version of the $Λ$XCDM model (Grande et al., 2006). The latter is a composite running vacuum model (RVM) that involves a DE component $X$ (`cosmon') of generic nature. We compute the effective equation of state of $Λ$XCDM and use state-of-the-art techniques to fit this model to two standard sets of cosmological data, one involving SNIa from Pantheon$+$ and the other SNIa from DES-Dovekie, in addition to BAO data from DESI DR2 and the CMB data from Planck PR4. We do not use large scale structure formation data for this analysis nor the SH0ES calibration of $H_0$. We find that $Λ$XCDM naturally performs the crossing of the phantom divide as observed by DESI near $z\simeq 0.4$ using the $w_0w_a$CDM parameterization, a feature well favored by existing model-agnostic analyzes of the same data (González-Fuentes & Gómez-Valent:2025; 2026). It turns out that the cosmon $X$ behaves as `phantom matter' (PM) near the present, which in contrast to usual phantom DE satisfies the strong energy condition (as ordinary matter) and furnishes positive pressure ($P_X>-ρ_X>0$) at the expense of negative energy density ($ρ_X<0$). $Λ$XCDM provides a better fit than $w_0w_a$CDM and, as a bonus, alleviates the cosmic coincidence problem. Given that PM appears in stringy versions of the RVM (Mavromatos & Solà Peracaula 2021 a,b) , the $Λ$XCDM appears to be a composite DDE model with a good chance of explaining the crossing of the phantom divide from first principles, therefore providing theoretical support to the DESI observations inferred from generic parameterizations of the DE.

astro-ph.CO↗

arXiv2026

Towards a unified quantum field theory of dark energy and inflation: unstable de Sitter vacuum and running vacuum

Inflation is a necessary cosmic mechanism to cure basic inconsistencies of the standard model of cosmology. These problems are usually `fixed' by postulating the existence of a scalar field (called the ``inflaton''). However, other less ad hoc options are possible. In the running vacuum model (RVM) framework, the vacuum energy density (VED) $ρ_{\rm vac}$ is a function of the Hubble rate $H$ and its time derivatives. In this context, the VED is dynamical (there is no rigid cosmological constant $Λ$). In the FLRW epoch, $ρ_{\rm vac}$ evolves very slowly with expansion. In contrast, in the very early universe the vacuum fluctuations induce higher powers $H^N$ capable of unleashing fast inflation in a short period in which $H\simeq$ const. We call this mechanism `RVM-inflation'. It does not require an inflaton field since inflation is brought about by pure quantum field theory (QFT) effects on the dynamical background. It is different from Starobinsky's inflation, in which $H$ is never constant. In this work, we study a closely related scenario: the decay of the exact de Sitter vacuum into FLRW spacetime in its radiation epoch and the subsequent impact on the current universe, and compare with the RVM. We find that in both cases inflation is driven by $H^4$ powers together with subleading contributions of order $H^2$ that ease a graceful-exit transition into the radiation-dominated epoch, where the FLRW regime starts and ultimately develops a mildly evolving VED in the late universe: $δρ_{\rm vac}\sim {\cal O}(m_{\rm Pl} ^2 H^2)$. The proposal presented here aims at a unified QFT approach to inflation and dark energy (conceived as dynamical vacuum energy) with potentially measurable phenomenological consequences in the present universe, and constitutes a first step toward establishing its full theoretical and phenomenological consistency.

gr-qc↗

arXiv2026

Running Vacuum in the expanding Universe: a unified QFT paradigm for Inflation and Dark Energy

The concordance $Λ$CDM model, based on a rigid $Λ$-term for the entire cosmic history, has been in crisis for a long time. In our expanding Universe, an evolving $Λ$ with the expansion is intuitively much more reasonable. In the running vacuum model (RVM) framework, based on quantum field theory (QFT) in curved spacetime, quantum effects induce a vacuum energy density (VED) $ρ_{\rm vac}=Λ/(8πG)$ which is a function of the Hubble rate $H$ and its time derivatives, $ρ_{\rm vac}=ρ_{\rm vac}(H, \dot{H},\ddot{H},\dots)$. Currently, $ρ_{\rm vac}$ evolves very slowly with the expansion, $δρ_{\rm vac}\sim {\cal O}(m_{Pl} ^2 H^2)$, and this fact provides a possible fundamental origin of dark energy (DE), conceived as dynamical vacuum energy. In the RVM, Newton's $G$ is also evolving, but much more slowly (logarithmically with $H$): $G=G(\ln H)$. In the very early universe, the vacuum fluctuations induce higher (even) powers, e.g. $\sim H^4$, capable of triggering fast inflation in a very short period, in which $H$ is very large and approximately constant. This is the mechanism of `RVM-inflation'. It does not require an `inflaton' field since inflation is brought about by pure QFT effects on the dynamical background. It differs from Starobinsky's inflation, where $H$ is never constant. Furthermore, the dynamics of $ρ_{\rm vac}(H)$ and $G(H)$ can also have implications on the frequently discussed possibility that the fundamental `constants' of Nature can be mildly evolving with the cosmic expansion. Putting things together, a unified QFT framework of dark energy and inflation ensues as a realistic theory for the description of the universe as a whole on fundamental grounds. In it, dynamical VED is predicted and is much welcomed, since it fits in with current DESI measurements, preferring dynamical DE over a rigid $Λ$ term.

gr-qc↗

arXiv2026

Dynamical Dark Energy models in light of the latest observations

In this paper, we study several models and parameterizations of dynamical dark energy (DE) that have been studied already in the past, in conjunction with the recently proposed model $w$XCDM, the running vacuum model (RVM) with and without a threshold at $z=1$ and two variants of it, the RRVM and the ``flipped RVM'', and compare them all with the concordance $Λ$CDM model and the popular $w_0w_a$CDM parameterization. We use two standard sets of cosmological data, one including distant supernovae from Pantheon$+$ and the other from DES-Y5. The rest of the data (BAO from DESI DR2 and CMB from Planck PR4) are shared by the two sets. They are analyzed using the state-of-the-art techniques. No structure formation data are utilized for this analysis and no use is made of the SH0ES calibration of $H_0$. Even so, we find that the flipped RVM and to a lesser extent the $w$XCDM and the RVM with threshold, point to significant evidence of dynamical DE, at a level comparable to $w_0w_a$CDM, more conspicuously for the dataset that involves DES-Y5 observations. We also find that while more traditional models studied in the past, in which there is an exchange between vacuum energy and cold dark matter (through e.g. an interactive source proportional either to the density of dark matter or to that of vacuum) still hint at dynamical DE, the strength of the statistical signal (which we assess through information criteria and other estimators) is nevertheless less pronounced. Finally, we discuss the ability of the various models to explain the data by performing an analysis of their effective equation-of-state parameters and corresponding evolution of their dark energy densities.

astro-ph.CO↗

arXiv2025

Running vacuum and H^4-inflation

Recent studies of QFT in cosmological spacetime indicate that the speeding up of the present universe may not just be associated with a rigid cosmological term but with a running one that evolves with the expansion rate: $Λ=Λ(H)$. This running is inherited from the cosmic evolution of the vacuum energy density (VED), $ρ_{\rm vac}$, which is sensitive to quantum effects in curved spacetime that ultimately trigger that running. The VED is a function of the Hubble rate and its time derivatives: $ρ_{\rm vac}=ρ_{\rm vac}(H, \dot{H},\ddot{H},...)$. Two nearby points of the cosmic evolution during the FLRW epoch are smoothly related as $δρ_{\rm vac}\sim {\cal O}(H^2)$. In the very early universe, in contrast, the higher powers of the Hubble rate take over and bring about a period of fast inflation. They originate from quantum effects on the effective action of vacuum, which we compute. Herein we focus on the lowest possible power for inflation to occur: $H^4$. During the inflationary phase, $H$ remains approximately constant and very large. Subsequently, the universe enters the usual FLRW radiation epoch. This new mechanism (`RVM-inflation') is not based on any supplementary `inflaton' field, it is fueled by pure QFT effects on the dynamical background and is different from Starobinsky's inflation, in which $H$ is never constant.

gr-qc↗

arXiv2025

Composite Dark Energy and the Cosmological Tensions

The standard cosmological model currently in force, aka $Λ$CDM, has been plagued with a variety of tensions in the last decade or so, which puts it against the wall. At the core of the $Λ$CDM we have a rigid cosmological term, $Λ$, for the entire cosmic history. Recently, the results from the DESI collaboration suggested the possibility that dark energy (DE) should be dynamical rather than just a cosmological constant. Using a generic $w_0w_a$CDM parameterization, DESI favors quintessence behavior at $\sim 3σ$ c.l. However, to alleviate the tensions the DE needs more features. In the proposed $w$XCDM model of [45] the DE is actually a composite cosmic fluid with two components $(X,Y)$ acting sequentially: first $X$ (above a transition redshift $z_t$) and second $Y$ (below $z_t$). Fitting the model to the data, we find that the late component $Y$ behaves as quintessence, like DESI. However, to cure the $H_0$ and growth tensions, $X$ must behave as `phantom matter' (PM), which in contrast to phantom DE provides positive pressure at the expense of negative energy density. Using the SNIa (considering separately Pantheon$+$ and DESY5), cosmic chronometers, transversal BAO, LSS data, and the full CMB likelihood from Planck 2018, we find that both tensions can be completely cut down. We also compare the $w$XCDM with our own results using the standard $w$CDM and $w_0w_a$CDM parameterizations of the DE. In all cases, model $w$XCDM performs much better. Finally, we have repeated our analysis with BAO 3D data (replacing BAO 2D), and we still find that the main dynamical DE models (including composite ones) provide a much better fit quality compared to $Λ$CDM. The growth tension is alleviated again, but in contrast, the $H_0$-tension remains significant, which is most likely reminiscent of the internal conflict in the BAO sector.

astro-ph.CO↗

arXiv2024

Composite running vacuum in the Universe: implications on the cosmological tensions

The possibility that the vacuum energy density (VED) could be time dependent in the expanding Universe is intuitively more reasonable than just a rigid cosmological constant for the entire cosmic history. The framework of the running vacuum model (RVM) is a salient example derived from QFT in curved spacetime, wherein the VED appears as a power series of the Hubble rate, $H(t)$, and its derivatives. The RVM contributes to alleviate the cosmological tensions and at a more fundamental level it also helps to smooth out certain hardcore aspects of the cosmological constant problem. Composite dark energy (DE) extensions of the RVM are possible, in which the DE is a mixed fluid made out of running vacuum and an entity X called ``phantom matter'' which, I should stress, is radically different from phantom DE, since the former produces positive pressure like ordinary matter (therein its name). The prototype is the old $Λ$XCDM model from 2006. Recently a simplified version of the latter, called the $w$XCDM, has proven capable to yield a truly outstanding fit to the cosmological data with dramatic implications for mitigating the cosmological tensions. The analysis is sensitive to the kind of BAO data used, transversal (2D) or anisotropic (3D). For both BAO types the overall fit quality is substantially better than for the standard $Λ$CDM model, and the growth tension becomes alleviated. Nevertheless, only for 2D BAO the $H_0$-tension can be completely cut down. Most notably, this scenario favours (at $3.3σ$ c.l.) quintessence-like behavior around our time, as recently reported by the DESI Collaboration. In what follows I will summarize in very simple phenomenological terms the essentials of the RVM idea and its composite extensions, as well as the remarkable practical implications ensuing from such scenarios.

astro-ph.CO↗

arXiv2024

Vacuum energy and cosmological constant in QFT in curved spacetime

The cosmological constant term (CC), $Λ$, is a pivotal ingredient in the standard model of cosmology or $Λ$CDM, but it is a rigid quantity for the entire cosmic history. This is unnatural and inconsistent. Different theoretical and phenomenological conundrums suggest that the $Λ$CDM necessitates further theoretical underpinning to cope with modern observations. An interesting approach is the framework of the `running vacuum model' (RVM). It endows $Λ$ with cosmic dynamics within a fundamental framework since it is based on QFT. In the RVM, the vacuum energy density (VED) appears as a series of powers of the Hubble function and its derivatives, $ρ_{\rm vac} (H, \dot{H},...)$. In the current universe, $ρ_{\rm vac}$ changes as $\sim H^2$. Higher order effects ${\cal O}(H^4)$, on the other hand, can be responsible for a new mechanism of inflation (RVM-inflation). On the practical side the RVM can alleviate the cosmological tensions on $σ_8$ and $H_0$. An intriguing smoking gun signature of the RVM is that its equation of state can mimic quintessence, as recently observed by DESI, so the vacuum can be the sought-for dynamical DE. At a deeper theoretical level, the RVM-renormalized form of the VED can avoid extreme fine tuning related to the well-known cosmological constant problem. Overall, the RVM has the capacity to impinge positively on relevant theoretical and practical aspects of modern cosmology.

hep-th↗

arXiv2023

Stringy Running Vacuum Model and current Tensions in Cosmology

We discuss the potential alleviation of both the Hubble and the growth of galactic structure data tensions observed in the current epoch of Cosmology in the context of the so-called Stringy Running Vacuum Model (RVM) of Cosmology. This is a gravitational field theory coupled to matter, which, at early eras, contains gravitational (Chern-Simons (CS) type) anomalies and torsion, arising from the fundamental degrees of freedom of the massless gravitational multiplet of an underlying microscopic string theory. The model leads to RVM type inflation without external inflatons, arising from the quartic powers of the Hubble parameter that characterise the vacuum energy density due to primordial-gravitational-wave-induced anomaly CS condensates, and dominate the inflationary era. In modern eras, of relevance to this work, the gravitational anomalies are cancelled by chiral matter, generated at the end of the RVM inflationary era, but cosmic radiation and other matter fields are still responsible for a RVM energy density with terms exhibiting a quadratic-power-of-Hubble-parameter dependence, but also products of the latter with logarithmic $H$-dependencies, arising from potential quantum-gravity and quantum-matter loop effects. In this work, such terms are examined phenomenologically from the point of view of the potential alleviation of the aforementioned current tensions in Cosmology. Using standard information criteria, we find that these tensions can be substantially alleviated in a way consistent not only with the data, but also with the underlying microscopic theory predictions, associated with the primordial dynamical breaking of supergravity that characterise a pre-RVM-inflationary phase of the model.

gr-qc↗

arXiv2023

The dynamics of vacuum, gravity and matter: Implications on the fundamental constants

The possibility that the vacuum energy density (VED), $ρ_{\rm vac}$, could be time dependent in the expanding Universe is intuitively more reasonable than just a rigid cosmological constant for the entire cosmic history. The dynamics of $ρ_{\rm vac}=ρ_{\rm vac}(H)$ as a function of the Hubble rate, $H(t)$, most likely contributes to alleviate cosmological problems and tensions, having also implications on the so-called fundamental `constants' of Nature, which should be slowly drifting with the cosmic expansion owing to the fluctuations of the quantum vacuum. This includes the gravitational `constant' $G$, but also the gauge and Yukawa couplings as well as the particle masses themselves (both of dark matter and baryonic matter). The subtle exchange of energy involved is the basis for the ``micro and macro connection''. Herein, I discuss not only this connection as a possibility but show that it is in fact a generic prediction of QFT in cosmological spacetime which is fully compatible with general covariance. This fact has not been pointed out until recently when an appropriate renormalization framework for the VED has been found which is free from the usual conundrums associated with the cosmological constant problem.

gr-qc↗

arXiv2023

String-inspired running-vacuum cosmology, quantum corrections and the current cosmological tensions

In the context of a string-inspired running vacuum model (RVM) of cosmology with anomalies and torsion-induced axion-like fields, we discuss quantum corrections to the corresponding energy density, in approximately de Sitter eras, during which the Hubble parameter $H(t)$ varies very slowly with the cosmic time $t$. Such corrections arise either from graviton loops in the corresponding gravitational theory, or from path integration of massive quantum matter fields. They depend logarithmically on $H(t)$, in the form $H^n\, {\rm ln}(H^2)$, $n \in 2Z^+$. In the modern eras, for which the $n=2$ terns are dominant, such corrections may contribute to an alleviation of the currently observed cosmological $H_0$ and structure-growth tensions. In particular, we argue that such an effect is accomplished for a (dynamically-broken) supergravity-based RVM cosmological model. In the current de-Sitter era, for this case, rather surprisingly, the quantum graviton corrections dominate those due to matter fields, provided the scale of the primordial (pre-RVM inflation) dynamical breaking of local supersymmetry lies near the reduced Planck scale, which is a natural assumption in the context of the model.

gr-qc↗

arXiv2018

Density perturbations for running vacuum: a successful approach to structure formation and to the $σ_8$-tension

Recent studies suggest that dynamical dark energy (DDE) provides a better fit to the rising affluence of modern cosmological observations than the concordance model ($Λ$CDM) with a rigid cosmological constant, $Λ$. Such is the case with the running vacuum models (RVMs) and to some extent also with a simple XCDM parametrization. Apart from the cosmic microwave background (CMB) anisotropies, the most crucial datasets potentially carrying the DDE signature are: i) baryonic acoustic oscillations (BAO), and ii) direct large scale structure (LSS) formation data (i.e. the observations on $f(z)σ_8(z)$ at different redshifts). As it turns out, analyses mainly focusing on CMB and with insufficient BAO+LSS input, or those just making use of gravitational weak-lensing data for the description of structure formation, generally fail to capture the DDE signature, whereas the few existing studies using a rich set of 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 DDE might well be encoded in the current cosmological observations. Being the issue so pressing, here we explain both analytically and numerically the origin of the possible hints of DDE in the context of RVMs, which arise at a significance level of $3-4σ$. By performing a detailed study on the matter and vacuum perturbations within the RVMs, and comparing with the XCDM, we show why the running vacuum fully relaxes the existing $σ_8$-tension and accounts for the LSS formation data much better than the concordance model.

astro-ph.CO↗