arXiv subjects

Cristian Moreno-Pulido

Publications and source records attributed to Cristian Moreno-Pulido.

At least 19 recordsLinked to original sources

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↗

arXiv2025

Approximate solutions to the shrinking core model and their interpretation

The shrinking core model describes the reaction of a spherical solid particle with a surrounding fluid. In this work, we revisit the SCM by deriving it from the underlying physical processes and performing a careful non-dimensionalisation, which highlights the limitations of the commonly used pseudo-steady-state approximation, particularly in liquid-solid systems where fluid and solid densities are comparable. To address these limitations, we derive approximate analytical solutions using a perturbation method that improves upon the pseudo-steady-state model. We also obtain a small-time solution capturing early transient behavior. A semi-implicit finite difference scheme is implemented to solve the full model numerically and benchmark the analytical approximations. We demonstrate that the perturbation solution provides significantly improved accuracy over the pseudo-steady-state model, especially in diffusion-limited regimes. Finally, we propose a simple fitting procedure combining the perturbation with the early-time solutions to estimate physical parameters from experimental data at minimal computational cost.

cond-mat.other↗

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↗

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

Dark energy in quantum field theory: Implications on modern cosmology

In this dissertation, the nature of Dark Energy (DE) is examined from both theoretical and phenomenological perspectives. The possibility of DE being a dynamic quantity in quantum field theory (QFT) in curved spacetime is studied. The primary aim is to go beyond the usual approach that relies on ad hoc fields and instead treat DE as a quantum vacuum under appropriate QFT renormalization. Specifically, the dynamic behavior of DE could arise from quantum vacuum fluctuations in the Universe, evolving alongside the background expansion. Thus, the evolution of the vacuum energy density can be expressed in terms of the Hubble function and its derivatives, $ρ_{\rm vac} =ρ_{\rm vac}(H)$. This approach yields a significant revelation: the equation of state of the quantum vacuum, derived from first principles, deviates from its traditional constant value of $w_{\rm vac}=-1$. Additionally, a new inflationary mechanism emerges in this context, rooted in the quantum effects in curved spacetime. Moreover, the thesis displays a phenomenological exploration of two related models that go beyond the $Λ$CDM model: the Brans-Dicke model with a cosmological constant and the Running Vacuum Model, which is related to the QFT calculations. These models have been tested under different datasets and scenarios to determine the constraints on their free parameters. The results of the fits are presented and discussed in relation to cosmological tensions concerning $H_0$ and $σ_8$. The conclusions drawn from this thesis indicate promising signals of the dynamic behavior of quantum vacuum, potentially impacting the cosmological constant problem and the cosmological tensions.

gr-qc↗

arXiv2023

Running vacuum in QFT in FLRW spacetime: The dynamics of $ρ_{\rm vac}(H)$ from the quantized matter fields

Phenomenological work in the last few years has provided significant support to the idea that the vacuum energy density (VED) is a running quantity with the cosmological evolution and that this running helps to alleviate the cosmological tensions afflicting the $Λ$CDM. On the theoretical side, recent devoted studies have shown that the properly renormalized $ρ_{\rm vac}$ in QFT in FLRW spacetime adopts the "running vacuum model" (RVM) form. While in three previous studies by two of us (CMP and JSP) such computations focused solely on scalar fields non-minimally coupled to gravity, in the present work we compute the spin-$1/2$ fermionic contributions and combine them both. The calculation is performed using a new version of the adiabatic renormalization procedure based on subtracting the UV divergences at an off-shell renormalization point $M$. The quantum scaling of $ρ_{\rm vac}$ with $M$ turns into cosmic evolution with the Hubble rate, $H$. As a result the "cosmological constant" $Λ$ appears in our framework as the nearly sustained value of $8πG(H)ρ_{\rm vac}(H)$ around (any) given epoch $H$, where $G(H)$ is the gravitational coupling, which is also running, although very mildly (logarithmically). We find that the VED evolution at present reads $δρ_{\rm vac}(H)\sim ν_{\rm eff}\, m_{\rm Pl}^2 \left(H^2-H_0^2 \right)\ (|ν_{\rm eff}|\ll 1)$. The coefficient $ν_{\rm eff}$ receives contributions from all the quantized fields, bosons and fermions. Remarkably, there also exist higher powers ${\cal O}(H^{6})$ which can trigger inflation in the early universe. Finally, the equation of state (EoS) of the vacuum receives also quantum corrections from bosons and fermion fields, shifting its value from -1. The striking consequence is that the EoS of the quantum vacuum may nowadays effectively appears as quintessence.

gr-qc↗

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↗

arXiv2023

Renormalizing the vacuum energy in cosmological spacetime: implications for the cosmological constant problem

The renormalization of the vacuum energy in quantum field theory (QFT) is usually plagued with theoretical conundrums related not only with the renormalization procedure itself, but also with the fact that the final result leads usually to very large (finite) contributions incompatible with the measured value of $Λ$ in cosmology. Herein, we compute the zero-point energy (ZPE) for a nonminimally coupled (massive) scalar field in FLRW spacetime using the off-shell adiabatic renormalization technique employed in previous work. The general off-shell result yields a smooth function $ρ_{\rm vac}(H)$ made out of powers of the Hubble rate and/or of its time derivatives involving different (even) adiabatic orders $\sim H^N$ ($N=0,2,4,6,...)$, i.e. it leads, remarkably enough, to the running vacuum model (RVM) structure. We have verified the same result from the effective action formalism and used it to find the $β$-function of the running quantum vacuum. No undesired contributions $\sim m^4$ from particle masses appear and hence no fine-tuning of the parameters is needed in $ρ_{\rm vac}(H)$. Furthermore, we find that the higher power $\sim H^6$ could naturally drive RVM-inflation in the early universe. Our calculation also elucidates in detail the equation of state of the quantum vacuum: it proves to be not exactly $-1$ and is moderately dynamical. The form of $ρ_{\rm vac}(H)$ at low energies is also characteristic of the RVM and consists of an additive term (the so-called `cosmological constant') together with a small dynamical component $\sim νH^2$ ($|ν|\ll1$). Finally, we predict a slow ($\sim\ln H$) running of Newton's gravitational coupling $G(H)$. The physical outcome of our semiclassical QFT calculation is revealing: today's cosmic vacuum and the gravitational strength should be both mildly dynamical.

gr-qc↗

arXiv2022

Equation of state of the running vacuum

Recent studies of quantum field theory in FLRW spacetime suggest that the cause of the speeding up of the universe is the running vacuum (RV). Appropriate renormalization of the energy-momentum tensor shows that the vacuum energy density is a smooth function of the Hubble rate and its derivatives: $ρ_{\rm vac}=ρ_{\rm vac}(H, \dot{H},\ddot{H},...)$. This is because in QFT the quantum scaling of $ρ_{\rm vac}$ with the renormalization point turns into cosmic evolution with $H$. As a result, any two nearby points of the cosmic expansion during the standard FLRW epoch are smoothly related through $δρ_{\rm vac}\sim {\cal O}(H^2)$. In our approach, what we call the `cosmological constant' $Λ$ is just the nearly sustained value of $8πG(H)ρ_{\rm vac}(H)$ around (any) given epoch, where $G(H)$ is the running gravitational coupling. In the present study, after summarizing the main QFT calculations supporting the RV approach, we focus on the calculation of the equation of state (EoS) of the RV for the entire cosmic history within such a QFT framework. In particular, in the very early universe, where higher (even) powers $ρ_{\rm vac}\sim{\cal O}(H^N)$ ($N=4,6,\dots$) triggered inflation during a short period in which $H=$const, the vacuum EoS is very close to $w_{\rm vac}=-1$. This ceases to be true during the FLRW era, where it adopts the EoS of matter during the relativistic ($w_{\rm vac}=1/3$) and non-relativistic ($w_{\rm vac}=0$) epochs. Interestingly enough, we find that in the late universe the EoS becomes mildly dynamical and mimics quintessence, $w_{\rm vac}\gtrsim-1$. It finally asymptotes to $-1$ in the remote future, but in the transit the RV helps alleviating the $H_0$ and $σ_8$ tensions.

gr-qc↗

arXiv2021

Renormalized $ρ_{\rm vac}$ without $m^4$ terms

The cosmological constant term, $Λ$, in Einstein's equations has been for three decades a building block of the concordance or standard $Λ$CDM model of cosmology. Although the latter is not free of fundamental problems, it provides a good phenomenological description of the overall cosmological observations. However, an interesting improvement in such a phenomenological description and also a change in the theoretical status of the $Λ$-term occurs upon realizing that the vacuum energy density, $ρ_{\textrm{vac}}$, is actually a "running quantity" in quantum field theory in curved spacetime. Several works have shown that this option can compete with the $Λ$CDM with a rigid $Λ$term. The so-called, "running vacuum models" (RVM) are characterized indeed by a $ρ_{\textrm{vac}}$ which is evolving with time as a series of even powers of the Hubble parameter and its time derivatives. This form has been motivated by renormalization group arguments in previous works. Here we review a recent detailed computation by the authors of the renormalized energy-momentum tensor of a non-minimally coupled scalar field with the help of adiabatic regularization. The final result is noteworthy: $ρ_{\textrm{vac}}(H)$ takes the precise structure of the RVM, namely a constant term plus a dynamical component $\sim H^2$ (which may be detectable in the present universe) including also higher order effects $\mathcal{O}(H^4)$ which can be of interest during the early stages of the cosmological evolution. Besides, it is most remarkable that such renormalized form of $ρ_{\textrm{vac}}$ does not carry dangerous terms proportional to $m^4$, the quartic powers of the masses of the fields, which are a well-known source of exceedingly large contributions to the vacuum energy density and are directly responsible for fine tuning in the context of the CC problem.

gr-qc↗

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.

astro-ph.CO↗

arXiv2020

Cosmology Intertwined III: $f σ_8$ and $S_8$

The standard $Λ$ Cold Dark Matter cosmological model provides a wonderful fit to current cosmological data, but a few tensions and anomalies became statistically significant with the latest data analyses. While these anomalies could be due to the presence of systematic errors in the experiments, they could also indicate the need for new physics beyond the standard model. In this Letter of Interest we focus on the tension of the Planck data with weak lensing measurements and redshift surveys, about the value of the matter energy density $Ω_m$, and the amplitude or rate of the growth of structure ($σ_8,fσ_8$). We list a few interesting models for solving this tension, and we discuss the importance of trying to fit with a single model a full array of data and not just one parameter at a time.

astro-ph.CO↗

arXiv2020

Cosmology Intertwined II: The Hubble Constant Tension

The current cosmological probes have provided a fantastic confirmation of the standard $Λ$ Cold Dark Matter cosmological model, that has been constrained with unprecedented accuracy. However, with the increase of the experimental sensitivity a few statistically significant tensions between different independent cosmological datasets emerged. While these tensions can be in portion the result of systematic errors, the persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the need for new physics. In this Letter of Interest we will focus on the $4.4σ$ tension between the Planck estimate of the Hubble constant $H_0$ and the SH0ES collaboration measurements. After showing the $H_0$ evaluations made from different teams using different methods and geometric calibrations, we will list a few interesting new physics models that could solve this tension and discuss how the next decade experiments will be crucial.

astro-ph.CO↗

arXiv2020

Cosmology Intertwined I: Perspectives for the Next Decade

The standard $Λ$ Cold Dark Matter cosmological model provides an amazing description of a wide range of astrophysical and astronomical data. However, there are a few big open questions, that make the standard model look like a first-order approximation to a more realistic scenario that still needs to be fully understood. In this Letter of Interest we will list a few important goals that need to be addressed in the next decade, also taking into account the current discordances present between the different cosmological probes, as the Hubble constant $H_0$ value, the $σ_8 - S_8$ tension, and the anomalies present in the Planck results. Finally, we will give an overview of upgraded experiments and next-generation space-missions and facilities on Earth, that will be of crucial importance to address all these questions.

astro-ph.CO↗

arXiv2020

Cosmology Intertwined IV: The Age of the Universe and its Curvature

A precise measurement of the curvature of the Universe is of primeval importance for cosmology since it could not only confirm the paradigm of primordial inflation but also help in discriminating between different early Universe scenarios. The recent observations, while broadly consistent with a spatially flat standard $Λ$ Cold Dark Matter ($Λ$CDM) model, are showing tensions that still allow (and, in some cases, even suggest) a few percent deviations from a flat universe. In particular, the Planck Cosmic Microwave Background power spectra, assuming the nominal likelihood, prefer a closed universe at more than 99\% confidence level. While new physics could be in action, this anomaly may be the result of an unresolved systematic error or just a statistical fluctuation. However, since a positive curvature allows a larger age of the Universe, an accurate determination of the age of the oldest objects provides a smoking gun in confirming or falsifying the current flat $Λ$CDM model.

astro-ph.CO↗

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↗

arXiv2020

Running vacuum in quantum field theory in curved spacetime: renormalizing $ρ_{vac}$ without $\sim m^4$ terms

The $Λ$-term in Einstein's equations is a fundamental building block of the `concordance' $Λ$CDM model of cosmology. Even though the model is not free of fundamental problems, they have not been circumvented by any alternative dark energy proposal either. Here we stick to the $Λ$-term, but we contend that it can be a `running quantity' in quantum field theory (QFT) in curved spacetime. A plethora of phenomenological works have shown that this option can be highly competitive with the $Λ$CDM with a rigid cosmological term. The, so-called, `running vacuum models' (RVM's) are characterized by the vacuum energy density, $ρ_{vac}$, being a series of (even) powers of the Hubble parameter and its time derivatives. Such theoretical form has been motivated by general renormalization group arguments, which look plausible. Here we dwell further upon the origin of the RVM structure within QFT in FLRW spacetime. We compute the renormalized energy-momentum tensor with the help of the adiabatic regularization procedure and find that it leads essentially to the RVM form. This means that $ρ_{vac}(H)$ evolves as a constant term plus dynamical components ${\cal O}(H^2)$ and ${\cal O}(H^4)$, the latter being relevant for the early universe only. However, the renormalized $ρ_{vac}(H)$ does not carry dangerous terms proportional to the quartic power of the masses ($\sim m^4$) of the fields, these terms being a well-known source of exceedingly large contributions. At present, $ρ_{vac}(H)$ is dominated by the additive constant term accompanied by a mild dynamical component $\sim νH^2$ ($|ν|\ll1$), which mimics quintessence.

gr-qc↗