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, $\rho_{\rm vac} =\rho_{\rm vac}(H)$. 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 $\Lambda$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 $\sigma_8$. 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.Comment: PhD Thesi

Geostatistical Evaluation of Rock-Quality Designation and its link with Linear Fracture Frequency

International audienceRock Quality Designation (RQD) is an important attribute used in geotechnics for quantifying the rock quality. It measures the borehole core recovery percentage incorporating only pieces of solid core that are longer than 100 mm measured along the centerline of the core. The presentation examines the behavior of this attribute in a Chilean porphyry copper deposit by analyzing more than 60,000 1.5-meter long samples.The drill holes have different directions and the nature of RQD requires accounting for the sample direction if the fracture network is anisotropic, a concept different from the geostatistical anisotropy which measures the variability along a direction set by two samples. A directional analysis is conducted and shows different variograms associated with different sample direction classes. This leads to different maps, calling into question the usual practices which do not account for the sample direction.The second part of the presentation concerns the link with the linear Fracture Frequency (FF), another important attribute which measures the number of discontinuities per meter. Under the assumption that the discontinuities along a line follow a Poisson process, Priest &Hudson established in 1976 a formula which expresses RQD as a function of FF. This formula is compared to E[RQF|FF], the mathematical expectation of RQD given FF, deduced from the data. The result of the comparison depends on whether FF is or is not corrected by the sinus of the angle between the sample direction and the fracture, as recommended by Terzaghi in 1965. When applied to FF, this correction breaks a natural correlation with RQD which appears when no correlation is applied. In the latter case, the Priest & Hudson formula is acceptable, in the first case it is not. So there is a dilemma: on the one hand, Terzaghi looks necessary to correctly calculate FF, on the other, the correction systematically increases FF and reduces the relative influence of RQD when both attributes are incorporated into an overall rating like the Rock Mass Rating (RMR), for example.A discussion follows, pointing out that RQD is subject to the same directional bias as FF and should be corrected in the same way by a sinus of the angle between the sample and the fracture, but such a correction is difficult, if not impossible; tests are presented. Finally, a correction of RQD is proposed, based on the Priest & Hudson formula

Running vacuum in quantum field theory in curved spacetime: renormalizing ρvac without ∼m4 terms

The Λ-term in Einstein's equations is a fundamental building block of the 'concordance' ΛCDM 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 space time. A plethora of phenomenological works have shown that this option can be highly competitive with the ΛCDM with a rigid cosmological term. The, so-called, 'running vacuum models' (RVM's) are characterized by the vacuum energy density, ρvac, 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 ρvac(H) evolves as a constant term plus dynamical components O(H2) and O(H4), the latter being relevant for the early universe only. However, the renormalized ρvac(H) does not carry dangerous terms proportional to the quartic power of the masses (∼m4) of the fields, these terms being a well-known source of exceedingly large contributions. At present, ρvac(H) is dominated by the additive constant term accompanied by a mild dynamical component ∼νH2 (|ν|≪1), which mimics quintessence

Cosmological constant and equation of state of the quantum vacuum

Recent studies of quantum field theory in FLRW spacetime suggest that the cause of the speeding up of the universe is the quantum vacuum, no need of ad hoc quintessence fields. Appropriate renormalization of the energy-momentum tensor shows that the vacuum energy density is a smooth function of the Hubble rate and its derivatives: $\rho_{\rm vac}=\rho_{\rm vac}(H, \dot{H},\ddot{H},...)$. This is because in QFT the quantum scaling of $\rho_{\rm vac}$ 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 $\delta\rho_{\rm vac}\sim {\cal O}(H^2)$. In this scenario, no fine tuning is needed at all. What we call the `cosmological constant' $\Lambda$ is just the nearly sustained value of $8\pi G(H)\rho_{\rm vac}(H)$ around (any) given epoch, where $G(H)$ is the running gravitational coupling. In the very early universe, higher (even) powers $\rho_{\rm vac}\sim{\cal O}(H^N)$ ($N=4,6,..$) triggered fast inflation during a short period in which $H=$const, no need of ad hoc inflatons. In that period, the equation of state (EoS) of the vacuum is very close to $w_{\rm vac}=-1$, but this ceases to be true during the FLRW era. Amazingly, the quantum vacuum acts as a formidable cosmic chameleon: it subsequently adopts the EoS of matter during the relativistic ($w_{\rm vac}=1/3$) and non-relativistic ($w_{\rm vac}=0$) epochs, and in the late universe it mimics quintessence, $w_{\rm vac}\gtrsim-1$, only to tend again to $-1$ in the remote future. In the transit, the quantum vacuum helps to solve the $H_0$ and $\sigma_8$ tensions.Comment: Extended discussion, references adde

Biological organization and cross-generation functions

International audienceThe organizational account of biological functions interprets functions as contributions of a trait to the maintenance of the organization that, in turn, maintains the trait. As has been recently argued, however, the account seems unable to provide a unified grounding for both intra- and cross-generation functions, since the latter do not contribute to the maintenance of the same organization which produces them. To face this 'ontological problem', a splitting account has been proposed, according to which the two kinds of functions require distinct organizational definitions. In this article, we propose a solution for the ontological problem, by arguing that intra- and cross-generation functions can be said to contribute in the same way to the maintenance of the biological organization, characterized in terms of organizational self-maintenance. As a consequence, we suggest maintaining a unified organizational account of biological functions

Sistematización de experiencias del Colectivo Tierra de Sueños

Servicio Social ComunitarioSe trata de una sistematización de las experiencias en las dimensiones comunitarias, artísticas y pedagógicas de la comunidad de Tierra de Sueños realizada a través de procedimientos de investigación cualitativa como elaboración de diarios de campo, observaciones participantes y no participantes, entrevistas y grupos focales como también de la elaboración de una categorización con la información obtenida.PregradoPsicólog

An Organizational Account of Biological Functions

International audienceIn this paper, we develop an organizational account that defines biological functions as causal relations subject to closure in living systems, interpreted as the most typical example of organizationally closed and differentiated self-maintaining systems. We argue that this account adequately grounds the teleological and normative dimensions of functions in the current organization of a system, insofar as it provides an explanation for the existence of the function bearer and, at the same time, identifies in a non-arbitrary way the norms that functions are supposed to obey. Accordingly, we suggest that the organizational account combines the etiological and dispositional perspectives in an integrated theoretical framework