arXiv subjects

Spyros Basilakos

Publications and source records attributed to Spyros Basilakos.

At least 73 records · Page 4Linked to original sources

arXiv2020

Quantum Anomalies in String-Inspired Running Vacuum Universe: Inflation and Axion Dark Matter

In this letter, we elaborate further on a Cosmological "Running-Vacuum" type model for the Universe, suggested previously by the authors within the context of a string-inspired effective theory in the presence of a Kalb-Ramond (KR) gravitational axion field which descends from the antisymmetric tensor of the massless gravitational string multiplet. In the presence of this field, which has anomalous CP violating interactions with the gravitons, primordial gravitational waves induce gravitational anomalies, which in turn are responsible for the appearance of $H^2$ and $H^4$ contributions to the vacuum energy density, these terms being characteristic of generic "running-vacuum-model (RVM) type", where $H$ is the Hubble parameter. In this work we prove in detail the appearance of the $H^4$ terms due to gravitational-anomaly-induced condensates in the energy density of the primordial Universe, which can self-consistently induce inflation, and subsequent exit from it, according to the generic features of RVM. We also argue in favour of the robustness of our results, which were derived within an effective low-energy field theory approach, against Ultra Violet completion of the theory. During the radiation and matter-dominated eras, gravitational anomalies cancel, as required for the consistency of the quantum matter/radiation field theory. However, chiral and QCD-axion-type anomalies survive and have important consequences for both cosmic magnetogenesis and axionic dark matter in the Universe. Finally, the stringy RVM scenario presented here predicts quintessence-like dynamical dark energy for the current Universe, which is compatible with the existing fitting analyses of such model against observations

gr-qc↗

arXiv2020

Growth index of matter perturbations in the light of Dark Energy Survey

We study how the cosmological constraints from growth data are improved by including the measurements of bias from Dark Energy Survey (DES). In particular, we utilize the biasing properties of the DES Luminous Red Galaxies (LRGs) and the growth data provided by the various galaxy surveys in order to constrain the growth index ($γ$) of the linear matter perturbations. Considering a constant growth index we can put tight constraints, up to $\sim 10\%$ accuracy, on $γ$. Specifically, using the priors of the Dark Energy Survey and implementing a joint likelihood procedure between theoretical expectations and data we find that the best fit value is in between $γ=0.64\pm 0.075$ and $0.65\pm 0.063$. On the other hand utilizing the Planck priors we obtain $γ=0.680\pm 0.089$ and $0.690\pm 0.071$. This shows a small but non-zero deviation from General Relativity ($γ_{\rm GR}\approx 6/11$), nevertheless the confidence level is in the range $\sim 1.3-2σ$. Moreover, we find that the estimated mass of the dark-matter halo in which LRGs survive lies in the interval $\sim 6.2 \times 10^{12} h^{-1} M_{\odot}$ and $1.2 \times 10^{13} h^{-1} M_{\odot}$, for the different bias models. Finally, allowing $γ$ to evolve with redshift [Taylor expansion: $γ(z)=γ_{0}+γ_{1}z/(1+z)$] we find that the $(γ_{0},γ_{1})$ parameter solution space accommodates the GR prediction at $\sim 1.7-2.9σ$ levels.

astro-ph.CO↗

arXiv2020

Gravitational and Chiral Anomalies in the Running Vacuum Universe and Matter-Antimatter Asymmetry

We present a model for the Universe in which quantum anomalies are argued to play an important dual role: they are responsible for generating matter-antimatter asymmetry, but also provide time-dependent contributions to the vacuum energy density of "running-vacuum" type, which drive the Universe evolution. During the inflationary phase of a string-inspired Universe, and its subsequent exit, the existence of primordial gravitational waves induce gravitational anomalies, which couple to the Kalb-Ramond (KR)axion field of the massless gravitational string multiplet. Such anomalous CP violating interactions have two important effects: first, they lead to contributions to the vacuum energy density of the form appearing in the "running vacuum model" (RVM) framework, proportional to both the square and the fourth power of the effective Hubble parameter $H$, the $H^4$ terms being associated with anomaly-induced inflation within the RVM framework. Second, there is an undiluted KR axion at the end of inflation, which plays an important role in generating matter-antimatter asymmetry through baryogenesis via leptogenesis in models with heavy right handed neutrinos. As the Universe exits inflation and enters a radiation dominated era, the generation of chiral fermionic matter is responsible for the cancellation of gravitational anomalies, thus restoring diffeomorphism invariance for the matter/radiation (quantum) theory, as required for consistency. Chiral U(1) anomalies may remain uncompensated, though, during matter/radiation dominance, providing RVM-like $H^2$ and $H^4$ contributions to the Universe energy density. Finally, in the current era, gravitational anomalies resurface, leading to much weaker RVM-like $H_0^2$ contributions to the vacuum energy density.Our model favours axionic Dark Matter,the source of which can be the KR axion.

hep-ph↗

arXiv2020

Dynamics and cosmological evolution in Λ-varying cosmology

We study the dynamical properties of a large body of varying vacuum cosmologies for which dark matter interacts with vacuum. In particular, performing the critical point analysis we investigate the existence and the stability of cosmological solutions which describe de-Sitter, radiation and matter dominated eras. We find several cases of varying vacuum models that admit stable critical points, hence they can be used in describing the cosmic history.

gr-qc↗

arXiv2019

Scalar Field Theory Description of the Running Vacuum Model: the Vacuumon

We investigate the running vacuum model (RVM) in the framework of scalar field theory.This dynamical vacuum model provides an elegant global explanation of the cosmic history, namely the universe starts from a non-singular initial de Sitter vacuum stage, it passes smoothly from an early inflationary era to a radiation epoch ("graceful exit") and finally it enters the dark matter and dark energy (DE) dominated epochs, where it can explain the large entropy problem and predicts a mild dynamical evolution of the DE. Within this phenomenologically appealing context, we formulate an effective {\it classical} scalar field description of the RVM through a field $ϕ$, called the {\it vacuumon}, which turns out to be very helpful for an understanding and practical implementation of the physical mechanisms of the running vacuum during both the early universe and the late time cosmic acceleration. In the early universe the potential for the vacuumon may be mapped to a potential that behaves similarly to that of the scalaron field of Starobinsky-type inflation at the {\it classical} level, whilst in the late universe it provides an effective scalar field description of DE. The two representations, however, are not physically equivalent since the mechanisms of inflation are entirely different. Moreover, unlike the scalaron, vacuumon is treated as a classical background field, and not a fully fledged quantum field, hence cosmological perturbations will be different between the two pictures of inflation.

gr-qc↗

arXiv2019

Bayesian analysis of $f(T)$ gravity using $fσ_8$ data

We use observational data from Supernovae (SNIa) Pantheon sample, from direct Hubble constant measurements with cosmic chronometers (CC), from the Cosmic Microwave Background shift parameter $\text{CMB}_{\text{shift}}$, and from redshift space distortion ($fσ_8$) measurements, in order to constrain $f(T)$ gravity. We do not follow the common $γ$ parameterization within the semi-analytical approximation of the growth rate, in order to avoid model-dependent uncertainties. Up to our knowledge this is the first time that $f(T)$ gravity is analyzed within a Bayesian framework, and with background and perturbation behaviour considered jointly. We show that all three examined $f(T)$ models are able to describe adequately the $fσ_8$ data. Furthermore, applying the Akaike, Bayesian and Deviance Information Criteria, we conclude that all considered models are statistically equivalent, however the most efficient candidate is the exponential model, which additionally presents a small deviation from $Λ$CDM paradigm.

astro-ph.CO↗

arXiv2019

Dynamical system analysis at background and perturbation levels: Quintessence in severe disadvantage comparing to $Λ$CDM

We perform for the first time a dynamical system analysis of both the background and perturbation equations, of $Λ$CDM cosmology and quintessence scenario with an exponential potential. In the former case the perturbations do not change the stability of the late-time attractor of the background equations, and the system still results in the dark-energy dominated, de Sitter solution, having passed from the correct dark-matter era with $γ\approx6/11$. However, in the case of quintessence the incorporation of perturbations changes the stability and properties of the background evolution, and the only conditionally stable points present either an exponentially increasing matter clustering not favored by observations, or Laplacian instabilities, and thus not physically interesting. This result is a severe disadvantage of quintessence cosmology comparing to $Λ$CDM paradigm.

gr-qc↗

arXiv2019

Dynamics of Einstein-Aether Scalar field Cosmology

We study the cosmological evolution of the field equations in the context of Einstein-Aether cosmology by including a scalar field in a spatially flat Friedmann--Lema\^ıtre--Robertson--Walker spacetime. Our analysis is separated into two separate where a pressureless fluid source is included or absent. In particular, we determine the critical points of the field equations and we study the stability of the specific solutions. The limit of general relativity is fully recovered, while the dynamical system admits de Sitter solutions which can describe the past inflationary era and the future late-time attractor. Results for generic scalar field potentials are presented while some numerical behaviours are given for specific potential forms.

gr-qc↗

arXiv2019

Independent Cosmological Constraints from high-z HII Galaxies

We present new high spectral resolution observations of 15 high-z ($1.3 \leq$ z $\leq 2.5$) HII Galaxies (HIIG) obtained with MOSFIRE at the Keck Observatory. These data, combined with already published data for another 31 high-z and 107 z $\leq 0.15$ HIIG, are used to obtain new independent cosmological results using the distance estimator based on the established correlation between the Balmer emission line velocity dispersion and luminosity for HIIG. Our results are in excellent agreement with the latest cosmological concordance model ($Λ$CDM) published results. From our analysis, we find a value for the mass density parameter of $Ω_m=0.290^{+0.056}_{-0.069}$ (stat). For a flat Universe we constrain the plane $\lbraceΩ_m;w_0\rbrace = \lbrace 0.280^{+0.130}_{-0.100} ; -1.12^{+0.58}_{-0.32}\rbrace $ (stat). The joint likelihood analysis of HIIG with other complementary cosmic probes (Cosmic Microwave Background and Baryon Acoustic Oscillations) provides tighter constraints for the parameter space of the Equation of State of Dark Energy that are also in excellent agreement with those of similar analyses using Type Ia Supernovae instead as the geometrical probe.

astro-ph.GA↗

arXiv2019

Do we Come from a Quantum Anomaly?

We present a string-based picture of the cosmological evolution in which (CP-violating) gravitational anomalies acting during the inflationary phase of the universe cause the vacuum energy density to "run" with the effective Hubble parameter squared, $H^2$, thanks to the axion field of the bosonic string multiplet. This leads to baryogenesis through leptogenesis with massive right-handed neutrinos. The generation of chiral matter after inflation helps in cancelling the anomalies in the observable radiation- and matter- dominated eras. The present era inherits the same "running vacuum" structure triggered during the inflationary time by the axion field. The current dark energy is thus predicted to be mildly dynamical, and dark matter should be made of axions. Paraphrasing Carl Sagan: "we are all anomalously made from starstuff " .

hep-th↗

arXiv2019

Quantum Anomalies, Running Vacuum and Leptogenesis: an Interplay

We discuss a connection between gravitational-wave physics, quantum theory anomalies, right-handed (sterile) neutrinos, (spontaneous) CPT Violation and Leptogenesis, within the framework of string-inspired cosmological models. In particular, we present a scenario, according to which (primordial) gravitational waves induce gravitational anomalies during inflation. This, in turn, results in the existence of an undiluted (at the exit of inflation/beginning of radiation era) bakcground of the Kalb-Ramond (KR) axion of the massless bosonic string gravitational multiplet. The latter may violate spontaneously CP and CPT symmetries, and induce leptogenesis during the radiation-dominated era in models involving right-handed neutrinos. The so-generated lepton asymmetry may then be communicated to the baryon sector by appropriate baryon-minus-lepton-number (B - L)-conserving, but (B + L)-violating, (sphaleron) processes in the Standard Model sector, thus leading to matter dominance over antimatter in the Universe.In the current (approximately de Sitter) era, the KR axion background may provide a source for an axionic dark matter in the Universe, through its mixing with other axions that are abundant in string models. As an interesting byproduct of our analysis, we demonstrate that the anomalies contribute to the vacuum energy density of the Universe terms of 'running-vacuum' type, proportional to the square of the Hubble parameter, $H^2$.

hep-ph↗

arXiv2019

Dark energy and dark matter unification from dynamical space time: observational constraints and cosmological implications

A recently proposed Dynamical Space-time Cosmology (DSC) that unifies dark energy and dark matter is studied. The general action of this scenario includes a Lagrange multiplier, which is coupled to the energy momentum tensor and a scalar field which is different from quintessence. First for various types of potentials we implement a critical point analysis and we find solutions which lead to cosmic acceleration and under certain conditions to stable late-time attractors. Then the DSC cosmology is confronted with the latest cosmological data from low-redshift probes, namely measurements of the Hubble parameter and standard candles (Pantheon SnIa, Quasi-stellar objects). Performing an overall likelihood analysis and using the appropriate information criteria we find that the explored DSC models are in very good agreement with the data. We also find that one of the DSC models shows a small but non-zero deviation from $Λ$ cosmology, nevertheless the confidence level is close to $\sim 1.5σ$.

gr-qc↗

arXiv2019

Testing dynamical vacuum models with CMB power spectrum from Planck

The cosmic expansion is computed for various dynamical vacuum models $Λ(H)$ and confronted to the Cosmic Microwave Background (CMB) power spectrum from Planck. We also combined CMB in a joint analysis with other probes in order to place constraints on the cosmological parameters of the dynamical vacuum models. We find that all $Λ(H)$ models are very efficient and in very good agreement with the data. Considering that the interaction term of the dark sector is given in terms of matter and radiation densities, we find that the corresponding $Λ(H)$ model shows a small but non-zero deviation from $Λ$ cosmology, nevertheless the confidence level is close to $\sim 2.5σ$.

astro-ph.CO↗

arXiv2019

Constraining the Asymptotically Safe Cosmology: cosmic acceleration without dark energy

A recently proposed Asymptotically Safe cosmology provides an elegant mechanism towards understanding the nature of dark energy and its associated cosmic coincidence problem. The underlying idea is that the accelerated expansion of the universe can occur due to infrared quantum gravity modifications at intermediate astrophysical scales (galaxies or galaxy clusters) which produce local anti-gravity sources. In this cosmological model no extra unproven energy scales or fine-tuning are used. In this study the Asymptotically Safe model is confronted with the most recent observational data from low-redshift probes, namely measurements of the Hubble parameter, standard candles (Pantheon SnIa, Quasi-stellar objects), Baryonic Acoustic Oscillations (BAOs) and high redshift probes (CMB shift parameters). Performing an overall likelihood analysis we constrain the free parameters of the model and we test its performance against the concordance model (flat $Λ$CDM) utilizing a large family of information criteria. We find that the Asymptotically Safe model is statistically equivalent with $Λ$CDM, hence it can be seen as a viable and very efficient cosmological alternative.

astro-ph.CO↗

arXiv2018

Cosmological constraints on $γ$-gravity models

In this paper we place observational constraints on the well-known $γ$-gravity $f(R)$ model using the latest cosmological data, namely we use the latest growth rate, Cosmic Microwave Background, Baryon Acoustic Oscillations, Supernovae type Ia and Hubble parameter data. Performing a joint likelihood analysis we find that the $γ$-gravity model is in very good agreement with observations. Utilizing the AIC statistical test we statistically compare the current $f(R)$ model with $Λ$CDM cosmology and find that they are statistically equivalent. Therefore, $γ$-gravity can be seen as a useful scenario toward testing deviations from General Relativity. Finally, we note that we find somewhat higher values for the $f(R)$ best-fit values compared to those mentioned in the past in the literature and we argue that this could potential alleviate the halo-mass function problem.

astro-ph.CO↗

arXiv2018

Constraining the dark energy models with H(z) data: an approach independent of $H_{0}$

We study the performance of the latest $H(z)$ data in constraining the cosmological parameters of different cosmological models, including that of Chevalier-Polarski-Linder $w_{0}w_{1}$ parametrization. First, we introduce a statistical procedure in which the chi-square estimator is not affected by the value of the Hubble constant. As a result, we find that the $H(z)$ data do not rule out the possibility of either non-flat models or dynamical dark energy cosmological models. However, we verify that the time varying equation of state parameter $w(z)$ is not constrained by the current expansion data. Combining the $H(z)$ and the Type Ia supernova data we find that the $H(z)$/SNIa overall statistical analysis provides a substantial improvement of the cosmological constraints with respect to those of the $H(z)$ analysis. Moreover, the $w_{0}-w_{1}$ parameter space provided by the $H(z)$/SNIa joint analysis is in a very good agreement with that of Planck 2015, which confirms that the present analysis with the $H(z)$ and SNIa probes correctly reveals the expansion of the Universe as found by the team of Planck. Finally, we generate sets of Monte Carlo realizations in order to quantify the ability of the $H(z)$ data to provide strong constraints on the dark energy model parameters. The Monte Carlo approach shows significant improvement of the constraints, when increasing the sample to 100 $H(z)$ measurements. Such a goal can be achieved in the future, especially in the light of the next generation of surveys.

astro-ph.CO↗

arXiv2018

Comparison of the linear bias models in the light of the Dark Energy Survey

The evolution of the linear and scale independent bias, based on the most popular dark matter bias models within the $Λ$CDM cosmology, is confronted to that of the Dark Energy Survey (DES) Luminous Red Galaxies (LRGs). Applying a $χ^2$ minimization procedure between models and data we find that all the considered linear bias models reproduce well the LRG bias data. The differences among the bias models are absorbed in the predicted mass of the dark-matter halo in which LRGs live and which ranges between $\sim 6 \times 10^{12} h^{-1} M_{\odot}$ and $1.4 \times 10^{13} h^{-1} M_{\odot}$, for the different bias models. Similar results, reaching however a maximum value of $\sim 2\times 10^{13} h^{-1} M_{\odot}$, are found by confronting the SDSS (2SLAQ) Large Red Galaxies clustering with theoretical clustering models, which also include the evolution of bias. This later analysis also provides a value of $Ω_{m}=0.30\pm 0.01$, which is in excellent agreement with recent joint analyses of different cosmological probes and the reanalysis of the Planck data.

astro-ph.CO↗

arXiv2018

Tachyon warm inflation with the effects of Loop Quantum Cosmology in the light of Planck 2015

We investigate the observational signatures of quantum cosmology in the Cosmic Microwave Background data provided by Planck collaboration. We apply the warm inflationary paradigm with a tachyon scalar field to the loop quantum cosmology. In this context, we first provide the basic cosmological functions in terms of the tachyon field. We then obtain the slow-roll parameters and the power spectrum of scalar and tensor fluctuations respectively. Finally, we study the performance of various warm inflationary scenarios against the latest Planck data and we find a family of models which are in agreement with the observations.

gr-qc↗