Dynamical Vacuum Research Articles

Phenomenology of cosmological particle creation, Dirac sea and all that

We constructed the conformally invariant model for scalar particle creation induced by strong gravitational fields. Starting from the “usual” hydrodynamical description of the particle motion written in the Eulerian coordinates we substituted the particle number conservation law (which enters the formalism) by “the particle creation law”, proportional to the square of the Weyl tensor (following the famous result by Ya.B. Zel’dovich and A.A. Starobinsky). Then, demanding the conformal invariance of the whole dynamical system, we have got both the (Weyl)-conformal gravity and the Einstein-Hilbert gravity action integral with dilaton field. Thus, we obtained something like the induced gravity suggested first by A.D. Sakharov. It is shown that the resulting system is self-consistent. We considered also the vacuum equations. It is shown that, beside the “empty vacuum”, there may exist the “dynamical vacuum”, which is nothing more but the Dirac sea. The latter is described by the unexpectedly elegant equation which includes both the Bach and Einstein tensors and the cosmological terms. All the vacuum solutions violate the local conformal invariance.

Scalar field vacuum expectation value induced by gravitational wave background

We show that a massless scalar field in a gravitational wave background can develop a non-zero vacuum expectation value. We draw comparisons to the generation of a non-zero vacuum expectation value for a scalar field in the Higgs mechanism and with the dynamical Casimir vacuum. We propose that this vacuum expectation value, generated by a gravitational wave, can be connected with particle production from gravitational waves and may have consequences for the early Universe where scalar fields are thought to play an important role.

The Effects of Thermal and Dynamical Parameters and Vacuum Application on Porosity in High-Pressure Die Casting of A383 Al-Alloy

High-Pressure Die Casting (HPDC) is a near-net shape manufacturing method for the non-ferrous materials casting process, and it is widely used in the automotive industry. In the HPDC method, mold (TM) and casting temperature (TC), first- (V1) and second (V2)-phase injection velocity, injection pressure (P3), mold and casting material, mold design and vacuum application are all important parameters for producing high-quality products, as they directly affect the mechanical and metallurgical properties and micro–macroporosity of the product parts. In this study, the effects of thermal (mold and casting temperature) injection parameters and dynamic (pressure, velocity and vacuum application) injection parameters on the mechanical properties and porosity of the samples were studied. In the experiments and simulations performed as part of this study, A383 was selected as the casting material and DIN 1,2344 as the hot work tool steel. The mold cavity was shaped according to the tensile test specimen. The experiments were repeated under 64 different conditions, using different dynamic and thermal parameters and vacuum application drawn in the mold cavity. The samples obtained from the experiments were subjected to tensile tests to identify their mechanical properties. A gas pycnometer was used to measure and calculate porosity rates. All the tests were simulated using FLOW-3D software proper to the experimental conditions. In conclusion, the most useful thermal parameters obtained from the experimental study were found to be 1063 K for casting temperature and 493 K for mold temperature, while the optimum dynamic parameter values were determined to be 1.7 m/s injection velocity and 10–20 MPa injection pressure for vacuum application drawn in the mold cavity.

Dynamical dark energy vs. 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 ΛCDM, 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 signs of DDE using a simple XCDM parameterization. These signs can be enhanced up to near 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 -tension, which is one of the intriguing phenomenological problems that has not been possible to solve within the ΛCDM so far. This fact contributes to strengthen the possibility that dynamical vacuum energy, or in general DDE, could be presently favored by the observations.

De Sitter stability and coarse graining

We present a 4-dimensional back reaction analysis of de Sitter space for a conformally coupled scalar field in the presence of vacuum energy initialized in the Bunch–Davies vacuum. In contrast to the usual semi-classical prescription, as the source term in the Friedmann equations we use expectation values where the unobservable information hidden by the cosmological event horizon has been neglected i.e. coarse grained over. It is shown that in this approach the energy-momentum is precisely thermal with constant temperature despite the dilution from the expansion of space due to a flux of energy radiated from the horizon. This leads to a self-consistent solution for the Hubble rate, which is gradually evolving and at late times deviates significantly from de Sitter. Our results hence imply de Sitter space to be unstable in this prescription. The solution also suggests dynamical vacuum energy: the continuous flux of energy is balanced by the generation of negative vacuum energy, which accumulatively decreases the overall contribution. Finally, we show that our results admit a thermodynamic interpretation which provides a simple alternate derivation of the mechanism. For very long times the solutions coincide with flat space.

Dark Energy and Spacetime Symmetry

The Petrov classification of stress-energy tensors provides a model-independent definition of a vacuum by the algebraic structure of its stress-energy tensor and implies the existence of vacua whose symmetry is reduced as compared with the maximally symmetric de Sitter vacuum associated with the Einstein cosmological term. This allows to describe a vacuum in general setting by dynamical vacuum dark fluid, presented by a variable cosmological term with the reduced symmetry which makes vacuum fluid essentially anisotropic and allows it to be evolving and clustering. The relevant solutions to the Einstein equations describe regular cosmological models with time-evolving and spatially inhomogeneous vacuum dark energy, and compact vacuum objects generically related to a dark energy: regular black holes, their remnants and self-gravitating vacuum solitons with de Sitter vacuum interiors—which can be responsible for observational effects typically related to a dark matter. The mass of objects with de Sitter interior is generically related to vacuum dark energy and to breaking of space-time symmetry. In the cosmological context spacetime symmetry provides a mechanism for relaxing cosmological constant to a needed non-zero value.

First Evidence of Running Cosmic Vacuum: Challenging the Concordance Model

Abstract Despite the fact that a rigid -term is a fundamental building block of the concordance ΛCDM model, we show that a large class of cosmological scenarios with dynamical vacuum energy density together with a dynamical gravitational coupling G or a possible non‐conservation of matter, are capable of seriously challenging the traditional phenomenological success of the ΛCDM. In this paper, we discuss these “running vacuum models” (RVMs), in which 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 SN Ia+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 RVMs appears significantly more favored than the ΛCDM, namely, at an unprecedented level of . Furthermore, the Akaike and Bayesian information criteria confirm that the dynamical RVMs are strongly preferred compared to the conventional rigid -picture of the cosmic evolution.

Particle creation phenomenology, Dirac sea and the induced Weyl and Einstein-dilaton gravity

We constructed the conformally invariant model for scalar particle creation induced by strong gravitational fields. Starting from the “usual” hydrodynamical description of the particle motion written in the Eulerian coordinates we substituted the particle number conservation law (which enters the formalism) by “the particle creation law”, proportional to the square of the Weyl tensor (following the famous result by Ya.B. Zel'dovich and A.A. Starobinsky). Then, demanding the conformal invariance of the whole dynamical system, we have got both the (Weyl)-conformal gravity and the Einstein-Hilbert gravity action integral with dilaton field. Thus, we obtained something like the induced gravity suggested first by A.D. Sakharov. It is shown that the resulting system is self-consistent. We considered also the vacuum equations. It is shown that, beside the “empty vacuum”, there may exist the “dynamical vacuum”, which is nothing more but the Dirac sea. The latter is described by the unexpectedly elegant equation which includes both the Bach and Einstein tensors and the cosmological terms.

Cosmological constant vis-à-vis dynamical vacuum: Bold challenging the ΛCDM

Next year we will celebrate 100 years of the cosmological term, [Formula: see text], in Einstein’s gravitational field equations, also 50 years since the cosmological constant problem was first formulated by Zeldovich, and almost about two decades of the observational evidence that a nonvanishing, positive, [Formula: see text]-term could be the simplest phenomenological explanation for the observed acceleration of the Universe. This mixed state of affairs already shows that we do no currently understand the theoretical nature of [Formula: see text]. In particular, we are still facing the crucial question whether [Formula: see text] is truly a fundamental constant or a mildly evolving dynamical variable. At this point the matter should be settled once more empirically and, amazingly enough, the wealth of observational data at our disposal can presently shed true light on it. In this short review, I summarize the situation of some of these studies. It turns out that the [Formula: see text]. hypothesis, despite being the simplest, may well not be the most favored one when we put it in hard-fought competition with specific dynamical models of the vacuum energy. Recently, it has been shown that the overall fit to the cosmological observables SNIa+BAO+H(z)+LSS+BBN+CMB do favor the class of “running” vacuum models (RVM’s) — in which [Formula: see text] is a function of the Hubble rate — against the “concordance” [Formula: see text]CDM model. The support is at an unprecedented level of [Formula: see text] and is backed up with Akaike and Bayesian criteria leading to compelling evidence in favor of the RVM option and other related dynamical vacuum models. I also address the implications of this framework on the possible time evolution of the fundamental constants of Nature.

Unification of dark energy and dark matter based on the Petrov classification and space-time symmetry

The Petrov classification of stress-energy tensors provides a model-independent definition of a vacuum by the algebraic structure of its stress-energy tensor and implies the existence of vacua whose symmetry is reduced as compared with the maximally symmetric de Sitter vacuum associated with the Einstein cosmological term. This allows to describe a vacuum in general setting by dynamical vacuum dark fluid, presented by a variable cosmological term with the reduced symmetry which makes vacuum dark fluid essentially anisotropic and allows it to be evolving and clustering. The relevant regular solutions to the Einstein equations describe regular cosmological models with time-evolving and spatially inhomogeneous vacuum dark energy, and compact vacuum objects generically related to a dark energy through the de Sitter vacuum interior: regular black holes, their remnants and self-gravitating vacuum solitons — which can be responsible for observational effects typically related to a dark matter. The mass of objects with de Sitter interior is generically related to vacuum dark energy and to breaking of space-time symmetry.

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 perturbation equations for a general system consisting of self-conserved matter, ρm, and self-conserved dark energy (DE), ρD. 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 ρD(H) consists of a constant term, C0, 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 their fit quality with that of the concordance ΛCDM model. Those with C0=0 include the so-called ``entropic-force'' and ``QCD-ghost'' DE models, as well as the pure linear model ρD∼H, all of which appear strongly disfavored. The models with C0≠0 , in contrast, emerge as promising dynamical DE candidates whose phenomenological performance is highly competitive with the rigid Λ-term inherent to the ΛCDM.

HINTS OF DYNAMICAL VACUUM ENERGY IN THE EXPANDING UNIVERSE

Recently there have been claims of 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 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 indicate a significantly better agreement as compared to the concordance ΛCDM model, thus supporting the possibility of a dynamical cosmic vacuum.

The $\bar{\Lambda}{\rm CDM}$ 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. This is a very welcome fact, namely if we care at all about our own existence and fate. However, we do not have a minimally satisfactory explanation for our good fortune and for the failing of the SM at that crucial point. To start with, an expanding universe is not expected to have a static vacuum energy density. We should rather observe a mildly dynamical behavior δΛ(t) ~ R ~ H2(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 GUT) 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 standard ΛCDM regime during the radiation epoch. Here, instead, we consider a class of dynamical vacuum models which incorporate in a single vacuum structure [Formula: see text] the rapid stage of inflation, followed by the radiation and cold matter epochs, until achieving our dark energy universe. The early behavior of such "running vacuum model" [Formula: see text] bares resemblance with Starobinsky's inflation in the early universe and is very close to the concordance model for the entire post-inflationary history. Most remarkably, the inflationary period in the [Formula: see text] terminates with "graceful exit" and the large entropy problem can be solved. The model is compatible with the latest cosmological data on Hubble expansion and structure formation, and at the same time presents distinctive observational features that can be tested in the near future.

Number counts and dynamical vacuum cosmologies

We study non-linear structure formation in an interacting model of the dark sector of the Universe in which the dark energy density decays linearly with the Hubble parameter, $\rho_{\Lambda} \propto H$, leading to a constant-rate creation of cold dark matter. We derive all relevant expressions to calculate the mass function and the cluster number density using the Sheth-Torman formalism and show that the effect of the interaction process is to increase the number of bound structures of large masses ($M \gtrsim 10^{14} M_{\odot}h^{-1}$) when compared to the standard $\Lambda$CDM model. Since these models are not reducible to each other, this number counts signature can in principle be tested in future surveys.

Dynamical vacuum energy in the expanding Universe confronted with observations: a dedicated study

Despite the many efforts, our theoretical understanding of the ultimate nature of the dark energy component of the universe still lags well behind the astounding experimental evidence achieved from the increasingly sophisticated observational tools at our disposal. While the canonical possibility is a strict cosmological constant, or rigid vacuum energy density ρΛ = const., the exceeding simplicity of this possibility lies also at the root of its unconvincing theoretical status, as there is no explanation for the existence of such constant for the entire cosmic history. Herein we explore general models of the vacuum energy density slowly evolving with the Hubble function H and/or its time derivative, ρΛ = ρΛ(H,Ḣ). Some of these models are actually well-motivated from the theoretical point of view and may provide a rich phenomenology that could be explored in future observations, whereas some others have more limitations. In this work, we put them to the test and elucidate which ones are still compatible with the present observations and which ones are already ruled out. We consider their implications on structure formation, in combination with data on type Ia supernovae, the Cosmic Microwave Background, the Baryonic Acoustic Oscillations, and the predicted redshift distribution of cluster-size collapsed structures. The relation of these vacuum models on possible evidence of dynamical dark energy recently pointed out in the literature is also briefly addressed.

Experimental determination of parameters of multichannel hydrogen diffusion in solid probe

The multi-channel diffusion mechanism is modeled by diffusion equations.The method for analysis of experimental dynamical curves of vacuum hot extraction (VHE) is developed, which allows to determine the binding energy and diffusion constants of hydrogen in the probe under study.The experimental data have been obtained using industrial hydrogen analyzer, which allows to carry out the absolute measurements of the dynamical curves vacuum hot extraction of the hydrogen from a solid probe. Experimental verification of mathematical models gives adequate results.The differences of the proposed approach with the method of TDS are discussed.

Entropic-force dark energy reconsidered

We reconsider the entropic-force model in which both kinds of Hubble terms, $\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{H}$ and ${H}^{2}$, appear in the effective dark energy (DE) density affecting the evolution of the main cosmological functions, namely, the scale factor, deceleration parameter, matter density, and growth of linear matter perturbations. However, we find that the entropic-force model is not viable at the background and perturbation levels due to the fact that the entropic formulation does not add a constant term in the Friedmann equations. On the other hand, if on mere phenomenological grounds we replace the $\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{H}$ dependence of the effective DE density with a linear term $H$ without including a constant additive term, we find that the transition from deceleration to acceleration becomes possible, but the recent structure formation data strongly disfavor this cosmological scenario. Finally, we briefly compare the entropic-force models with some related DE models (based on dynamical vacuum energy) which overcome these difficulties and are compatible with the present observations.

Deviation from the standard uncertainty principle and the dark energy problem

Quantum fluctuations of a real massless scalar field are studied in the context of the Generalized Uncertainty Principle (GUP). The dynamical finite vacuum energy is found in spatially flat Friedmann-Robertson- Walker (FRW) spacetime which can be identified as dark energy to explain late time cosmic speed-up. The results show that a tiny deviation from the standard uncertainty principle is necessary on cosmological ground. By using the observational data we have constraint the GUP parameter even more stronger than ever.

Effective equation of state for running vacuum: 'mirage' quintessence and phantom dark energy

Past analyses of the equation of state (EoS) of the Dark Energy (DE) were not incompatible with a phantom phase near our time. This has been the case in the years of WMAP observations, in combination with the remaining cosmological observables. Such situation did not completely disappear from the data collected from the Planck satellite mission. In it the EoS analysis may still be interpreted as suggesting w<-1, and so a mildly evolving DE cannot be discarded. In our opinion the usual ansatzs made on the structure of the EoS for dynamical DE models (e.g. quintessence and the like) are too simplified. In this work we examine in detail some of these issues and suggest that a general class of models with a dynamical vacuum energy density could explain the persistent phantom anomaly, despite there is no trace of real phantom behavior in them. The spurious or "mirage" effect is caused by an attempt to describe them as if the DE would be caused by fundamental phantom scalar fields. Remarkably, the effective DE behavior can also appear as quintessence in transit to phantom, or vice versa.

Expansion history with decaying vacuum: a complete cosmological scenario

We propose a novel cosmological scenario with the space-time emerging from a pure initial de Sitter stage and subsequently evolving into the radiation, matter and dark energy dominated epochs, thereby avoiding the initial singularity and providing a complete description of the expansion history and a natural solution to the horizon problem. The model is based on a dynamical vacuum energy density which evolves as a power series of the Hubble rate. The transit from the inflation into the standard radiation epoch is universal, giving a clue for a successful description of the graceful exit. Since the resulting late time cosmic history is very close to the concordance LCDM model, the new unified framework embodies a more complete past cosmic evolution than the standard cosmology.