In previous work, q-theory was introduced to describe the gravitating macroscopic behavior of a conserved microscopic variable q. In this article, the gluon condensate of quantum chromodynamics is considered in terms of q-theory. The remnant vacuum energy density (i.e., cosmological constant) of an expanding universe is estimated as K_{QCD}^3 / E_{Planck}^2, with string tension K_{QCD} \approx (10^2 MeV)^2 and gravitational scale E_{Planck} \approx 10^{19} GeV. The only input for this estimate is general relativity, quantum chromodynamics, and the Hubble expansion of the present Universe.
In the same way as the realization of some of the famous gedanken experiments imagined by the founding fathers of quantum mechanics has recently led to the current renewal of the interpretation of quantum physics, it seems that the most recent progresses of observational astrophysics can be interpreted as the realization of some cosmological gedanken experiments such as the removal from the universe of the whole visible matter or the cosmic time travel leading to a new cosmological standard model. This standard model involves two dark components of the universe, dark energy and dark matter. Wh…
We describe a link between the cosmological constant problem and the problem of time in quantum gravity. This arises by examining the relationship between the cosmological constant and vacuum energy in light of non-perturbative formulations of quantum gravity.
We estimate the entropy associated to a background of squeezed cosmic gravitons, and we argue that the process of cosmological pair production from the vacuum may explain the large amount of entropy of our present universe.
We point out a potential relevance between the Krein-Gupta-Bleuler (KGB) vacuum leading to a fully covariant quantum field theory for gravity in de Sitter (dS) spacetime and the observable smallness of the cosmological constant. This may provide a formulation of linear quantum gravity in a framework amenable to developing a more complete theory determining the value of the cosmological constant.
A spacetime-independent variable is introduced which characterizes a Lorentz-invariant self-sustained quantum vacuum. For a perfect (Lorentz-invariant) quantum vacuum, the self-tuning of this variable nullifies the effective energy density which enters the low-energy gravitational field equations. The observed small but nonzero value of the cosmological constant may then be explained as corresponding to the effective energy density of an imperfect quantum vacuum (perturbed by, e.g., the presence of thermal matter).
We study gravitational waves generated during the inflationary epoch in presence of a decaying cosmological parameter on a 5D geometrical background which is Riemann flat. Two examples are considered, one with a constant cosmological parameter and the second with a decreasing one.
We propose a suitable vacuum state for a quantized Dirac field interacting with a classical scalar field in a Friedman-Robertson-Walker-Lemaitre spacetime. This state is constructed from an extended version of the adiabatic expansion of the associated Fourier modes of the field. We compute the full renormalized stress-energy tensor. For any given initial time, this instantaneous vacuum can always be chosen to have vanishing Fourier modes for the renormalized stress-energy tensor. We check the consistency of the solution for several inflationary models.
It is known that the unregularized expressions for the stress-energy tensor components corresponding to subhorizon and superhorizon vacuum fluctuations of a massless scalar field in a Friedmann-Robertson-Walker background are characterized by the equation of state parameters w=1/3 and w=-1/3, which are not sufficient to produce cosmological acceleration. However, the form of the adiabatically regularized finite stress-energy tensor turns out to be completely different. By using the fact that vacuum subhorizon modes evolve nearly adiabatically and superhorizon modes have w=-1/3, we approximatel…
The $q$-theory approach to the cosmological constant problem is reconsidered. The new observation is that the effective classical $q$-theory gets modified due to the backreaction of quantum-mechanical particle production by spacetime curvature. Furthermore, a Planck-scale cosmological constant is added to the potential term of the action density, in order to represent the effects from zero-point energies and phase transitions. The resulting dynamical equations of a spatially-flat Friedmann-Robertson-Walker universe are then found to give a steady approach to the Minkowski vacuum, with attracto…
We develop an stochastic approach to study gravitational waves produced during the inflationary epoch under the presence of a decaying cosmological parameter, on a 5D geometrical background which is Riemann flat. We obtain that the squared tensor metric fluctuations depend strongly on the cosmological parameter $Λ(t)$ and we finally illustrate the formalism with an example of a decaying $Λ(t)$.
We study the vacuum polarisation effects of the Dirac fermionic field induced by a pointlike global monopole located in the cosmological de Sitter spacetime. First we derive the four orthonormal Dirac modes in this background in a closed form. Quantising the field using these modes, we then compute the fermionic condensate, $\langle 0| \overlineΨ Ψ| 0\rangle$, as well as the vacuum expectation value of the energy-momentum tensor for a massive Dirac field, regularised in a particular way. We have used the Abel-Plana summation formula in order to extract the global monopole contribution to these…
We present the study of a two 3-branes system embedded in a 5-dimensional space-time in which the fifth dimension is compacted on a $S^{1}/Z_{2}$ orbifold and the space between the two branes is vacuum. Assuming isotropic and homogeneous space time, we probe that the dynamics of the visible brane is affected by the dynamics of the hidden brane and the possible components which fills it. We study two different cases: assuming a constant equation of state and inflationary scalar field, and its possible consequences in the brane evolution.
In linearized quantum gravity, a shift of the average energy-momentum can be compensated by a shift of the average gravitational field. This allows a renormalization scheme that naturally removes the contribution of quantum vacuum fluctuations to the cosmological constant, solving the old cosmological-constant problem for weak gravitational fields.
The physical Hamiltonian of a gravity-matter system depends on the choice of time, with the vacuum naturally identified as its ground state. We study the expanding universe with scalar field in the volume time gauge. We show that the vacuum energy density computed from the resulting Hamiltonian is a non-linear function of the cosmological constant and time. This result provides a new perspective on the relation between time, the cosmological constant, and vacuum energy.
We propose a model with a compensating scalar field whose back reaction to the cosmological curvature cancels possible vacuum energy density down to the terms of the order of the time dependent critical energy density. Thus the model simultaneously solves the mystery of the compensation of vacuum energy with the accuracy of 120 orders of magnitude and explains existence of the observed dark energy. At an early stage the suggested cosmological model might experience exponential expansion without an additional inflaton field.
One hope to solve the cosmological constant problem is to identify a symmetry principle, based on which the cosmological constant can be reduced either to zero, or to a tiny value. Here, we note that requiring that the vacuum state is Lorentz invariant significantly reduces the theoretical value of the vacuum energy density. Hence, this also reduces the discrepancy between the observed value of the cosmological constant and its theoretical expectation, down from 123 orders of magnitude to 56 orders of magnitude. We find that, at one loop level, massless particles do not yield any contribution …
A mechanism for suppressing the cosmological constant is developed, based on an analogy with a superconducting phaseshift in which free fermions coupled perturbatively to a weak gravitational field are in an unstable false vacuum state. The coupling of the fermions to the gravitational field generates fermion condensates with zero momentum and a phase transition induces a nonperturbative transition to a true vacuum state by producing a positive energy gap $Δ$ in the vacuum energy, identified with $\sqrtΛ$, where $Λ$ is the cosmological constant. In the strong coupling limit a large cosmologica…
We construct infinite dimensional families of non-singular stationary space times, solutions of the vacuum Einstein equations with a negative cosmological constant.
It is argued that quantum states of geometry, like those of particles, should be coherent on light cones of any size. An exact classical solution, the gravitational shock wave of a relativistic point particle, is used to estimate gravitational drag from coherent energy flows, and the expected gravitational effect of virtual transverse vacuum energy fluctuations on surfaces of causal diamonds. It is proposed that the appropriately spacetime-averaged gravitational effect of the Standard Model vacuum state leads to the observed small nonzero value of the cosmological constant, dominated by gravit…
The cosmological constant problem arises at the intersection between general relativity and quantum field theory, and is regarded as a fundamental problem in modern physics. In this paper we describe the historical and conceptual origin of the cosmological constant problem which is intimately connected to the vacuum concept in quantum field theory. We critically discuss how the problem rests on the notion of physical real vacuum energy, and which relations between general relativity and quantum field theory are assumed in order to make the problem well-defined.
It is generally accepted that the entropy of an asymptotically de Sitter universe is bounded by the area, in Planck units, of the de Sitter horizon. Based on an analysis of the entropy associated to the vacuum quantum fluctuations, we suggest that the existence of such a holographic bound constitutes a possible explanation for the observed value of the cosmological constant, theoretically justifying a relation proposed 35 years ago by Zel'dovich.
The one-loop effective action for non-minimal scalar modified gravity on de Sitter background with a constant scalar field is found. The corresponding induced cosmological constant is evaluated. It is shown that quantum effects in non-minimal modified gravity may induce an early-time de Sitter universe even in the situation when such solution does not occur on the classical level. Classical reconstruction of the theory is presented in such a way that the resulting theory has a cosmological solution unifying early-time inflation with late-time acceleration.
Recently, we have proposed a definition for the vacuum and suggest a mechanism for symmetry breaking. In this mechanism extra massless fields, vacuum fields, arise. We apply our method to the standard model of particle physics and obtain vacuum sector of this model. Vacuum field of the standard model is a four-vector with electromagnetic nature. This field is invariant under $ U(1)_{em} $ gauge transformation. We speculate that it could be responsible for many cosmological phenomena like inflation, dark energy and cosmic magnetic fields.
An speculative solution for the cosmological constant problem is proposed. It is argued that while the true quantum vacuum energy density is of the order of $M_P^4$, the observed classical vacuum energy density may be much smaller due to the huge amount of scattering process in the vacuum state of quantum gravity.
It will be argued here that the cosmological constant problem exists because of the way the vacuum is defined in quantum field theory. It has been known for some time that for QFT to be gauge invariant certain terms--such as part of the vacuum polarization tensor--must be eliminated either explicitly or by some form of regularization followed by renormalization. It has recently been shown that lack of gauge invariance is a result of the way the vacuum is defined, and redefining the vacuum so that the theory is gauge invariant may also offer a solution to the cosmological constant problem.
Current astronomical observations are successfully explained by the present cosmological paradigm based on the concordance model ($Λ_0$CDM + Inflation). However, such a scenario is composed of a heterogeneous mix of ingredients for describing the different stages of cosmological evolution. Particularly, it does not give an unified explanation connecting the early and late time accelerating inflationary regimes which are separated by many aeons. Other challenges to the concordance model include: a singularity at early times or the emergence of the Universe from the quantum gravity regime, the "…
It is widely believed that as one of the candidates for dark energy, the cosmological constant should relate directly with the quantum vacuum. Despite decades of theoretical effects, however, there is still no quantitative interpretation of the observed cosmological constant. In this work, we consider the quantum state of the whole universe including the quantum vacuum. Everett's relative-state formulation, vacuum quantum fluctuations and the validity of Einstein's field equation at macroscopic scales imply that our universe wave function might be a superposition of states with different cosmo…
We point out that the expansion of the universe leads to a cosmological time evolution of the vacuum expectation of the Higgs boson. Within the standard model of particle physics, the cosmological time evolution of the vacuum expectation of the Higgs leads to a cosmological time evolution of the masses of the fermions and of the electroweak gauge bosons while the scale of Quantum Chromodynamics (QCD) remains constant. Precise measurements of the cosmological time evolution of $μ=m_e/m_p$, where $m_e$ and $m_p$ are respectively the electron and proton mass (which is essentially determined by th…
In the context of colliding brane worlds I discuss a toy cosmological model, developed in collaboration with E. Gravanis, which arguably produces inflation and a relaxing to zero cosmological ``constant'' hierarchically small as compared to the supersymmetry breaking (TeV) scale. Supersymmetry breaking is induced by compactification of the brane worlds on magnetized tori. The crucial ingredient is the non-criticality (non conformality) of string theory on the observable brane world induced at the collision, which is thus viewed as a cause for departure from equilibrium in this system. The hier…
Introducing a variable cosmological function $Λ(t)$ in a geometrical manner from a 5D Riemann-flat metric, we investigate the possibility of having a geometrical criterion to choose a suitable cosmological function $Λ(t)$ for every 4D dynamical hypersurface capable of generate phantom cosmologies.
The false vacuum decay in a brane world model is studied in this work. We investigate the vacuum decay via the Coleman-de Luccia instanton, derive explicit approximative expressions for the Coleman-de Luccia instanton which is close to a Hawking-Moss instanton and compare the results with those already obtained within Einstein's theory of relativity.
In the framework of Inflationary theory, the assumption that the quantum state of the perturbations is a non-vacuum state leads to a difficulty: non-vacuum initial states imply, in general, a large energy density of inflaton field quanta, not of a cosmological term type, that could prevent the inflationary phase. In this short note, we discuss in detail why this is so, keeping an eye on possible non-Gaussian features due to considering generic non-vacuum initial states.
The current vacuum energy density observed as dark energy ${ ρ}_{ \rm dark }\simeq 2.5\times10^{-47}\ {\rm GeV^{4}}$ is unacceptably small compared with any other scales. Therefore, we encounter serious fine-tuning problem and theoretical difficulty to derive the dark energy. However, the theoretically attractive scenario has been proposed and discussed in literature: In terms of the renormalization-group (RG) running of the cosmological constant, the vacuum energy density can be expressed as ${ ρ}_{ \rm vacuum }\simeq m^{2}H^{2}$ where $m$ is the mass of the scalar field and rather dynamical …
In this paper, the generation of topological energy in models with large extra dimensions is investigated. The origin of this energy is attributed to a topological deformation of the standard Minkowski vacuum due to compactification of extra dimensions. This deformation is seen to give rise to an effective, finite energy density due to massive Kaluza-Klein modes of gravitation. It's renormalized value is seen to depend on the size of the extra dimensions instead of the UV cut-off of the theory. It is shown that if this energy density is to contribute to the observed cosmological constant, ther…
Whereas the total energy in zero-point fluctuations of the particle physics vacuum gives rise to the cosmological constant problem, differences in the vacuum give rise to real physical phenomena, such as the Casimir effect. Hence we consider the zero-point energy bound between two parallel conducting plates -- proxy for a solid slice of cosmological constant -- as a convenient laboratory in which to investigate the gravitation and inertia of vacuum energy. We calculate the Casimir effect in a weak gravitational field, obtaining corrections to the vacuum stress-energy and attractive force on th…
Bianchi type V viscous fluid cosmological model for barotropic fluid distribution with varying cosmological term $Λ$ is investigated. We have examined a cosmological scenario proposing a variation law for Hubble parameter $H$ in the background of homogeneous, anisotropic Bianchi type V space-time. The model isotropizes asymptotically and the presence of shear viscosity accelerates the isotropization. The model describes a unified expansion history of the universe indicating initial decelerating expansion and late time accelerating phase. Cosmological consequences of the model are also discusse…
We explore an effective 4D cosmological model for the universe where the variable cosmological constant governs its evolution and the pressure remains negative along all the expansion. This model is introduced from a 5D vacuum state where the (space-like) extra coordinate is considered as noncompact. The expansion is produced by the inflaton field, which is considered as nonminimally coupled to gravity. We conclude from experiental data that the coupling of the inflaton with gravity should be weak, but variable in different epochs of the evolution of the universe.
The standard model of elementary particle physics and the theory of general relativity can be extended by the introduction of a vacuum variable which is responsible for the near vanishing of the present cosmological constant (vacuum energy density). The explicit realization of this vacuum variable can be via a three-form gauge field, an aether-type velocity field, or any other field appropriate for the description of the equilibrium state corresponding to the Lorentz-invariant quantum vacuum. The extended theory has, without fine-tuning, a Minkowski-type solution of the field equations with sp…
Decaying vacuum cosmological models evolving smoothly between two extreme (very early and late time) de Sitter phases are capable to solve or at least to alleviate some cosmological puzzles, among them: (i) the singularity, (ii) horizon, (iii) graceful-exit from inflation, and (iv) the baryogenesis problem. Our basic aim here is to discuss how the coincidence problem based on a large class of running vacuum cosmologies evolving from de Sitter to de Sitter can also be mollified. It is also argued that even the cosmological constant problem become less severe provided that the characteristic sca…
We proposes an alternative model of duality symmetry, based on the previously obtained divergence theory, including an scalar field, an internal vector and a metric signature. At some small scale an effective scalar field equation has appeared whose potential acts like a Higgs one, where the metric signature plays the role of an order parameter. Non-vanishing Vacuum condensation of this Higgs field occurs once a signature change from Euclidean to Lorentzian is formed. The mass scale of Higgs field excitations around this vacuum may contribute, in the Lorentzian sector, to the cosmological cons…
We construct a vacuum of string theory in which the magnitude of the vacuum energy is $< 10^{-123}$ in Planck units. Regrettably, the sign of the vacuum energy is negative, and some supersymmetry remains unbroken.
In this Letter we show that the vacuum polarization of quantum fields in an anti-de Sitter space naturally gives rise to a small but nonzero cosmological constant in a brane world living in it. To explain the extremely small ratio of mass density in the cosmological constant to the Planck mass density in our universe (\approx 10^{-123}) as suggested by cosmological observations, all we need is a four-dimensional brane world (our universe) living in a five-dimensional anti-de Sitter space with a curvature radius r_0 \sim 10^{-3}cm and a fundamental Planck energy M_P \sim 10^9 GeV, and a scalar …
We propose a simple model that provides a dynamical cancellation mechanism of the vacuum energy density appearing either in the form of a bare cosmological constant, quantum fluctuations of matter fields or the result of phase transitions. This `conformal compensator model' is based on a conformal coupling $A(φ)$ between the Einstein and the Jordan frames. This couples a second scalar field $λ$ to the trace of the matter energy-momentum tensor, including the bare cosmological constant, and serves as a dynamical Lagrange multiplier. As a result, the scalar $λ$ relaxes to a value which cancels t…
We have found for the general class of Modified Gravity Models f(R,G) a new instability which can arise in vacuum for the scalar modes of the cosmological perturbations if the background is not de Sitter. In particular, the short-wavelength modes, if stable, in general have a group velocity which depends linearly in k, the wave number. Therefore these modes will be in general superluminal. We have also discussed the condition for which in general these scalar modes will be ghost-like. There is a subclass of these models, defined out of properties of the function f(R,G) and to which the f(R) an…
General non-singular accelerating cosmological solutions for an initial cosmic period of pure vacuum birth era are derived. This vacuum era is described by a varying cosmological "constant" suggested by the Renormalisation Group flow of Asymptotic Safety scenario near the ultraviolet fixed point. In this scenario, natural exit from inflation to the standard decelerating cosmology occurs when the energy scale lowers and the cosmological "constant" becomes insignificant. In the following period where matter is also present, cosmological solutions with characteristics similar to the vacuum case a…
We examine the Weyl double copy relation for vacuum solutions of the Einstein equations with a cosmological constant using the approach we previously described, in which the spin-1/2 massless free-field spinors (Dirac-Weyl fields) are regarded as basic units. Based on the exact non-twisting vacuum type N and vacuum type D solutions, the finding explicitly shows that the single and zeroth copies fulfill conformally invariant field equations in conformally flat spacetime. In addition, irrespective of the presence of a cosmological constant, we demonstrate that the zeroth copy connects Dirac-Weyl…
We study the quantum vacuum zero point energy in the Schwarzschild black hole as well as in the Nariai limit of the dS-Schwarzschild backgrounds. We show that the regularized vacuum energy density near the black hole and also in the Nariai setup match exactly with the corresponding value in the flat background, scaling with the fourth power of the mass of the quantum field. The horizon radius of the dS space created from the vacuum zero point energy introduces a new length scale which should be compared with the black hole horizon radius. There is an upper limiting mass for the black hole imme…
We outline the class of globally regular spherically symmetric solutions to the minimally coupled GR equations asymptotically de Sitter in the origin and asymptotically Schwarzschild at infinity. A source term connects smoothly de Sitter vacuum at the regular center with Minkowski vacuum at infinity and corresponds to anisotropic spherically symmetric vacuum defined macroscopically by the algebraic structure of its stress-energy tensor invariant under boosts in the radial direction. De Sitter-Schwarzschild geometry describes a vacuum nonsingular black hole which evolves, in the course of Hawki…
We construct a large class of new singularity-free static Lorentzian four-dimensional solutions of the vacuum Einstein equations with a negative cosmological constant. The new families of metrics contain space-times with, or without, black hole regions. Two uniqueness results are also established.
In the framework of the induced matter theory of gravity, we derive a 5D solution of the field equations that can describe a 4D cosmological scenario where the dark fluid (dark matter plus dark energy) equation of state has a geometrical origin. There is no a natural separation of the dark sector into different components, and then the model may provide a geometrical explanation for the existence of a dark degeneracy in cosmological scenarios.
Vacuum energy changes during cosmological phase transitions and becomes relatively important at epochs just before phase transitions. For a viable cosmology the vacuum energy just after a phase transition must be set by the critical temperature of the next phase transition, which exposes the cosmological constant problem from a different angle. Here we propose to experimentally test the properties of vacuum energy under circumstances different from our current vacuum. One promising avenue is to consider the effect of high density phases of QCD in neutron stars. Such phases have different vacuu…
We consider a single field governed expansion of the universe from a five dimensional (5D) vacuum state. Under an appropiate change of variables the universe can be viewed in a effective manner as expanding in 4D with an effective equation of state which describes different epochs of its evolution. In the example here worked the universe fistly describes an inflationary phase, followed by a decelerated expansion. Thereafter, the universe is accelerated and describes a quintessential expansion to finally, in the future, be vacuum dominated.
We study the dynamics of the FLRW flat cosmological models in which the vacuum energy density varies with time, $Λ(t)$. In particular, we investigate the dynamical properties of a generalized vacuum model and we find that under certain conditions the vacuum term in the radiation era varies as $Λ(z) \propto (1+z)^{4}$, while in the matter era we have $Λ(z)\propto (1+z)^{3}$ up to $z\simeq 3$ and $Λ(z)=Λ$ for $z\le 3$. The confirmation of such a behavior would be of paramount importance because it could provide a solution to the cosmic coincidence problem as well as to the fine tuning problem, w…
Were studied classical oscillations of background metric in the multidimensional anisotropic model of Kazner in the de-Sitter stage. Obtained dependence of fluctuations on dimension of space-time with infinite expansion. Stability of the model could be achieved when number of space-like dimensions equals or more then four. Were calculated contributions to the density of "vacuum energy", that are providing by proper oscillations of background metric and compared with contribution of cosmological arising of particles due to expansion. As it turned out, contribution of gravitational oscillation o…
We consider the cosmology of a 3-brane universe, in a five dimensional space time (Bulk). We present some solutions to the five-dimensional Einstein equation, where a perfect fluid is confined to the 3-brane. We investigate the evolution of brane for two models of bulk. We choose ordinary and modified polytropic gas in brane and it is seen that these models have some new features.
In BP models with hundreds of fluxes, we compute the effects of cosmological dynamics on the probability distribution of landscape vacua. Starting from generic initial conditions, we find that most fluxes are dynamically driven into a different and much narrower range of values than expected from landscape statistics alone. Hence, cosmological evolution will access only a tiny fraction of the vacua with small cosmological constant. This leads to a host of sharp predictions. Unlike other approaches to eternal inflation, the holographic measure employed here does not lead to "staggering", an exc…
In the first part of this paper we consider expanding vacuum cosmological spacetimes with a free $T^N$-action. Among them, we give evidence that Gowdy spacetimes have AVTD (asymptotically velocity term dominated) behavior for their initial geometry, in any dimension. We then give sufficient conditions to reach a similar conclusion about a $T^2$-invariant four dimensional nonGowdy spacetime. In the second part of the paper we consider vacuum cosmological spacetimes with crushing singularities. We introduce a monotonic quantity to characterize Kasner spacetimes. Assuming scale-invariant curvatur…
