In the Friedmann equations, an infinite initial density is avoided only when the universe is spatially flat. With such equations being then valid when the scale factor $a=0$, the universe must also be in the state of vacuum when $a$ is infinitesimal. Matter creation thus largely comes from nonlinearity of the Friedmann equations at a finite $a$. For the correspondence between such an initial vacuum and the Friedmann universe, therefore, a vacuum density is suggested to be gravitationally significant only when it could also have a matter phase. The effective cosmological term is then due to such a vacuum density.
Calculations are presented for creation of massive and massless scalar particles coupled to Gauss-Bonnet type curvature in Friedmann cosmological models. It has been shown that, for fields of mass~$m$, the effect of the coupling parameter $ζ$ with the Gauss-Bonnet invariant is insignificant if $ζm^2 \ll 1 $. In all cases under consideration, the created particle number is compatible by order of magnitude with the number of causally disconnected space-time regions by the Compton time, corresponding $1/m$ or $\sqrtζ$.
The problem of particle creation in cosmology concerning whether the results for the number of particles are infinite or finite is discussed for scalar and spinor particles in Friedmann expanding Universe. It is shown that the results are always finite if one puts in case of scalar particles creation a special term into the Lagrangian. Numerical estimates of the effect of particle creation are made. The role of creation of superheavy particles in cosmology is discussed.
The theoretical and observational consequences of thermodynamics of open systems, which allow particle creation are investigated in modified $f(R,T)$ ($R$ is the Ricci scalar and $T$ is the trace of energy-momentum tensor) theory of gravity within the framework of a flat Friedmann-Robertson-Walker line element. A simplest particular model $f(R,T)=R+2f(T)$ and "gamma-law" equation of state $p=(γ-1)ρ$ are assumed to explore the role of particle creation in the early and late time evolution of the universe. A power-law expansion model is proposed for $0\leq γ\leq 2$ by considering the natural phe…
By way of a complete integration of the Friedmann equations, in terms of observables, it is shown that for the cosmological constant $Λ> 0$ there exist non-flat FLRW models for which the total density parameter $Ω$ remains $\sim 1$ throughout the entire history of the universe. Further, it is shown that in a precise quantitative sense these models are not finely tuned. When observations are brought to bear on the theory, and in particular the WMAP observations, they confirm that we live in just such a universe. The conclusion holds when the classical notion of $Λ$ is extended to dark energy.
I briefly review the flatness problem within the context of classical cosmology and examine some of the debate in the literature with regard to its definition and even the question whether it exists. I then present some new calculations for cosmological models which will collapse in the future; together with previous work by others for models which will expand forever, this allows one to examine the flatness problem quantitatively for all cosmological models. This leads to the conclusion that the flatness problem does not exist, not only for the cosmological models corresponding to the current…
Results on particle creation from vacuum by the gravitational field of the expanding Friedmann Universe are presented. Finite results for the density of particles and the energy density for created particles are given for different exact solutions for different regimes of the expansion of the Universe. The results are obtained as for conformal as for nonconformal particles. The hypothesis of the origination of visible matter from the decay of created from vacuum superheavy particles identified with the dark matter is discussed.
Beyond Comoving Volume: Horizon Flux and Matter Creation in Modified Cosmology from the Unified First Law of Thermodynamics
Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso（瓦尔帕莱索大学科学与物理天文学研究所）；Facultad de Física, Universidad Veracruzana（韦拉克鲁斯大学物理学院）；Instituto de Física, Pontificia Universidad Católica de Valparaíso（瓦尔帕莱索天主教大学物理研究所）
We recently formulated a model of the universe based on an underlying W3-symmetry. It allows the creation of the universe from nothing and the creation of baby universes and wormholes for spacetimes of dimension 2, 3, 4, 6 and 10. Here we show that the classical large time and large space limit of these universes is one of exponential fast expansion without the need of a cosmological constant. Under a number of simplifying assumptions our model predicts that w=-1.2 in the case of four-dimensional spacetime. The possibility of obtaining a w-value less than -1 is linked to the ability of our mod…
The evolution of the closed Friedmann Universe with a packet of short scalar waves is considered with the help of the Wheeler--DeWitt equation. The packet ensures conservation of homogeneity and isotropy of the metric on average. It is shown that during tunneling the amplitudes of short waves of a scalar field can increase catastrophically promptly if their influence to the metric do not take into account. This effect is similar to the Rubakov-effect of catastrophic particle creation calculated already in 1984. In our approach to the problem it is possible to consider self-consistent dynamics …
We analyse a generalisation of general relativity that incorporates a cosmic time-variation of the velocity of light in vacuum, $c,$ and the Newtonian gravitation 'constant', $G,$ proposed by Albrecht and Maguejo. We find exact solutions for Friedmann universes and determine the rate of variation of $c$ required to solve the flatness and classical cosmological constant problems. Potential problems with this approach to the resolution of the flatness and classical cosmological constant problems are highlighted. Reformulations are suggested which give the theory a more desirable limit as a theor…
New results on finite density of particle creation for nonconformal massive scalar particles in Friedmann Universe as well as new counterterms in dimensions higher than 5 are presented. Possible role of creation of superheavy particles for explaining observable density of visible and dark matter is discussed.
Different models of the role of creation of superheavy particles in the early Friedmann Universe with their subsequent decay on light particles are investigated. The observable numbers of baryon and entropy are predicted. The possible role of superheavy particles in creation of cold dark matter is discussed.
We discuss the dynamical effects of bulk viscosity and particle creation on the early evolution of the Friedmann -Robertson -Walker model in the framework of open thermodynamical systems. We consider bulk viscosity and Particle creation as separate irreversible processes. Exact solutions of the Einstein field equations are obtained by using the "gamma-law" equation of state $p=(γ-1)ρ$, where the adiabatic parameter $γ$ varies with scale factor of the metric. We consider the cosmological model to study the early phases of the evolution of the universe as it goes from an inflationary phase to a …
The paper deals with non-equilibrium thermodynamics based on adiabatic particle creation mechanism with the motivation of considering it as an alternative choice to explain the recent observed accelerating phase of the universe. Using Friedmann equations, it is shown that the deceleration parameter ($q$) can be obtained from the knowledge of the particle production rate ($Γ$). Motivated from thermodynamical point of view, cosmological solutions are evaluated for the particle creation rates in three cosmic phases, namely, inflation, matter dominated and present late time acceleration. The decel…
In this work we present the cosmological dynamics of interacting dark energy models in the framework of particle creation mechanism. The particle creation mechanism presented here describes the true non equilibrium thermodynamics of the universe. In spatially flat Friedmann-Lema{\^ı}tre-Robertson-Walker universe considered here, the dissipative bulk viscous pressure is due to the non conservation of particle number. For simplicity, we assume that the creation of perfect fluid particles to be isentropic (adiabatic) and consequently the viscous pressure obeys a linear relationship with particle …
We present a dynamical analysis of the (classical) spatially flat and negative curved Friedmann-Lameitre-Robertson-Walker (FLRW) universes evolving, (by assumption) close to the thermodynamic equilibrium, in presence of a particles creation process, described by means of a realiable phenomenological approach, based on the application to the comoving volume (i. e. spatial volume of unit comoving coordinates) of the theory for open thermodynamic systems. In particular we show how, since the particles creation phenomenon induces a negative pressure term, then the choice of a well-grounded ansatz …
Whereas the nature of dark components in the Universe remains unknown, alternative models of gravity have been developed to offer a geometric explanation to the origin of such components. In this work we use the Minimal Geometric Deformation approach to study extensions of the theory of General Relativity in a cosmological context. This is possible since such approach allows the decoupling of gravitational sources, and the Einstein field equations can be analytically solved with the presence of a new gravitational sector once a known GR solution is considered. In particular, we implement such …
A scenario for gravitational particle creation in a stiff matter dominated flat Friedmann-Robertson-Walker universe is presented. The primary creation of scalar particles is calculated using quantum field theory in curved spacetime and it is found to be strongly dependent on the scalar mass and the expansion parameter of the universe. The particle creation is most effective for a very massive scalar field and large expansion parameter. We apply the results to cosmology and calculate an upper bound for the equilibrium temperature of the secondary particles created by the scalar field decay.
In this paper, we have studied flat Friedmann--Lema\^ıtre--Robertson--Walker (FLRW) model with modified Chaplygin gas (MCG) having equation of state $p_{m}=Aρ-% \frac{B}{ρ^{γ}}$, where $0\leq A\leq 1$, $0\leq γ\leq 1$ and $B$ is any positive constant in }${\footnotesize f(R,T)}${\footnotesize \ gravity with particle creation. We have considered a simple parametrization of the Hubble parameter $H$ in order to solve the field equations and discussed the time evolution of different cosmological parameters for some obtained models showing unique behavior of scale factor. We have also discussed the…
In extended models of gravity a nonminimal coupling to matter has been assumed to lead to irreversible particle creation. In this paper we challenge this assumption. We argue that a non-minimal coupling of the matter and gravitational sectors results in a change in particle-momentum on a cosmological timescale, irrespective of particle creation or decay. We further argue that particle creation or decay associated with a non-minimal coupling to gravity could only happen as a result of significant deviations from a homogeneous Friedmann-Robertson-Walker geometry on microscopic scales, and provid…
The Einstein field equations for any spherically symmetric metric and a geodesic perfect fluid source are cast in a canonical simple form, both for Lorentzian metrics and for instantons. Both kinds of metrics are explicitly written for the Lema{\^ı}tre-Tolman-Bondi family and for a general $Λ$-Friedmann-Lema{\^ı}tre-Robertson-Walker universe. In the latter case (including of course the instanton version) we study whether the probability of quantum creation of our Universe vanishes or not. It is found, in accordance with previous results, that only the closed model can have a nonzero probabilit…
We consider a model in which the universe is the direct product of a (3+1)-dimensional Friedmann, Robertson-Walker (FRW) space and a compact hyperbolic manifold (CHM). Standard Model fields are confined to a point in the CHM (i.e. to a brane). In such a space, the decay of massive Kaluza-Klein modes leads to the injection of any initial bulk entropy into the observable (FRW) universe. Both Kolmogoro-Sinai mixing due to the non-integrability of flows on CHMs and the large statistical averaging inherent in the collapse of the initial entropy onto the brane smooth out any initial inhomogeneities …
We test the Yilmaz theory of gravitation by working out the corresponding Friedmann-type equations generated by assuming the Friedmann-Robertson-Walker cosmological metrics. In the case that space is flat the theory is consistent only with either a completely empty universe or a negative energy vacuum that decays to produce a constant density of matter. In both cases the total energy remains zero at all times, and in the latter case the acceleration of the expansion is always negative. To obtain a more flexible and potentially more realistic cosmology, the equation of state relating the pressu…
Geometry of the universe has always intrigued mathematicians and cosmologists. Recent results from the Wilkinson Microwave Anisotropy Project (WMAP) indicate that the visible universe is incredibly flat. This apparent flatness could be due to the fact that only a small part of the universe is visible, thus indicating that the geometry of the universe is still an open and an interesting problem in cosmology. Assuming a profound connection between Friedmann, Robertson, Walker (FRW) geometries and thermodynamics, we construct a parameter free exploratory model that allows us to predict the geomet…
We consider an isotropic and homogeneous universe in loop quantum cosmology. We assume that the matter content of the universe is dominated by dust matter in early time and a phantom matter at late time which constitutes the dark energy component. The quantum gravity modifications to the Friedmann equation in this model indicate that the classical big bang singularity and the future big rip singularity are resolved and are replaced by quantum bounce. It turns out that the big bounce and recollapse in the herein model contribute to a cyclic scenario for the universe. We then study the quantum t…
The recently proposed states of low energy provide a well-motivated class of reference states for the quantized linear scalar field on cosmological Friedmann-Robertson-Walker spacetimes. The low energy property of a state is localized close to some value of the cosmological time coordinate. We present calculations of the relative cosmological particle production between a state of low energy at early time and another such state at later time. In an exponentially expanding Universe, we find that the particle production shows oscillations in the spatial frequency modes. The basis of the method f…
We deduce general expressions for the line element of universe models with negative and vanishing spatial curvature described by conformally flat spacetime coordinates. The empty Milne universe model and models with dust, radiation and vacuum energy are exhibited. Discussing the existence of particle horizons we show that there is continual creation of space, matter and energy when conformal time is used in Friedmann-Robertson-Walker models with negative spatial curvature.
The description of spacetime is an fundamental problem of cosmology. We explain why the current assignments of spacetime geometries for $Ω_k$ of the Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) model are probably incorrect and suggest more useful descriptions. We show that $Ω_k$ represents not only curvature but the influence of matter density on the extent of spacetime between massive objects. Recent analyses of supernovae type Ia (SNe Ia) and HII/GEHR data with the FLRW model present the best fits with a small value for $Ω_m$ and a large $Ω_k$. These results are consistent with our Universe …
The classical Friedmann-Lemaître equations are solved using a corrected version of Planck's radiation law. The function curves of the scale parameter a(t) and the variations with temperature a(T) and t(T) are given. It is shown that a reasonable cosmological evolution is only possible in case of flat spatial slices (k=0). The initial singularity is avoided. Horizon and flatness problems do not exist. For low temperatures compared with the Planck Temperatur, the equations yield the usual course of expansion of the standard FLRW model for a radiation universe with k=0 and p=u(T)/3.
Particle Creation and Variable Generalized Chaplygin Gas in $\mathcal{F}(\mathcal{R},Σ,\mathcal{T})$ Gravity
L. N. Gumilyov Eurasian National University（列夫·尼基塔维奇·古米廖夫欧亚国立大学）；S.P.M. Science and Gilani Arts, Commerce College（S.P.M.科学与吉拉尼文理学院）；Pacif Institute of Cosmology and Selfology (PICS)（太平洋宇宙学与自我学研究所（PICS））；Centre for Cosmology, Astrophysics and Space Science (CCASS), GLA University（GLA大学宇宙学、天体物理学与空间科学中心）；Jubail Industrial College（朱拜勒工业学院）
The matter creation model of Prigogine--Géhéniau--Gunzig--Nardone is revisited in terms of a redefined creation pressure which does not lead to irreversible adiabatic evolution at constant specific entropy. With the resulting freedom to choose a particular gas process, a flat FRWL cosmological model is proposed based on three input characteristics: (i) a perfect fluid comprising of an ideal gas, (ii) a quasi-adiabatic polytropic process, and (iii) a particular rate of particle creation. Such model leads to the description of the late-time acceleration of the expanding Universe with a natural t…
Dynamical systems analysis of a cosmological model with interacting Umami Chaplygin fluid in adiabatic particle creation mechanism: Some bouncing features
Creation of Cold Dark Matter (CCDM), in the context of Einstein Field Equations, leads to a negative creation pressure, which can be used to explain the accelerated expansion of the Universe. Recently, it has been shown that the dynamics of expansion of such models can not be distinguished from the concordance $Λ$CDM model, even at higher orders in the evolution of density perturbations, leading at the so called "dark degeneracy". However, depending on the form of the CDM creation rate, the inclusion of spatial curvature leads to a different behavior of CCDM when compared to $Λ$CDM, even at ba…
We deduce general expressions for the line element of universe models with positive spatial curvature described by conformally flat spacetime coordinates. Models with dust, radiation and vacuum energy are exhibited. Discussing the existence of particle horizons we show that there is continual annihilation of space, matter and energy in a dust and radiation dominated universe, and continual creation in a LIVE domined universe when conformal time is used in Friedmann-Robertson-Walker models with positive spatial curvature. A general procedure is given for finding coordinates to be used in Penros…
We study the quantum vacuum fluctuations around closed Friedmann-Robertson-Walker (FRW) radiation-filled universes with nonvanishing cosmological constant. These vacuum fluctuations are represented by a conformally coupled massive scalar field and are treated in the lowest order of perturbation theory. In the semiclassical approximation, the perturbations are governed by differential equations which, properly linearized, become generalized Lame equations. The wave function thus obtained must satisfy appropriate regularity conditions which ensure its finiteness for every field configuration. We…
We consider the quantum Friedmann equations which include one-loop vacuum fluctuations due to gravitons and scalar field matter in a FLRW background with constant $ε=-{\dot{H}}/{H^2}$. After several field redefinitions, to remove the mixing between the gravitational and matter degrees of freedom, we can construct the one loop correction to the Friedmann equations. Due to cosmological particle creation, the propagators needed in such a calculation are typically infrared divergent. In this paper we construct the graviton and matter propagators, making use of the recent construction of the infrar…
Cosmological adiabatic particle creation results in the generation of irreversible entropy. The evolution of this entropy is examined in a flat Friedmann--Robertson--Walker universe at late times, using a dissipative model with a power-law term (proportional to the power of the Hubble parameter $H$). In a dissipative universe, the irreversible entropy included in the Hubble volume is found to be proportional to $H^{-1}$, unlike for the case of the Bekenstein--Hawking entropy on the horizon of the universe. In addition, the evolution of the horizon entropy is examined, extending the previous an…
In a flat Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) geometry, we consider the expansion of the universe powered by the gravitationally induced `adiabatic' matter creation. To demonstrate how matter creation works well with the expanding universe, we have considered a general creation rate and analyzed this rate in the framework of dynamical analysis. The dynamical analysis hints the presence of a non-singular universe (without the big bang singularity) with two successive accelerated phases, one at the very early phase of the universe (i.e. inflation), and the other one describes the curren…
Bianchi I cosmological models consisting of a fluid with both bulk and shear viscosity are studied. It is shown how the dynamical importance of the shear and the fluid density change in the course of evolution. Exact solutions with an equation of state $ρ= p$ for a stiff fluid are also obtained in several special cases, assuming the viscosity coefficients to be the power functions of the density. The results are, in some relevant cases, compared with those of Belinskii and Khalatnikov (1976) in the asymptotic limits and are seen to agree with them in that the models start with $ρ= 0$ at the be…
Cosmology with a Non-minimally Coupled Dark Matter Fluid I. Background Evolution
SISSA（斯issa国际高等研究学院）；Institute for Fundamental Physics of the Universe (IFPU)（宇宙基础物理研究所）；INFN-Sezione di Trieste（的里雅斯特意大利国家核物理研究所）；IRA-INAF（博洛尼亚天文台研究所）
We construct a model for the universe based on the existence of quantum fields at finite temperature in the background of Robertson-Walker spacetime in presence of a non-zero cosmological constant. We discuss the vacuum regime in the light of the results obtained through previous studies of the back-reaction of massless quantum fields in the static Einstein universe, and we argue that an adiabatic vacuum state and thermal equilibrium is achieved throughout this regime. Results shows that such a model can explain many features of the early universe as well as the present universe. The model is …
In this paper it is studied the cosmology of a homogeneous and isotropic spacetime endorsed with a conformally coupled massless scalar field. We find six different solutions of the Friedmann equation that represent six different types of universes, all of them are periodically distributed along the complex time axis. From a classical point of view, they are then isolated, separated by Euclidean regions that represent quantum mechanical barriers. Quantum mechanically, however, there is a non-zero probability for the state of the universes to tunnel out through a Euclidean instanton and suffer a…
We investigate the interaction between dark energy and dark matter in the framework of irreversible thermodynamics of open systems with matter creation/annihilation. We consider dark energy and dark matter as an interacting two component (scalar field and "ordinary" dark matter) cosmological fluid in a homogeneous spatially flat and isotropic Friedmann-Robertson-Walker (FRW) Universe. The thermodynamics of open systems as applied together with the gravitational field equations to the two component cosmological fluid leads to a generalization of the elementary dark energy-dark mater interaction…
In this paper it is presented the model of a multiverse made up of universes which are created in entangled pairs that conserve the total momentum conjugated to the scale factor. For the background spacetime it is assumed a FRW metric with a scalar field with mass $m$ minimally coupled to gravity. For the fields that propagate in the entangled spacetimes it is considered the perturbations of the spacetime and the scalar field, whose quantum states become entangled too. They turn out to be in a quasi thermal state and the corresponding thermodynamical magnitudes are computed. Three observables …
We present the solution of the problem of the $1/\Box, \Box \to 0,$ asymptotic terms discovered in the one-loop form factors of the gravitational effective action. Owing to certain constraints among their coefficients, which we establish, these terms cancel in the vacuum stress tensor and do not violate the asymptotic flatness of the expectation value of the metric. They reappear, however, in the Riemann tensor of this metric and stand for a new effect: a radiation of gravitational waves induced by the vacuum stress. This coherent radiation caused by the backreaction adds to the noncoherent ra…
We investigate the interaction between scalar fields and radiation in the framework of warm inflationary models by using the irreversible thermodynamics of open systems with matter creation/annihilation. We consider the scalar fields and radiation as an interacting two component cosmological fluid in a homogeneous, spatially flat and isotropic Friedmann-Robertson-Walker (FRW) Universe. The thermodynamics of open systems as applied together with the gravitational field equations to the two component cosmological fluid leads to a generalization of the elementary scalar field-radiation interactio…
We investigate the creation of dark matter particles as a result of the decay of the scalar field in the framework of warm inflationary models, by using the irreversible thermodynamics of open systems with matter creation/annihilation. We consider the scalar fields, radiation and dark matter as an interacting three component cosmological fluid in a homogeneous and isotropic Friedmann-Lemaitre-Robertson-Walker (FLRW) Universe, in the presence of the curvature terms. The thermodynamics of open systems as applied together with the gravitational field equations to the three component cosmological …
