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We show that short-range interactions between the fundamental particles in the universe can drive a period of accelerated expansion. This description fits the early universe. In the present day universe, if one postulates short-range interactions or a sort of "shielded gravity", the picture may repeat., 4 pages, 1 figure. Discussion expanded. Some references added. Matches version in print. To appear in Phys. Lett. A

We show how the Szekeres form of the line element is naturally adapted to study Penrose limits in classical string backgrounds. Relating the "old" colliding wave problem to the Penrose limiting procedure as employed in string theory we discuss how two orthogonal Penrose limits uniquely determine the underlying target space when certain symmetry is imposed. We construct a conformally deformed background with two distinct, yet exactly solvable in terms of the string theory on R-R backgrounds, Penrose limits. Exploiting further the similarities between the two problems we find that the Penrose limit of the gauged WZW Nappi-Witten universe is itself a gauged WZW plane wave solution of Sfetsos and Tseytlin. Finally, we discuss some issues related to singularity, show the existence of a large class of non-Hausdorff solutions with Killing Cauchy Horizons and indicate a possible resolution of the problem of the definition of quantum vacuum in string theory on these time-dependent backgrounds., Some misprints corrected. Matches the version in print. To appear in Classical & Quantum Gravity

We consider cosmological solutions of string and M-theory compactified to four dimensions by giving a general prescription to construct four-dimensional modular cosmologies with two commuting Killing vectors from vacuum solutions. By lifting these solutions to higher dimensions we analyze the existence of cosmological singularities and find that, in the case of non-closed Friedmann-Robertson-Walker universes, singularities can be removed from the higher-dimensional model when only one of the extra dimensions is time-varying. By studying the moduli space of compactifications of M-theory resulting in homogeneous cosmologies in four dimensions we show that U-duality transformations map singular cosmologies into non-singular ones., 21 pages, harvmac. No figures. Minor changes. Typos corrected, a footnote added in Sec. 3 and two words added to the title. Final version to appear in Nuclear Physics B

We discuss the entropy generation in quantum tunneling of a relativistic particle under the influence of a time-varying force with the help of squeezing formalism. It is shown that if one associates classical coarse grained entropy to the phase space volume, there is an inevitable entropy growth due to the changes in position and momentum variances. The entropy change can be quantified by a simple expression ΔS=ln cosh 2r, where r, is the squeeze parameter measuring the “height” and “width” of the potential barrier. We suggest that the universe could have acquired its initial entropy in a quantum squeeze from “nothing” and briefly discuss the implications of our proposal.

We present a solution to the vacuum Einstein Equations which represents a collapse of a gravitational wave in 5 dimensions. Depending on the focal length of the wave, the collapse results, either in a black string covered by a horizon, or in a naked singularity which can be removed., Comment: Minor changes. A reference added. Matches the print version. To appear in Phys. Lett. A

We look at the possibility of superinflationary behavior in a class of anisotropic Type-IIB superstring cosmologies in the context of Pre-big Bang scenario and find that there exists a rather narrow range of parameters for which these models inflate. We then show that, although in general this behavior is left untouched by the introduction of a Ramond-Ramond axion field through a SL(2,R) rotation, there exists a particular class of axions for which inflation disappears completely. Asymptotic past initial conditions are briefly discussed, and some speculations on the possible extension of Pre-big Bang ideas to gravitational collapse are presented., harvmac, epsf. 3 figures included

Pineal gland cysts are typically asymptomatic, benign cysts most commonly found incidentally in adults. In rare cases, a large pineal gland cyst can be complicated by intracystic hemorrhage, which could then manifest with neurological symptoms. We report a new complication of intracystic hemorrhage in a large pineal gland cyst in a 40-year-old man with new onset seizures.

Objective: To report a case of spontaneous tumor lysis syndrome (STLS) of a solid tumor in a patient who had undiagnosed metastatic hepatocellular carcinoma. Clinical Presentation and Intervention: A 70-year-old man with a medical history of alcohol abuse, withdrawal seizure and hypertension presented to the emergency department after being found unresponsive by his landlord. The patient had a bulky mass in the liver, classic laboratory abnormalities, oliguric renal failure and elevated alpha fetoprotein. He had never been treated with cytotoxic therapy. He was treated aggressively with fluid resuscitation and sodium bicarbonate, but he continued to be oliguric and the deterioration of his renal function also continued. Due to a minimal response to treatment and a poor prognosis, he was discharged to hospice for palliative care. Conclusion: This case showed that STLS should be in the differential diagnosis of a patient who has malignant disease and has developed classic laboratory abnormalities and renal failure even without previous cytotoxic therapy.

Some exact solutions for the Einstein field equations corresponding to inhomogeneous $G_2$ cosmologies with an exponential-potential scalar field which generalize solutions obtained previously are considered. Several particular cases are studied and the properties related to generalized inflation and asymptotic behaviour of the models are discussed., Comment: 21 pages LaTeX, 3 figures appended as a uuencoded compressed tar file

A solution is presented which describes the collision of gravitational waves in a closed Friedmann--Robertson--Walker background. The matter field present is taken to be that of a stiff perfect fluid. It is shown that, unlike the situations in which the background is open or spatially flat, the solution in this case contains a future space-like curvature singularity.

We present a family of exact solutions to the Einstein field equations which describe inhomogeneous generalizations of spatially flat FRW cosmologies. The self-interacting scalar field with an exponential potential of the form V approximately ek phi serves as a source for the expansion. We show that these inhomogeneous models never inflate and briefly discuss the implications of our results on the genericity of inflation and isotropization at late times.

We obtain exact solutions for the Einstein equations with an exponential-potential scalar field (\(V=\Lambda e^{k\phi}\)) which represent simple inhomogeneous generalizations of Bianchi I cosmologies. Studying these equations numerically we find that in most of the cases there is a certain period of inflationary behaviour for \(k^22\) the solutions homogenize generically at late times. Yet, {\em none of the solutions} isotropize. For some particular values of the integration constants we find a multiple inflationary behaviour for which the deceleration and the inflationary phases interchange each other several times during the history of the model., Comment: 18 pages, Plain LaTeX, 2 Figures to be sent on request, to appear in Phys. Rev. D., report 2

We obtain a general exact solution of the Einstein field equations for the anisotropic Bianchi type I universes filled with an exponential-potential scalar field and study their dynamics. It is shown, in agreement with previous studies, that for a wide range of initial conditions the late-time behavior of the models is that of a power-law inflating Friedmann-Robertson-Walker (FRW) universe. This property does not hold, in contrast, when some degree of inhomogeneity is introduced, as discussed in our following paper.

The authors consider some anisotropic model universes filled with a homogeneous self-interacting scalar field with exponential potential (V approximately ealpha phi ). Specifically, by assuming power law behaviour for the scale factors they are able to find exact solutions of Einstein equations for homogeneous Bianchi III and VI cosmologies. They compare the behaviour of these models with that of flat and open FRW solutions with similar scalar fields and show that neither of the obtained anisotropic solutions inflate.

The authors construct an infinite-dimensional family of solutions to Einstein's field equations representing collisions between plane gravitational waves with variable polarization. The boundary conditions on the null hypersurfaces are studied and presented as a set of algebraic conditions for the constant coefficients of the Legendre functions. The behaviour of the gravitational field near the focusing two-surface is also analysed and some of the issues related to the strong cosmic censorship conjecture are briefly discussed.

In this paper we construct and briefly study the 5D time-dependent solutions of general relativity obtained via double analytic continuation of the black hole (Myers-Perry) and of the black ring solutions with a double (Pomeransky-Senkov) and a single rotation (Emparan-Reall). The new solutions take the form of a generalized Einstein-Rosen cosmology representing gravitational waves propagating in a closed universe. In this context the rotation parameters of the rings can be interpreted as the extra wave polarizations, while it is interesting to state that the waves obtained from Myers-Perry Black holes exhibit an extra boost-rotational symmetry in higher dimensions which signals their better behavior at null infinity. The analogue to the C-energy is analyzed., Comment: 18 pages, 4 figures. References added, introduction and conclusions are amended, some issues related to singularity structure and symmetries are discussed. Matches the print version to appear in JHEP

We study the possibility that a generalised real scalar field minimally coupled to gravity could explain both the galactic and the cosmological dark components of the universe. Within the framework of Einstein's Relativity we model static galactic halos by considering the most general action built from the scalar field and its first derivatives. Although the gravitational configuration is static, the scalar field may be either static, or homogeneous and linear in time. In the case of the static scalar field, the models we look at inevitably posses unphysical negative energies, and we are led to a sort of no-go result. In the case of the homogeneous scalar field, on the contrary, we find that compact objects with flat rotational curves and with the mass and the size of a typical galaxy can be successfully modeled and the Tully-Fisher relation recovered. We further show that the homogeneous scalar field deduced from the galactic halo spacetimes has an action compatible with the kinetic Unified Dark Matter models recently proposed by Scherrer. Therefore, such a homogeneous kinetic Unified Dark Matter not only may correctly mimic the galactic dynamics, but could also be used to model the present-day accelerated expansion in the universe., Minor cahanges, some misprints corrected. References added. Matches the print version. To appear in Phys. Rev. D

We study spatially flat isotropic universes driven by k-essence. It is shown that Friedmann and k-field equations may be analytically integrated for arbitrary k-field potentials during evolution with a constant baryotropic index. It follows that there is an infinite number of dynamically different k-theories with equivalent kinematics of the gravitational field. We show that there is a large "window" of stable solutions, and that the dust-like behaviour separates stable from unstable expansion. Restricting to the family of power law solutions, it is argued that the linear scalar field model, with constant function F, is isomorphic to a model with divergent speed of sound and this makes them less suitable for cosmological modeling than the non-linear k-field solutions we find in this paper., Comment: Revised version. A detailed discussion relating the power-law solutions with the linear K-essence field and inverse square potential, on one hand, and the models with divergent sound volocity, on the other, is added

Modeling the potential by an inverse square law in terms of the tachyon field ($V(T)=\beta T^{-2}$) we find exact solution for spatially flat isotropic universe.We show that for $\beta>2\sqrt{3}/3$ the model undergoes power-law inflation. A way to construct other exact solutions is specified and exemplified., Comment: References added. Matches the version in print. To appear in PRD

Recently we proposed a scenario in which pre-big-bang universes are created in the interaction region of two colliding plane waves. The waves interact gravitationally leading generically to gravitational collapse resulting in a singularity with Kasner-like structure. The analytic relations between the Kasner exponents and the initial data can be explicitly evaluated. The conditions for pre-big-bang inflation are discussed.

We construct a broad family of exact solutions to the five-dimensional Einstein equations coupled to a scalar field with an exponential potential. Embedding a three-brane in these bulk space-times in a particular way we obtain a class of self-tuned curved brane worlds in which the vacuum energy on the brane is gravitationally idle, the four-dimensional geometry being insensitive to the value of the brane tension. This self-tuning arises from cancellations, enforced by the junction conditions, between the scalar field potential, the brane vacuum energy and the matter on the brane. Finally, we study some physically relevant examples and their dynamics., Comment: v2: 10 pages, RevTeX4. Minor changes. Typos corrected and references added. New paragraph included in the conclusions discussing the role of the singularities in the self-tuning mechanism. Final version to appear in Physical Review D

We analyze the possible resolution of cosmological singularities in M-theory cosmology by mapping singular into regular solutions using U-duality transformations.

We propose a picture, within the pre-big-bang approach, in which the universe emerges from a bath of plane gravitational and dilatonic waves. The waves interact gravitationally breaking the exact plane symmetry and lead generically to gravitational collapse resulting in a singularity with the Kasner-like structure. The analytic relations between the Kasner exponents and the initial data are explicitly evaluated and it is shown that pre-big-bang inflation may occur within a dense set of initial data. Finally, we argue that plane waves carry zero gravitational entropy and thus are, from a thermodynamical point of view, good candidates for the universe to emerge from., Comment: 18 pages, LaTeX, epsfig. 3 figures included. Minor changes; paragraph added in the introduction, references added and typos corrected. Final version published in Classical and Quantum Gravity

We study inflation and entropy generation in a recently proposed pre-big-bang model universe produced in a collision of gravitational and dilaton waves. It is shown that enough inflation occurs provided the incoming waves are sufficiently weak. We also find that entropy in this model is dynamically generated as the result of the nonlinear interaction of the incoming waves, before the universe enters the phase of dilaton driven inflation. In particular, we give the scaling of the entropy produced in the collision in terms of the focusing lengths of the incoming waves., Comment: 10 pages, latex. 1 postscript figure included. Bound on the incoming focal lengths extended to strong-coupling terminated inflation. Section 3 expanded. Typos corrected and reference added. Final version to appear in Phys. Lett. B

Families of anisotropic and inhomogeneous string cosmologies containing non-trivial dilaton and axion fields are derived by applying the global symmetries of the string effective action to a generalized Einstein-Rosen metric. The models exhibit a two-dimensional group of Abelian isometries. In particular, two classes of exact solutions are found that represent inhomogeneous generalizations of the Bianchi type VI_h cosmology. The asymptotic behaviour of the solutions is investigated and further applications are briefly discussed., Comment: Minor extension of concluding section; 18 pages, to appear in Phys.Rev. D

Global symmetries of the string effective action are employed to generate tilted, homogeneous Bianchi type VI_h string cosmologies from a previously known stiff perfect fluid solution to Einstein gravity. The dilaton field is not constant on the surfaces of homogeneity. The future asymptotic state of the models is interpreted as a plane wave and is itself an exact solution to the string equations of motion to all orders in the inverse string tension. An inhomogeneous generalization of the Bianchi type III model is also found., Comment: 9 pages, Standard Latex Source. To appear in Physics Letters B Minor change: Authors now alphabetically listed

We look at the transition to the semiclassical behaviour and the decoherence process for the inhomogeneous perturbations in the inflationary universe. Two different decoherence mechanisms appear: one dynamical, accompanied with a negligible, if at all, entropy gain, and the other, effectively irreversible dephasing, due to a rapid variation in time of the off-diagonal density matrix elements in the post-inflationary epoch. We thus settle the discrepancies in the entropy content of perturbations evaluated by different authors., Comment: LaTeX2e with the epsf package

We present a general algorithm which permits to construct solutions in string cosmology for heterotic and type-IIB superstrings in four dimensions. Using a chain of transformations applied in sequence: conformal, T-duality and SL(2,R) rotations, along with the usual generating techniques associated to Geroch transformations in Einstein frame, we obtain solutions with all relevant low-energy remnants of the string theory. To exemplify our algorithm we present an inhomogeneous string cosmology with S^3 topology of spatial sections, discuss some properties of the solution and point out some subtleties involved in the concept of homogeneity and isotropy in string cosmology., Comment: 14 pages, TeX harvmac. Some misprints corrected and one reference added

Reduction of initial spatial curvature by gravi-solitons is studied in the framework of composite universe. These models have asymptotic regions with different homogeneity groups matched by the gravitational pulses. They admit only initial singularities and ultimately evolve into the Bianchi type I regime.

It is shown that the ambiguity which appears in the evolution of spacetimes with colliding waves arises from an incomplete physical treatment of the problem in which only the gravitational field is considered. For several representative cases, it is shown that the ambiguity disappears completely when the underlying physical equations for the matter content are taken into consideration. It is also argued that the transformation of colliding null dusts into other forms of matter is unrealisable.

Algorithms for generating new exact solutions of the Einstein-Klein-Gordon field equations, which describe inhomogeneous universes with S3 topology of spatial sections, are developed. The known exact vacuum and stiff-fluid solutions with S3 topology are used as an input. The methods developed are further applied to derive inhomogenous generalizations of Bianchi type IX solutions and inhomogeneous S3 Gowdy models with gravitational and scalar waves. It is shown that the new solutions, which are generalizations of the Bianchi type IX models, permit identification of the scalar field with the velocity potential of the stiff irrotational fluid. The latter result is further used to study the growth rate of density perturbations of the isotropic and anisotropic Bianchi type IX universes in a fully nonlinear relativistic regime. The role of anisotropy of the rate of growth of density perturbations is studied in detail.

Next exact solutions of the Einstein-massless scalar field equations are presented. The solutions describe inhomogeneous universes with S 3 topology. They include, as particular homogeneous cases, the familiar Taub universe and the Bianchi type IX models with a stiff irrotational fluid.

We discuss the most general solution describing the collision of plane gravitational waves with constant polarization. Among these solutions there is an infinite-dimensional family of metrics free of curvature singularities and analytically extendable across the ``focusing'' hypersurface. These regular solutions describe collisions between two incoming plane waves with different amplitudes for which u-v symmetry is broken. Boundary conditions on the null hypersurfaces u=0, v=0 are discussed and it is shown that any solution describing the scattering of plane gravitational waves with constant polarization has to include at least two solitary terms each of which stabilizes the behavior of the gravitational field on the null boundaries.

The results of two different approaches to the late-time behavior of primordial gravitational radiation are compared. Using asymptotic expansion in time of the tetrad components of the Riemann tensor one finds that initially chaotic behavior transfers itself into a radiative gravitational field of Petrov typeN ast→∞. On the other hand, to accomplish the physical picture, we study the high-frequency behavior of the field variables in the same formalism. We show that the background spacetime is of general Petrov type I, and then calculate the tetrad components of the stress-energy tensor induced by the “disappeared” radiation with the help of Newman-Penrose equations.

The authors present a new exact solution of the Einstein field equations which describes an inhomogeneous cosmological model with a perfect fluid source. The equation of state of the fluid is p= rho /3. Some of the main geometrical and kinematical properties of the solution are briefly discussed.

The asymptotic behavior in time of the Reimann tensor for a large class of inhomogeneous universes is discussed, and the cosmological analog of the familiar peeling-off theorem from general relativity is formulated.

We construct a solution to the Einstein field equations which describes the collision between a variably polarized gravitational wave and a shell of null dust. Depending on the choice of the parameters the variably polarized wave can have an arbitrarily smooth wavefront. The shell of null dust is followed by a constantly polarized plane gravitational wave.

The exact solution of the Einstein equations describing a pair of gravitational solitons superimposed on a spatially flat expanding background is considered. Using the high-frequency approximation we show that the soliton-''antisoliton'' pair carries an energy density which might be expressed as a simple function of intrinsic solitary parameters: epsilon = (m/sup 2//16..pi..t) ln(lambda/sup 2/+l/sup 2/)/lambda/sup 2/, where m is the solitary amplitude, lambda is the width of each soliton, and l denotes the initial spatial separation of the pair.