WE CONSIDER A FIVE-DIMENSIONAL EINSTEIN?CHERN?SIMONS ACTION WHICH IS COMPOSED OF A GRAVITATIONAL SECTOR AND A SECTOR OF MATTER WHERE THE GRAVITATIONAL SECTOR IS GIVEN BY A CHERN?SIMONS GRAVITY ACTION INSTEAD OF THE EINSTEIN?HILBERT ACTION AND WHERE THE MATTER SECTOR IS GIVEN BY THE SO-CALLED PERFECT FLUID. IT IS SHOWN THAT (I) THE EINSTEIN?CHERN?SIMONS (ECHS) FIELD EQUATIONS SUBJECT TO SUITABLE CONDITIONS CAN BE WRITTEN IN A SIMILAR WAY TO THE EINSTEIN?MAXWELL FIELD EQUATIONS; (II) THESE EQUATIONS HAVE SOLUTIONS THAT DESCRIBE AN ACCELERATED EXPANSION FOR THE THREE POSSIBLE COSMOLOGICAL MODELS OF THE UNIVERSE, NAMELY, SPHERICAL EXPANSION, FLAT EXPANSION, AND HYPERBOLIC EXPANSION WHEN ?, A PARAMETER OF THE THEORY, IS GREATER THAN ZERO. THIS RESULT ALLOWS US TO CONJECTURE THAT THESE SOLUTIONS ARE COMPATIBLE WITH THE ERA OF DARK ENERGY AND THAT THE ENERGY?MOMENTUM TENSOR FOR THE FIELD HA, A BOSONIC GAUGE FIELD FROM THE CHERN?SIMONS GRAVITY ACTION, CORRESPONDS TO A FORM OF POSITIVE COSMOLOGICAL CONSTANT. IT IS ALSO SHOWN THAT THE ECHS FIELD EQUATIONS HAVE SOLUTIONS COMPATIBLE WITH THE ERA OF MATTER: (I) IN THE CASE OF AN OPEN UNIVERSE, THE SOLUTIONS CORRESPOND TO AN ACCELERATED EXPANSION (?>0) WITH A MINIMUM SCALE FACTOR AT INITIAL TIME THAT, WHEN TIME GOES TO INFINITY, THE SCALE FACTOR BEHAVES AS A HYPERBOLIC SINE FUNCTION. (II) IN THE CASE OF A FLAT UNIVERSE, THE SOLUTIONS DESCRIBE AN ACCELERATED EXPANSION WHOSE SCALE FACTOR BEHAVES AS AN EXPONENTIAL FUNCTION OF TIME. (III) IN THE CASE OF A CLOSED UNIVERSE THERE IS FOUND ONLY ONE SOLUTION FOR A UNIVERSE IN EXPANSION, WHICH BEHAVES AS A HYPERBOLIC COSINE FUNCTION OF TIME.

WE PERFORM A BAYESIAN MODEL SELECTION ANALYSIS FOR INTERACTING SCENARIOS OF DARK MATTER AND MODIFIED HOLOGRAPHIC RICCI DARK ENERGY (MHRDE) WITH LINEAR INTERACTING TERMS. WE USE A COMBINATION OF SOME OF THE LATEST COSMOLOGICAL DATA SUCH AS TYPE IA SUPERNOVAE, COSMIC CHRONOMETERS, THE LOCAL VALUE OF THE HUBBLE CONSTANT, BARYON ACOUSTIC OSCILLATIONS MEASUREMENTS AND COSMIC MICROWAVE BACKGROUND THROUGH THE ANGULAR SCALE OF THE SOUND HORIZON AT LAST SCATTERING. WE FIND MODERATE/STRONG EVIDENCE AGAINST ALL THE MHRDE INTERACTING SCENARIOS STUDIED WITH RESPECT TO ?CDM WHEN THE FULL JOINT ANALYSIS IS CONSIDERED.

IN THIS PAPER WE STUDY FINITE TIME FUTURE SINGULARITIES IN ANISOTROPIC BIANCHI TYPE I MODELS. IT IS SHOWN THAT THERE EXIST FUTURE SINGULARITIES SIMILAR TO BIG RIP ONES (WHICH APPEAR IN THE FRAMEWORK OF PHANTOM FRIEDMANN-ROBERTSON-WALKER COSMOLOGIES). SPECIFICALLY, IN AN ELLIPSOIDAL ANISOTROPIC SCENARIO OR IN A FULLY ANISOTROPIC SCENARIO, THE THREE DIRECTIONAL AND AVERAGE SCALE FACTORS MAY DIVERGE AT A FINITE FUTURE TIME, TOGETHER WITH ENERGY DENSITIES AND ANISOTROPIC PRESSURES. WE CALL THESE SINGULARITIES ?ANISOTROPIC BIG RIP SINGULARITIES.? WE SHOW THAT THERE ALSO EXIST BIANCHI TYPE I MODELS FILLED WITH MATTER, WHERE ONE OR TWO DIRECTIONAL SCALE FACTORS MAY DIVERGE. ANOTHER TYPE OF FUTURE ANISOTROPIC SINGULARITIES IS SHOWN TO BE PRESENT IN VACUUM COSMOLOGIES, I.E., KASNER SPACETIMES. THESE SINGULARITIES ARE INDUCED BY THE SHEAR SCALAR, WHICH ALSO BLOWS UP AT A FINITE TIME. WE CALL SUCH A SINGULARITY ?VACUUM RIP.? IN THIS CASE ONE DIRECTIONAL SCALE FACTOR BLOWS UP, WHILE THE OTHER TWO AND AVERAGE SCALE FACTORS TEND TO ZERO.

IN THIS PAPER WE CONSIDER A SPATIALLY FLAT FRIEDMANN-ROBERTSON-WALKER (FRW) COSMOLOGICAL MODEL WITH COSMOLOGICAL CONSTANT, CONTAINING A STIFF FLUID AND A CLASSICAL DIRAC FIELD. THE PROPOSED COSMOLOGICAL SCENARIO DESCRIBES THE EVOLUTION OF EFFECTIVE DARK MATTER AND DARK ENERGY COMPONENTS REPRODUCING, WITH THE HELP OF THAT EFFECTIVE MULTIFLUID CONFIGURATION, THE QUINTESSENTIAL BEHAVIOR. WE FIND THE VALUE OF THE SCALE FACTOR WHERE THE EFFECTIVE DARK ENERGY COMPONENT CROSSES THE PHANTOM DIVIDE. THE MODEL WE INTRODUCE, WHICH CAN BE CONSIDERED AS A MODIFIED ?CDM ONE, IS CHARACTERIZED BY A SET OF PARAMETERS WHICH MAY BE CONSTRAINED BY THE ASTROPHYSICAL OBSERVATIONS AVAILABLE UP TO DATE.

WE CONSTRUCT A PIECEWISE MODEL THAT GIVES A PHYSICAL VIABLE REALIZATION OF FINITE-TIME FUTURE SINGULARITY FOR A SPATIALLY FLAT FRIEDMANN-ROBERTSON-WALKER UNIVERSE WITHIN THE INTERACTING DARK MATTER--DARK ENERGY FRAMEWORK, WITH THE LATTER ONE IN THE FORM OF A VARIABLE VACUUM ENERGY. THE SCALE FACTOR SOLUTIONS PROVIDED BY THE MODEL ARE ACCOMMODATED IN SEVERAL BRANCHES DEFINED IN FOUR REGIONS DELIMITED BY THE SCALE FACTOR AND THE EFFECTIVE ENERGY DENSITY. A BRANCH STARTS FROM A BIG BANG SINGULARITY AND DESCRIBES AN EXPANDING MATTER-DOMINATED UNIVERSE UNTIL THE SUDDEN FUTURE SINGULARITY OCCURS. THEN, AN EXPANDING BRANCH EMERGES FROM A PAST SINGULARITY, REACHES A MAXIMUM, REVERSES ITS EXPANSION AND POSSIBLY COLLAPSES INTO ITSELF WHILE ANOTHER EXPANDING BRANCH EMERGES FROM THE LATTER SINGULARITY AND HAS A STABLE DE SITTER PHASE WHICH IS INTRINSICALLY STABLE. WE OBTAIN A DIFFERENT PIECEWISE SCALE FACTOR WHICH DESCRIBES A CONTRACTING DE SITTER UNIVERSE IN THE DISTANT PAST UNTIL THE FINITE-TIME FUTURE SINGULARITY HAPPENS. IT EMERGES AND CONTINUES IN A CONTRACTING PHASE, BOUNCES AT THE MINIMUM, REVERSES, AND ENTERS INTO A STABLE DE SITTER PHASE WITHOUT A DRAMATIC FINAL. ALSO, WE EXPLORE THE AFORESAID COSMIC SCENARIOS BY FOCUSING ON THE LEADING CONTRIBUTIONS OF SOME PHYSICAL QUANTITIES NEAR THE SUDDEN FUTURE SINGULARITY AND APPLYING THE GEOMETRIC TIPLER AND KR\OLAK CRITERIA IN ORDER TO INSPECT THE BEHAVIOR OF TIMELIKE GEODESIC CURVES AROUND SUCH SINGULARITY.

WE RECONSIDER THE COSMOLOGICAL MODEL DISCUSSED BY SUNG-WON KIM IN THE CONTEXT OF FRIEDMANN-ROBERTSON-WALKER COSMOLOGIES WITH A TRAVERSABLE WORMHOLE, WHERE IT IS ASSUMED THAT THE MATTER CONTENT IS DIVIDED INTO TWO PARTS: THE COSMIC PART THAT DEPENDS ON TIME ONLY AND THE WORMHOLE PART THAT DEPENDS ON SPACE ONLY. THE COSMIC PART OBEYS THE BAROTROPIC EQUATION OF STATE PC=??C. THE COMPLETE ANALYSIS REQUIRES FURTHER CARE AND REVEALS MORE INTERESTING RESULTS THAN WHAT WAS PREVIOUSLY SHOWN BY THE AUTHOR. THEY CAN BE READILY APPLIED TO THE EVOLUTION OF A LARGE CLASS OF COSMOLOGICAL MODELS WHICH ARE MORE GENERAL THAN FRIEDMANN-ROBERTSON-WALKER MODELS.

IT IS WELL KNOWN THAT KASNER-TYPE COSMOLOGIES PROVIDE A USEFUL FRAMEWORK FOR ANALYZING THE THREE-DIMENSIONAL ANISOTROPIC EXPANSION BECAUSE OF THE SIMPLIFICATION OF THE ANISOTROPIC DYNAMICS. IN THIS PAPER RELATIVISTIC MULTI-FLUID KASNER-TYPE SCENARIOS ARE STUDIED. WE FIRST CONSIDER THE GENERAL CASE OF A SUPERPOSITION OF TWO IDEAL COSMIC FLUIDS, AS WELL AS THE PARTICULAR CASES OF NON-INTERACTING AND INTERACTING ONES, BY INTRODUCING A PHENOMENOLOGICAL COUPLING FUNCTION Q( T). FOR TWO-FLUID COSMOLOGICAL SCENARIOS THERE EXIST ONLY COSMOLOGICAL SCALING SOLUTIONS, WHILE FOR THREE-FLUID CONFIGURATIONS THERE EXIST NOT ONLY COSMOLOGICAL SCALING ONES, BUT ALSO MORE GENERAL SOLUTIONS. IN THE CASE OF TRIPLY INTERACTING COSMIC FLUIDS WE CAN HAVE ENERGY TRANSFER FROM TWO FLUIDS TO A THIRD ONE, OR ENERGY TRANSFER FROM ONE COSMIC FLUID TO THE OTHER TWO. IT IS SHOWN THAT BY REQUIRING THE POSITIVITY OF ENERGY DENSITIES THERE ALWAYS IS A MATTER COMPONENT WHICH VIOLATES THE DOMINANT ENERGY CONDITION IN THIS KIND OF ANISOTROPIC COSMOLOGICAL SCENARIOS.

IN THIS PAPER, WE STUDY CLASSICAL GENERAL RELATIVISTIC STATIC WORMHOLE CONFIGURATIONS WITH PSEUDOSPHERICAL SYMMETRY. WE SHOW THAT, IN ADDITION TO THE HYPERBOLIC WORMHOLE SOLUTIONS DISCUSSED BY LOBO AND MIMOSO IN [PHYS. REV. D 82, 044034 (2010)], THERE EXISTS ANOTHER WORMHOLE CLASS, WHICH IS A TRULY PSEUDOSPHERICAL COUNTERPART OF SPHERICAL MORRIS?THORNE WORMHOLE (CONTRARY TO THE LOBO?MIMOSO WORMHOLE CLASS), SINCE ALL CONSTRAINTS ORIGINALLY DEFINED BY MORRIS AND THORNE FOR SPHERICALLY SYMMETRIC WORMHOLES ARE SATISFIED. WE SHOW THAT, FOR BOTH CLASSES OF HYPERBOLIC WORMHOLES, THE ENERGY DENSITY, AT THE THROAT, IS ALWAYS NEGATIVE, WHILE THE RADIAL PRESSURE IS POSITIVE, CONTRARY TO THE SPHERICALLY SYMMETRIC MORRIS?THORNE WORMHOLE. SPECIFIC HYPERBOLIC WORMHOLES ARE CONSTRUCTED AND DISCUSSED BY IMPOSING DIFFERENT CONDITIONS FOR THE RADIAL AND LATERAL PRESSURES, OR BY CONSIDERING RESTRICTED CHOICES FOR THE REDSHIFT AND THE SHAPE FUNCTIONS. IN PARTICULAR, WE SHOW THAT A HYPERBOLIC WORMHOLE CANNOT BE SUSTAINED AT THE THROAT BY PHANTOM ENERGY AND THAT THERE ARE PSEUDOSPHERICALLY SYMMETRIC WORMHOLES SUPPORTED BY MATTER WITH ISOTROPIC PRESSURE AND CHARACTERIZED BY SPACE SECTIONS WITH AN ANGLE DEFICIT (OR EXCESS).

IN THIS PAPER WE STUDY (N+1)-DIMENSIONAL EVOLVING WORMHOLES SUPPORTED BY ENERGY SATISFYING A POLYTROPIC EQUATION OF STATE. THE CONSIDERED EVOLVING WORMHOLE MODELS ARE DESCRIBED BY A CONSTANT REDSHIFT FUNCTION AND GENERALIZES THE STANDARD FLAT FRIEDMANN?ROBERTSON?WALKER SPACETIME. THE POLYTROPIC EQUATION OF STATE ALLOWS US TO CONSIDER IN (3+1)-DIMENSIONS GENERALIZATIONS OF THE PHANTOM ENERGY AND THE GENERALIZED CHAPLYGIN GAS SOURCES.

WE PRESENT A FAMILY OF STATIC AND EVOLVING SPHERICALLY SYMMETRIC LORENTZIAN WORMHOLE SOLUTIONS IN N+1 DIMENSIONAL EINSTEIN GRAVITY. IN GENERAL, FOR STATIC WORMHOLES, WE REQUIRE THAT AT LEAST THE RADIAL PRESSURE HAS A BAROTROPIC EQUATION OF STATE OF THE FORM PR=?R?, WHERE THE STATE PARAMETER ?R IS CONSTANT. ON THE OTHER HAND, IT IS SHOWN THAT IN ANY DIMENSION N?3, WITH ?(R)=?=0 AND ANISOTROPIC BAROTROPIC PRESSURE WITH CONSTANT STATE PARAMETERS, STATIC WORMHOLE CONFIGURATIONS ARE ALWAYS ASYMPTOTICALLY FLAT SPACETIMES, WHILE IN 2+1 GRAVITY THERE ARE NOT ONLY ASYMPTOTICALLY FLAT STATIC WORMHOLES AND ALSO MORE GENERAL ONES. IN THIS CASE, THE MATTER SUSTAINING THE THREE-DIMENSIONAL WORMHOLE MAY BE ONLY A PRESSURELESS FLUID. IN THE CASE OF EVOLVING WORMHOLES WITH N?3, THE PRESENCE OF A COSMOLOGICAL CONSTANT LEADS TO AN EXPANSION OR CONTRACTION OF THE WORMHOLE CONFIGURATIONS: FOR POSITIVE COSMOLOGICAL CONSTANT WE HAVE WORMHOLES WHICH EXPAND FOREVER AND, FOR NEGATIVE COSMOLOGICAL CONSTANT WE HAVE WORMHOLES WHICH EXPAND TO A MAXIMUM VALUE AND THEN RECOLLAPSE. IN THE ABSENCE OF A COSMOLOGICAL CONSTANT THE WORMHOLE EXPANDS WITH CONSTANT VELOCITY, I.E WITHOUT ACCELERATION OR DECELERATION. IN 2+1 DIMENSIONS THE EXPANDING WORMHOLES ALWAYS HAVE AN ISOTROPIC AND HOMOGENEOUS PRESSURE, DEPENDING ONLY ON THE TIME COORDINATE.

WE INVESTIGATE A FAMILY OF INHOMOGENEOUS AND ANISOTROPIC GRAVITATIONAL FIELDS EXHIBITING A FUTURE SINGULARITY AT A FINITE VALUE OF THE PROPER TIME. THE STUDIED SPHERICALLY SYMMETRIC SPACETIMES ARE ASYMPTOTICALLY FRIEDMANN-ROBERTSON-WALKER AT SPATIAL INFINITY AND DESCRIBE WORMHOLE CONFIGURATIONS FILLED WITH TWO MATTER COMPONENTS: ONE INHOMOGENEOUS AND ANISOTROPIC FLUID AND ANOTHER ISOTROPIC AND HOMOGENEOUSLY DISTRIBUTED FLUID, CHARACTERIZED BY THE SUPERNEGATIVE EQUATION OF STATE \OMEGA=P/\RHO < -1. IN PREVIOUSLY CONSTRUCTED WORMHOLES, THE NOTION OF THE PHANTOM ENERGY WAS USED IN A MORE EXTENDED SENSE THAN IN COSMOLOGY, WHERE THE PHANTOM ENERGY IS CONSIDERED A HOMOGENEOUSLY DISTRIBUTED FLUID. SPECIFICALLY, FOR SOME STATIC WORMHOLE GEOMETRIES THE PHANTOM MATTER WAS CONSIDERED AS AN INHOMOGENEOUS AND ANISOTROPIC FLUID, WITH RADIAL AND LATERAL PRESSURES SATISFYING THE RELATIONS PR/?

IN THIS PAPER WE DERIVE NEW STATIC PHANTOM TRAVERSABLE WORMHOLES BY ASSUMING A SHAPE FUNCTION WITH A QUADRATIC DEPENDENCE ON THE RADIAL COORDINATE R. WE MAINLY FOCUS OUR STUDY ON WORMHOLES SUSTAINED BY EXOTIC MATTER WITH POSITIVE ENERGY DENSITY (AS SEEN BY ANY STATIC OBSERVER) AND A VARIABLE EQUATION OF STATE PR/?