IN THIS WORK WE STUDY LINEAR AND NONLINEAR COSMOLOGICAL INTERACTIONS, WHICH DEPEND ON DARK MATTER AND DARK ENERGY DENSITIES IN THE FRAMEWORK OF GENERAL RELATIVITY. BY USING THE AKAIKE INFORMATION CRITERION (AIC) AND THE BAYESIAN INFORMATION CRITERION (BIC) WITH DATA FROM SNIA (UNION 2.1 AND BINNED JLA), H(Z), BAO AND CMB WE COMPARE THE INTERACTING MODELS AMONG THEMSELVES AND ANALYZE WHETHER MORE COMPLEX INTERACTING MODELS ARE FAVORED BY THESE CRITERIA. IN THIS CONTEXT, WE FIND SOME SUITABLE INTERACTIONS THAT ALLEVIATE THE COINCIDENCE PROBLEM.

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.

WE PERFORM A BAYESIAN MODEL SELECTION ANALYSIS FOR DIFFERENT CLASSES OF PHENOMENOLOGICAL COUPLED SCENARIOS OF DARK MATTER AND DARK ENERGY WITH LINEAR AND NON-LINEAR INTERACTING TERMS. WE USE A COMBINATION OF SOME OF THE LATEST COSMOLOGICAL DATA SUCH AS TYPE IA SUPERNOVAE (SNE IA), COSMIC CHRONOMETERS (CC), COSMIC MICROWAVE BACKGROUND (CMB) AND TWO SETS OF BARYON ACOUSTIC OSCILLATIONS MEASUREMENTS, NAMELY, 2-DIMENSIONAL ANGULAR MEASUREMENTS (BAO2) AND 3-DIMENSIONAL ANGLE-AVERAGED MEASUREMENTS (BAO3). WE FIND WEAK AND MODERATE EVIDENCE AGAINST TWO-THIRDS OF THE INTERACTING SCENARIOS CONSIDERED WITH RESPECT TO ?CDM WHEN THE FULL JOINT ANALYSIS IS CONSIDERED. ABOUT ONE-THIRD OF THE MODELS PROVIDE A DESCRIPTION TO THE DATA AS GOOD AS THE ONE PROVIDED BY THE STANDARD MODEL. OUR RESULTS ALSO INDICATE THAT EITHER SNE IA, CC OR BAO2 DATA BY THEMSELVES ARE NOT ABLE TO DISTINGUISH AMONG INTERACTING MODELS OR ?CDM BUT THE STANDARD BAO3 MEASUREMENTS AND THE COMBINATION WITH THE CMB DATA ARE INDEED ABLE TO DISCRIMINATE AMONG THEM. WE FIND THAT EVIDENCE DISFAVORING INTERACTING MODELS IS WEAKER WHEN WE USE BAO2 (DATA CLAIMED TO BE ALMOST MODEL-INDEPENDENT) INSTEAD OF THE STANDARD BAO3 MEASUREMENTS. THESE RESULTS HELP SELECT CLASSES OF VIABLE AND NON-VIABLE INTERACTING MODELS IN LIGHT OF CURRENT DATA.

WE STUDY AN INTERMEDIATE INFLATIONARY STAGE IN A JORDAN-BRANS-DICKE THEORY. IN THIS SCENARIO WE ANALYZE THE QUANTUM FLUCTUATIONS CORRESPONDING TO ADIABATIC AND ISOCURVATURE MODES. OUR MODEL IS COMPARED TO THAT DESCRIBED BY USING THE INTERMEDIATE MODEL IN EINSTEIN GENERAL RELATIVITY THEORY. WE ASSESS THE STATUS OF THIS MODEL IN LIGHT OF THE SEVEN-YEAR WMAP DATA.

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.

WE REVISE THE DYNAMICAL PROPERTIES OF A CLASS OF COSMOLOGICAL MODELS WHERE THE DARK SECTOR INTERACTS THROUGH AN INTERACTING TERM THAT CHANGES SIGN DURING EVOLUTION. IN PARTICULAR, WE OBTAIN THE CRITICAL POINTS AND INVESTIGATE THE EXISTENCE AND STABILITY CONDITIONS FOR COSMOLOGICAL SOLUTIONS, DESCRIBING RADIATION, MATTER AND DARK ENERGY DOMINATED ERAS. WE FIND THAT ALL THE STUDIED MODELS ADMIT A STABLE CRITICAL POINT CORRESPONDING TO AN ACCELERATED PHASE. WE USE BACKGROUND DATA TO FIND THE BEST FIT PARAMETERS FOR ONE OF THE STUDIED MOD- ELS, RESULTING IN AN INTERACTING PARAMETER WITH A DEFINITE SIGN WITHIN THE 1? CONFIDENCE LEVEL, CONSISTENT WITH THE RESULTS OF THE DYNAMICAL SYSTEM ANALYSIS. WE COMPUTE THE STATEFINDER PARAMETERS AND PLOT THE R - Q AND R - S PLANES, WHERE WE OBSERVE DIFFERENT EVOLUTIONARY TRAJECTORIES FOR THE INTERACTING SCENARIOS, ALLOWING IN THIS SENSE TO DISCRIMINATE AMONG DARK ENERGY MODELS.

WE PERFORM A DYNAMICAL SYSTEM ANALYSIS AND A BAYESIAN MODEL SELECTION FOR A NEW SET OF INTERACTING SCE- NARIOS IN THE FRAMEWORK OF MODIFIED HOLOGRAPHIC RICCI DARK ENERGY (MHR-IDE) MODELS. THE DYNAMICAL ANALYSIS SHOWS A MODIFIED RADIATION EPOCH AND A LATE-TIME ATTRACTOR CORRE- SPONDING TO AN ACCELERATED PHASE OF HOLOGRAPHIC DARK ENERGY. IN THE BAYESIAN COMPARISON, WE USE A COMBINATION OF BACK- GROUND DATA INCLUDING TYPE IA SUPERNOVAE, COSMIC CHRONOME- TERS, THE LOCAL VALUE OF THE HUBBLE CONSTANT, BARYON ACOUS- TIC OSCILLATIONS, AND COSMIC MICROWAVE BACKGROUND DATA. WE FIND EVIDENCE AGAINST ALL THE MHR-IDE SCENARIOS STUDIED IN COMPARISON WITH THE ?CDM WHEN THE FULL JOINT ANALYSIS IS CONSIDERED.

IN THIS WORK WE PRESENT A GENERALIZED BRANS-DICKE LAGRANGIAN INCLUDING A NON-MINIMALLY COUPLED GAUSS-BONNET TERM WITHOUT IMPOSING THE VANISHING TORSION CONDITION. IN THE FIELD EQUATIONS OBTAINED THE TORSION IS CLOSED RELATED TO THE DYNAMICS OF THE SCALAR FIELD, I.E. IF NON-MINIMALLY COUPLED TERMS ARE PRESENT IN THE THEORY, THE TORSION MUST BE PRESENT. FOR THE STUDIED LAGRANGIAN WE ANALYSE THE COSMOLOGICAL CONSEQUENCES OF CONSIDER AN EFFECTIVE TORSIONAL FLUID IN THE DESCRIPTION OF THE UNIVERSE. WE SHOW THAT IN THE PROPOSED SCENARIO THE EFFECTIVE TORSIONAL FLUID CAN BE RESPONSIBLE FOR THE CURRENT ACCELERATION OF THE UNIVERSE.

WE INVESTIGATE AN INTERACTING TWO-FLUID MODEL IN A SPATIALLY FLAT FRIEDMANN?LEMAÎTRE?ROBERTSON?WALKER (FLRW) UNIVERSE, WHEN THE ENERGY TRANSFER BETWEEN THESE TWO DARK COMPONENTS IS PRODUCED BY A FACTORIZABLE NONLINEAR SIGN-CHANGEABLE INTERACTION DEPENDING LINEARLY ON THE ENERGY DENSITY AND QUADRATICALLY ON THE DECELERATION PARAMETER. WE SOLVE THE SOURCE EQUATION AND OBTAIN THE EFFECTIVE ENERGY DENSITIES OF THE DARK SECTOR AND THEIR COMPONENTS. WE SHOW THAT THE EFFECTIVE EQUATION OF STATE OF THE DARK SECTOR INCLUDES SOME OF THE SEVERAL KINDS OF CHAPLYGIN GAS EQUATIONS OF STATE AS WELL AS A GENERALIZATION OF THE POLYTROPIC EQUATION OF STATE. WE USE BAYESIAN STATISTICS METHODS TO CONSTRAIN FREE PARAMETERS IN THE MODELS DURING THE MOST RECENT EVOLUTION CONSIDERING SUPERNOVAE TYPE IA AND MEASUREMENTS OF THE HUBBLE EXPANSION RATE. THE RESULTING CONSTRAINTS PROVIDE NEW INFORMATION ON SIGN-CHANGEABLE INTERACTIONS, ITS EQUIVALENCES AND COMPATIBILITY WITH PREVIOUS MODELS AND NOVEL LATE TIME UNIVERSE DYNAMICS.

WE OBTAIN CONDITIONS FOR THE EXISTENCE OF AN ATTRACTOR IN THE SYSTEM OF EQUATIONS DESCRIBING A TACHYON WARM INFLATIONARY MODEL WITH BULK VISCOSITY TAKEN INTO ACCOUNT. WHEN THESE CONDITIONS ARE MET THE EVOLUTION APPROACHES SLOW-ROLL REGIME. WE PRESENT THE PRIMORDIAL POWER SPECTRUM FOR THE TACHYON FIELD BY CONSIDERING A DISSIPATION COEFFICIENT DEPENDING ON THE SCALAR FIELD AND TEMPERATURE.