Phantom Cosmology And Gravitational Back Reaction

With the development of science and technique, the cosmology has entered an era of accurate measurement rather than being a speculative science. Observations have verified the standard cosmological model on one hand, but on the other hand, they also make us face some new problems. At 1998 Ia supernova data reveal that our universe is undergoing an accelerating expansion, and this discovery is confirmed by many other observational data and as a result cosmology is becoming one of the most fascinating subjects in physics. In order to explain the present cosmic accelerating expansion some physicists think that the observed acceleration is a signal of breakdown of our understanding of the laws of gravitation and propose to modify the Einstein's gravity theory. But many other physicists believe that the present cosmic acceleration is caused by an exotic energy, called as dark energy, whose pressure is negative. Observations indicate that dark energy consists of 70% of total energy in our universe. The simplest candidate of dark energy is the cosmological constant, which can explain the present cosmic accelerating expansion but at the same time it also leaves some theoretical problems unresolved. Inspired by inflation theory, physicists use certain scalar fields, such as quintessence and phantom, to explain the present cosmic acceleration.Phantom field has a negative kinetic term in its Lagrangian, which leads to some new features, such as Big Rip. In this paperfirstly we discuss the dynamics of phantom dark energy dominated universe and the destroy of gravitational bound system by phantom dark energy as well as phantom inflation. Then we study the gravitational back reaction of long wavelength cosmo-logical perturbations in phantom dark energy dominated universe and phantom inflation era respectively. We find that in phantom dark energy dominated universe the back reaction can avoid the appearance of Big Rip, but in phantom inflation scenario, if we want to obtain enough inflationary e-folds and appropriate density perturbation spectra, the effect of back reaction is too weak to terminate the inflation.Secondly we study the constraint of observational data on dark energy model. By the maximum likelihood technique we use the 194 la supernova data (SNela) and the 26 X-ray gas mass fractions in cluster of galaxies to constrain a variable dark energy model with the equation of state w — u>o/(l + 61n(l -f z)), where b is a positive constant, and find that the combination of SNela and galaxy clusters X-ray data gives rise to strong restrictions on Q,m and wq but weak ones on b.Thirdly we examine two kinds of dark energy: phantom and quintom by the statefinder diagnosis. Our results show that the statefinder diagnostic can differentiate quintom and phantom models from other dark energy models.Finally the effect of trans-Planck physics on relic gravitational waves is studied. We find that the spectrum of energy den-sities of total relic gravitational waves acquires corrections due to the consideration of trans-Planckian physics and these corrections depend sensitively on the vacuum state that was actually realized at the beginning of the inflation.