Constraints on the nature of cosmological constant - From big bang nucleosyntheis and CMB anisotropies

Numerous recent observations strongly suggest that the expansion of the universe is now accelrating due to the effect of some unknown form of "dark energy" with negative pressure. The most physically plausible candidate for the dark energy is the quintessence scalar field, which tracks the evolution of the dominant energy component in the universe and acts like a cosmological constant only after the onset of the matter dominated epoch. A generic feature of such fields is the possibility of having a significant energy density throughout the thermal history of the universe, which can affect the cosmic expansion and physical processes in the early universe. These models can therefore be effectively constrained by using the well established theories of primordial nucleosynthesis (BBN) and the cosmic microwave background (CMB) anisotropies in comparison to the latest observations. In the present work we analyze some popular forms for the quintessence field and show that primordial ⁴He and D abundances put one of the most stringent constraints on the field in the early universe while CMB anisotropies put constraints on its nature at later epochs. We find that the simplest quintessence models can be ruled out from current observations. We also deduce constraints on the epoch of matter creation at the end of quintessential inflation through the effects of gravitational waves on the CMB anisotropies.