Axionlike Dark Energy and Particle Decay in theFuture of the Accelerating Universe
The 1998 discovery that the universe was accelerating in its expansion has yet to be explained theoretically, meriting the continual theoretical and observational study of this phenomena. In this thesis, we undergo a phenomenological study of the cosmological implications of this “dark energy” in two different ways. In the first part of this thesis, we examine the cosmological evolution of ultralight axionlike (ULA) scalar fields with potentials of the form V (φ) = m2f 2 [1 − cos (φ/f)]2 , with particular emphasis on the deviation in their behavior from the corresponding small-φ powerlaw approximations to these potentials: V (φ) ∝ φ 2n . We show that in the slow-roll regime, when φ˙2/2 V (φ), the full ULA potentials yield a more interesting range of possibilities for quintessence than do the corresponding power law approximations. For rapidly oscillating scalar fields, we derive the equation of state parameter and oscillation frequency for the ULA potentials and show how they deviate from the corresponding power-law values. We derive an analytic expression for the equation of state parameter that better approximates the ULA value than does the pure power-law approximation. In the second part, we study particle decay in the future of the accelerating universe. We generalize the result that in a cosmological constant dominated universe, the decay of matter into relativistic particles can never cause radiation to once again dominate over matter. We study both models of dark energy comprised of quintessence and cosmologies ending in a “big rip” in this context.