We consider a massless, minimally coupled scalar with a quartic self-interaction which is released in Bunch-Davies vacuum in the locally de Sitter background of an inflating universe. It was shown, in this system, that quantum effects can induce a temporary phase of super-acceleration, causing a violation of the Weak Energy Condition on cosmological scales. In this paper, we investigate the system's stability by studying the behavior of linearized perturbations in the quantum-corrected effective field equation at one-and two-loop order. We show that the amplitude of the quantum-corrected mode function is reduced in time, starting from its initial classical (Bunch-Davies) value. This implies that the linear perturbations do not grow, hence the model is stable. The decrease in the amplitude is in agreement with the system developing a positive (growing) mass-squared due to quantum processes.The induced mass, however, remains perturbatively small and does not go tachyonic.This ensures the stability.

the LIGO interferometers detected the gravitational wave (GW) signal (GW170817) from the coalescence of binary neutron stars. This signal was also simultaneously seen throughout the electromagnetic (EM) spectrum from radio waves to gamma-rays. We point out that this simultaneous detection of GW and EM signals rules out a class of modified gravity theories, termed "dark matter emulators," which dispense with the need for dark matter by making ordinary matter couple to a different metric from that of GW. We discuss other kinds of modified gravity theories which dispense with the need for dark matter and are still viable. This simultaneous observation also provides the first observational test of Einstein's Weak Equivalence Principle (WEP) between gravitons and photons. We estimate the Shapiro time delay due to the gravitational potential of the total dark matter distribution along the line of sight (complementary to the calculation in [1]) to be about 400 days. Using this estimate for the Shapiro delay and from the time difference of 1.7 seconds between the GW signal and gamma-rays, we can constrain violations of WEP using the parameterized post-Newtonian (PPN) parameter γ, and is given by |γGW − γEM| < 9.8 × 10 −8 .

We compute the one loop self-mass of a charged massless, minimally coupled scalar in a locally de Sitter background geometry. The computation is done in two different gauges: the noninvariant generalization of Feynman gauge which gives the simplest expression for the photon propagator and the de Sitter invariant gauge of Allen and Jacobson. In each case dimensional regularization is employed and fully renormalized results are obtained. By using our result in the linearized, effective field equations one can infer how the scalar responds to the dielectric medium produced by inflationary particle production. We also work out the result for a conformally coupled scalar. Although the conformally coupled case is of no great physical interest the fact that we obtain a manifestly de Sitter invariant form for its self-mass-squared establishes that our noninvariant gauge introduces no physical breaking of de Sitter invariance at one loop order.

For many quantum field theory computations in cosmology it is not possible to use the flat space trick of obtaining full, interacting states by evolving free states over infinite times. State wave functionals must be specified at finite times and, although the free states suffice to obtain the lowest order effects, higher order corrections necessarily involve changes of the initial state. Failing to correctly change the initial state can result in effective field equations which diverge on the initial value surface, or which contain tedious sums of terms that redshift like inverse powers of the scale factor. In this paper we verify a conjecture from 2004 that the lowest order initial state correction can indeed absorb the initial value divergences and all the redshifting terms of the two loop expectation value of the stress tensor of a massless, minimally coupled scalar with a quartic self interaction on nondynamical de Sitter background.

We compute the one loop corrections from quantum gravity to the selfmass-squared of a massless, minimally coupled scalar on a locally de Sitter background. The calculation was done using dimensional regularization and renormalized by subtracting fourth order BPHZ counterterms. Our result should determine whether quantum gravitational loop corrections can significantly alter the dynamics of a scalar inflaton.