A novel integrated Sachs-Wolfe effect from Early Dark Energy

We study the nonlinear effects of minimally coupled, massless, cosmological scalar fields on the cosmic microwave background (CMB). These fields can exhibit post-recombination parametric resonance and subsequent nonlinear evolution leading to novel contributions to the gravitational potential. We compute the resulting contributions to the CMB temperature anisotropies through the time-variation of the gravitational potential (i.e., the integrated Sachs-Wolfe (ISW) effect). We find that fields that constitute 5% of the total energy density and become dynamical at $z_c \simeq 10^{4}$ can produce marginally observable ISW signals at multipoles $\ell \simeq 2000$. Fields that become dynamical at earlier times and/or have initial displacements at a flatter part of their potential, produce ISW contributions that are significantly larger and at higher multipoles. We calculate these dynamics and the resulting evolution of gravitational perturbations using analytic estimates alongside detailed nonlinear lattice simulations, which couple scalar fields and cosmological fluids to a perturbed metric. Finally, we discuss the possibility of detecting these features with future high-resolution CMB observations.Comment: 12 pages, 6 figures, comments welcom

Numerical Simulations of Nonlinear Physics in the Universe

I address a variety of problems in contemporary cosmology, both with the models and theories that compose it—such as the Hubble Tension and General Relativity—as well as the methods used to perform it—like Numerical Relativity and the Well-Posedness problem. Through these issues, we will come to understand that nonlinear physics, though complex, difficult to understand / work with, and overlooked by the majority of physicists, is a crucial component of the dynamics of our Universe. The way the world works is often not the same as the way that physics works. Physics describes the arc that a baseball follows by describing the forces acting on it, or its change in energy, and deriving a mathematical description of its path. The ball, however, just flies. Sometimes we are tempted to see these mathematical descriptions as the ‘way’ the world is, because those descriptions are what is necessary to solve the problem at hand. As the Universe expands or moves or evolves, it can do so in complex and convoluted ways which are not reducible to palatable equations of motion. In these situations, it is necessary to investigate the complicated, nonlinear motion, and describing it mathematically becomes all that we can do for now. We must thoroughly investigate claims made about the effects of non-linearity as these dynamics are unpredictable and yet nontrivial. We conclude that we must do the problem, no matter how difficult, to claim its outcome

A Well-Posed UV Completion for Simulating Scalar Galileons

The Galileon scalar field theory is a prototypical example of an effective field theory that exhibits the Vainshtein screening mechanism, which is incorporated into many extensions to Einstein gravity. The Galileon describes the helicity zero mode of gravitational radiation, the presence of which has significant implications for predictions of gravitational waves from orbiting objects, and for tests of gravity sensitive to additional polarizations. Because of the derivative nature of their interactions, Galileons are superficially not well-posed as effective field theories. Although this property is properly understood merely as an artifact of the effective field theory truncation, and is not theoretically worrisome, at the practical level it nevertheless renders numerical simulation highly problematic. Notwithstanding, previous numerical approaches have successfully evolved the system for reasonable initial data by slowly turning on the interactions. We present here two alternative approaches to improving numerical stability in Galileon numerical simulations. One of these is a minor modification of previous approaches, which introduces a low pass filter that amounts to imposing a UV cutoff together with a relaxation method of turning on interactions. The second approach amounts to constructing a (numerical) UV completion for which the dynamics of the high momentum modes is under control, and for which it is unnecessary to slowly turn on nonlinear interactions. We show that numerical simulations of the UV theory successfully reproduce the correct Galileon dynamics at low energies, consistent with the low-pass filter method and with previous numerical simulations.Comment: 10 pages, 6 figures; fixed missing reference; added reference