arXiv subjects

Alexander Cassem

Publications and source records attributed to Alexander Cassem.

2 recordsLinked to original sources

arXiv2026

Theoretical and Observational Bounds on Dynamical Chern-Simons Gravity as an Effective Field Theory

Gravitational effective theories are essential for characterizing the space of deviations from General Relativity (GR). Testing these theories against fundamental principles, such as causality and unitarity, can yield constraints on the corresponding parameters. In this paper, we perform such an analysis on the very interesting dynamical Chern-Simons (dCS) gravity. This is a parity violating correction to GR wherein a new scalar field couples to the Pontryagin density $^*R\,R$. It has generated significant interest, including possible new gravitational wave shapes for LIGO/Virgo and new phenomena from cosmic inflation. In this work, we begin by deriving the dispersion relation and wave packet speed on top of a gravitational wave background in dCS gravity. This alters the corresponding Shapiro time delay (which we compute to second order), potentially giving superluminality. Causality then demands a bound on the dCS coupling constant, which we find to be moderately sharper than, but compatible with, standard estimates. We then examine a UV completion in the form of a set of $N$ fermions with a (pseudo) Yukawa coupling. By imposing perturbativity and a gravitational species bound, we find that the dCS coupling constant is constrained significantly more, depending on the choice of scale of the species bound. We also identify higher order operators generated from the UV completion. Overall, we find that any dCS corrections to gravitational dynamics should likely be very small on macroscopic systems of observational interest, such as in late-time merging black holes.

hep-th↗

arXiv2024

Unitarity, Causality, and Solar System Bounds May Significantly Limit the Use of Gravitational Waves to Test General Relativity

The prospect of detecting/constraining deviations from general relativity by studying gravitational waves (GWs) from merging black holes has been one of the primary motivations of GW interferometers like LIGO/Virgo. Within pure gravity, the only possible way deviations can arise is from the existence of higher order derivative corrections, namely higher powers of the Riemann curvature tensor, in the effective action. Any observational bounds imply constraints on the corresponding Wilson coefficients. At the level of the action, one can imagine the coefficients are sufficiently large so as to be in principle detectable. However, from the point of view of some fundamental principles, namely causality and unitarity, this is much less clear, as we examine here. We begin by reviewing certain known bounds on these coefficients, which together imply a low cut off on the effective theory. We then consider a possible mechanism to generate such terms, namely in the form of many scalars, minimally coupled to only gravity, that can be integrated out to give these higher order operators. We show that a by product of this is the generation of quantum corrections to Newton's potential, whose observable consequences are already ruled out by solar system tests. We point out that over 7 orders of magnitude of improvement in interferometer sensitivity would be required to avoid such solar system constraints. We also mention further constraints from Hawking radiation from black holes.

hep-th↗