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Gravitational-wave propagation in arbitrary background

Omar Contigiani (CITA) // October 12, 2021

Abstract: The propagation of gravitational waves (GWs) probes the existence of exotic scalar waves predicted by extended models of gravity designed to explain the nature of dark energy. In General Relativity and beyond, linearized waves on top of a background metric are commonly studied under one of two assumptions: either the background is fixed to a specific form, or a well-behaved separation between wave and background must be imposed. This is traditionally done by requiring that the wavelength of the first is smaller than the typical length scale of the second. However, this naturally limits us to the geometric optics regime, where the ratio between the two is <<1, and the energies involved are far higher than the typical cosmological scales. In this somewhat technical but accessible blackboard seminar, I will introduce a new way to achieve this separation for every GW wavelength. To better explain how this formalism can be used to interpolate between low- and high-energy regimes, I will expand on the definition and implications of a well-behaved separation of scales. Finally, I will conclude by presenting a simple application of this formalism to the propagation of scalar waves. Our results show for the first time that interferometers on Earth cannot detect scalar waves for a wide class of extended models.

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