A geometric probe of cosmology – I. Gravitational lensing time delays and quasar reverberation mapping

Abstract

We present a novel, purely geometric probe of cosmology based on measurements of differential time delays between images of strongly lensed quasars due to finite source effects. Our approach is solely dependent on cosmology via a ratio of angular diameter distances, the image separation, and the source size. It thereby entirely avoids the challenges of lens modelling that conventionally limit time delay cosmography, and instead entails the lensed reverberation mapping of the quasar Broad Line Region. We demonstrate that differential time delays are measurable with short cadence spectroscopic monitoring of lensed quasars, through the timing of kinematically identified features within the broad emission lines. This provides a geometric determination of an angular diameter distance ratio complementary to standard probes, and as a result is a potentially powerful new method of constraining cosmology.

Gravitational microlensing time delays at high optical depth: image parities and the temporal properties of fast radio bursts

“…In Taylor expanding the time arrival surface about the macroimage location, we find that (e.g. Paczynski 1986;Ng & Lewis 2020);…”

“…Zwicky 1937;Schneider et al 1992). Due to differing paths between the source and the observer, photons can travel different distances and experience different gravitational potentials, leading to a time delay between the images that can be used as a cosmological probe (see Refsdal 1964Refsdal , 1966Wong et al 2019;Ng & Lewis 2020).…”