Abstract
High-precision cosmological probes have revealed a small but significant tension between the parameters measured with different techniques, among which there is one based on time delays in gravitational lenses. We discuss a new way of using time delays for cosmology, taking advantage of the extreme precision expected for lensed fast radio bursts, which are short flashes of radio emission originating at cosmological distances. With coherent methods, the achievable precision is sufficient for measuring how time delays change over the months and years, which can also be interpreted as differential redshifts between the images. It turns out that uncertainties arising from the unknown mass distribution of gravitational lenses can be eliminated by combining time delays with their time derivatives. Other effects, most importantly relative proper motions, can be measured accurately and disentangled from the cosmological effects. With a mock sample of simulated lenses, we show that it may be possible to attain strong constraints on cosmological parameters. Finally, the lensed images can be used as galactic interferometer to resolve structures and motions of the burst sources with incredibly high resolution and help reveal their physical nature, which is currently unknown.
Highlights
The gravitational lens effect (Refsdal 1964a) can deflect light so strongly that it produces multiple images of a single source. Refsdal (1964b) argued that time delays can be used to determine distances and, the Hubble constant (H0), at a time when it was not even clear that the effect would ever be seen in observations
We propose an alternative route for cosmology with lensed fast radio bursts (FRBs)
This is better than for the relative change of the time delay because the relevant baseline is the time between bursts, which can be many years, and which increases without limit if the FRB is repeating persistently
Summary
The gravitational lens effect (Refsdal 1964a) can deflect light so strongly that it produces multiple images of a single source. Refsdal (1964b) argued that time delays (light travel time differences between images) can be used to determine distances and, the Hubble constant (H0), at a time when it was not even clear that the effect would ever be seen in observations. We argue that with coherent time-delay measurements, we can achieve levels of accuracy that are much better than the burst duration and does this allow us to disentangle cosmological effects from the proper motion, but we can even eliminate the unknown mass distributions of the lensing galaxies as the main source of potential systematic errors. Finding the time delay between the waves is made much easier when we have a good estimate from the intensity correlation
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