Time Delay Cosmography (1605.05333v1)

Abstract: Gravitational time delays, observed in strong lens systems where the variable background source is multiply-imaged by a massive galaxy in the foreground, provide direct measurements of cosmological distance that are very complementary to other cosmographic probes. The success of the technique depends on the availability and size of a suitable sample of lensed quasars or supernovae, precise measurements of the time delays, accurate modeling of the gravitational potential of the main deflector, and our ability to characterize the distribution of mass along the line of sight to the source. We review the progress made during the last 15 years, during which the first competitive cosmological inferences with time delays were made, and look ahead to the potential of significantly larger lens samples in the near future.

Time Delay Cosmography

The paper "Time Delay Cosmography" by Tommaso Treu and Philip J. Marshall explores the use of gravitational time delays in strong lens systems as a method for measuring cosmological distances. This approach is complementary to other cosmographic probes and provides avenues to gain insights into the expansion dynamics of the universe. The paper covers significant advancements in this technique over fifteen years and anticipates future developments fueled by larger samples of lensing systems.

Gravitational lensing occurs when a massive foreground object, such as a galaxy or galaxy cluster, alters the path of light from a background source, like a quasar, resulting in multiple images of this source. When the background source is variable, the time delays between the arrival of light from different images can be measured. These time delays are directly related to the cosmological distances involved in the bending of light. This method offers a means to derive cosmological parameters, particularly the Hubble constant ($H_0$), with a precision that can be comparable to, or even surpass, traditional methods.

The efficacy of time delay cosmography hinges on several key components:

• The selection and size of an appropriate sample of lensed quasars or supernovae.
• Precise time delay measurements between the multiple images.
• Accurate modeling of the gravitational potential of the deflecting mass, typically a massive galaxy.
• Comprehensive understanding of the mass distribution along the line of sight to the background source.

The authors review the historical progress of time delay cosmography leading up to the competitive cosmological inferences made in recent years. The derivation of cosmological distances from time delays hinges on robust theoretical foundations and astute application of Fermat's principle in gravitational optics. The time delay distance can then be expressed as a combination of angular diameter distances, tied to cosmological parameters.

A substantial part of the research efforts has focused on tackling systematic errors and uncertainties associated with modeling the gravitational lens potential and characterizing external mass distributions affecting the lensing. These include considering effects from structures along the line of sight, and obtaining high-resolution imaging and detailed observations of the lens environment.

The paper underscores the significance of combining time delays with additional cosmological data to break degeneracies and improve precision. The methodology is currently limited by sample sizes, but forthcoming surveys are expected to discover many more lensed systems, providing the opportunity to refine distance estimates further.

Looking ahead, advancements in observational technology, such as the James Webb Space Telescope and thirty-meter-class telescopes, promise to enhance imaging and spectroscopic capabilities. These improvements will enable better modeling of lens mass distributions and potentially leverage additional information from time delay and angular diameter distances.

Cosmological implications of this research not only hinge on the precision measurements of $H_0$, but also in probing scenarios beyond the standard $\Lambda$CDM model, including dynamic dark energy models or deviations from spatial flatness.

Overall, this paper systematically reviews and projects the promising avenue that time delay cosmography presents for further understanding of cosmic acceleration and cosmological parameters, with the anticipation of significantly contributing to the body of cosmological evidence in the coming decade.