Modified Gravity

Background

General Relativity (GR) is our most precisely tested theory of gravity, yet on the largest scales in the universe, we need to invoke dark energy to explain the observed accelerated expansion. One alternative is to ask: what if gravity itself behaves differently on cosmological scales? Theories of Modified Gravity (MG) replace or extend GR with modified field equations that can naturally produce accelerated expansion, without requiring a new energy component like $\Lambda$.

Distinguishing modified gravity from dark energy is one of the central challenges in modern cosmology. The key insight is that the two scenarios leave distinct imprints on the growth of structure. In GR, the rate at which galaxies cluster together is fully determined by the expansion history: once you know $H(z)$, you know how structure grows. In modified gravity theories, the relationship between the expansion history and the growth rate is broken: the growth can be enhanced or suppressed relative to the GR prediction.

The growth rate is observationally accessible through redshift-space distortions (RSD), the apparent anisotropy in galaxy clustering caused by the peculiar motions of galaxies. The key observable combination is $f\sigma_8$, where $f = d\ln D / d\ln a$ is the logarithmic growth rate and $\sigma_8$ is the amplitude of matter fluctuations. A measurement of $f\sigma_8$ as a function of redshift, combined with the geometric BAO measurement, gives a joint test of GR on cosmological scales.

My Work

I am contributing to the development of the analysis pipeline for the DESI DR2 Modified Gravity key paper. The goal is to use the full DESI dataset, combining BAO measurements (geometry) and RSD measurements (growth) across multiple galaxy samples spanning a wide redshift range, to perform a state-of-the-art test of General Relativity on cosmological scales, and to place constraints on the most commonly studied MG parameterisations, such as the $(\mu, \Sigma)$ and growth-index $\gamma$ frameworks.

This work is ongoing, and a paper is in preparation as part of the DESI DR2 results program.