Stuart tells us about his recent work exploring how to use peculiar velocity measurements to constrain modified gravity (specifically f(R) and DGP models). He finds that even using just linear scales we would be able to detect or rule out model parameter regions that would be entirely consist with current measurements of the background expansion.
He does this by predicting the auto and cross spectra between galaxy overdensity and peculiar velocity, using just linear theory – and then analysing those observables in modified gravity simulations to measure the parameters used in the simulations.
At this point it is just a proof of concept, as for each modified gravity scenario the method is only constraining the linear galaxy bias and one model parameter (other cosmological parameters are known) and also not taking into account additional observational uncertainties involved in measuring peculiar velocities in the real world.
Still, the method does work, at least when the effect produces a large enough deviation from LCDM so the proof of concept works. Future careful work with 6DF and SDSS data (and, one day soon, DESI) should allow the method to bring about real constraints on these models.
Cosmology Talks 