Probing early modification of gravity with Planck, ACT and SPT

We consider a model of early modified gravity (EMG) that was recently proposed as a candidate to resolve the Hubble tension. The model consists of a scalar field sigma with a nonminimal coupling (NMC) to the Ricci curvature of the form F(sigma) = M2pl + xi sigma 2 and an effective mass induced by a quartic potential V (sigma) = lambda sigma 4/4. We present the first analyses of the EMG model in light of the latest ACT DR4 and SPT-3G data in combination with full Planck data, and find a greater than or similar to 2 sigma preference for a non-zero EMG contribution from a combination of primary CMB data alone, mostly driven by ACT-DR4 data. This is different from popular 'Early Dark Energy' models, which are detected only when the high-l information from Planck temperature is removed. We find that the NMC plays a key role in controlling the evolution of density perturbations that is favored by the data over the minimally coupled case. Including measurements of supernovae luminosity distance from Pantheon+, baryonic acoustic oscillations and growth factor from BOSS, and CMB lensing of Planck leaves the preference unaffected. In the EMG model, the tension with SH0ES is alleviated from -6 sigma to -3 sigma. Further adding SH0ES data raises the detection of the EMG model above 5 sigma.