Implications for the Hubble tension from the ages of the oldest astrophysical objects

We use the ages of old astrophysical objects (OAO) in the redshift range 0≲z≲8 as stringent tests of the late-time cosmic expansion history. Since the age of the Universe at any redshift is inversely proportional to H0, requiring that the Universe be older than the oldest objects it contains at any redshift, provides an upper limit on H0. Using a combination of galaxies imaged from the CANDELS program and various high-z quasars, we construct an age-redshift diagram of ≳100 OAO up to z∼8. Assuming the ΛCDM model at late times, we find the 95% confidence level upper limit H0<73.2km/s/Mpc, in slight disagreement with a host of local H0 measurements. Taken at face value, and assuming that the OAO ages are reliable, this suggests that ultimately a combination of pre- and post-recombination (z≲10) new physics might be required to reconcile cosmic ages with early-time and local H0 measurements. In the context of the Hubble tension, our results motivate the study of either a) combined global pre- and post-recombination modifications to ΛCDM, or b) local new physics which only affects the local H0 measurements.