The dimensionless age of the Universe: a riddle for our time
  We present the interesting coincidence of cosmology and astrophysics that points toward a dimensionless age of the universe H_0*t_0 that is close to one. Despite cosmic deceleration for 9 Gyr and acceleration since then, we find H_0t*_0 = 0.96 +/- 0.01 for the LCDM model that fits SN Ia data from Pan-STARRS, CMB power spectra, and baryon acoustic oscillations. Similarly, astrophysical measures of stellar ages and the Hubble constant derived from redshifts and distances point to H_0*t ~ 1.0 +/- 0.1$. The wide range of possible values for H_0*t_0 realized during comic evolution means that we live at what appears to be a special time. This "synchronicity problem" is not precisely the same as the usual Coincidence problem because there are combinations of Omega_Matter and Omega_Lambda for which the usual coincidence problem holds but for which H_0*t_0 is not close to 1. 
Arturo Avelino (1) and Robert P. Kirshner (1 and 2) ((1) Harvard   University, (2) Gordon and Betty Moore Foundation)
astro-ph.CO gr-qc