The inverse of the Hubble constant. The Hubble time, also called the Hubble age or the Hubble period, provides an estimate for the age of the universe by presuming that the universe has always expanded at the same rate as it is expanding today.
Figure: The Hubble time is the amount of time one predicts by projecting back assuming the universe has always been expanding at its current rate. If the universe used to be expanding much faster then the Hubble time overestimates the age of the universe. If the universe is expanding faster today than it used to be, the Hubble time underestimates the true age of the universe.