linguistic:voice_change
Hubble time

The Hubble constant H has units of inverse time; the Hubble time t is simply defined as the inverse of the Hubble constant, i.e.

t H ≡ 1 H 0 = 1 67.8 (k m / s) / M p c = 4.55 × 10 17 s = 14.4 billion years. {\displaystyle t_{H}\equiv {\frac {1}{H_{0}}}={\frac {1}{67.8\mathrm {~(km/s)/Mpc} }}=4.55\times 10^{17}\mathrm {~s} =14.4{\text{ billion years}}.}

This is slightly different from the age of the universe, which is approximately 13.8 billion years. The Hubble time is the age it would have had if the expansion had been linear, and it is different from the real age of the universe because the expansion is not linear; it depends on the energy content of the universe (see § Derivation of the Hubble parameter).

We currently appear to be approaching a period where the expansion of the universe is exponential due to the increasing dominance of vacuum energy. In this regime, the Hubble parameter is constant, and the universe grows by a factor e each Hubble time:

H ≡ a ˙ a = constant ⟹ a ∝ e H t = e t t H {\displaystyle H\equiv {\frac {\dot {a}}{a}}={\textrm {constant}}\quad \Longrightarrow \quad a\propto e^{Ht}=e^{\frac {t}{t_{H}}}}

Likewise, the generally accepted value of 2.27 Es means that (at the current rate) the universe would grow by a factor of e in one exasecond.

Over long periods of time, the dynamics are complicated by general relativity, dark energy, inflation, etc., as explained above.