What is the Hubble radius?

The Hubble radius (or Hubble sphere) is the distance at which the recession velocity equals the speed of light: d_H = c / H0.

Is the Hubble time constant?

No. As the universe evolves the Hubble parameter changes, so the Hubble time changes too. The Hubble constant is its present-day value.

For a matter-dominated universe the age is about 2/3 of the Hubble time. With dark energy the age is close to the Hubble time.

Calculate the Hubble time t_H = 1/H0, the characteristic timescale of cosmic expansion, in seconds and gigayears.

Hubble Constant (H0)

km/s/Mpc

Hubble Time

440,857,142,857,142,850.00 s

Hubble Time (Gyr)13.9689 Gyr

Hubble Radius4,282.86 Mpc

Hubble Time vs Hubble Constant (H0)

The Hubble time is the reciprocal of the Hubble constant, giving the time scale of the present expansion.

t_H = 1 / H0

To convert, note that 1 km/s/Mpc = 1 / (3.086 x 10¹⁹ s).

The Hubble radius c / H0 defines the distance at which the recession velocity equals the speed of light.

H0 = 70 km/s/Mpc.