What is the Hubble constant?

It measures the current expansion rate of the universe. Current estimates range from about 67 to 73 km/s/Mpc depending on the method.

Why is there a Hubble tension?

Measurements from the early universe (CMB) give H0 ≈ 67, while local distance-ladder methods give ≈73. The discrepancy is not yet resolved.

No. Every point in an expanding universe sees all other points receding. There is no centre.

Estimate the distance to a galaxy from its recession velocity using Hubble's law: d = v / H0.

Recession Velocity

km/s

Hubble Constant (H0)

km/s/Mpc

Distance

100.00 Mpc

Distance (Mly)326.16 Mly

Distance vs Recession Velocity

v = H0 × d or equivalently d = v / H0

*v* = recession velocity (km/s)

*H0* = Hubble constant (≈70 km/s/Mpc)

*d* = distance in megaparsecs

This linear relation holds well for galaxies not too far away (z < 0.1). For larger distances, cosmological models are needed.

A galaxy recedes at 7 000 km/s with H0 = 70 km/s/Mpc.

Convert redshift to velocity.

Estimate how far back in time you are looking.

Explore distance measurement methods.