Measuring the cosmic proper distance from fast radio bursts

Understand

The cosmic proper distance $d_P$ is a fundamental distance in the Universe.

• Unlike the luminosity and angular diameter distances, which correspond to the angular size, the proper distance is the length of light path from the source to observer.
• However, the proper distance has not been measured before.
• The recent redshift measurement of a repeat fast radio burst (FRB) can shed light on the proper distance.