National Astronomical Observatory of Japan



Expressing the distance to remote objects

In general, the distances to astronomical object beyond about a couple billion light-years are estimated using redshift -- a change in the wavelength of light from the object. The relation between the redshift and the distance depends on the history of the Universe: how fast (or slow) the Universe has been expanding. This is determined by models (based on physics) and certain characteristic parameters. These parameters are still being determined, so their estimated values change from year to year. Also, there are different ways to define distances (co-moving distances, luminosity distances, ...), which give different values. Because of the complexity, sometimes different astronomers find different distances for the same object.

In NAOJ website articles for press releases or public explanations, we adopt the following policies:

  • We use the cosmological model with the most widely accepted estimates at the time for the cosmological constant and cold dark matter (so-called Λ-CDM model).
  • We use values for the cosmological parameters given in the most recent published results by the major collaborations (*1).
  • We use the light travel distance, which is the product of the speed of light and the travel time between the emission of the light at the object and the detection of the light at Earth.

Note that there are some exceptions, like the "Uchiu-zu" site.

*1 As of 2015, we use Planck 2013: H0 = 67.3 km/s/Mpc, Ωm = 0.315、ΩΛ = 0.685.
Planck Collaboration et al. (2014) "Planck 2013 results. XVI. Cosmological parameters"

Reference: Web-based Cosmological Calculator