Hubble’s Law

What is Hubble’s Law?

Hubble’s law statement is given as

The for distant galaxies, redshift in the spectra is directly proportional to their distance.

Hubble’s law is also known as Hubble-Lemaitre law, studied in physical cosmology. Only with the help of Hubble’s law the universe can expand and fulfil the cosmological principle.

Hubble’s Law Formula

Hubble’s law formula is given in the following table:

\(v = H_{0}d\)


  • v is the velocity of the galaxy in km/s
  • H0 is Hubble constant in km/s/Mpc
  • d is the distance of the galaxy in Mpc

Limitations of Hubble’s law

By determining the shift in observed light into redshift in the spectrum, one can determine the distance of the galaxy from us using Hubble’s equation after measuring the recession velocity. Following are the limitations of Hubble’s law which makes the measurement challenging:

  • Because of the intrinsic motion of galaxies, observed velocity gets influenced.
  • Galaxy orbiting due to gravitational movements.

What is Hubble Constant?

Hubble constant is defined as the unit of measurement which is used for describing the expansion of the universe. Hubble‘s constant H is 160 km/s per million-light-years.

What is Redshift?

Redshift is the phenomenon in which an object’s wavelength increases due to electromagnetic radiation. Blueshift is opposite to redshift where the energy increases due to shorter wavelengths which are also known as negative redshift. The main reasons for redshift are as follows:

  • Doppler effect ie; the movement of objects either closer or apart from each other in space.
  • Strong gravitational force leads to gravitational redshift.
  • Cosmological redshift is an expansion of space such that objects are separated without the change in their position.

Redshift formula

Redshift formula is given in the below table:

\(z = \frac{\Delta \lambda }{\lambda }\)


  • z is the redshift
  • Δλ is the shift in wavelength in the spectra
  • λ is the wavelength

Example of Hubble’s Law

Q1. For some cluster, velocity is measured as v = 103 km/s. What is the distance? Assume H0 = 60km/s/Mpc
Ans: We know,
v = 103 km/s
H0 = 60km/s/Mpc
The formula used is:
d = v/H0
=(103 km/s)/(60km/s/Mpc)
=16.7 Mpc

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