Mattig formula

Mattig's formula is one of the most important formulae in observational cosmology and extragalactic astronomy which gives relation between radial coordinate and redshift of a given source. It depends on the cosmological model being used and is needed to calculate luminosity distance in terms of redshift.[1]

Without dark energy

Derived by W. Mattig in a 1958 paper,[2] the mathematical formulation of the relation is,[3]

r_1 = \frac{c}{R_0 H_0} \frac{q_0z+(q_0-1)(-1+\sqrt{1+2q_0z})}{q_0^2(1+z)}

Where, r_1=\frac{d_p}{R}=\frac{d_c}{R_0} is the radial coordinate distance (proper distance at present) of the source from the observer while d_p is the proper distance and d_c is the comoving distance.

q_0=\Omega_0/2 is the deceleration parameter while \Omega_0 is the density of matter in the universe at present.
R_0 is scale factor at present time while R is scale factor at any other time.
H_0 is Hubble's constant at present and
z is as usual the redshift.

This equation is only valid if q_0 > 0. When q_0 \le 0 the value of r_1 cannot be calculated. From this radius we can calculate luminosity distance using the following formula,

D_L \ = \ R_0r_1(1+z) = \frac{c}{H_0q_0^2} \left[q_0z+(q_0-1)(-1+\sqrt{1+2q_0z})\right]

When q_0=0 we get another expression for luminosity distance using Taylor expansion,

D_L = \frac{c}{H_0}\left(z+\frac{z^2}{2}\right)

But in 1977 Terrell devised a formula which is valid for all q_0 \ge 0,[4]

D_L = \frac{c}{H_0}z\left[1+\frac{z(1-q_0)}{1+q_0z+\sqrt{1+2q_0z}}\right]

References

  1. Observations in Cosmology, Cambridge University Press
  2. Mattig, W. (1958), "Über den Zusammenhang zwischen Rotverschiebung und scheinbarer Helligkeit", Astronomische Nachrichten 284: 109, Bibcode:1958AN....284..109M, doi:10.1002/asna.19572840303
  3. Bradley M. Peterson, "An Introduction to Active Galactic Nuclei", p. 149
  4. Terrell, James (1977), "The luminosity distance equation in Friedmann cosmology", Am. J. Phys., 45(9): 869–870, Bibcode:1977AmJPh..45..869T, doi:10.1119/1.11065
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