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Physics Quiz

The Expanding Universe





Cosmological Redshift

Photons propagating through the expanding space are stretched between their point of emission emission (em) and their point of observation (obs), creating an increase in wavelength: the cosmological redshift.

redshift z
This effect is prescribed by the current cosmological model as an observable manifestation of the time-dependent cosmic scale factor a(t) in the following way:

redshift z

(1+z) is the factor by which the universe has expanded while the photon was travelling towards the observer. If tobs is the present time, then a(tobs)=1, and the cosmic scale factor at the time of emission is




e. g.:
for a
cosmological redshift of z=3 the cosmic scale factor at the time of emission was a=1/4.

The discovery of the linear relationship between redshift, interpreted as recessional velocity, and distance yields a straightforward mathematical expression for Hubble's Law as follows:

Hubble Law

v = recession velocity, in km/sec
H0 = Hubble Parameter (today), H0 = (70 +/- 7) km/sec/Mpc
D = (comoving) distance from the galaxy to the observer, in Mpc

H0 may be interpreted as the rate of expansion of the Universe, and the "Hubble time" 1/H0 as the current age of the universe.

Web Links

Hubble's law (Wikipedia)

Redshift (Wikipedia)

Rotverschiebung (Wikipedia)

The Expanding Universe (Ohio State University)



2001-2023 J. Giesen

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Updated: 20.10.2023