Ask A Scientist© Astronomy Archive

Question:  Could the red shift be caused by any other phenonenon"something" other than
velocity of recession. ? In other words, could the mass of all
the matter beyond  the source of the emitted red shifted light
cause a red shift?
ken e anderson

Answer 1:  Yes. There is also a gravitational redshift as well as a Doppler redshift.
However, it is only significant for light emitted from extremely massive
bodies like quasars or black holes (light CAN be emitted from NEAR a
black hole, as long as it's outside the black hole's event horizon).
This phenomenon can be understood if you assume that photons have an
equivalent rest mass proportional to their energy,
m=E/c^2=h*nu/c^2 where h=Planck's constant; nu=photon's frequency.
If we let g=GM/R be the gravitational potential where the photon is
emitted, then the photon loses energy = delta_E = g*m when it flies out
of the gravitational potential well of the mass M at radius R. If you
work out the math, you'll find the photon's changes in frequency and
wavelength to be:
delta_nu/nu = g/c^2  and  delta_lambda/lambda = 1/( c^2/g - 1 )
In astronomy we call delta_lambda/lambda the redshift of the object
emitting the photon (usually designated by the letter Z). If g=c^2,
then the redshift is infinite, and this would correspond to a photon
being emitted right at an event horizon.
See my next response for a modest attempt at explaining
COSMOLOGICAL REDSHIFT.
hawley

Answer 2:  Cosmological redshift is an OBSERVATIONAL phenomenon discovered by
V. M. Slipher in 1912 when taking spectra of distant galaxies. Edwin
Hubble and M. Humason later showed that the amount of redshift was
proportional to the faintness of the galaxy which presumably varies
with the galaxy's distance. This relationship has come to be known as
"Hubble's Law" and is expressed as z=H*r/c where z is the redshift
(see previous response); H is the so-called Hubble constant; r is the
distance to the galaxy; and c is, as always, the speed of light.
There has been much controversy surrounding the origin of the cosmo-
logical redshift -- whether it is due to Doppler redshift, gravitational
redshift, or something as yet unknown. Current thinking favors the
Doppler explanation and is used as evidence that the universe is
expanding which, in turn, led to the Big Bang Theory of cosmology.
(There is other eveidence that also supports the Big Bang, e.g.,
 "primordial nucleosynthesis."
One of the main goals of the Hubble Space Telescope is to pin down
the exact value of the Hubble constant (another is to look for other
stellar systems with planets.) If the Big Bang theory of cosmology
and Einstein's Theory of General Relativity are both correct, then
the value of the Hubble constant determines whether the universe is
open or closed, i.e., whether the universe will continue to expand
forever or eventually collapse upon itself.
hawley