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Cosmological red shift

  1. Jul 31, 2004 #1
    What is the cosmological red shift? I've seen it before and couldn't figure out what exactly it was. All I know is that it has something to do with the effects of gravity and how it relates to photon frequency.
  2. jcsd
  3. Aug 1, 2004 #2
    I ripped this from http://wikipedia.org

  4. Aug 1, 2004 #3

    That tells me that a photon experiences a red or blueshift with strong gravitational fields, but it still doesn't answer my question of what a red(or blue)shift is.
  5. Aug 1, 2004 #4
    When we look at other galaxies, we see their lights "redder". That's because they're going away from us. You know the Doppler effect? When you approach a wave emitter, the frequency of waves seems as if higher to you. And if you go away from the emitter, you find the frequency lowered. Thinking of light as an electromagnetic wave, the light of the far galaxies are shifted to red, because the wavelength of red light is longer.
  6. Aug 1, 2004 #5
    A naive question:
    How can the Doppler effect happen for light in the first place, if light speed is not additive?

    In the case of sound waves, for instance, the time spent to travel the peak-to-peak distance (time between one peek striking the ear and the next) can vary because of the relative velocity between emitter and receiver; but I thought c to be a constant for any reference frame, thus such addition won't serve as an explanation for frequency shift.

    On the other hand, if the redshift happens due to time "curving" along a geodesic, then I have yet another naive question:
    Since no time curving will make light going faster than c, then can we predict, from this fact alone, that there will be no blueshifts? (And there goes the idea of an expanding universe down the toilet :biggrin: .)
    Last edited: Aug 1, 2004
  7. Aug 1, 2004 #6


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    AFAIK, the term "redshift" originated in spectroscopy. In early (and many modern) spectroscopes, the light from an object first passed through a slit, and was then dispersed; a single frequency (wavelength) thus appeared as a 'line' in the resulting spectrum. Atomic spectra are full of discrete lines, corresponding to the electronic transitions in excited atoms or ions. If the source of the light is in motion wrt the spectroscope, with a component towards or away from it, a particular atomic line will be seen at a different frequency/wavelength - if the source is moving towards the spectroscope, the line will appear at a higher frequency/shorter wavelength; and if away at a lower frequency/longer wavelength. As red light has a lower frequency than blue, the 'shifts' caused by relative radial motion came to be called blueshift and redshift.

    When the first spectroscopic surveys of (non-local) galaxies was undertaken, Hubble noticed that there appeared to be a relationship between the distance of a galaxy and the redshift of the lines in its spectrum (the most prominent of which are absorption lines of atoms and ions of elements such as Mg and Ca in the combined light of the common stars in the galaxy). This was called the Hubble relationship.

    Around the same time, Einstein's theory of General Relativity was applied to the universe as a whole, and it was discovered that the easiest solutions to the equations implied a universe that was expanding. Scientists of the day put two and two together, and concluded that the Hubble relationship was evidence for an expanding universe, which - consistent with GR - means that space itself is expanding. Redshifts which arise from this effect are called 'cosmological redshifts', to distinguish them from redshifts which arise from the light source being in a gravitational well (cf the spectroscope) - 'gravitational redshift' - and those from the relative bulk motion of the object and spectroscope.
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