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Red Shift and Receding Galaxies

  1. Jul 13, 2012 #1

    I understand the idea of red shift and how that explains that galaxies are receding, etc.

    I just don't understand how the measurement is made.

    I know that, in labs, we can shine white light at hydrogen and analyze the absorption spectrum. Some black lines are going to show up, due to the fact that specific wavelength corresponds to a frequency, which corresponds to an energy that is equal to one of the change in energy levels of that element.

    But what do they measure/analyze in order to get the red shifted spectrum? I am terribly confused! :redface:

  2. jcsd
  3. Jul 13, 2012 #2


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    Astronomers look for the same absorption lines of hydrogen and other elements in the spectra of galaxies as the absorption lines produced by these elements in the lab. They find that the absorption lines for the galaxies occur at longer wavelengths than those in the lab.

    You can find lots of info by just doing a web search on "redshift galaxies". For example,
  4. Jul 13, 2012 #3
    I see! Thank you.

    Oh and, if you don't mind, could you help me with another problem I am having?

    I understand how the red shift of light from distant galaxies can be used as evidence for an expanding universe. But, how does the fact that the galaxies further away are moving faster serve as evidence too?
  5. Jul 13, 2012 #4


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    The redshift of galaxies is due to the expansion of space itself. A galaxy farther away will increase its distance from us faster than a nearby galaxy because there is more space between us and the distant galaxy to expand. An analogy would be to put a cookie sheet in an hot oven. The sheet will expand while its temperature is increasing. The speed at which two points of the sheet move away from one another while the sheet is expanding is proportional to the distance separating them.

    However, analogies can be misleading. The redshift of light from galaxies is not due to the doppler effect. Rather, the wavelength of light from distant galaxies is stretched by the expansion of space itself. At least that's my understanding.
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