The Cosmological Red Shift
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Critical Thinking Questions
(Think about these carefully before you consult the answers below)
1) Why is it the "Red Shift?" How come no one talks about a "Blue Shift?" <answer>
2) I've heard that there are processes that will cause the red shift, not just the expansion of the Universe. Couldn't these things becausing the red shifts we detect from distant galaxies, which wouldn't necessarily mean the that Universe is expanding. <answer>
3) What about the "Gravitational Red Shift?" Could that cause the apparent red shift of galaxies, and not a universal expansion? <answer>
4) How can it be that astronomers seem so confident in the idea of the "Cosmological Red Shift," yet they do not know the exact rate of expansion of the Universe called for by the Hubble Law? <answer>
ANSWERS
1) Blue shifts are certainly possible, and some of the nearer galaxies do indeed have velocities of approach that give them a slight blue shift. However, astronomers have found that virtually every galaxy beyond our local group exhibits a red shift, which appears to be in direct corrolation with the distance between those galaxies and the Milky Way. So red shifts are far more common and give evidence to a profound cosmological concept: the Universe is expanding.
2) There are several other processes that can affect the light coming from distant galaxies. Some such as reddening from interstellar dust can be readily eliminated because it does not affect the wavelength of spectral lines. One theory, commonly called the "tired light" theory, advocates the idea that light slowly loses energy as it passes through space, resulting in a lengthening of wavelengths similar to red shifts. The farther the light travels, the more red shift there would be. It would not require that the Universe be expanding. However, the idea of tired light requires the existence of hitherto unknown material in intergalactic space, and poses other problems as well. In general, it is not as simple or elegant an idea as the Doppler red shift.
3) Very strong gravitational fields can cause a red shift, known also an an Einstein shift because it was predicted in Einstein's General Theory of Relativity. However, the amount of mass needed to produce such shifts would be enormous, and would be otherwise detectable between the galaxies, which it is not. (The "missing matter" believed to exist cannot be evoked to explain such enormous graviational fields throughout the Universe to cause galactic red shifts.)
4) The exact nature of the Hubble Constant depends on our knowing the distances to the galaxies. The less precise our knowledge of the distances, the less precise the Hubble constant will be and consequently the less precise our knowledge of the expansion rate. The distances to only a relative few nearby stars are known precisely through stellar parallax. Through our knowledge of these, we have developed the idea of spectral classes and the ability to guess a star's distance. We base the distances to nearby galaxies on these ideas. Then based on our knowledge of the nearby galaxies we can develop a classification system for galaxies and guess at the distances to even farther galaxies. Even the realtive brightnesses of supernovae and the rotation rate of galaxies can help us guess at their distances. The bottom line is that the distant determination for objects very far away is based on a series of guesses. Most of those guesses are probably pretty good, so we hav some confidence that our determinations of the distances to galaxies are reasonable. But because they are based on a series of guesses and assumptions, they are not as accurate at the moment as we would like. That doesn't invalidate the idea of the red shift or that the Universe is expanding. But it forces us, in effect, to make educated guesses of the rate, at least for now.
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