The point is, arguments about analogies (and, by extension, the proper words in which to translate some well-accepted scientific phenomenon) are not "right" or "wrong." The analogies are simply "useful" or "useless," "helpful" or "misleading." And which of these categories they fall into may depend on the context. These are only arguments about an analogy, i.e. These are not arguments about the theory €” everyone agrees on what GR predicts for observables in cosmology. Sean Carroll expressed this well in his blog post: The main thing to realize about all this is that we’re talking purely about a question of interpretation: everybody (at least, everybody sane) agrees on the physics - the argument is only about what words to wrap around the physics. Hogg and I argue for the rehabilitation of the idea that the galaxies are moving and the observed redshift can be regarded as a Doppler shift. In the cosmological context, though, people often say that the observed redshift has a different explanation: they say that the galaxies aren’t “really” moving, but rather that space itself is expanding, which causes the light to be stretched out in wavelength. Most of the time, when you see a redshift, it’s a Doppler shift - that is, it’s caused by the fact that the observed object is moving away from you. This fact is the observational basis for the idea of the expanding Universe. The most important fact in cosmology is that the light from distant galaxies is redshifted. I’ll quickly recap the big idea, but then I want to comment on some followups to our paper: a blog post by Sean Carroll (from way back when we first posted our paper) and a recent paper by MichaÅ‚ Chodorowski.įirst the background. I said a bit about the argument of the paper in a previous post. The talk is based on the paper David Hogg and I wrote last year.
REDSHIFT EQUATION HOW TO
I’m giving a talk here in Paris tomorrow on the question of how to interpret the cosmological redshift. Frequently Asked Questions about Black Holes.Frequently Asked Questions About Black Holes.We can then use a simple equation to calculate the redshift value. We can then calculate the wavelength from the observed spectra based on the spectrum (for this example the observed line is at 675 nm). We can then use this to calculate the exact redshift.įrom the graphic above we can take the Hydrogen Alpha emission line at 656.2 nm. Once we find a known spectral line we can work out it's wavelength in the spectra. This indicates a redshift and we can tell that the galaxies are moving away from us (or we are moving away from the galaxies). When we compare the two we see a correlation between the Hydrogen lines of the Sun and of the distant galaxies, the only difference is that the absorption lines in the galaxies are all moved up (towards the red). One from out Sun (a known spectra - we know each of the absorption lines) and one from a supercluster of distant galaxies. In the diagram above you can see two spectra. A very common element in space is hydrogen. To determine the redshift features in the spectrum (such as absorption lines, emission lines, or other variations in light intensity) are searched for and if found compared with known features in the spectrum of various elements. The spectrum of light coming from a distant object can be measured through spectroscopy. In 1901 Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors. In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.1 Å in the red. In 1868, British astronomer William Huggins was the first to determine the velocity of a star moving away from the Earth by this method. The effect is sometimes called the "Doppler-Fizeau effect". The first doppler redshift was described in 1848 by French physicist Armand-Hippolyte-Louis Fizeau, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. The hypothesis was tested and confirmed for sound waves by the Dutch scientist Christoph Hendrik Diederik Buys Ballot in 1845. The Doppler effect is named after Christian Andreas Doppler who offered the first known physical explanation for the phenomenon in 1842. Wavelength increases up towards the red and beyond (frequency decr History of Redshift and Blueshift Absorption lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the optical spectrum of the Sun (left).
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