As a light-producing body in space, say a star, moves away from the earth and the perceived frequency of light waves decreases as the waves “stretch out”, the light we see will be redder. Thus the name “redshift.”
However, the differences in light frequency are often so miniscule that they are hard to observe. This is where spectroscopy comes into play. Astronomers rely on the redshift observable in a star or galaxy’s spectrograph. The chemical composition of stellar bodies can be recorded as tiny black lines in a spectral analysis of the colors we can see when looking at the bodies. The resulting diagram is known as a spectrograph. Since elements are known to absorb certain light spectra, or colors, looking at the places where no color is received (the black lines) can tell us what elements exist in the star. Since these lines have consistent locations in the color spectrum based on the elements present, if they are all shifted down the spectrum from their expected positions, the star is said to be redshifted and we can tell that it is moving away.
Why not just call redshift the Doppler Effect?
The Doppler Effect refers specifically to the relationship between a stationary and a moving object and is, therefore, dependent on velocity. The most common type of redshift (Type II), however, results between two stationary objects and is dependent on the distance between them. The space between any two cosmological objects is expanding, leading to an increase in wavelength and decrease in frequency of light that traverses that space.
That is not to say that redshift does not result from objects moving relative to one another as well. In fact, Type I redshift results from the motion of galaxies relative to their neighboring galaxies. As our Milky Way heads on collision course with the relatively nearby Andromeda galaxy, the resulting increase in frequency and decrease in wavelength from our perspective as the galaxies draw closer is colloquially known as “blueshift.” The third type of redshift is much subtler. Gravitational, or Type III, redshift results as gravitational forces from massive special bodies cause the light to shift ever so slightly, warping it or causing it to deviate from its course in an observable way. The verification of gravity’s effect on light also helped validate Einstein’s theory of general relativity.
Why does redshift matter?
Knowing the length of different colored light waves allows astronomers to calculate the distance of an object from earth, as well as determine whether it is moving relatively away from or towards us. Other than the obvious application of measuring distance to deep-space objects, redshift is used to map the expansion of our universe, to determine the relative age of galaxies, and even to find exoplanets. It was through the Edwin Hubble’s observation that the redshifts for objects were larger the farther away they were (implying that the further away a galaxy was, the faster it was moving away from us), that cosmologists came to the initial conclusion that our universe was expanding uniformly. If a star is alternately exhibiting redshift and blueshift (moving away from and then towards us) then an outside gravitational force must be acting upon it. This “wiggle” in a star indicates that it is being orbited by a planet which is exerting a gravitational force pulling the star ever so slightly away from its center of rotation. A tried and true method proving its mettle even at the cutting edge of science, spectrographic redshift analysis was also just used to find the most distant galaxy known to date, a 13.2 billion year old galaxy towards the outer edges of the known universe.
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