What is redshift?
Everyone is familiar with the sound a passing siren makes: the distant noise, the buildup, the peak in pitch as it passes you, and finally the fading of the siren as the vehicle races into the distance. This phenomenon is known as the Doppler Effect and is analogous to the principle astronomers use to calculate how far deep-space objects (galaxies, stars, nebulae) are from earth.
The Doppler Effect, named for Christian Andreas Doppler, occurs as sound waves “bunch up” in front of a moving object. This “bunching up” is actually an increase in the frequency of the sound waves directly in front of the object. As the frequency of sound waves increases, the pitch we hear increases as well. Frequency of sound waves is, in turn, dependent on the velocity of an object; therefore, as the noise-making object approaches the observer, the frequency of the sound wave increases. This results in a higher perceived pitch. The waves “stretch out” and the frequency decreases as the object moves away from the listener, who subsequently hears a decreasing pitch.
The same effect occurs with light. Light — just like sound — has a wave form. However, as the frequency of the light waves increases, the color we see changes. Visible light occurs along a rainbow spectrum – in fact, what we think of as a rainbow is the many wavelengths contained in white light from our sun being refracted, or bent and spread out, by water in the atmosphere. In the rainbow spectrum of visible light, red light is at one end with the lowest frequency and longest wave length and violet light is at the other end, with the highest frequency and shortest wave length. As a light-emitting body in space, say a star, moves away from the Earth and the wavelength of light emitted from it “stretches out,” the light that reaches us on Earth will look redder. Thus the name “redshift.”
However, the differences in light frequency are often so minuscule 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 spectrum to tell its distance and its velocity relative to Earth. The spectrum of an light received from an object can also tell us, among other things, the chemical composition of the object. Each element and isotope emits energy at a very specific and unique wavelength when excited and, likewise, only absorbs energy of very specific wavelengths. Therefore, spikes (emission) and sharp dips (absorption) in the continuum of wavelengths received from an object indicate which elements are present and around it.
The absorption line for neutral hydrogen, for example, has an innate wavelength of 21 centimeters. So, when astronomers notice that the 21-cm line in a spectrum is showing up at a slightly longer wavelength than 21 centimeters, they can tell that the wavelength of light traveling from some star or galaxy through the neutral hydrogen is being “stretched” to a longer wavelength and redshift is occurring. And it’s not just the 21-cm line that gets redshifted. In a spectrum, if the innate emission and absorption features of elements are all shifted to longer wavelengths, the object is moving away from Earth (redshift), and if the lines are shifted to shorter wavelengths, the object is moving towards the Earth (blueshift).
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 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 decrease in Andromeda’s 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 bend 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 Edwin Hubble’s 1929 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. This realization that size of redshift is proportional to distance is why “redshift” is commonly used as a distance metric among astronomers.
Redshift even has striking implications for the discovery planets orbiting other stars. 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 can indicate that it is being orbited by a planet which is exerting a gravitational force that pulls 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.