Astronomy Essentials

What do redshifts tell astronomers?

Redshifts: Concentric circles, wide and red on the left, narrow and blue on the right.
Image via Wikimedia Commons.

Astronomers use redshifts to measure how the universe is expanding, and thus to determine the distance to our universe’s most distant (and therefore oldest) objects. What is a redshift? It’s often compared to the high-pitched whine of an ambulance siren coming at you, which drops in pitch as the ambulance moves past you and then away from you. That change in the sound of an ambulance is due to what’s called the Doppler effect. It’s a good comparison because both sound and light travel in waves, which are affected by their movement through air and space.

Sound can only move so fast through the air; sound travels at about 750 miles (1,200 kilometers) per hour. As an ambulance races forward and blares its siren, the sound waves in front of the ambulance get squished together. Meanwhile, the sound waves behind the ambulance get spread out. This means the frequency of the sound waves is higher ahead of the ambulance (more sound waves will strike a listener’s ear, over a set amount of time) and lower behind it (fewer sound waves will strike a listener’s ear, over a set amount of time). Our brains interpret changes in the frequency of sound waves as changes in pitch

Like sound, light is also a wave traveling at a fixed speed: 186,000 miles (300,000 km) per second, or some one billion kilometers per hour. Light, therefore, plays by similar rules as sound. 

But, in the case of light, we perceive changes in wave frequency as changes in color, not changes in pitch. 

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Animated diagram showing waves closer together in front of moving car and wider apart behind it.
As a vehicle moves, sound waves in front of it get squished up while those behind get spread out. This changes the perceived frequency and we hear the pitch change as the vehicle goes by. Credit: Wikipedia
Redshift and blueshift
Similar to sounds from a moving vehicle, as a star moves away from us, the light becomes redder. As it moves towards us, the light becomes bluer. Image via Wikipedia.

For example, if a lightbulb were to move very rapidly through space, the light would appear blue as it approaches you and then become red after it passes. Measuring such slight changes in the frequency of light lets astronomers measure the speed – and therefore the distance – of everything in the universe! That’s true because the faster an object moves away from us, the farther away it is.

Thus, in our expanding universe, a measurement of speed translates to a measurement of distance.

Here’s a recent example. Astronomers said in early January 2020 that the most distant quasar known at this time – quasar J0313-1806 – has a record-setting redshift of z = 7.64. In accordance with astronomers’ interpretations of redshift, we’re seeing quasar J0313-1806 – a highly luminous galaxy nucleus in the early universe, thought to be powered by a supermassive black hole – just 670 million years after the Big Bang, or more than 13 billion light-years away.

Read more: A new record for the most distant quasar

Or consider an even more distant object, not a very bright quasar, but instead just a regular galaxy in the early universe. GN-z11 is a high-redshift galaxy found in the direction of the constellation Ursa Major, the Great Bear. GN-z11 is currently the oldest and most distant known galaxy in the observable universe, with a redshift of z = 11.09. That redshift corresponds to a distance of 13.4 billion light-years. So we see this object as it existed 13.4 billion years ago, just 400 million years after the Big Bang.

Very many distant galaxies, looking like a star field, with inset of irregular pink blob.
The object in the inset is currently the most distant known galaxy, called GN-z11. Its name is derived from its location in the GOODS-North field of galaxies – captured by the Hubble Space Telescope – and its high cosmological redshift number (GN + z11). It’s thought this galaxy will remain the most distant galaxy known until the James Webb Space Telescope is launched, hopefully in late 2021, and begins observing at even greater distances. Image via NASA.

Of course, making these measurements is a bit trickier than just saying “that star looks redder than it should be.” Instead, astronomers make use of markers in the spectrum of starlight. This is the study of spectroscopy. If you shine a flashlight beam through a prism, a rainbow comes out the other side. But if you place a clear container filled with hydrogen gas between the flashlight and the prism, gaps appear in the smooth rainbow of colors, places where the light literally goes missing.

Vertical columns of rainbow light with dark lines across them at different heights.
The dark absorption lines of a star at rest (left) get shifted towards red if the star is moving away from Earth (right). Image via Wikipedia.

The hydrogen atoms are tuned to absorb very specific frequencies of light. When a beam of light consisting of many colors passes through the gas, those frequencies get removed – absorbed – from the beam. The rainbow becomes littered with what astronomers call absorption lines. Replace the hydrogen with helium, and you get a completely different pattern of absorption lines. Every atom and molecule has a distinct absorption fingerprint that allows astronomers to tease out the chemical makeup of distant stars and galaxies.

When we pass starlight through a prism (or similar device suitable for telescopes, such as diffraction gratings), we see a forest of absorption lines from hydrogen, helium, sodium, and so on. However, if that star is hurtling away from us, all those absorption lines undergo a Doppler shift and move toward the red part of the rainbow. This is what we call a redshift. For stars heading toward us, the opposite happens, and the lines are shifted toward the blue end of the spectrum; they are blueshifted (generally, astronomers only use the term redshift to simplify things, and just put a negative sign in front of it if it’s a blueshift).

By measuring how far away the lines are located from where they’re supposed to be in the spectrum, astronomers can calculate the speed of a star or a galaxy relative to Earth, and even how a galaxy rotates: by measuring a different redshift for one side of the galaxy compared to the other, you can see which side is moving away from you and which side is moving toward you.

With this tool, the motion of the universe is revealed and a host of new questions can be investigated.

And galaxies aren’t the only things that can be investigated with redshifts. Astronomers have learned to tease out the subtle tug of a distant planet on its parent star, thus revealing the planet to astronomers. If a star in our Milky Way galaxy has a hidden planet – and if astronomers see that the star sometimes exhibits a slight redshift and other times a slight blueshift – the astronomers infer that star is alternating between moving toward and away from us. They refer to this movement as a “wobble” of the star in space. Something must be pulling on the star, causing it to wobble. By measuring how far the absorption lines shift, an astronomer can determine the mass of the invisible companion and its distance from the star, and come to the conclusion that a planet is in orbit around the star!

Two positions of distant star showing its rays shorter and longer depending on movement.
As a planet orbits a star, it tugs the star back and forth with tiny movements. Astronomers see the star wobbling as an alternating red and blueshift of its spectrum. Image via ESO.

In addition to finding other worlds, redshifts also led to one of the most important discoveries of the 20th century. In the 1910s, astronomers at Lowell Observatory and elsewhere noticed that the light from nearly every galaxy was redshifted: most galaxies in the universe were racing away from us! A Belgian scientist, Georges Lemaître, who was also a priest, recognized that the recession velocities of the galaxies could be explained by a startling truth: the universe is expanding! In 1929, American astronomer Edwin Hubble matched up redshifts with distance estimates to the galaxies and uncovered something remarkable: the farther away a galaxy, the faster it’s receding. This relation, the Hubble law, was renamed in 2018 by the International Astronomical Union to the Hubble–Lemaître law.

What came to be known as the cosmological redshift was the first piece of the Big Bang theory, and ultimately a description of the origin of our universe.

The list of the most distant astronomical objects is always changing as astronomers find higher and higher redshifted objects on the brink of the observable universe. Galaxies, quasars and even gamma-ray bursts travel for eons across the cosmos, delivering their faint red light, and revealing a little more of the secrets of the universe.

Graph with line of plus signs from lower left to upper right.
Edwin Hubble and colleagues found a correlation between distance to a galaxy (horizontal axis) and how quickly it’s moving away from Earth (vertical axis). The movement of galaxies in a nearby cluster adds some “noise” to this plot. Image via William C. Keel/ Wikipedia.

Bottom line: A redshift reveals how an object in space (star/planet/galaxy) is moving compared to us. It lets astronomers measure a distance for the most distant (and therefore oldest) objects in our universe.

January 24, 2021
Astronomy Essentials

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Christopher Crockett

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