What is a redshift?

Subtle changes in the color of starlight let astronomers find planets, measure the speeds of galaxies, and track the expansion of the universe.

Astronomers use redshifts to track the rotation of our galaxy, tease out the subtle tug of a distant planet on its parent star, and measure the expansion rate of the universe. What is a redshift? It’s often compared to the way a police officer catches you when you’re speeding. But, in the case of astronomy, these answers all come from our ability to detect miniscule changes in the color of light.

Police and astronomers both rely on a principle called the Doppler shift.  It’s something you’ve experienced while standing near a passing train.  As the train approaches, you hear the horn blowing at a particular pitch.  Suddenly, as the train passes, the pitch drops.  Why does the horn pitch depend on where the train is?

Sound can only move so fast through the air – about 1,200 kilometers per hour (about 750 miles per hour).  As the train rushes forward and blows its horn, the sound waves in front of the train get squished together.  Meanwhile, the sound waves behind the train get spread out.  This means the frequency of the sound waves is now higher ahead of the train and lower behind it. Our brains interpret changes in the frequency of sound as changes in pitch.  To a person on the ground, the horn starts off high as the train approaches and then goes low as the train recedes.

Animation of the Doppler effect

As a car 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 car goes by.  Credit: Wikipedia

Light, like sound, is also a wave stuck at a fixed speed – one billion kilometers per hour – and therefore plays by the same rules.  Except, in the case of light, we perceive changes in frequency as changes in color.  If a lightbulb moves very rapidly through space, the light appears blue as it approaches you and then becomes red after it passes.

Measuring these slight changes in the frequency of light lets astronomers measure the speed of everything in the universe!

Redshift and blueshift

Just like sounds from a moving car, as a star moves away from us, the light becomes redder.  As it moves towards us, the light becomes bluer.  Credit: Wikipedia

Of course, making these measurements is little trickier than just saying “that star looks redder than it should be.”  Instead, astronomers make use of markers in the spectrum of starlight.  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, the rainbow changes!  Gaps appear in the smooth continuum of colors – places where the light literally goes missing.

Redshift of absorption lines

The dark absorption lines of a star at rest (left) get shifted towards red if the star is moving away from Earth (right). Credit: Wikipedia

The hydrogen atoms are tuned to absorb very specific frequencies of light.  When light consisting of many colors tries to pass through the gas, those frequencies get removed 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), 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 towards the red part of the rainbow – a process called redshifting. If the star turns around and now comes flying towards us, the opposite happens. This is called, not surprisingly, blueshifting.

By measuring how far the pattern of lines moves from where it’s supposed to be, astronomers can precisely calculate the speed of the star relative to Earth!  With this tool, the motion of the universe is revealed and a host of new questions can be investigated.

Take the case where the absorption lines of a star regularly alternate between blueshift and redshift. This implies the star is moving towards us and away from us – over and over and over. It tells us the star is wobbling back and forth in space. This could only happen if something unseen was pulling the star around.  By carefully measuring how far the absorption lines shift, an astronomer can determine the mass of the invisible companion and its distance from the star. And that’s how astronomers have found nearly 95% of the nearly 800 known planets orbiting other stars!

Finding planets with doppler shifts

As a planet orbits a star, it tugs the star back and forth.  Astronomers see the star’s movement as an alternating red and blueshift of its spectrum. Credit: ESO

In addition to finding roughly 750 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.  For some reason, most galaxies in the universe were racing away from us!  In 1929, American astronomer Edwin Hubble matched up these redshifts with distance estimates to these galaxies and uncovered something remarkable: the farther away a galaxy, the faster it’s receding.  Hubble had stumbled upon a startling truth: the universe was uniformly expanding!  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.

Hubble's Law

Edwin Hubble 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.  Credit: William C. Keel (via Wikipedia)

Redshifts, the subtle movement of tiny dark lines in a star’s spectrum, are a fundamental part of the astronomer’s toolkit.  Isn’t it remarkable that the principle behind something as mundane as the changing pitch of a passing train horn underlies our ability to watch galaxies spin, find hidden worlds, and piece together the entire history of the cosmos?

Christopher Crockett