How red light revealed that the Universe is expanding

We have known for some time that the universe is expanding, and that its expansion is accelerating. But how do we know that? We'll show you how Edwin Hubble revealed that the universe is getting away from us.

One of the biggest shocks, when studying science history, is much our view of the universe has changed in the last century. Less than a hundred years ago, brilliant and educated astronomers were still debating whether or not the Milky Way made up the entire universe. A decade after that, we still sure that the universe was at a steady state. Then we thought it was expanding, but would eventually collapse or slow to a standstill. Now we see that the universe is undergoing ever accelerating expansion. But how did astronomers look out at a sea of bright dots and come to these conclusions?


Hubble and the Expanding Universe

In the 1920s, Edwin Hubble got his hands on two things that would let him revolutionize the way people saw the universe. One was the biggest telescope ever yet built. The other was an interesting finding from fellow astronomer Vesto Slipher. Slipher had been watching what he called nebulae - but what we now know were galaxies - and been intrigued by their light. The light was a great deal redder than he had expected it to be. He attributed this to redshift.

Imagine you and another person are standing along a length of rope. Every second you pluck the rope. Each wave that this creates travels to the other person, and so they see the rope twitch once per second. If you were to start walking away from the other person at a steady pace, the distance you cover each second would add to the distance the wave in the rope had to travel, and so, although you plucked the rope once a second, they would see it move every 1.1 seconds. The faster you walked, the more time for the other person would pass between rope movements. The same thing happens with waves of light. As a light source recedes from it observer, the peaks of the waves of light grow more and more spaced out from each other. This shifts them to the red end of the light spectrum. Slipher concluded that the light from the "nebulae" was red because they were moving away from Earth.


Hubble took his shiny new telescope and looked for redshift. He found it everywhere, but it certain stars seemed far more affected than others. Some stars and galaxies were only slightly shifting towards red, but sometimes the redshift was massive. After gathering a lot of data, Hubble sat down and made a graph that compared the redshift of an object to its distance from Earth. The farther away an object was, the more the light from it was redshifted. The universe looked like the rubber skin of a balloon as it was being blown up - as it got bigger, every point retreated from every other point.

Faster Universe! Kill! Kill!

So it was well established at the beginning of the twentieth century that the universe was expanding. Most scientists assumed, looking at the data, that it was slowing in its expansion. Some thought it would gradually stretch toward a finish line it never actually reached. Others thought it would collapse back. Astronomers agreed that they needed to get a really good look at the universe. For that they needed better telescopes, which they got, and a little help from the universe, which came in the form of the type 1a supernova.


A white dwarf is a star doesn't have quite enough matter in it to make a supernova, and should die a quiet death. White dwarfs that go supernova have a companion nearby. When the companion star gets too close, the white dwarf steals its material and goes off in a supernova the instant it gets enough mass to do so. Because all of these supernovae go off when they're at approximately the same mass every stage of the explosion, including the aftereffects, looks the same from one type 1a supernova to the next. They all have the same brightness. Because we know how brightness varies with distance, as long as we can measure the brightness of the type 1a supernova we know how far away it is from us, and so how long the light has been traveling. And when we look at the redshift of the light, we know how much the universe expanded while the star has been traveling.


When astronomers looked at distant, and so ancient stars, they noticed the distance didn't match up with the amount of expansion. They light from the stars had been traveling longer than expected to account for the expansion factor. It was as though the expansion was slower in the past. The universe's expansion wasn't slowing down. It was speeding up.

This is the reason behind the famous "dark energy" that physicists are wondering about. It's the energy (or whatever) that is stretching, pulling, pushing or otherwise embiggening the universe. Until we know what it is, we don't have the full story behind the universe's baffling expansion. We just know that it's expanding.


Top Image: Rogelio Bernal Andreo

Sombrero Galaxy Image: Hubblesite

Type 1a Supernova: Chandra X-Ray Observatory

Via Sloan Digital Sky Survey, UCLA, Cornell, ABC, Dark Energy Survey, UCLA.


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