To examine the way the universe behaved in the past, astronomers look at extremely distant objects, such as supernovae in galaxies billions of light-years away. But how does that work? How can astronomers look out into space and see the universe back in time?
The answer lies in the speed of light. Light waves move very fast, about 186,000 miles per second (300,000 km/s). Light moves so fast that as you go about your daily life, it appears to travel instantaneously from one place to another. For example, it takes only a few billionths of a second for light to travel across your bedroom when you turn on a lamp.
In space, however, the distances are so immense that the time that light takes to travel is noticeable.
Going the Distance
The Moon is Earth’s closest companion, at about 239,000 miles (390,000 km) away. Light takes around 1.3 seconds to travel that distance.
The Sun is 93 million miles (150 million km) away, far enough that the light it emits needs about 500 seconds to travel to Earth. We call the distance light takes to travel in a second a light-second, the distance it takes to travel in a minute a light-minute, and so on. So the Sun is about eight light-minutes away from Earth. The light shining on you right now first left the Sun eight minutes earlier.
Across our Milky Way galaxy, distances are measured in terms of how many years it takes light to travel. The nearest star is over four light-years away. So when we look at that nearest star, we see it not as it is today, but as it was four years ago. We are seeing the light that left that star four years previously and is just reaching us now.
The diameter of our galaxy is 100 thousand light-years. So when we look at even more distant stars, we see them as they were thousands to tens of thousands of years ago, depending on how far away they are and thus the distance their light has had to travel.
A Galaxy Far, Far Away
Galaxies are yet farther away in both space and time. Our nearest large neighbor galaxy, Andromeda, is about two and a half million light-years away. The Virgo Cluster of galaxies is the largest nearby collection of galaxies, at about 60 million light-years from the Milky Way. The light we see today from galaxies in the Virgo Cluster started on its path toward us at the same time as the age of the dinosaurs was ending on Earth. If you were in a Virgo Cluster galaxy today, and you had a telescope powerful enough to study Earth, you would be able to see the prehistoric reptiles.
Very distant galaxies are billions of light-years away. At that distance, their light tells what the universe was like billions of years ago. Since the age of the universe is about 14 billion years, these distant observations allow astronomers to measure changes over the lifetime of the universe. So when astronomers look out into space, they are essentially also looking back into time.
This fact was vital to the teams studying the expansion of space, because their goal was to compare the speed of the universe’s expansion in the past with the speed of the universe today. By studying extremely distant supernovae in faraway galaxies, they were able to judge the speed of the universe’s expansion in the early universe.