By looking far away, we can look back in time. This simple yet mind-blowing fact allows astronomers to observe snapshots of the universe at different times, using them to piece together the complex history of cosmic evolution. With each new telescope we build, we can see farther and earlier into the history of the universe. The James Webb Space Telescope (JWST) hopes to peer all the way back to when the first galaxies were forming.
The notion that looking out corresponds to looking back is relatively young. It comes from Einstein’s theory of special relativity, which asserts—among other things—that light travels at the speed of light, and that nothing travels faster than that. On an everyday basis, we almost never experience the consequences of this concept, because the speed of light is so large (300,000 km/s, or about a million times faster than a jet plane) that this “travel time” hardly matters. If we turn on the light or someone sends us an email from Europe, we perceive these events (we see the light bulb go on, or receive the email) as instantaneous, because light takes only a tiny fraction of a second to travel through a room or even around the entire Earth. But on an astronomical scale, the finiteness of the speed of light has profound implications.
The sun is about 150 million km away, which means that light from the sun takes about 8 minutes and 20 seconds to reach us. When we look at the sun, we see a picture that is 8 minutes old. Our nearest neighboring galaxy, Andromeda, is about 2.5 million light years away; when we look at Andromeda, we are looking at it as it was 2.5 million years ago. This may sound like a lot on human time-scales, but it’s a really short time as far as galaxies are concerned; our “stale” picture is probably still a good representation of how Andromeda looks today. However, the sheer vastness of the universe ensures that there are many cases for which light’s travel time matters. If we look at a galaxy one billion light years away, we are seeing it as it was one billion years ago, enough time for a galaxy to change significantly.
So just how far back in time can we see? The answer to this question is determined by three different factors. One is the fact that the universe is “only” 13.8 billion years old, so we can’t look back in time to an epoch more remote than the beginning of the universe, known as the Big Bang. Another issue—at least if we are concerned with astrophysical objects such as galaxies—is that we need something to look at. The primordial universe was a scalding soup of elementary particles. It took some time for these particles to cool down and cohere into atoms, stars and galaxies. Finally, even once these objects were in place, seeing them from Earth many billions of years afterwards requires extremely powerful telescopes. The brightness of physical sources decreases rapidly with distance, and trying to spot a galaxy at a distance of 1 billion light years is as challenging as trying to spot a car’s headlight about 60,000 miles away. Trying to spot the same galaxy at a distance of 10 billion light years is 100 times harder.
So far, this has been the driving factor in limiting the distance to the farthest galaxies that we can see. Until the 1980s, all of our telescopes were based on the ground, where the Earth’s atmosphere and light pollution hinder their performance. Nonetheless, we were already aware of galaxies over 5 billion light years away. The launch of the Hubble Space Telescope in 1990 allowed us to smash this distance record many times and, as I write this, the farthest known galaxy is located a staggering 13.4 billion years in the past.
This brings us to one of the key issues of modern astronomy: what properties of these faraway galaxies can we actually measure? While observations of nearby galaxies show their shapes and colors in great detail, often the only piece of information that we can collect about the most distant galaxies is their overall brightness. But by looking at them with telescopes that are sensitive to frequencies of light beyond the visible range, such as ultraviolet, radio and infrared, we can uncover clues about the stellar populations of the galaxy, as well as about its distance from us.
By observing galaxies at as many different frequencies as possible, we can create a spectrum, which shows how bright the galaxy is in each type of light. Because the universe is expanding, the electromagnetic waves that are detected by our telescopes have been stretched along the way, and it so happens that the amount of stretch in the spectra is proportional to the distance of the galaxy from us. This relationship, called Hubble’s Law, allows us to measure how far away these galaxies are. Spectra can also reveal other properties, such as the total amount of mass in stars, the rate at which the galaxy is forming stars and the age of the stellar populations.
Only a few months ago, a team of astronomers from the U.S. and Europe used observations from the Hubble Space Telescope and the Spitzer infrared space telescope to discover the farthest galaxy known to date, GN-z11. Observed only 400 million years after the Big Bang (“when the universe was only 3 percent of its current age,” according to principal investigator Pascal Oesch) it has a mass of one billion suns combined together, about 1/25th of our own Milky Way.
GN-z11 is forming stars about 20 times faster, at the remarkable rate of 25 new suns per year. “It’s amazing that a galaxy so massive existed only 200 million to 300 million years after the very first stars started to form. It takes really fast growth, producing stars at a huge rate, to have formed a galaxy that is a billion solar masses so soon,” explains Garth Illingworth, another investigator on the discovery team.
The existence of such a massive object at such an early time clashes with current scenarios of cosmic assembly, posing new challenges for scientists who work on modeling galaxy formation and evolution. “This new discovery shows that the Webb telescope (JWST) will surely find many such young galaxies reaching back to when the first galaxies were forming,” says Illingworth.
JWST is scheduled for launch in 2018 and will orbit around the sun/Earth system from a special location 900,000 miles away from us. Like Hubble, JWST will carry several instruments, including powerful cameras and spectrographs, but it will have enhanced sensitivity: its primary mirror will be almost seven times larger, and its frequency range will extend much further into the infrared region. The different range of frequencies will allow JWST to detect spectra with higher stretch, belonging to farther objects. It will also have the unique capability to take spectra of 100 objects simultaneously. With JWST, we expect to push the distance barrier even farther, to an epoch only 150 million years after the Big Bang, and to discover the very first galaxies ever formed. JWST will help us understand how the shapes of galaxies change with time, and what factors govern galaxy interactions and mergers.
But JWST will not just look at galaxies. By peering at the universe in infrared light, we will be able to see through the thick curtains of dust that enshroud newly born stars and planets, providing a window onto the formation of other solar systems. Furthermore, special instruments called coronagraphs will enable imaging of planets around other stars, and hopefully lead to the discovery of several Earth-like planets able to host life. For anyone who has ever looked at the sky and wondered what’s out there, the next decade is going to be a very exciting time.