How Close to the Big Bang Can We See?
As astronomical records go, one is in constant jeopardy: Last week, headlines trumpeted yet another galaxy as the most distant—and therefore oldest—ever seen. Light reaching us from this latest galactic-distance champion, which bears the unfortunate moniker of z8_GND_5296, left there 13.1 billion years ago. That means the picture we see of it now comes from just 700 million years after the big bang.
Similar announcements seem to crop up every few months, bringing up the question of just how much further back in time and deeper into space we will ever be able to look. (Because other galaxies are moving away from us, the most distant are also the oldest.) Should z8_GND_5296 settle in for a nice, long reign as the farthest confirmed galaxy, or is it possible to see even more distant objects by spotting light that dates even closer to big bang? How close to the big bang scientists possibly see?
The short answer is that z8_GND_5296's days are numbered. Over the next several years, new observing methods and telescopes should turn up plenty of contenders. And before the decade is out, we might even succeed in looking all the way back into the universe's first few hundred million years, when the very first stars and galaxies are thought to have formed. But the big question is: What will these ancient cosmic objects be like?
"If galaxies are there and are bright enough to see, we will see them... but are they bright enough?" says Steven Finkelstein, an assistant professor of astronomy at the University of Texas at Austin and lead author of the Nature paper describing z8_GND_5296.
Jennifer Lotz, an astronomer with the Space Telescope Science Institute (STScI) in Baltimore, agrees: "We don't know what's happening in the first half-billion years of the universe."
STScI helps operate the Hubble Space Telescope, which has led the way in discovering far-off galaxies and other objects. Very little light from ancient galaxies makes its way to Earth, giving telescopes like Hubble a needle-in-the-haystack task. "It's that little pinprick of light coming from the very first galaxies that we can hope to see," Lotz says.
And with so little evidence, it's hard to be sure what you're seeing. Z8_GND_5296 turned up in one of Hubble's sky surveys, and over time these searches have netted many contenders for the title of most distant galaxy. But many of these candidates have not been confirmed through spectroscopy—the gold standard for measuring astronomical distances.
Spectrometers identify the signatures of chemical elements in light, which can help to tell you what a star or galaxy is made of. However, as the universe expands, light waves get stretched out, and their signatures move deeper into the redder portion of the electromagnetic spectrum where light waves have longer wavelengths. The extent to which these signatures are "redshifted" reveals the distance the light has traveled.
To confirm z8_GND_5296's distance, Finkelstein's group used a sensitive infrared spectrometer on the W.M. Keck Observatory in Hawaii. But measuring redshift for galaxies out in z8_GND_5296's neck of the cosmic woods is tough work. Light carrying a preferred spectroscopic signature known as a Lyman-alpha emission line—from hydrogen, and made in abundance when stars form—tends to get absorbed by hydrogen gas between us and the emitting galaxy, especially when that galaxy is very distant. In fact, 42 of the 43 candidates Hubble turned up in its survey displayed no Lyman-alpha lines. Only z8_GND_5296 had one.
Deeper and Deeper
To give astronomers a leg up in finding and confirming these candidate distant galaxies, NASA recently inaugurated its Frontier Fields project to hunt for galaxies up to 100 times fainter than what Hubble and other space observatories can normally glimpse. Frontier Fields will take advantage of naturally occurring cosmic "zoom lenses," created by the gravitational warping of background light by foreground galaxy clusters.
"We'll be looking at these really whopping massive clusters of galaxies that act like natural telescopes and magnify galaxies behind them," says Lotz, who is also a Frontier Fields principal investigator. "These objects will appear much brighter than they actually are, and we might get lucky that some are bright enough to do spectroscopic confirmation with Lyman-alpha."
If not, astronomers might try using other spectroscopic lines to nail down galactic distances. One of these being tested is a carbon line, which astronomers are using at a giant new ground-based facility called the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. Five of the six galaxy clusters Frontier Fields will probe were specifically selected to fall within ALMA's field of view.
Yet the real game changer in hunting for ever more distant objects will be the James Webb Space Telescope. The biggest, baddest next-generation space telescope is slated to launch in 2018. James Webb should be able to look back as far as perhaps 100 million years after the big bang, easily scrounging up examples of the first galaxies theorized to have taken shape about 400 million years into the universe's existence. And James Webb will not be alone in plumbing such cosmic depths. Three mammoth ground-based telescopes—the Giant Magellan Telescope, the Thirty Meter Telescope, and the European Extremely Large Telescope, all scheduled for first light in the early 2020s—are built to peer into the beginnings of the universe.
As astronomers keep probing further back in time, at some point there will be no galaxies to see at all. That will be because scientists will reach a point at which the universe has not yet spawned them. At that point, the goal becomes seeing the first stars. "We think the first stars formed sort of on their own, not in galaxies," Finkelstein says.
Yet no matter how advanced our telescopes become, it's an open question whether scientists could detect something as small as a star at such vast distances. Galaxies contain millions of stars. Spotting a galaxy as opposed to a single sun, then, it like spotting a city at night versus seeing a single lit window. Fortunately, the very first stars are expected to have been extremely massive and thus ended their lives as cataclysmic, super-bright supernovas, which we do have a prayer of seeing, Finkelstein says.
And though we may spy flickers of the first stars, you don't have to go much deeper into cosmic history to encounter the ultimate limit to how far back we can ever see. As it turns out, we have already "seen" it. This cosmic dead end occurs at about 380,000 years post-big bang and is known as the epoch of recombination. Before this time, the universe was still too hot for electrons and protons to pair up and form the most basic atom, hydrogen. And unbound electrons scatter light. So, until those first hydrogen atoms came onto the scene, scientists think, the cosmos was an opaque soup of energy. Only afterward did the universe became transparent to light—light that was free to stream into our telescopes.
The relic light from this recombination event is known as the cosmic microwave background radiation. We observe it as a diffuse, essentially featureless, uniform glow from all over the sky, and it was most recently charted by the European Space Agency's Planck spacecraft.
"In terms of seeing light, we've seen the cosmic microwave background," Finkelstein says. "So, technically, we've seen all the way back."