An ionized hydrogen atom, consisting of a proton shorn of its associated electron, cannot undergo the 21-centimeter transition discussed above, since that transition depends on the relative spins of the electron-proton pair. Astronomers, therefore, look for signs of reionization by determining when 21-centimeter emissions start to turn off — evidence that stars are, simultaneously, starting to turn on. Their tactic, in other words, is to seek out the effects of the first stars rather than the stars themselves.
This predicted shutdown in 21-centimeter radio signals will not occur everywhere at once, Loeb says. To picture what happens, he suggests imagining a block of Swiss cheese. The holes in the cheese represent places around stars and galaxies where UV radiation has ionized hydrogen atoms, bringing 21-centimeter emissions to a halt. The solid parts of the cheese represent places farther removed from the radiation, where neutral hydrogen still exists.
Over time, Loeb says, the holes keep growing and eventually overlap until there is no “cheese” (neutral hydrogen) left. We know this is the case because the universe is almost completely reionized today and has been for more than 12 billion years. In fact, Loeb claims that 99.99 percent of the universe’s volume was reionized during the 950 million years that followed the Big Bang.
When, during that time, did the stars switch on? One observer inspired to find out was Aaron Parsons at the University of California, Berkeley. Parsons is co-principal investigator on the Precision Array to Probe the Epoch of Reionization (PAPER), a 128-antenna telescope in South Africa’s Karoo Desert. Parsons explains his goals this way: The first stars generated UV radiation, and at some point these stars produced enough radiation to ionize the gas lying between galaxies. “The question is, ‘When did that happen?’ ” he says. Instead of looking at the stars, Parsons is trying to catch the moment when the 21-centimeter signal disappears, which should correspond to the time when most of the hydrogen became ionized.
For starters, Parsons and his collaborators are essentially looking at many single, two-dimensional slices of “Swiss cheese” and counting the number of holes to get a statistical measure of how patchy the cheese is — the number of holes indicating how far along reionization is. If that approach proves successful, the plan is to expand PAPER or build a new and improved instrument that could directly map in three dimensions the distribution of neutral hydrogen and the “cheese holes” where neutral hydrogen is absent. That would provide a fuller picture, and chronology, of reionization throughout the cosmos. Knowing when reionization kicks in will allow scientists to deduce precisely when the earliest stars flicked on.
The Next Frontier
Loeb, meanwhile, is also working on the next big frontier in cosmology, using neutral hydrogen observations to explore an even earlier chapter in cosmic history: the dark ages before star formation. He says this might be the most interesting epoch of all — a time when the primordial clumps of hydrogen took shape, becoming the clouds from which the first stars and galaxies would eventually form.
Toward that end, Loeb is an investigator on the Dark Ages Radio Explorer (DARE) mission, which aims to put a radio antenna on a spacecraft orbiting the moon. Flying above the Earth’s ionosphere, which blocks or scrambles certain electromagnetic frequencies, DARE would deliver much cleaner measurements than today’s antennas. Loeb helped optimize the instrument’s design but stresses that DARE is still just an idea, and an unfunded one at that.
Even without DARE up and running, existing projects are starting to gather new data. For instance, the Hubble Space Telescope recently spotted a galaxy that flicked its lights on just 380 million years after the Big Bang. Hubble’s successor, the James Webb Space Telescope, will sport a mirror nearly three times the diameter and seven times the collecting area of Hubble’s. This 6.5-meter mirror should enable it to detect even fainter (and therefore older) galaxies.
Loeb and his university, Harvard, are also partners in the Giant Magellan Telescope (GMT), a 24.5-meter instrument being built on a Chilean mountaintop. The GMT is aiming to begin observations within the next decade, and since it’s about five times bigger in area than today’s biggest optical telescopes, it should drastically accelerate the search for the universe’s first galaxies.
Meanwhile, two even more grandiose projects — the Thirty Meter Telescope in Hawaii and the 39-meter European Extremely Large Telescope in Chile — are also moving forward, although funding remains a challenge.
To Loeb, these high-profile developments offer a welcome change from the quiet, data-poor field he entered two decades ago. And he’s already making advance preparations, devising strategies to process and interpret the deluge of data he expects to see in the coming years. Although it’s impossible to know what discoveries these data will yield, Loeb hopes the revised account of first light that he’s helped draft will hold up in the wake of the newly acquired information — but he says he will be equally happy if the stars surprise him yet again.
During a recent, reflective moment, Loeb decided to grade the Bible’s creation story. He gave it a B-plus, taking into account the work’s age.
“The insight that the universe had a beginning is very impressive,” he says, “but the subsequent details are flawed.” With the benefit of modern technology, plus two decades’ worth of theoretical input, Loeb is optimistic that he and his colleagues can turn that B-plus into an A (or at least an A-minus).
[This article originally appeared in print as "First Light"]