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See Inside May 2009

Mapping the Universe with Helium

A new way to squeeze information from the microwave background

Cosmologists talk about the cosmic microwave background radiation, their snapshot of the universe at the tender age of 400,000 years, so much that it might seem pretty well mined out by now. After all, the European Space Agency intends for its new Planck satellite to extract “essentially all the information available” in the radiation’s spatial patterns. But cosmologists looking beyond Planck say the radiation has a barely explored aspect that, if it could be observed with enough precision, would reveal new details about the early universe: its spectrum.

Astronomers routinely use the rainbow of colors emitted by the sun and other stars to determine their composition. At the American Astronomical Society meeting this past January, renowned astrophysicist Rashid Sunyaev of the Max Planck Institute for Astrophysics in Garching, Germany, argued that a successor to Planck might pick up similar fingerprints in the background radiation, the spectrum of which currently seems completely featureless and generic.

In the conventional picture, the background radiation consists of photons produced in the earliest moments of the big bang. They scattered off protons and electrons in a game of cosmological pachinko until everything cooled enough for protons to latch onto electrons and form hydrogen atoms—a process known as recombination. Being electrically neutral, the atoms were less prone to scatter photons. So the photons began to stream across space in more or less straight lines. The pachinko had thoroughly mashed their spectrum, and the only thing cosmologists can glean from it is the overall density of matter.

Yet this picture glosses over two nuances. First, it took a while for protons to get a firm hold on electrons. Their grip was tentative at first. To tighten up, a newly formed atom had to lose energy by emitting photons, and it did so in its own good time. Further complicating matters, a photon from one atom tended to knock the electron off another. Like crabs in a bucket, atoms thwarted one another. What overcame their mutual antagonism was cosmic expansion, which sapped the photons’ energy and gradually tilted the balance in favor of atom formation over destruction. The commonly cited time frame of 400,000 years is just a convenient milestone; recombination actually took as long as a couple of million years to run to completion.

The second nuance is that although the universe consisted mostly of hydrogen, it also had a fair amount of helium. With twice the electric charge of protons, helium nuclei had more latching power and formed atoms earlier, catching their first electron at about 15,000 years and their second at 100,000 years. What is more, they avoided the crab bucket syndrome. A small advance guard of hydrogen atoms acted as a moderator, intercepting the photon emitted by one helium atom before it could destroy another. So helium atoms formed snappily.

The photons emitted by hydrogen and helium added some compositional fingerprints to the primordial soup. Measuring the number of photons emitted by helium would nail exactly how much helium the universe synthesized—a quantity that now must be extrapolated, with difficulty, from the amount of helium in stars. “This is an absolutely clear way to find the primordial abundance of helium,” Sunyaev says. In addition, photons from helium date to an age before the release of the microwave background. They might bear the stamp of processes now hidden from us, such as exotic particle decays.

The trouble is that the helium photons are outnumbered a billion to one by primordial photons. Fortunately, because helium atoms formed so rapidly, the photons they emitted are sharply concentrated at certain frequencies, known as spectral lines. Sunyaev and Jens Chluba of the Canadian Institute for Theoretical Astrophysics have urged a new mission to sweep across frequencies looking for a spike in the number of photons, like drawing your finger across a surface to feel a bump too small to measure with a ruler. “To observe these lines, you have to observe a fixed position and scan across frequency,” says José Alberto Rubiño-Martín of the Institute of Astrophysics of the Canary Islands. In contrast, existing missions, including the Planck satellite, observe a fixed frequency and scan across position.

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