State of the Universe: Microwave glow powers cosmic insights

New observations of the oldest light in the universe have enabled astronomers to determine the age of the cosmos with unprecedented precision, infer the existence of a vast sea of neutrinos, and better gauge the start and duration of the long-ago era when the first stars switched on.

The findings come from an analysis of 5 years of observations of the cosmic microwave background—the radiation left over from the Big Bang—using NASA’s Wilkinson Microwave Anisotropy Probe (WMAP).

The glow was generated at the birth of the universe, but WMAP sees the radiation as it appeared when the universe was about 380,000 years old. That’s when the cosmos became cool enough for electrons and ions to combine into neutral atoms, releasing the radiation these charged particles had trapped. A snapshot of the early universe, the radiation is riddled with regions slightly hotter or colder than average-markings of the primordial lumps that grew into galaxies and galaxy clusters. The microwave background also carries the fingerprints of what it has encountered during its multibillion-year journey to Earth.

MICRO MAP. Temperature fluctuations (red denotes hottest, blue coldest) in the remnant radiation from the Big Bang reveal density variations that were seeds of galaxies. A scheduled launch of the Planck mission will hone the resolution of this WMAP picture. WMAP Science Team

By using WMAP to measure the size of the hot and cold spots as they appear on the sky today, along with knowledge of their size when the radiation was first released, researchers have pegged the age of the universe to 13.73 billion years, give or take 0.12 billion.

According to theory, immediately after the Big Bang, positrons and electrons collided and annihilated, producing both photons and vast numbers of nearly massless elementary particles called neutrinos. The neutrinos would have slightly smoothed out variations in the microwave background. For the first time, WMAP data reveal just such a smoothing. “Had we not seen this, it would have implied that that was something missing in our understanding of the first second after the Big Bang,” says WMAP scientist David Spergel of Princeton University.

He and his colleagues unveiled their findings last week in online articles.

The team also more accurately determined when the first stars were born. Soon after those stars turned on, they emitted enough ultraviolet light to reionize the universe, stripping atoms of their electrons. These electrons created a thin fog that scattered the microwave background radiation and polarized it. WMAP measurements indicate that the first stars began to shine when the universe was no older than 430 million years. Combined with ground-based surveys of ancient quasars, the findings also indicate that reionization was an extended process, lasting for half a billion years.

That provides a guide for future telescopes—such as Hubble’s proposed successor, the James Webb Space Telescope—as to “when in time they need to begin looking for the first stars,” says Spergel.

The results provide “another milestone in precision cosmology,” comments theorist Max Tegmark of the Massachusetts Institute of Technology. Though the findings mostly confirm previous results based on 3 years of WMAP data (SN: 3/18/06, p. 163), the added precision is critical for testing models for the origin of the universe and the formation of galaxies, says Nick Gnedin of the Fermi National Accelerator Laboratory in Batavia, Ill.

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