Ultimate Retro: Modern echoes of the early universe

Two teams of astronomers have for the first time detected the surviving notes of a cosmic symphony created just after the Big Bang, when the universe was a foggy soup of matter and radiation. The discoverers say that the survival of the acoustic imprint from this early epoch, 13.7 billion years ago, provides compelling new evidence that the blueprint for the present distribution of galaxies was set at the time of the Big Bang by random subatomic fluctuations.

SOUND REASONING. Distribution of modern-day galaxies as revealed by the Sloan Digital Sky Survey (top). Lower Images show, left to right, a typical patch of sky in the survey, the telescope used for the survey, and a depiction of a primordial sound wave detectable today (also presented as red circle in sky survey). SDSS

In 1999, researchers detected a specific pattern of acoustic oscillations in the faint, ancient whisper of radiation—the cosmic microwave background—left over from the Big Bang. This week, Shaun Cole of the University of Durham in England and his colleagues announced that they had discerned remnants of that pattern while analyzing data from the Two-Degree Field Redshift Gravity Survey, a large-scale analysis of 220,000 galaxies. The map covers one-twentieth the area of the sky out to a distance of 2 billion light-years from Earth.

Another team, led by Daniel Eisenstein of the University of Arizona in Tucson, examined a subset of 46,000 galaxies from another sky map, the Sloan Digital Sky Survey, which covers one-quarter of the sky.

Each team used a different method of analysis but found the same acoustic pattern. The groups reported their findings this week at a meeting of the American Astronomical Society in San Diego.

The signals are so weak that, to detect them even in large-galaxy surveys, “both groups had to work quite hard,” notes cosmologist David N. Spergel of Princeton University. “The result is another important milestone in establishing a standard model for cosmology.”

The early universe rang like a bell, notes Spergel. As gravity drew together clumps of atomic matter, radiation—then tightly bound to that matter—exerted an outward pressure. The tug-of-war between gravity’s pull and radiation’s push generated pressure waves, or acoustic oscillations.

About 400,000 years after the Big Bang, the universe had cooled sufficiently for the radiation to break free from matter and travel unimpeded into space. Now in the form of microwaves, this radiation pervades the universe and provides a snapshot of the cosmos at that early time, ripples and all.

The small size of the fluctuations, both in the microwave background and the galaxy distribution today, provides additional evidence that most of the mass of the universe is composed of dark matter—an exotic, invisible, and primordial material that has never interacted with light and so had never generated sound waves, notes Spergel.

Eisenstein notes that, using the length of the sound waves as a cosmic ruler, astronomers can calculate the universe’s expansion. Both of the new studies agree with earlier reports that cosmic expansion is speeding up (SN: 5/22/04, p. 330: Dark Doings). The universe’s first symphony therefore provides independent evidence that the cosmos is filled with dark energy, which causes the acceleration.

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