The 2009-2013 observational run of the European Space Agency’s Planck satellite observatory produced a wealth of data that in many ways completes a cosmological program that began half a century earlier. This was made possible thanks to a large international Planck Scientist Collaboration.
In the early 1960s, the field of cosmology consisted mostly of speculation, not science. The accidental discovery of an idiosyncratic radio signal in 1964, however, began the field’s transformation from metaphysics to physics. That fortuitous detection by two Bell Labs researchers working on microwave transmissions turned out to be consistent with a theoretical prediction: A universe born in a “big bang” would have left a fossil radiation imprint on space in every direction. But the detection also presented a deep conundrum. According to theory, the relic radiation would contain the seeds of the structures we see in the universe today—stars, galaxies, us—in the form of temperature variations. According to the 1964 observation, though, the universe was completely smooth—unless the radio detector that discovered the Cosmic Microwave Background wasn’t sensitive enough to reveal those details.
Nearly thirty years later, the Cosmic Background Explorer (COBE) made the first foray into space in search of those details. In 1992 the COBE team triumphantly announced both that the cosmological background spectral energy distribution followed a Planck function with an extremely high accuracy as predicted. Furthermore, the small fluctuations were also detected (at a millionth part of the average value as predicted). But this new picture, while sufficient to vindicate the Big Bang interpretation of cosmology, was still imprecise enough to warrant further investigation.
A decade later, a space observatory with far greater sensitivity, the Wilkinson Microwave Anisotropy Probe (WMAP), produced a map of the microwave background that revealed the telltale temperature variations in exquisite detail, providing the first reliable observational accounting of the universe’s contents, structure, and evolution.
(One of the Principal Investigators on COBE, John Mather, along with the COBE team received the 2006 Gruber Prize in Cosmology. The PI for WMAP, Charles Bennett, and the WMAP collaboration received the 2012 Gruber Prize in Cosmology.)
The Planck observatory, though, would push technology to limits that science would not soon surpass, like the cooling of detectors in space at a tenth of a degree above absolute zero.
The experiment consisted of two detectors, each attuned to a portion of the electromagnetic spectrum invisible to the eye. The High Frequency Instrument, under the direction of Jean-Loup Puget, studied the universe in far-infrared light; the Low Frequency Instrument, under the direction of Nazzareno Mandolesi, mostly observed in microwaves. The teams behind the two experiments operated independently, each thereby serving as a check on the other, and together they assembled a composite map of the early universe that is an order of magnitude more sensitive than WMAP.
Planck’s most significant achievements include:
• Providing a new census of the universe—26.8 percent dark matter, 68.3 percent dark energy, and 4.9 percent ordinary matter (such as atoms);
• Finding extremely robust evidence that the geometry of the universe is “flat”—that parallel lines truly never meet—a pre-condition for leading theories of the initial state of the universe: inflation paradigm and theories of structures formation;
• Reaching the precision threshold for measuring the CMB’s extraordinarily subtle palette of temperature differences; and
• Reinforcing, through those temperature differences, the quantum interpretation of the universe’s evolution—specifically, by filling in observational details regarding the theoretical moment, one trillionth of a trillionth of a trillionth of a second after the universe came into existence, during which space went through an “inflationary” stage that stretched its size a trillion-fold. (Alan Guth and Andrei Linde received the 2004 Gruber Prize in Cosmology for developing the theory of cosmic inflation.)
- Testing with high accuracy the other generic predictions of inflation concerning the deviation from equal amplitudes of fluctuations when looking at the small and large angular scales on the sky.
- Detecting with high accuracy the polarization on large scale which contains information on the reionization of the universe by the first stars and galaxies.
That data, released in 2013, has already transformed theoretical research in cosmology, and such is the nature of its precision that it will feed further research into the contents and evolutions of the universe for decades to come.