An Eye in the Biggest of Skies

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On July 5, the European Space Agency finally released the long-awaited data from the Planck satellite, which has been studying the fading afterglow of the Big Bang that gave birth to the universe 13.7 billion years ago. The satellite's two highly sensitive detectors have measured the intensity of microwave radiation emanating from space, which will allow scientists to construct a more complete and accurate heat map of the universe. As researchers eagerly pour over these new clues, they are looking for an answer to the most fundamental cosmological question: how the universe came into existence.

Cosmic microwave background (CMB) radiation was discovered in the 1960s. Two decades ago, a satellite named COBE (for Cosmic Background Explorer) first detected the telltale “seeds” of the cosmos's large-scale structure in the form of slight variations in the CMB. These indicated that 380,000 years after the Big Bang, matter had indeed begun clumping together. The slightly denser regions show up as marginally hotter relative to their surroundings.

Now the Planck satellite has mapped the temperature (that is, density) fluctuations with much finer resolution, producing a stunning all-sky picture (see above). By carefully mining this data, scientists can build up a detailed understanding of what the universe is made of, how fast it is expanding, and what its ultimate fate might be. The significance of the CMB is that it has travelled across space almost undisturbed since about 380,000 years after the Big Bang, when its temperature was several thousand degrees. In effect, Planck provides a snapshot of what the universe looked like as an infant, long before the formation of stars or galaxies.

The most striking feature of the CMB is that it is distributed remarkably evenly across the sky. The temperature of the radiation varies by only about one part in 100,000, confirming a fundamental fact about the state of the early universe: it was extraordinarily simple, consisting of hydrogen and helium gas spread with almost precise uniformity through space, expanding at the same rate everywhere and equally fast in all directions. The source of this primordial simplicity was long a puzzle to cosmologists. After all, most explosions are messy affairs. What made the universe erupt into being in such an exquisitely orchestrated and orderly manner?

The favored explanation is a theory called inflation, which theorizes that just after the cosmic origin, a powerful pulse of antigravity propelled matter apart at an accelerating rate, causing the universe to leap in size by an enormous factor in a mere trillion trillion trillionth of a second. When inflation ended a split second later, almost all initial irregularities had been “stretched to death,” leaving the universe smooth and pristine. But perfect uniformity would be a disaster, as it would preclude the existence of galaxies, stars, and planets with living observers like us. For galaxies to grow in the face of the overall expansion of the universe, it is necessary for them to get a head start in the form of “seeds,” or clumps of matter, to act as gravitational foci.

The Planck satellite also provides a crucial window on the epoch of inflation, because events that took place at that very early time should have left subtle clues in the structure of the universe that would be mirrored in the distribution of the radiation. At the top of the list for study are those all-important temperature fluctuations in the CMB, without which we would not exist. Planck is able to detect variations as little as one part in a million and down to angular sizes as small as 10 arc minutes (roughly a third of the size of the moon) across almost the entire sky. Naturally everyone wants to know how the fluctuations originated. Were they magically imprinted on the universe at the outset, or did they arise as the result of physical processes that theorists can model mathematically?

If cosmic inflation fluctuated in strength very slightly from place to place, it could have created the crucial density variations that served as the “seeds” of the large-scale structure. One popular idea is that those variations have their source in quantum mechanics, that weird branch of physics normally reserved for the atomic realm. At the time of inflation, the universe was, roughly speaking, squeezed to subatomic dimensions, so quantum effects could have played a key role. The hallmark of quantum physics is uncertainty, which can translate into random spontaneous fluctuations manifested in all forms of movement, including the expansion of the universe. If this theory is correct, then the hot and cold blobs in the Planck satellite's all-sky picture are ultramicroscopic quantum fluctuations hailing from the very threshold of cosmic creation, writ large and frozen in the sky for all to see!

It is often remarked that the Big Bang was the biggest particle-physics experiment in history. The searing heat of the universe’s fiery birth would have created every sort of subatomic particle and slammed them together with a power unattainable in even the world’s largest accelerator. The flood of data coming from the Planck satellite will push back the time horizon for theorizing and move us closer to linking the science of the largest physical systems in the universe with that of the very smallest. Cosmologists are convinced that the most basic features of the universe were forged in that first fleeting surge of energy, so it may well be that the properties of the smallest entities in nature in fact determined the very structure of the life-friendly cosmos we now inhabit.

Paul Davies is a physicist, cosmologist, and astrobiologist at Arizona State University, where he is director of Beyond: Center for Fundamental Concepts in Science. The latest of his many books is The Eerie Silence: Renewing Our Search for Alien Intelligence.