paul_j_steinhardt's picture
Albert Einstein Professor in Science, Departments of Physics and Astrophysical Sciences, Princeton University; Coauthor, Endless Universe
The Big Bang Cannot Be What We Thought It Was

A year and half ago, the scientific community and the press trumpeted the claim by a team of scientists that they had found definitive proof that the universe began with a Big Bang followed by a period of accelerated expansion, known as inflation. Their proof was that the light produced in the infant universe and collected by their detectors exhibited a distinctive pattern of polarization that could only be explained if the large scale structure of the universe was set when the temperature and density of the universe were extraordinarily high, just as posited in the Big Bang inflationary picture.

Over the ensuing year, though, it became clear that the claim was a blunder: in searching for a cosmic signal from the distant universe, the team had not taken proper account of the polarization of light that occurred nearby when it passed through the dust in our Milky Way on the way to their detectors. The new claim from the team, published in recent months, is that there is no sign of the cosmic polarization they had been seeking despite an extensive search with extraordinarily sensitive detectors.

The retraction received considerable attention but the full import of the news has not been appreciated: we now know that the Big Bang cannot be what we thought it was.

The prevailing view has been that the Big Bang was a violent high-energy event during which space, time, matter and energy were suddenly created from nothing in a distorted, non-uniform distribution. To account for the undistorted nearly uniform universe we actually observe, many cosmologists hypothesize a period of rapid stretching (inflation) just after the bang when the concentration of energy and matter was still very high. If there were inflationary stretching only, the universe would become perfectly smooth, but there is always quantum physics in addition to stretching and quantum physics resists perfect smoothness.

At the high concentrations of energy required for inflation, random quantum fluctuations keep generating bumps and wiggles in the shape of space and the distribution of matter and energy that should remain when inflation ends. The quantum-generated irregularities should appear today as hot spots and cold spots in the pattern of light emanating from the early universe, the so-called cosmic background radiation. Indeed, the hot and cold spots have been observed and mapped in numerous experiments since the COBE satellite detected the first spatial variations in the cosmic background radiation temperature in 1992.

The problem is that, when the concentration of energy is high, the quantum-generated distortions in space should modify the way light scatters from matter in the early universe and imprint a spiraling pattern of polarization across the cosmos. It was the detection of this spiraling pattern (referred to as B-mode) that was claimed as proof of the Big Bang inflationary picture, and then retracted. The failure to detect the B-mode pattern means that there is something very wrong with the picture of a violent Big Bang followed by a period of high energy-driven inflation. Whatever processes set the large-scale structure of the universe had a to be a gentler, lower-energy process than has been supposed.

Simply lowering the energy concentration at which inflation starts, as some theorists have suggested, only leads to more trouble. This leaves more time after the Big Bang for the non-uniform distribution of matter and energy to drive the universe away from inflation. Starting inflation after the Big Bang and having enough inflation to smooth the universe becomes exponentially less likely as the energy concentration is lowered. The universe is more likely to emerge as too rough, too curved, too inhomogeneous compared to what we observe.

Something more radical is called for. Perhaps an improved understanding of quantum gravity will enable us to understand how the Big Bang and inflation can be discarded in favor of gentler beginning. Or perhaps the Big Bang was actually a gentle bounce from a previous period of contraction to the current period of expansion. During a period of slow contraction, it is possible to smooth the distribution of space, matter and energy and to create hot spots and cold spots without creating any B-modes at all.

As the news sinks in, scientists will need to rethink depending on whether forthcoming, more sensitive efforts to detect a B-mode pattern find anything at all. Whatever is found, our view of the Big Bang will be changed, and that is newsworthy.