M. Kornmesser/ESO

Cosmic constant. An artist's impression of a quasar, the glowing heart of an ancient galaxy.

Fundamental Constant May Depend on Where in the Universe You Are

A fundamental physical constant akin to the charge of the electron or the speed of light may depend on where in the universe you are, a team of astronomers reports. If true, that observation would overturn scientists' basic assumption that the laws of physics are the same everywhere in the universe. Other researchers are skeptical, however.

The constant in question is the so-called fine-structure constant. A number with a value of about 1/137, the constant dictates the strength of the electromagnetic force and, hence, determines the exact wavelengths of light an atom will absorb. The idea that the constant may have changed over the age of the universe isn't new. Astrophysicist John Webb of the University of New South Wales in Sydney, Australia, and his colleagues first rang that bell in 1998, using data from the 10-meter telescope at the W. M. Keck Observatory on Mauna Kea, Hawaii, which peers into the Northern Hemisphere.

Back then, the team looked at the brightly shining centers of ancient galaxies known as quasars. Light from the quasars must pass through clouds of gas on its several-billion-year journey to Earth, and the atoms in the gas absorb light of specific wavelengths. So the spectrum of the light reaching Earth is missing these wavelengths and looks a bit like a bar code. The overall shift of the lines tells researchers how far away a gas cloud is and, hence, how long ago the light passed through it. The relative spacing of the lines lets them estimate the fine-structure constant at that time. Analyzing such data, Webb and colleagues argued that the fine-structure constant was about 1 part in 100,000 smaller 12 billion years ago than it is today. That was a radical proposition, as the laws of physics are supposed to be the same no matter where you are in the universe.

The result was not universally accepted, however. In 2004, Patrick Petitjean, an astronomer at the Institute for Astrophysics in Paris, and colleagues used observations of 23 clouds from the Very Large Telescope (VLT) on Cerro Paranal in Chile, which peers into the southern sky, and found no discernible variation in the fine-structure constant.

Case closed? Not quite. Now Webb and his colleagues have scoured the southern sky themselves using the VLT. Their 153 clouds suggested a difference of 1 part in 100,000 in the fine-structure constant 12 billion years ago. Except in the southern sky, the constant seems to be larger. Connecting the two extremes with a line, the team found that absorption patterns in the clouds along that line are consistent with the fine-structure constant changing slowly through space—smaller in the distant northern sky and larger on the southern side.

"The result is thrilling," says atomic physicist Wim Ubachs of the Free University of Amsterdam, who wasn't involved in the work. "It might be an indicator that the universe is different from what we thought it to be." Ubachs says he's open to the idea that fundamental constants might actually change over time and position, as scientists don't have a decent explanation for why the fundamental constants have their particular values anyway. Still, the huge claim that a constant changes demands weighty evidence—which the new data are not, as even Webb's team agrees. They say the chances that random statistical fluctuations in the data could produce a fake signal as big are less than 1 in 15,000, the team reported online 31 October in Physical Review Letters. To qualify as hard evidence, those odds must drop to 1 in 2 million.

Not surprisingly, Petitjean finds the suggestion that the fine-structure constant changes across space "very difficult to believe." He argues that taken by themselves, the Webb team's VLT data wouldn't be interesting. Webb admits that the chances that random fluctuations in the new VLT data could produce a fake trend are a fairly large 1 in 34. But he argues that the data are compelling because two independent telescopes, pointing in different directions, saw the fine-structure constant changing at the same rate and in the same direction. As for why Petitjean's group didn't see the increase in its own data from the VLT, Webb says Petitjean and colleagues were looking in the wrong direction. The 23 clouds Petitjean's team studied don't run along the line through the universe where the fine-structure constant appears to change, Webb says, so it's no surprise that they didn't see the same trend.

Petitjean sees the agreement differently. He says that the results match because in light up to about 10 billion years old, his team and Webb's see the same thing: no change. Only Webb's group analyzed the older light, and that is the source of its trend. Until it is confirmed independently by others, he warns, "everybody should be careful about the result."

If it stands up, Webb says, the claim might help answer a grand conceptual question: Why do the fundamental constants take on values that permit life to exist when tiny changes would make life impossible? If the fundamental constants vary over the potentially infinite extent of the universe, our place in the universe would naturally be where the constants are tuned just right to make our existence possible—a version of the so-called anthropic principle. In some circles, however, the anthropic principle raises eyebrows even higher than the idea of changing physical constants.

Posted in Physics