Tag Archives: hydrogen absorption

It’s time to declare a ceasefire in the fight to find out whether the constants of nature vary. What was supposed to be a new superweapon in the battle has turned into something of a damp squib.

Some observations of how hydrogen gas in space absorbs light at ultraviolet wavelengths have hinted that the fine structure constant, responsible for the strength of electromagnetism, is not the same throughout the universe. That would point to exotic new physics, including the existence of extra dimensions and universes other than our own.

But the measurement is tricky, and researchers had hoped that studying how hydroxyl molecules emit and absorb light at radio wavelengths would give a more precise, independent measurement of the effect.

In theory, radio instruments can measure wavelengths 50 to 100 times more accurately than those that detect hydrogen absorption, says Nissim Kanekar at the National Centre for Radio Astrophysics in Pune, India.

But he and colleagues discovered the reality is more complicated. They observed the emission and absorption of radio waves from hydroxyl molecules in a gas cloud 6.7 billion light years from Earth that was absorbing light from a more distant galaxy.

No silver bullet

Quantum mechanics predicts that a particular set of emission and absorption lines in the hydroxyl molecule should be mirror images of each other, but in this case they found that was not true. Kanekar thinks the puzzling observation may be due to a second hydroxyl cloud lying along the same line of sight. It may absorb some of the radio waves, fouling the measurement.

“We thought we had the silver bullet, but it didn’t pan out,” says team member Chris Carilli of the National Radio Astronomy Observatory in Socorro, New Mexico.

Whatever is causing the odd measurement, the observation is simply not accurate enough to determine whether the fine structure constant is changing. “Their measurement is entirely consistent with our result ? and with zero,” says John Webb of the University of New South Wales in Sydney, Australia, who has previously found hints that the constant varies.

Carilli says it may take 20 years to make enough observations with the new technique to settle the question. Only a handful of gas clouds are known that exhibit the hydroxyl signal, but new surveys and radio arrays, such as the Square Kilometer Array, should turn up more examples.

Journal reference: arxiv.org/abs/1201.3372v1

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