Probe seeks changes in fine-structure constant
Mar 23, 2007
Cosmologists in the US have proposed a new way to measure the fine-structure constant as it was some 13 billion years ago -- and see if its value differed from that measured today. The method, which has yet to be verified using astronomical observations, involves measuring how hydrogen atoms absorbed photons from the cosmic microwave background. It could provide further evidence that this fundamental constant of nature -- which defines the strength of the electromagnetic interaction -- is not actually a constant after all (Phys. Rev. Lett. 98 11301).
Most measurements of the fundamental constants of nature have been made on Earth over the past one hundred years. However, it is possible that these quantities are different when measured elsewhere in the universe or at other times. Indeed, the ability of fundamental constants to vary over space and time plays an important role in some theories that attempt to unify gravity, electromagnetism, and the strong and weak nuclear forces.
Some physicists believe that the value of the fine-structure constant (α) has been growing since the universe was formed in the Big Bang some 13.5 billion years ago. Observations of light from distant quasars suggest that α may have been one part in 105 smaller some 11 billion years ago than it is today. Closer to home, measurements of α derived from studying the decay of radioactive isotopes on Earth suggest that the constant may have changed by one part in 107 over the past 4.6 billion years.
Now Benjamin Wandelt and Rishi Khatri of the University of Illinois at Urbana-Champaign have proposed a way of measuring α as it was 10-100 million years after the Big Bang – during the so-called “dark ages”, when the universe was cool enough for neutral hydrogen atoms to exist, but before stars and galaxies formed. During this period, the hydrogen atoms absorbed cosmic microwave background (CMB) radiation at a wavelength of about 21 cm, which corresponds to a transition between two atomic energy states. The result is an absorption line in the CMB that endures to this day.
Wandelt and Khatri have shown that the precise wavelength of the transition is very sensitive to changes in α. Since microwave radiation from the dark ages can be detected today, they reckon that the precise location of the 21-cm line and the relative strength of the absorption can be used to determine α as a function of time -- after correcting for the redshift caused by the expansion of the universe.
As hydrogen atoms absorbed photons throughout the dark ages, Wandelt and Khatri believe that it should be possible to track changes in α over a period of about 100 million years. Indeed, because hydrogen was ubiquitous during the dark ages, the researchers are confident that their technique could be used to create spatial maps of α, which could prove useful in the search for dark energy.
Unfortunately, the scheme cannot take advantage of the current generation of microwave telescopes -- such as the Wilkinson Microwave Anisotropy Probe (WMAP) satellite – which do not focus on the region of the microwave spectrum containing the 21 cm absorption line from the dark ages. However, Wandelt told Physics Web that the measurements could be made using the Long Wavelength Array (LWA) telescope that is currently being built in the US state of New Mexico.
Another complication is that the signal is buried beneath an intense background of radiation that originates from within our own galaxy. However, Wandelt is confident that this background can be subtracted to yield a measurement of α to an accuracy of about 0.1% within a decade.
About the author
Hamish Johnston is editor of Physics Web