Monday, January 28, 2008 ... //

Observing cosmic strings via the stretched hydrogen line?

Science Daily,
PhysOrg,
Wired Science

inform us that a group at University of Illinois has proposed a new method to search for signatures of cosmic strings in the skies. The project is based on the 21-centimeter Hydrogen line.

Recall that the Hydrogen line arises from transitions between two nearly ground states of the neutral Hydrogen atom that are split by the so-called hyperfine structure: its origin is in the interaction between the spins of the electron and the proton. The two states, distinguished by different total spins, differ by a very small energy whose corresponding photon has frequency of 1420 MHz or the wavelength of 21 centimeters.

A direct transition between these two states is highly suppressed and almost certainly unobservable in the terrestrial labs (the rate is less than 3 emissions per 10^{15} seconds). However, there is a lot of Hydrogen in the Cosmos so this 21-cm line is easily observable. However, the radiation whose wavelength is 21 cm today is not what the people want to observe.

Instead, they want to focus on the radiation whose wavelength was 21 cm right during the decoupling era. By the expansion of the Universe, the wavelength is now closer to 20 meters and they would need to build a network of powerful radio telescopes and try to see something. I might misunderstand something, but I wouldn't expect this stretched spectral line to be too sharp.

Such signals, if observed, could nevertheless not only identify the inhomogeneities caused by cosmic string networks - that are unobservable in the normal CMB spectrum - but even determine the string tension and perhaps some other features of such cosmic strings hypothetically imprinted into this portion of electromagnetic radiation. Note that cosmic strings appear in various unified theories, starting from grand unified theories and ending with superstring theory itself: cosmic strings can literally be fundamental strings from string theory stretched into astronomical distances.

It looks as a rather interesting and unexpected experimental idea that should be looked into very seriously. Such possibilities highlight that creative people may often solve questions that look too difficult at the beginning. They also emphasize how incredibly idiotic are the aggressive crackpots' proclamations that modern theoretical physics in general and string theory in particular is untestable.