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Optical physics

Physicists seek cosmic domain walls

17 Jan 2013
Bubble domain walls: Do similar structures exist in space?

Exotic structures known as cosmic domain walls could be observed from Earth by measuring the subtle effect of their magnetic-like fields as they pass through our galaxy. That is the conclusion of a team of physicists in the US, Canada and Poland that has proposed a new way of probing the nature of the mysterious dark matter and dark energy thought to permeate the universe.

The current standard Big Bang model of cosmology assumes that much of the energy in the universe is contained within two mysterious substances – dark matter and dark energy. Dark matter explains anomalies in the motion of galaxies and is thought to account for about 20% of the universe’s energy. Dark energy is invoked to explain the universe’s accelerating expansion and is reckoned to make up about 75%. Most direct searches assume that dark matter consists of some kind of particle, while dark energy is often taken to exist in the form of a “cosmological constant” that is added to the field equation for general relativity. A number of other possibilities have been put forward, however.

Walls and axions

One is the idea that dark matter and dark energy are instead contained within objects known as “domain walls”. These structures would form within an exotic kind of force field mediated by as-yet-undiscovered sub-atomic particles known as axions, which were originally proposed back in the 1970s as a way of accounting for the fact that the universe appears to contain much more matter than it does antimatter. In the hot early universe the strength of the field would have varied randomly in space, but as the cosmos expanded and cooled, the field would have settled down to single values within extended regions. The boundaries between these different regions would be the domain walls, with the sudden jump in field across the walls endowing them with energy.

In the latest work, a collaboration headed by theorist Maxim Pospelov of the University of Victoria in British Columbia and experimentalist Dmitry Budker of the University of California, Berkeley, set out to establish whether or not such walls could be detected using instruments on Earth. The researchers’ idea is to use magnetometers, devices made up of atoms whose spins are initially lined up and can then be rotated by an external magnetic field. An axion-like field would be “scalar”, which means that, unlike a magnetic field, it does not have a preferred direction in space and so ordinarily would not affect the output of a magnetometer. However, a change in the field strength, as would occur at a domain wall, would affect the spins of the atom in the device.

To work out whether or not this effect could be measured, Pospelov and colleagues assumed that domain walls would store a considerable fraction of either the universe’s dark matter or dark energy. On this basis they worked out both the effective magnetic field generated and the time it would take for the Earth to pass through a wall, assuming it to be moving relative to the network of domain walls at a typical galactic speed of one thousandth of the speed of light. The researchers found that for quite a range of possible values of the axion mass and coupling between the associated field and ordinary matter, both the effective magnetic field strength and the interaction time would be within the sensitivities of modern magnetometers. They also established that such interactions would take place at least once every few years.

Relatively rare sightings

As the researchers point out, however, such relatively rare sightings would be difficult to identify amid continuing background noise from the magnetometer itself, its shielding and a host of external sources such as power lines, passing cars or even magnetic storms in the Earth’s atmosphere. The solution they propose is to create a network of at least five such devices. Four would establish the speed and direction of travel of a passing wall. These data would be used to calculate the wall’s time of interaction with the fifth device. If prediction and measurement match up then, says Budker, “you can be more confident that you have seen a domain wall”.

The researchers have been assessing the performance of two prototype devices, one located at Berkeley and the other at the Jagiellonian University in Krakow. They have shown that they can correlate the signals from the two machines and that they can reject a significant fraction of the noise. They now hope to obtain up to about $10m of new funding to build the full-scale network, with other devices potentially located at California State University in East Bay and elsewhere in the US, as well as in other countries.

Not seeing not a failure

Budker concedes that the idea of domain walls is “a little bit exotic” and outside the mainstream when it comes to searching for dark matter and dark energy. He also acknowledges that the theoretical uncertainties make it hard to know what the chances of detection might be. But he maintains that detection should not be taken as the only measure of success. “It is very important to realize in the search for exotic physics that not seeing something is not a failure,” he says. “If instead you rule out a whole class of possible models then that is a success.”

Joana Oliveira of the University of Porto in Portugal cautions that a network of domain walls could only contribute significantly to dark energy if it were “frustrated”. This would mean the walls being almost static relative to one another, their only movement being the stretching caused by the universe’s expansion. “The difficulty in achieving this configuration is similar to that which exists in preventing foam from dissolving in a glass of beer,” she says.

Pospelov acknowledges the difficulty in devising a model that could tie up significant amounts of dark energy in domain walls. He points out that simple models proposed previously contained too few walls to achieve this, and as such is looking to develop more elaborate models. “To become a legitimate theory, the model has to be consistent with what we know about the evolution of the universe,” he says.

The research is published in Physical Review Letters.

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