Searching for ingredients of dark matter and dark energy

Two new reports advance efforts to identify components of dark matter and energy, which together comprise about 95% of the universe yet leave much to scientists' imaginations.

Both experiments illustrate how basic questions about the universe's development can be addressed by laboratory-scale experiments. In the first report, Elana Aprileand colleagues - members of the XENON Collaboration - report on their search for dark matter, a hypothetical matter, the existence of which is inferred from its gravitational effects on visible matter.

While gravitational processes have been thought to involve standard model particles like neutrinos and photons, more recent studies of the physical processes forming our universe suggest new particle types - like WIMPs (or weakly interacting massive particles) - are involved.

Experiments that test this notion look at how these proposed dark matter particles interact with standard particle types; for example, when WIMPs and standard model particles interact, they create recoiling charged particles visible in detectors like the underground XENON100 detector located in Italy.

Here, Aprile and colleagues used this instrument, a large tank of liquid xenon that forms a target for WIMPs, to detect distinct signals from recoil. With no evidence for particular signals, their results set limits on several types of dark matter candidates that have been proposed.

Paul Hamilton et al. searched for a hypothetical force called a chameleon field, one of the most prominent candidates for dark energy - a force thought to have propelled the expansion of the universe. For about a decade, scientists have been looking for a chameleon field, which modifies the wave functions of matter.

Here, in searching for a source for these fields, Hamilton et al. used a light-pulse atom interferometer. Their experiments greatly constrain existing theories of dark energy. A Perspective by Jorg Schmiedmayer and Hartmut Abele provides additional insights into both reports, which tackle some of the most pressing issues of current physics.

Article #13: "Atom-interferometry constraints on dark energy," by P. Hamilton; M. Jaffe; P. Haslinger; Q. Simmons; H. Muller at University of California, Berkeley in Berkeley, CA; H. Muller at Lawrence Berkeley National Laboratory in Berkeley, CA; J. Khoury at University of Pennsylvania in Philadelphia, PA.