Snapshots from the 2015 APS April Meeting

Scientists still don’t know what dark matter is, but they can locate it in the Universe by observing its gravitational pull on light emitted by distant galaxies. At the meeting, the Dark Energy Survey (DES) released a new sky map that provides the largest contiguous view to date of the dark matter distribution in the cosmos. The dark matter was mapped by observing its gravitational lensing effect on images of galaxies. These images were collected by a 570-megapixel camera mounted on the telescope of the Cerro Tololo Inter-American Observatory in Chile.

DES scientists Chihway Chang of the Swiss Federal Institute of Technology (ETH) in Zurich and Bhuvnesh Jain of the University of Pennsylvania, who presented the data, said the map is only a small fraction ( $3\%$) of the region the collaboration plans to cover by 2018. (The full survey will capture images of 1/8th of the full sky.) So far, the map is consistent with what cosmologists expect: dark matter is over 5 times more abundant than visible matter, and dark and visible matter tend to cluster together because of their mutual gravitational attraction. With a wider map of matter distribution, scientists will be able to derive information on dark energy, the mysterious force accelerating the expansion of the Universe.

ARPA-E, the federal agency that funds advanced energy technology, supports many high-risk/high-reward projects. In a session on ARPA-E, Program Director Howard Branz discussed some of the projects he has funded, including windows whose visible and infrared transmission can be controlled and several solar energy systems that can store heat for nighttime electricity generation.

Branz also discussed a device, developed by Stanford physicists, that cools itself to 5 degrees below the ambient air temperature without any power source when placed outside on a clear day. It works by reflecting almost all sunlight while also selectively emitting thermal radiation in the wavelength range of 8–13 micrometers, where the atmosphere is transparent and doesn’t radiate back. The researchers hope a future version could be placed on a roof to help cool a building.

Ari Glezer of Georgia Tech discussed his team’s ”solar vortex,” a cylindrical structure that generates a vortex powered by solar-heated air, similar to a natural dust devil. The rising and rotating air turns a turbine that will ultimately generate electricity. Based on a 5-meter-scale test structure in the Arizona desert and simulations, Glezer said the electricity cost will be competitive with wind and solar energy.

At an education session, two researchers discussed the challenges of learning undergraduate quantum mechanics. Chandralekha Singh of the University of Pittsburgh and her colleagues tested students at many universities with written conceptual exams and one-on-one interviews. The team found several “universal” misconceptions, such as the assumption that all quantum states are energy eigenstates. Many students can calculate correct answers to standard textbook problems but don’t understand the basic concepts.

Taking some cues from the large body of research on introductory-level courses, Singh’s solution is a series of interactive quantum mechanics tutorials she has developed around simulations produced by others. The tutorials directly address many of the common misconceptions. They provide visualizations and constantly ask students to predict the outcomes of virtual experiments before running them.

The other speaker, Antje Kohnle of the University of St Andrews in the UK, has developed several collections of interactive computer simulations of quantum mechanics based on extensive testing and feedback from students. The simulations, which she later said are complementary to Singh’s tutorials, provide guided exploration and interaction. For example, students can assess the inadequacy of hidden variable theories and explore the effects of quantum uncertainty on spin measurement outcomes.

In 2013, researchers from the IceCube neutrino telescope at the South Pole reported the detection of 26 neutrinos with the highest energies ever observed. The neutrinos likely come from outside our Galaxy, but the sources that accelerated the particles to such high energies remain a mystery. And to identify these sources, researchers estimate they will need to collect thousands of neutrino events. At the meeting, Frances Halzen, of the University of Wisconsin-Madison and the principal investigator at IceCube, described the design for a new detector that would capture roughly 10 times as many neutrinos.

IceCube is made of thousands of light-sensitive detectors suspended on 86 strings in a cubic kilometer of Antarctic ice. When a neutrino collides with an atom, it creates charged particles that generate a streak of Cherenkov radiation, which sets off the detectors in its path. The radiation is, however, absorbed by ice, so the detectors need to be close enough to a neutrino collision to record it. IceCube was therefore designed with a 125-meter spacing between the detectors. But Halzen explained that, after years of experiments, IceCube researchers now know that Antarctica’s deep ice is more transparent than previously thought. The detector spacing could thus exceed 250 meters, and a telescope 10 times larger than the current IceCube array could be built with a comparable number of detectors and at a similar cost.

–David Ehrenstein and Matteo Rini