The Dark Ages and Cosmic Dawn


In the storybook picture of the universe, it all began with the big bang. A brief era of inflation was followed by 300,000 years where the universe was filled with a radiation dominated plasma. Soon after this radiation faded to the point where matter (dark matter, protons, electrons, and small amounts of heavier nuclei) dominated the universe, the electrons and protons were able to recombine and form neutral hydrogen. This is known from studying the distant cosmic microwave background (CMB) radiation that is just now reaching us from the period of recombination.

What follows is less well understood. There is a delay between recombination and the formation of the first stars and galaxies. This period of time, referred to as the dark ages, is where baryonic matter first begins to fall into dark matter halos. The first galaxies, clusters, and superclusters all began to form at this time. Eventually stars form and explode in supernovae that populate the universe with incrementally more heavy elements. These heavy elements make it easier for gas clouds to cool and collapse, forming even more stars. The universe is now awaking from the dark ages to a period of time known as the cosmic dawn.

In order to study the process of galaxy formation it makes sense to look for the most distant galaxies possible. The further away a galaxy is, the further back in time we are seeing it. This is problematic because distant galaxies are typically found and studied with optical and near-infrared telescopes. At large distances, the bound-free opacity of neutral hydrogen shrouds these galaxies from view.

The LWA circumvents this problem by looking for highly redshifted 21 cm photons that are characteristic of neutral hydrogen. Instead of studying the proto-galaxies themselves, the LWA makes it possible to study the gas around these galaxies. How do galaxies form and develop into the wonderfully complex and beautiful systems we see today? How did they interact with the surrounding gas during this process? When did galaxy formation start and how long did it take? All of these questions are wide-open. However, with the LWA at Owen's Valley, we will begin to probe the answers to some of these questions.

The Transient Radio Sky

Time domain astronomy is a rich field with high potential for new discoveries, in all wavelength regimes. The success of transient searches in the optical (with the Palomar Transient Factory (PTF), the Catalina Real-time Transient Survey, the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), etc.) and in the X-ray and gamma-ray skies (with the Swift Gamma-Ray Burst Mission and the Fermi Gamma-ray Space Telescope) highlights the vast scientific yield and exciting nature of time domain astronomy. The variable and transient radio sky, however, remains relatively poorly sampled, due to the limited fields of view, sensitivity, and survey speeds of traditional radio interferometers, despite the evidence that radio transient phase space is equally as rich as its counterparts in other wavelengths.

The Owens Valley LWA will open up the field of radio transients -- with full cross-correlation of all of its 33,000 baselines and instantaneous imaging by a dedicated transient backend, the LWA will produce all-sky images every second with approximately 1˚ resolution in all 4 polarizations (IQUV), reaching less than 10 mJy RMS noise in a 1 hour integration. This all-sky sensitivity means we can perform targeted transient searches as well as conduct blind surveys to better sample the transient phase space and reveal new and exciting populations of radio transients.

The LWA transient search is particularly aimed at the detection of coherent radio emission from extrasolar planets, similar to the extremely bright electron cyclotron maser emission produced by magnetized planets in our own Solar System. The direct detection of extrasolar planets through their auroral radio emission would provide measurements of magnetic field strengths and rotation rate, as well as serving as an indirect probe of interior composition and dynamics.