From the dark ages to the first light
For hundreds of millions of years after the Big Bang, the cosmos was a dark and uniform place. Following a period of cooling, protons and electrons combined to form a dense fog of neutral hydrogen gas, initiating a period astronomers call the Cosmic Dark Ages. There were no stars or galaxies, and the universe was opaque to most forms of light. This era began about 379,000 years after the Big Bang.
The period that ended the Dark Ages is known as the Epoch of Reionization (EoR). It began as gravity slowly pulled the primordial hydrogen and helium into denser clumps, eventually igniting the first stars. These hypothetical Population III stars were unlike anything that exists today. Composed almost entirely of hydrogen and helium, they may have been 60 to 300 times more massive than our Sun. These stellar giants burned incredibly hot and died young, releasing high-energy ultraviolet radiation that stripped electrons from the surrounding hydrogen atoms. This process of reionization did not happen instantly. It started in bubbles around the first stars and galaxies, which grew and merged over time until the entire universe was once again a transparent, ionized plasma. This cosmic transformation is thought to have been largely complete about 1 billion years after the Big Bang.
Listening for a whisper from the dawn of time
Astronomers cannot see the first stars directly, but they can detect the faint radio signature of the neutral hydrogen gas that defined that era. Neutral hydrogen atoms naturally emit radiation at a very specific wavelength of 21 centimeters (a frequency of 1420 MHz) due to a "spin-flip" transition in their single electron. This signal, predicted in 1944 and first detected in 1951, allows radio astronomers to map hydrogen clouds in our own galaxy.
To see the 21 cm signal from the Epoch of Reionization, astronomers face two major challenges. First, the expansion of the universe has stretched this wavelength significantly. The signal from the EoR now arrives at Earth at low frequencies between 50 and 200 MHz, the same part of the radio spectrum used for FM radio and television broadcasts. Second, the signal is extraordinarily faint. This requires telescopes built in some of the most remote, radio-quiet places on Earth.
The coordinates for this Point of Interest are centered on the Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory, in Western Australia. This site hosts two telescopes designed specifically to hunt for the 21 cm signal: the Murchison Widefield Array (MWA) and the under-construction Square Kilometre Array (SKA-Low). The MWA consists of thousands of spider-like antennas spread over the desert. The future SKA-Low telescope will be an even larger instrument, with plans for 131,072 individual antennas spread out over 74 kilometers. These arrays are designed to filter out the radio noise from our own galaxy and Earth to isolate the faint signal from the universe's dawn.