Astronomy Colloquium

After stars form in the early Universe, their ultra-violet light alters the excitation state of the 21cm hyperfine line of neutral hydrogen gas in the intergalactic medium.  This initially causes the gas to absorb photons from the cosmic microwave background (CMB).  Later, energy deposited into the gas by the ultra-violet and X-ray emission from these early stars and their remnants heats the gas and eventually ionizes it.  These effects produce spectral features in the CMB observable today at frequencies below 200 MHz.  Our team recently reported the first detection of this redshifted 21cm signal in all sky radio observations acquired by our Experiment to Detect the Global EoR Signature (EDGES).  EDGES measures the all-sky radio spectrum between 50 and 200 MHz in the Outback of Western Australia.  We have found a flattened 21cm absorption profile in the sky-averaged radio spectrum centered at a frequency of 78 MHz with full width at half maximum of 19 MHz and an amplitude of 0.5 K.  The observed profile contains attributes that are both expected and surprising.  The low-frequency edge of the profile indicates that stars existed and created a strong background of Lyman-alpha photons by 180 million years after the Big Bang (z~20).   The high-frequency edge indicates that the hydrogen gas was heated above the radiation temperature by 270 million years (z~15).   These times are roughly consistent with astrophysical models of early star formation.  However, the best-fit amplitude of the observed profile is more than a factor of two greater than the largest standard predictions and suggests that the gas was either significantly colder than expected or the background radiation temperature was hotter than expected.  I will present the measurements along with results from instrumental verification tests that provide evidence that the observed feature is from the astronomical sky.