Cosmic Dawn is the era when the first stars and galaxies formed.    It was about 250 million years after the Big Bang – or about 13.5 billion years ago.  So far astronomers have very few observations of this era.  New observations of Cosmic Dawn are one of the highest priorities for astronomers and among the goals for NASA’s James Webb Space Telescope and other new large telescopes on the ground.

With OVRO-LWA, we will look for the gas between the first galaxies during Cosmic Dawn.  Stars in galaxies produce light spanning the electromagnetic spectrum, including infrared, visible, and ultra-violet – even X-rays.  Once galaxies form in the early universe, their starlight shines into the hydrogen gas between the galaxies.  The ultra-violet component of the light alters the excitation state of the hydrogen atoms.  This causes the atoms to absorb radio waves from the cosmic microwave background (CMB).  The radio waves are absorbed only at a very specific wavelength: 21 centimeters.  We should be able to see spectral distortions from this process in the CMB today, but they will be stretched to wavelengths longer than three meters by the expansion of the universe.  This is equivalent to radio frequencies below 100 MHz.  The distortions in the radio spectrum contain information about the first stars, galaxies, and black holes.  They also can help us understand how the temperature of the universe changed with time, possibly giving us new information about Dark Matter.   

In 2018, the Experiment to Detect the Global EoR Signature (EDGES) reported the first evidence for detection of a radio spectral distortion from Cosmic Dawn.  They found an absorption profile centered at about 78 MHz by averaging the radio intensity over a large area of the sky.  The frequency range of the signal reported by EDGES is contained in the LWA-OVRO observing band.  We will use OVRO-LWA to try to verify the signal and look for spatial fluctuations in it.  

The primary challenge for detecting the Cosmic Dawn signal is separating it from other astronomical radio sources in the foreground.  There are many strong sources of radio waves in the sky.  Our own Milky Way galaxy is the dominant radio emitter seen from Earth.  It emits radio waves primarily through synchrotron radiation, which is created by electrons moving at relativistic speeds around magnetic fields in the Galaxy.  This emission is more than 10,000 times stronger than the Cosmic Dawn signal.  Fortunately, the synchrotron spectrum is well-known and should be distinct from the expected Cosmic Dawn signal.  In principle, a good instrument will be able to distinguish between the two.  However, small instrumental imperfections can cause the synchrotron spectrum to look somewhat like the cosmological signal. Our recent upgrades to OVRO-LWA should help to overcome some of these instrumental effects.   

We can also compensate for some instrumental effects during the data analysis stage.  During this stage, raw time-ordered data are calibrated and reduced to an estimate of the 21 centimeter power spectrum from Cosmic Dawn. Many advances in mitigating instrumental effects during data analysis have occurred in the last decade.  OVRO-LWA is a particularly good instrument for comparing complementary analysis methods that have emerged. We will use two techniques to analyze observations: 1) a projected imaging approach and 2) an “m-mode” imaging technique. The projected imaging approach builds on methods pioneered by telescopes like LOFAR and MWA, which in turn build on standard processing techniques developed for historic radio interferometers like the VLA.  Using standard tools will allow us to make comparisons against other projects and, for some steps, use existing validated software codes.  The m-mode technique is more recent and takes advantage of the OVRO-LWA’s large field of view, stable performance, and continuous operation.  These features allow the raw measurements to be converted efficiently into spherical harmonic components (the name “m-mode” is derived from the letter “m” used to describe some of these components).  The spherical harmonic components are then used to generate full sky maps.  This technique was developed for the CHIME radio telescope and was used for our preliminary Cosmic Dawn analysis before we upgraded OVRO-LWA.