The term “cosmic dawn” refers to the epoch of early star formation that signals the end of the dark ages. If you put the history of the universe on a calendar where January 1 corresponds to the big bang, and December 31 corresponds to the present day, then January 5 is the cosmic dawn. During this period the very first stars light up and the very first galaxies form.
These stars and galaxies are firmly beyond the reach of the current generation of optical and infrared telescopes (for example, the Hubble Space Telescope). However it might be possible to detect the diffuse hydrogen gas from this era with a low frequency (long wavelength) radio telescope. Neutral hydrogen absorbs light at a characteristic wavelength of 21 centimeters. Because the universe is expanding, 21 centimeter photons emitted from distant clouds of hydrogen gas have been stretched by the time they reach earth. The amount by which the 21 centimeter photon has been stretched is a measure of how long the photon has been traveling and therefore how far away the corresponding hydrogen is. This technique can be used to map the three-dimensional distribution of hydrogen gas during the cosmic dawn.
Unfortunately the cosmological signature of distant hydrogen gas is very faint, and it will be very difficult to separate this signal from the blinding light of our own Milky Way galaxy and other sources of radio emission. Larger telescopes and sophisticated electronics can make this easier, but a topographical map of hydrogen is beyond the capabilities of any existing instrument.
I am using the Owens Valley Long Wavelength Array to measure the power spectrum of 21 centimeter brightness fluctuations during the cosmic dawn. The power spectrum is a statistical tool that is in many ways similar to shaking a box to guess the contents of a package without opening it. These measurements (the first of their kind at redshifts > 20) will provide insight into the early thermal history of the intergalactic medium, galaxy formation, and star formation.
The Owens Valley Long Wavelength Array (LWA) is a 288-antenna interferometer located at the Owens Valley Radio Observatory (OVRO) near Big Pine, California. The LWA detects radio waves with wavelengths between 3.5 meters and 10 meters. Longer wavelengths actually reflect off the Earth’s ionosphere so the radio waves detect by the LWA have the longest wavelength that is possible to observe without putting antennas in space.
251 of the 288 antennas are arranged within a 200 meter diameter core that provides excellent thermal sensitivity. 5 antennas are equipped with noise-switched receivers that allow for calibrated measurements of the total power detected by each of these antennas. The remaining 32 antennas are placed up to 2 kilometers away from the central core of the interferometer. These antennas are used to image the entire sky with 10 arcminute resolution.
We are used to seeing the plane of our galaxy, the Milky Way, as a faint strip of stars that is visible during a moonless night. In long wavelength radio waves the Milky Way is blindingly bright, and outshines even the sun (unless the sun is flaring). Supernova remnants (Cassiopeia A) and distant galaxies (Cygnus A) can also be brighter than the sun. At Caltech we are using the LWA to study the infant universe, detect stellar flares, find extra-solar planets, and characterize the Earth’s ionosphere.
BPJSpec is a 21 cm power spectrum routine developed for the Owens Valley LWA.
MLPFlagger is a flagging routine developed for the Owens Valley LWA.
CasaCore.jl is a Julia language wrapper of the
casacore library. I use CasaCore.jl
to interact with measurement sets and to perform coordinate system conversions.
This is an officially registered Julia package so it can be installed by simply
Pkg.add("CasaCore") from the Julia REPL.