Developing a Focal Plane Array at the GBT for 21 cm Astronomy

Abstract

Abstract. Progress has been made on the design of a 9 receiver array in the 700 to 945 MHz range to replace the single receiver in that frequency range currently in use at the Green Bank Telescope (GBT) in West Virginia. The new array will increase the rate of data collection ~9x in ongoing 21 cm intensity mapping, and it should also be a valuable resource for other users of the GBT. The following paper describes the science of 21 cm intensity mapping and the work that has been done in designing the receiver for the focal plane array. Cosmology with the 21 cm line of Neutral Hydrogen (HI)It has been observed that, on the largest scales, the universe is quite symmetric. If one considers scales much larger than galaxy clusters, the universe looks the same in every direction (isotropic) and also the same at every point (homogeneous). This allows cosmologists to make the simplifying approximation that, again only on very large scales, the matter and radiation density of the universe are constant. Combining this simplification with the rules of general relativity (GR), one can derive the Friedmann Equation, a differential equation which determines how the universe expands as a function of four things: the matter density, the radiation density, the curvature of space, and a mysterious component called dark energy (which is, in the currently favored ΛCDM model, Einstein's cosmological constant).As the universe expands, the wavelength of light traveling through the universe expands with it. It has also long been known that velocity curves of galaxies are inconsistent with gravitation from the visible luminous matter: objects are traveling too fast for the amount of matter that is seen. Therefore, rather than completely abandon the highly successful GR theory of gravity, cosmologists now postulate that most matter is 'dark matter,' which does not interact with or produce light. Studying dark matter and dark energy are two main goals of modern cosmology.Mapping the distribution of neutral hydrogen, using the 21 cm line, is a convenient way to study the distribution of dark matter and the equation of state of dark energy (to be discussed more fully later). Hydrogen's abundance makes it a good unbiased tracer of the underlying dark matter distribution (because of gravitational attraction, it should map the dark matter). The 21 cm line, being a (low energy) atomic transition, is always produced at a fixed wavelength (21 cm), and therefore any increase in wavelength over 21 cm is due to the expansion of the universe (cosmological redshift). The Friedmann equation allows one to use this redshift to calculate the radial position of the HI and the time in the past when the radiation was emitted. Aside from mapping the dark matter, the HI power spectrum should show the remnants of Baryon Acoustic Oscillations (BAO). BAOs were pressure waves in the primordial plasma of the early universe, created due to slight over densities and under densities in the matter of the universe (caused by random quantum fluctuations). Eventually the p...