Science: Cosmic Dawn
One of the most important gaps in our understanding of our Universe’s history is the “Cosmic Dawn.” The period from about 50 million years to one billion years after the Big Bang when the first stars, black holes, and galaxies in the Universe formed. One of the best ways to observe this era is with low-frequency radio telescopes, which can observe the “spin-flip” radiation from the hydrogen that pervades the Universe during the Cosmic Dawn. Astronomical observatories on the lunar surface and in cis-lunar space targeted at this background will be amongst the most sensitive probes of the early universe.
The requirements for these telescopes will push both technology boundaries and our knowledge of environmental effects at target destinations for human exploration. Such issues as trafficability in antenna deployment, space plasma effects, thermal shocking of electronics and mechanical systems, as well as power, survivability, and operation during lunar nights have direct applicability to exploration. The design of these observatories to conduct decadal-level research will provide some technology solutions for exploration. Thus, our research will enable both astrophysics and exploration.
What were the first objects to light up the Universe and when did they do it?
After the Big Bang, the universe was hot, dense, and nearly homogeneous. As the universe expanded, the material cooled, condensing after ~400,000 years into neutral atoms, freeing the cosmic microwave background (CMB). Fifty million years or so later, gravity drove the formation of the first luminous objects – stars and black holes – which ended the dark ages and initiated the cosmic dawn. These first stars likely differed dramatically from nearby stars, as they formed in vastly different environments. This transformative event marked the first emergence of complexity in our universe, but no currently planned mission can explore it. While the James Webb Space Telescope, the Wide-Field Infrared Survey Telescope, and a suite of ground-based facilities will observe the universe as it was just ~150 million years after the Big Bang, none now contemplate observing the true first stars and black holes. The most promising method to measure the properties of these first objects is the highly-redshifted spin-flip background. The “spin-flip” transition of neutral hydrogen emits a photon with a rest wavelength of 21-cm (or frequency 1420 MHz). This signal, while generated by an extraordinarily weak hyperfine transition, is nevertheless observable because neutral hydrogen pervades the universe at the onset of the cosmic dawn. Several ground-based instruments, such as the hydrogen epoch of reionization array, hope to observe the reionization era by the end of this decade. But probing the first stars, at much earlier times, is possible if we can reach very low frequencies (<100 MHz). A lunar farside radio telescope will open a unique window to this era.
What are optimal low-frequency antenna technologies for a lunar farside array?
We will investigate the antenna technologies for conducting 21-cm cosmology measurements from the lunar surface that provide both the required level of control of systematics and is amenable to deployment on the lunar surface. The primary challenge associated with exploiting the 21-cm cosmological signal is the control of systematics arising from the antenna. Indeed, in one of the earliest papers to consider the use of the 21 cm line to probe reionization called attention to this concern, noting that some aspects of the HI spectral feature could, in principle, be detected in integrations of only 24 hours and that “sensitivity is not an issue.” We will conduct trade studies of antenna technologies, with the aim of identifying one suitable for future development and testing during the course of the NESS work. Considerable work has been done on dipole antennas implemented as a conducting material deposited on a polyimide substrate. This approach has significant benefits, most notably small volumes for transport and multiple deployment approaches. However, this antenna would be deployed directly on the lunar surface, and the gain/beam of the antenna would be dependent upon the sub-surface characteristics at the deployment site. It is not clear that the sub-surface could be characterized sufficiently well to enable 21-cm cosmology measurements.
Key Astrophysical Processes
What are the key astrophysical processes driving the low-frequency radio signal from the Cosmic Dawn?
Due to the massive uncertainties in our understanding of galaxies during the Cosmic Dawn, NESS will undertake theoretical modeling to identify the most important and relevant science questions. NESS astronomers are developing “semi-analytic” models to describe the interaction of the first sources (stars, galaxies, & black holes) and the radiation backgrounds they generate - because those very radiation backgrounds affect subsequent generations of luminous structure. Armed with these models, we will develop fast, flexible algorithms to predict the spin-flip background that can be observed with a lunar telescope. Such predictions will guide the development of these instruments and bring the most relevant science questions into focus.
Additionally, while luminous sources have the most obvious effects on the spin-flip background, the signal before such sources form provides a precision probe of exotic physical processes in the early Universe, including primordial black hole formation, modified gravity models, and dark matter and dark energy physics. We will explore how these processes can be tested with future lunar or cis-lunar observations.
Astrophysical Parameters from Low-frequency Radio Measurements
How can we extract astrophysical parameters from low-frequency radio measurements of the Cosmic Dawn, alone and in combination with other future investigations?
Building upon codes developed to analyze data from the sky-averaged spin-flip background, we will develop algorithms to relate source models to spinflip background measurements from potential lunar farside interferometers. We will use state of the art Markov Chain Monte Carlo (MCMC) techniques, together with our efficient signal prediction, to determine the most critical parameters driving the background and whether unique, model-dependent, signatures exist. Additionally, Spin-flip observations are highly complementary to galaxy surveys, as the former are sensitive to the integrated emission from galaxies beyond the survey’s detection limit and/or emission mechanisms hidden from traditional galaxy surveys. Extracting the maximal information therefore requires simultaneous analysis of both techniques. We will extend our source models and statistical tools to include galaxy surveys, CMB measurements, and other observations, to determine the most efficient multi-wavelength strategies for generating robust constraints. Finally, we will use our analysis tools to perform a trade study of the collecting area, array configuration, and frequency range required to make meaningful constraints on the first luminous sources in the Universe with lunar farside measurements of the spin-flip background.
For additional information download NASA’s Program Annual Technology Report, Cosmic Origins Program Office (PDF, 25MB) from October 2017. In particular, read pages 51 and 61 for information on two low-frequency radio technology issues included in the new plan.