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Unravelling the Cosmic Reionization History

Final Report Summary - LOFARCORE (Unravelling the Cosmic Reionization History)

Following the explosive expansion of the Big Bang, the majority of the material in the Universe consisted of hot, ionized hydrogen which slowly cooled to form neutral hydrogen over the next few 100,000 years. These subsequent Dark Ages, so-called due to the lack of sources of light, are believed to have lasted for several hundred million years until the appearance of the first luminous stars and black holes which formed in small galaxies. This crucial phase in the evolution of the Universe, during which the majority of the material in the Universe went from being neutral hydrogen gas to an ionized state, is known as the Epoch of Reionization and probably took place between 300 and 800 million years after the Big Bang. The timing and nature of the Epoch of Reionization (EoR) is related to many fundamental questions in cosmology, such as properties of the first galaxies, the physics of quasars, and the formation of very metal-poor stars among a host of other important topics. Despite its importance, however, the EoR remains poorly understood and detecting observational evidence illuminating this crucial phase in the evolution of our Universe is currently one of the highest science priorities in modern cosmology.

The ultimate goal of the LOFARCORE project is to detect the extremely faint radio signal produced by the redshifted 21 cm line emission from neutral hydrogen in the earliest phases of the evolution of the universe only a few 100,000 years after the big bang. The project aims to detect these signals by acquiring and analyzing more than 3600 hours of observations with the LOFAR radio telescope in the largely unexplored frequency range from 120-180 MHz. The detection of the EoR has been one of the key science drivers for the design and construction of the LOFAR telescope since its inception. Given the extremely faint nature of the signal, it is also one of the most technically challenging. The accumulated data volume for the EoR campaign currently exceeds 5 petabytes and the processing of that data to reach a signal detection has required the development of entirely new algorithms and techniques for calibration, imaging, and processing. To apply these new, computationally expensive techniques the LOFARCORE team has constructed and operates a powerful dedicated compute cluster, with 1536 CPU cores and 128 GPUs located at the University of Groningen, and is steadily sifting through these many petabytes of data in search of the faint EoR signal.

Due to the unfortunate passing of Prof. de Bruyn, the LOFARCORE project has concluded earlier than originally planned and has not yet achieved a definitive detection of the EoR 21cm signal. The current team, however, contains all the requisite expertise, the observational data required is largely in hand, and the data processing and analysis infrastructure is functioning well. As part of the project, the LOFARCORE team have made the deepest radio continuum images, as well as the highest resolution images, ever made at these radio low frequencies. They have discovered that the synchrotron emission from our Galaxy is very highly polarized and produced unparalleled studies on the fine-scale structure and the dynamics of the ionosphere. At the same time, they have also developed some of the most advanced algorithms and techniques for processing radio data in the world and these tools are now regularly used by both the LOFAR Observatory and the wider scientific community. Finally, the team has currently achieved the most sensitive upper limit on the presence of the EoR signal to date and has all the data, infrastructure, and expertise necessary to answer fundamental questions in the coming years such as "When did the reionization of the Universe begin?", "How long did this process take?" and "What objects are responsible?".