The LOFAR-EoR Key Project is set to measure the neutral hydrogen fraction of the IGM as a function of redshift and angular position, tracing the EoR and mapping it, with arc-minute resolution (fine-tuned to the expected angular scale of the signal), as it progresses in space and time. The project will provide data-sets (i.e. cubes) over several hundred square degrees field-of- view (depending on depth and number of independent beams) and over the 115 -180 MHz frequency range (i.e. redshifts between 11.5 and 6.7).
Our ability to detect the 21 cm brightness temperature from the high redshift neutral hydrogen is challenged by the existence of a multi-component foreground contamination (see e.g. Figures 1 and 3): Ionospheric transients, synchrotron emission from the Galaxy, radio galaxies and free-free emission from distant galaxy clusters. To extract any information about the EoR from the data, it is essential to resort to statistical signal-processing techniques that are optimized to cope with the large systematic and random sources of errors.
Figure 3. The foregrounds of the 21 cm radiation. The figure shows the various Galactic and extra-galactic contaminants of the redshifted 21 cm radiation from the EoR. The difficulty posed by these foregrounds stems from the fact that their amplitude is about 3 orders of magnitude larger than the expected cosmological signal. (Courtesy of V. Jelić)
The challenges that these projects face are indeed very serious. For example, the redshifted 21 cm passes through a ~1000 times larger galactic and extra-galactic foregrounds that contaminate the data. The data is then further (often severely) influenced by the passing through the Earth's ionosphere and the instrument response. Hence an exquisite calibration of the telescope, in light of the constantly changing ionosphere and instrument response, is an absolute must.
Despite the many challenges, the near future will be very exciting time for this field as successful observation of this epoch will open a completely new area in cosmology. It will allow us to answer many of the important open questions about the Universe’s Dark Ages and the Epoch of Reionization, and bridge the huge observational gap we have in our knowledge of the Universe between 400,000 after the Big Bang - when the recombination occurred - up to 1 billion years later when the Universe became fully ionized.