One of the most accessible methods to probe the energy content and large-scale structure evolution of the Universe is though large galaxy surveys, which can be used as tracers of the underlying dark matter distribution. In the radio we can use HI galaxy surveys, which will provide a 3d view of the universe (using angle and redshift). These are threshold surveys in that they set a minimum flux above which galaxies can be individually detected. This will impose strong requirements on the telescope sensitivity and only with the phase 2 of the SKA will we be able to have HI galaxy surveys competitive at the cosmological level.
Instead we can consider measuring the integrated 21cm emission of several galaxies in one angular pixel on the sky and for a given frequency resolution. Each 3d pixel is expected to have several HI galaxies, so that their combined emission will provide a larger signal. By not requiring the detection of each galaxy, the specifications imposed on the telescope will be much less demanding. This is what has been commonly called as an “intensity mapping” experiment. It is similar to what is being planned for experiments aimed at probing the Epoch of Reionization (at z>6). The novel technique of HI intensity mapping, promises to deliver the largest 3-dimensional maps of the Universe ever made and push the limits of our understanding of cosmology.
At UWC, we are using the telescope MeerKAT to probe this signal and provide some of the very first detections of HI intensity mapping that can be used to understand our Universe. The possibilities opened by this new observational window promise a breakthrough in constraining cosmology in the very near future using telescopes such as the SKA.