Mapping the Universe

One of the most powerful tools we have in trying to understand the properties of the universe as a whole (e.g how much normal matter is in there, how much dark matter, how much dark energy? - i.e, the entire field of cosmology) are galaxy surveys - figuring out both how many galaxies there are at a particular redshift / time after the Big band and how close they are too each other. Traditionally, this has been done using large optical surveys like the Sloan Digital Sky Survey - which finds galaxies by detecting their star light. Galaxies also contain a lot of hydrogen gas, which when cold emits light at particular wavelength - 21 cm, in the radio band. It is possible to a galaxy survey by detecting this emission, which may be more sensitive to low mass galaxies with few stars than optical surveys. Such radio galaxy surveys are now feasible using telescopes like NRAO's 100m Green Bank Telescope and NAIA's Arecibo Telescope. To read more about these surveys, and how they can help determine the answer to such questions like "the nature of dark energy" read this and this and this article, with the full research articles here and here(Subscription, required).