The Universe is permeated by filaments of invisible dark matter that intersect at galaxies and other major structures. This is the conclusion of astronomers from the Cosmic Evolution Survey (COSMOS), who have created the first large-scale map of the distribution of dark matter. The apparent overlap of dark matter filaments with galaxies and other massive structures adds further weight to the theory that the Universe owes its structure to the gravitational pull of dark matter (Nature doi:10.1038/nature05497).
Dark matter is fundamentally different from normal “luminous” matter that makes up stars, planets and humans. It is invisible to modern telescopes, giving off no light or heat, and it seems to interact with normal matter only through gravity. Although dark matter has never been observed directly, most cosmologists believe dark matter plays a crucial role in how large structures such as galaxies emerged after the Big Bang.
The COSMOS team used the Hubble Space Telescope and several terrestrial instruments to chart the position of elusive dark matter in three dimensions. This was done by observing how light from distant galaxies is bent by the gravitational pull of dark matter in a process called gravitational lensing.
The map also reveals that regions of space containing large quantities of luminous matter almost always also contain large quantities of dark matter, which is exactly what physicists would expect to see if the gravitational collapse of dark matter was responsible for the structure in the Universe. “It’s reassuring how well our map confirms the standard theories for structure formation”, said lead researcher Richard Massey of the California Institute of Technology.
However, the survey also reveals areas with large quantities of dark matter with no corresponding luminous matter. In principle, this is also feasible because physicists believe that there is much more dark matter in the Universe than luminous matter.
While astronomers have already used gravitational lensing to map smaller regions surrounding individual galaxies, this is the first “wide-sky” survey that covers a region of the sky about the eight times the size of a full moon. The survey is also the first to look at dark matter in three dimensions – the third dimension being the distance (or time) travelled by the light after interacting with the dark matter. The distance was determined by combining observations made by Hubble and earthbound telescopes. This new ability to chart the evolution of dark matter through both space and time could also shed light on another elusive quantity – dark energy, which is believed to be accelerating the expansion of the Universe.
