Cosmology isn't my strong suit, but I remember exactly where and when it first sunk in that the universe was out of control. It was August 2000, and I was in Manchester, England, to cover the International Astronomical Union's XXIVth General Assembly.A parade of distinguished cosmologists weighed in on the recent revelation that the redshifts of the most distant supernovae could only be explained if the universe's expansion has been speeding up, rather than slowing down (as you'd expect due to its own self-gravity). This unexpected acceleration, they conjectured, must be due to some kind of force throughout space that opposes gravity itself on very large scales. It came to be called "dark energy."
The big thinkers still don't know what dark energy is, but they've come to accept its reality, thanks to key observational tests such as those from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) mission.
There's also no consensus on what makes up the dark matter — invisible stuff whose gravity holds galaxies together. But it isn't anything like the "normal," proton-neutron-electron (baryonic) matter that we're familiar with.
Whatever their true nature, dark energy (symbolized as Λ) and dark matter must dominate everything else by a large margin for the universe to "work." For those of you keeping score at home, the composition of our universe must be very nearly:
- 73% dark energy
- 23% non-baryonic dark matter
- 4% unseen baryonic matter
- <1% visible matter (stars, nebulae, and so forth)
By the way, this work was made possible by a little-known but astounding effort undertaken with the Hubble Space Telescope called the Cosmic Evolution Survey, or COSMOS. From October 2003 to November 2005, HST's Advanced Camera for Surveys acquired 38-minute-long exposures of 575 overlapping fields, collectively covering a patch of Sextans a full 1.3° square.
COSMOS took 1,000 hours of Hubble time to acquire — making it even more ambitious than observatory's famous, but much narrower, "deep fields." At full resolution, the final image is 100,800 pixels on a side — imagine a page in Sky & Telescope 28 feet on a side and you'll get the picture (so to speak).