 travelled since the CMB formed at recombination, and the geometry of space.
If we assume that the universe is permeated by a cosmological constant and contains a modicum of dark matter and atoms, we can pin down this distance very accurately by solving the equations of general relativity. But we do not really know if dark energy is the cosmological constant. If it is something with a much more complicated evolution, then we know very little about the distance to recombination. Without knowing how far away the hot and cold spots of the CMB are, we cannot predict how big they will be in a universe with negative, zero or positive curvature.
This means that we cannot pin down the geometry of space-time. We are then caught in a vicious circle: to know the geometry of the universe we need to pin down dark energy. Yet to determine how much dark energy there is, we need to know the geometry. Knowing one without the other is futile and a recipe for disaster.
These recent results are sobering. Having proselytised about the great discovery of the millennium, that the universe is flat, I now find myself backtracking. And there is a sense of foreboding that it may never be possible to know our cosmos as well as we'd like to.
There is a sense of foreboding that it may never be possible to understand our cosmos
Except that we seem to have entered a new, more ambitious, era in cosmology. While cosmology in the 1990s was very much driven by attempts to measure the geometry of the universe, as well as a host of other properties, the focus now is very much on pinning down the great unknown - dark energy.
How can we do this without knowing the geometry? Happily all is not lost: new experiments are being designed to probe the deepest recesses of the cosmos. The more information there is from different stages in the evolution of the universe, the easier it will be to figure out not only what dark energy is doing at different times, but also what the geometry of the universe is doing in different places.
Plans are afoot to build a gargantuan radio telescope called the Square Kilometre Array, either in Australia or Southern Africa, which would allow us to measure the positions of one billion galaxies. Galaxies that have clumped together can be seen as proxies for the hot and cold spots in the CMB, so the net result is akin to sampling the state of the universe at many different times, from which we can determine the geometry with greater accuracy.
And a new satellite mission, run by NASA and the European Space Agency, will measure how dark energy affects the evolution and interactions between galaxies out to tremendous distances.
These projects will be up and running within a decade. In the meantime, other smaller projects are chipping away at our ignorance. Indeed, this new age of discovery and exploration will bear fruits which, hopefully, will one day enable us to understand the geometry of the universe. It is just going to take a bit longer than we originally thought.
Pedro Ferreira is a cosmologist at the University of Oxford, UK, and author of The State of the Universe (Phoenix, 2006)
This article originally appeared on New Scientist.