n 1998 when two separate teams noted that distant supernovae were much dimmer than they should be. By far the simplest explanation is that the expansion of the universe is now accelerating.
Now, if the universe is accelerating then something must be pushing space-time apart. We have found that if 70 per cent of the universe is made up of an exotic, repulsive material called dark energy, then it could make up the shortfall of energy. So even though 96 per cent of the universe is invisible to us in the form of dark matter and dark energy, it does neatly explain the cosmos.
Einstein had proposed a possible candidate for this repulsive form of energy: the cosmological constant, or lambda, is a completely smooth form of energy which never dilutes as space expands. Lambda has been adopted wholesale by many cosmologists, so much so that the current, standard model is known as the lambda-cold dark matter model. And one of its main features is that the universe is flat.
But it's not that simple. To be brutally honest, we have no idea what lambda physically represents. Even our best guess gives us the wrong answer by more than a hundred orders of magnitude. So given our manifest ignorance, we have to allow for other possibilities: dark energy may not be constant in time and space. As long as our models predict the right amount of cosmic acceleration we are allowed to keep our options open.
So can we still assume the universe is flat? I thought so, until very recently, when a development shook my faith in the notion. Chris Clarkson and his collaborators at the University of Cape Town in Rondebosch, South Africa, decided to see how our knowledge of dark energy affected our measurement of the curvature of the universe and vice versa (Journal of Cosmology and Astroparticle Physics, DOI: 10.1088/1475-7516/2007/08/011).
To understand their results, remember how we measured curvature from the size of the hot and cold spots in the CMB. How big the spots appear to us - their angular size - depends on two unknowns: how far light has 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 gar