 new breed of microwave detectors that were far more sensitive than any used before. They were also each equipped with telescopes that were over an order of magnitude more accurate than those on the COBE satellite. Both of these features meant that they were able to accurately map out the fine details of the CMB.
There had already been strong hints of the coveted spots in 1997 from an experiment called MAT/Toco. But it was only after a few years of whittling away at the signals in early 2000 that we saw our own clear evidence of hot and cold spots with a typical size of 1 degree. This meant that the geometry of the universe was almost exactly flat. We were seeing direct and unambiguous evidence that we lived in a very special universe.
Together with later measurements from NASA's WMAP satellite, the results nailed down the geometry of the universe to within a few per cent. Suddenly life became much simpler for cosmologists who had been working on different theoretical models with different geometries. From then on there was one less free parameter to fiddle with and almost all the papers and textbooks stated up front that the universe was flat. Finally we had some certainty about the state of the universe.
Except that another dramatic narrative was unfolding in parallel. For decades, we had known that there simply was not enough stuff in the universe to balance the cosmic books. All the known atoms in stars, gas and dust spread out in the cosmos, add up to less than 4 per cent of the total matter and energy we knew had to exist for the universe to have developed to its current state.
Even taking into account the elusive dark matter which we believe bolsters galaxies and allows them to spin at breakneck speeds without flying apart, there still is not enough matter and energy to make it flat; the shortfall in the total energy of the universe seems to be about 70 per cent. And if this were so, the universe would have negative curvature. There must be something else out there to make up the deficit.
The answer came in 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