NEXT time you fancy doing something really frustrating, try balancing a pencil on its sharpened tip. Your efforts will succeed for a second at most. Yet the universe has been succeeding at a similar gravitational trick for the last 13.8 billion years.
The feat is embodied in its geometry. According to Einstein鈥檚 general theory of relativity, matter and energy bend space and time, and the amount of stuff the universe contains will determine its ultimate fate. If the universe is dense enough to curve space-time in on itself, all that gravity will eventually collapse it back down to nothing. If the universe鈥檚 density is low, it curves outwards 鈥� and the weakness of the gravitational pull will mean it expands forever.
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But our universe seems to fit in neither camp. The most powerful test of its geometry is the variation in the cosmic microwave background, the radiation emitted shortly after the big bang. According to measurements of this radiation, the density of matter and energy is such that the universe does not curve either way: it is perfectly flat. After an eternity, its expansion should grind to a halt with no subsequent collapse.
The plot thickened considerably in the late 1990s, when very distant exploding stars were inexplicably seen to be dimmer than expected. This suggested that the universe鈥檚 expansion was accelerating rather than slowing down. The proposed fix was to say that a large proportion of the universe exists as dark energy, a new ingredient allowing it to remain flat yet expand ever faster.
But devising a theory of dark energy has theorists sucking their pencils. 鈥淧rogress is pretty tough going,鈥� says Andrew Pontzen at University College London. There is no natural candidate for the phenomenon, and our best guess is that it is a vacuum energy of the sort allowed in quantum theory, resulting from particles popping in and out of existence. But simple calculations reveal that this would produce 120 orders of magnitude more dark energy than there seems to be, causing the universe to have ripped itself to pieces.
鈥淭here is no natural candidate to explain dark energy鈥�
We鈥檙e stuck. 鈥淯nless somebody has some amazing flash of insight, I think we are going to carry on not having a clue,鈥� says Pontzen. But there鈥檚 another possibility: that dark energy鈥檚 strength has changed over time. 鈥淎t that point, you鈥檙e in a very different situation. You will be able to very quickly narrow down the list of what it might be.鈥�
To break the impasse, the European Space Agency is building the Euclid telescope. Due for launch in 2020, it will map galaxies up to 10 billion light years away. By seeing how dark energy influences their arrangement and shape, the mission will be our best chance yet of seeing if its strength has changed. Only then will we begin to grasp the interplay of factors that keeps the universe so strangely flat.
This article appeared in print under the headline 鈥淭he universe is flat as a pancake鈥�
