I’ve been keeping a close eye on the clouds outside my office window today. There’s a squally wind blowing from the south that is whipping the trees around and threatening to deliver a thunderstorm to my back garden before the washing’s dry.

The flailing eucalypts and jostling date palms remind me of a science teacher I had at school who told us one day to think about the nature of wind as something that is visible only through its effects.

I can’t remember his exact words, but they were along these lines. If you look out of the window, you can’t see the wind directly but you know that it is there by looking at the things that it moves around. All of those waving branches, scudding clouds and chip packets tumbling down the gutter are evidence of the presence of wind.

This kind of inference is useful, but of course it won’t tell you the nature of the wind itself, which could be frustrating, particularly if you were trying to understand why all those normally placid plants suddenly began dancing around.

Cosmologists know the feeling. For most of the past decade they’ve been struggling to understand a remarkable force that is accelerating the expansion of the universe. They can see its effect but they have no idea what it is.

The discovery of this force came about in the late 1990s, when researchers were trying to get a grip on how fast the universe was expanding and whether or not its rate of expansion had started to slow down. They wanted to answer a question that had been troubling science since the 1920s, when astronomer Edwin Hubble discovered that all the galaxies we can see in the sky appear to be getting further away from us.

Hubble’s breakthrough revealed that the universe was expanding, but the question was whether that expansion would go on forever, or whether the gravity exerted by all the matter in the universe would eventually slow it down until it stopped and then began shrinking again.

To test all of this, two groups of researchers began looking at type 1a supernovas, extraordinarily bright objects that are formed when a dying star begins to draw matter into itself from another nearby star - eventually fuelling an enormous explosion. Cosmologists use the brightness of supernovas as ”standard candles” that allow them to measure how fast galaxies are moving apart. In 1998, both of those independent groups of researchers found something surprising. The supernovas were dimmer than expected, suggesting that the expansion of the universe was speeding up.

This shocked the cosmological community. Until then, the prevailing view had been that gravity worked as a kind of brake on expansion, inevitably slowing it down.

A well-known theoretical physicist, Michael Turner of the University of Chicago, gave a name to this anti-gravitational force, calling it ”dark energy”, in reference to another cosmological mystery known as ”dark matter”.

But naming a phenomenon is very different to understanding it, and the question now as then centres on what dark energy is exactly.

Coming to a better understanding of dark energy is one of the biggest efforts in modern cosmology. After all, calculations based on general relativity suggest that it makes up something like 74 per cent of the universe. How can we not know what it is?

Among the things we do know about dark energy is that its accelerating effects seem to have begun around 5 billion years ago. Late last year, Adam Riess from the Space Telescope Science Institute in Baltimore told the BBC to think of it as a tug-of-war. ”Imagine that you were having a tug-of-war and the other end of the rope disappears behind a curtain,” he said. ”Somebody else is tugging on the other end; we’ll call that dark energy.”

What the researchers showed in 1998 was that the thing behind the curtain was winning, but that wasn’t always the case. There was a time when ordinary matter was winning and the universe was decelerating.

Currently, scientists are divided about the detailed nature of dark energy. One suggestion is that it is a constant energy that fills all space to the same degree, making it a little like Einstein’s ”cosmological constant”, a universal force he invented in a misguided attempt to show that the universe was static. Another possible option is that it changes over time and space, which scientists call quintessence.

Sorting out the true nature of dark energy will require ever more precise measurements of the expansion of the universe. Some of that new evidence may come from the European Space Agency’s Planck satellite, which will measure with great accuracy variations in microwave radiation left behind by the big bang.

Plank, which is planned for launch next year, as well as several other projects around the world, aims to pin down the details of the dark energy mystery much more closely. Testing of the spacecraft began early this year and will continue until its launch in July next year. I hope by then my washing will be dry.

stephen.pincock@journalist.co.uk

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