杏吧原创

THE universe really is dominated by dark energy. Observations by an orbiting space telescope have confirmed claims that the expansion of the universe is accelerating. For that to happen, ordinary gravity must be being overpowered by some kind of repulsive force 鈥� which cosmologists call dark energy.

This acceleration was first spotted in the 1990s by looking at distant supernova explosions, and later backed up by the pattern of the cosmic microwave background. But both methods rely on uncertain physics.

Now a team of astrophysicists has taken a different approach using NASA鈥檚 Chandra telescope to measure X-rays coming from 26 galaxy clusters. 鈥淎 supernova is a complex thing,鈥� says team leader Steve Allen of the University of Cambridge. 鈥淏ut this is simple. It鈥檚 just the physics of hot gases.鈥�

Galaxy clusters are giant aggregations of hundreds of galaxies, containing vast amounts of gas at temperatures of around 100 million 掳C, hot enough to emit X-rays. By measuring the frequency and brightness of the X-rays, and knowing how much gas there is as a proportion of the total mass of a cluster, the team worked out the distance to each one. Then by plotting those distances against cosmic time, they found that the expansion of the universe began to accelerate about 6 billion years ago.

All three methods agree, and imply that about 75 per cent of the energy in the universe is in this repulsive form. 鈥淚t gives us much more confidence that dark energy is real,鈥� says Allen.