The universe is flat as a pancake. Coincidence?

 作者:刘衫     |      日期:2019-03-15 06:03:07
Image Source/Getty By Stuart Clark 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’s 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’s density is low, it curves outwards – and the weakness of the gravitational pull will mean it expands forever. 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’s expansion was accelerating rather than slowing down. The proposed fix was to say that a large proportion of the universe exists as dark energy,