How do we know the universe is expanding?

Albert Einstein's theory of General Relativity posits one of two possibilities: the universe is either expanding or contracting. It can't be static. Thanks to Edwin Hubble and every sophisticated instrument available since 1929, we know that most galaxies are moving away from us, and appear to be moving away from us faster and faster. The galaxies themselves are not getting larger, but the distance between the galaxies grows, like raisins on a rising loaf of bread. Another analogy is to think of the universe as an expanding balloon

Not so fast, says Christof Wetterich, a theoretical physicist at the University of Heidelberg in Germany, who has just published an alternative explanation. The universe may be static, but the mass of everything is growing exponentially. Wetterich's paper hasn't yet been peer-reviewed. Moreover, his idea can't even be tested.

However, according to Nature, no one is dismissing Wetterich out of hand. The physicist Arjun Berera at the University of Edinburgh, for one, likes the idea because it will keep scientists from getting too comfortable with one single explanation, so that they might "see if there are alternative explanations consistent with all known observation.”

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