New modification to gravity may explain the cosmological constant.
The vacuum of space isn't actually "empty"; it teems with particles that pop in and out of existence, giving the vacuum an energy of its own. But here's an embarrassing fact about that energy: it predicts that the cosmological constant (which provides a measure of the rate of the expansion of the Universe) should be 10120 times larger than we think it actually is.
Most scientists prefer things to be a bit more accurate than this. Still, the main question on cosmologists' minds is not why the predicted and real values appear to be so different, but how it is that the vacuum energy does so little. An answer of sorts has recently appeared in Physical Review Letters. But before we get to the paper, let's delve into the nature of the problem it's trying to solve.
An expanding Universe
When Einstein was first formulating a new theory of gravity, his solutions predicted that the Universe was expanding. At the time, the Universe was widely regarded to be static, so Einstein added a constant that counteracted the expansion and kept the Universe unchanging. Everyone rejoiced—electromagnetism, space, time, and gravity could all live together in harmony.
Later, Edwin Hubble took advantage of a new generation of telescopes to measure the speed at which distant galaxies were moving. He found that the further away a galaxy was, the faster away from us it was moving. The conclusion was inescapable: the Universe was expanding. Everyone chuckled over Einstein's big goof and got on with the business of crashing the economy and going to live in Hooverville.
Fast forward to the turn of the century, where yet another generation of telescopes—combined with an excellent understanding of how a particular type of supernova worked—allowed scientists to measure whether the rate at which the Universe expands is constant or not. Turns out it's not; every day, the Universe expands a bit faster than it did the day before. Inflation, it seems, is a physical as well as an economic universal, and Einstein's cosmological constant was back (albeit in altered form).
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