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In the beginning, all of space rang like a bell.

It was the immediate aftermath of the big bang, and the universe was filled with a torrid plasma—an energetic soup of particles and radiation. Although that plasma was remarkably smooth, it wasn’t completely smooth. There were slight density and pressure gradients that pushed material around, says Lloyd Knox, a cosmologist at the University of California, Davis, “and when stuff gets pushed around, those are sound waves.”

The ringing happened everywhere, so intensely that we can still sense it 13.8 billion years later. It has been detected directly in the cosmic microwave background, the afterglow left over from the big bang’s fading fireball, and it has been closely analyzed via the same basic physics used to study the structure of the sun. In fact, the primordial reverberation is so well measured and modeled that it has been used to deduce the precise rate at which the universe is expanding, a number known as the Hubble constant. That constant, in turn, is the cornerstone of our modern understanding of the size, age and structure of the cosmos.

But this seeming triumph has recently led Knox and his colleagues into controversy and confusion. If cosmologists’ prevailing theories of the universe are correct, then all the ways of calculating the Hubble constant in the modern era should give the same answer. The value derived by extrapolating from the ancient sound waves should match up exactly with the value derived from independent studies of the light from distant stars and galaxies. In reality, a series of studies show the two approaches yield a vexing disagreement—and the more diligently researchers attack the problem, the more definitive the conflict seems to be.

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Category: Science