科学美国人60秒:Space Supervoid Sucks Energy from Light

The universe is a dark, cold place. But it has a strange region that’s even colder than usual. Seen from Earth, it’s an area where the ambient cosmic microwave background light—the leftover thermal energy of the big bang—is much chillier than expected. Now astronomers say they’ve found in the same part of space a so-called supervoid—a large area mostly empty of galaxies. And they think the overlap is no coincidence. 

The supervoid extends 1.8 billion light-years across, making it perhaps the largest structure known in the cosmos, according to a report in the Monthly Notices of the Royal Astronomical Society. [István Szapudi et al, Detection of a supervoid aligned with the cold spot of the cosmic microwave background]

The supervoid’s relative lack of stuff could have drained energy from light that passed through it, explaining why the microwave background is colder there. Here’s how it works:

General relativity tells us that gravity bends spacetime, causing light to travel a curved path near massive objects, as if falling into a bowl. The supervoid, then, with its lack of mass, is akin to a hill. When light travels up that hill, it loses energy.

Normally it would regain the energy upon exiting the void—that is, when it comes down the other side of the hill. But because the expansion of space is accelerating, the hill the light tumbles down is less steep than it was when the light climbed up. And the flatter ride down means less energy recovered than was expended going up. Which translates to a low-energy region—a big chill in the remnant of the Big Bang.

—Clara Moskowitz