The largest structure in the universe is a whole lot of nothing, astronomers say
Researchers believe they have discovered the largest known structure in the universe, a 'supervoid' measuring some 1.8 billion light years across.
A partial image from the Planck telescope, with cold spot circled.
ESA and the Planck Collaboration
Researchers have discovered the largest known structure in the universe, but it isn鈥檛 really a thing so much as it is an absence.
Described last week by an international team of astronomers in last week in the Monthly Notices of the Royal Astronomical Society,聽this 'supervoid'聽spans 1.8 billion light years across, making it perhaps聽 known to humanity. But聽compared to the rest of the universe, it is remarkably empty.聽
The supervoid corresponds with a cold spot in the radiation left over by the Big Bang called the cosmic microwave background (CMB). As data from聽European Space Agency鈥檚 Planck observation mission show, the CMB is not uniform, with hot and cold 'patches.' But astronomers have struggled to explain the sheer size of this particular cold spot, which was first identified in 2004.
Previous investigations of the cold spot had found no voids in that direction. But as聽it turns out, astronomers just needed to look a little closer to home.
Istv谩n Szapudi, of the University of Hawaii at Manoa, led a targeted astronomical survey, believing the cold spot to be a result of a large void. Using Hawaii鈥檚 Pan-STARRS1 (PS1) telescope and NASA鈥檚 Wide Field Survey Explorer (WISE) satellite, Dr. Szapudi鈥檚 team observed a relatively close patch of sky from 3 billion light years away. It was there they found a huge gap where some 10,000 galaxies ought to have been.
The supervoid isn鈥檛 a vacuum in the traditional sense. It isn鈥檛 totally empty, rather it contains about 20 percent less matter than most parts of the universe. It represents a large and inexplicably homogenous region of a universe that is, for the most part, evenly distributed.
鈥淭he void itself I鈥檓 not so unhappy about. It鈥檚 like the Everest of voids 鈥� there has to be one that鈥檚 bigger than the rest,鈥� co-author Carlos Frenk, of Durham University, told . 鈥淏ut it doesn鈥檛 explain the whole cold spot, which we鈥檙e still in the dark about.鈥�
But while the supervoid doesn鈥檛 solve the cold spot problem, it does provide a possible 鈥� if partial 鈥� explanation.
Imagine a ball rolling up a hill. As the ball approaches the peak of the hill, it slows down, losing kinetic energy but gaining potential energy. As it crests the hill, gravity pulls it down on the other side, and it regains kinetic energy and loses potential energy.
All else being equal, something similar should happen to a photons as they pass through a void. As light leaves a denser region of the universe, which exerts a gravitational pull, it loses kinetic energy, not slowing down in this case, but increasing in wavelength and cooling. But as they approach the denser region on the other side of the void, they should regain that energy.
But that's not what actually happens, because, as the light made its way though the void, the universe expanded, preventing the light from converting all of its potential energy back into kinetic energy.
And if the supervoid and cold spot are correlated, it could support the accelerating universe theory 鈥� the notion that the universe is expanding at an increasing rate. Cosmologists have attributed that effect to 鈥渘egative pressure鈥� from dark energy.
鈥淭his is independent evidence, in case anyone doubts it, for the existence of dark energy,鈥� Frenk told the Guardian.