Starship Asterisk*

A team of astronomers at the University of Hawaii Institute for Astronomy (IfA) led by Dr. Istvan Szapudi has found direct evidence for the existence of "dark energy." Dark energy works against the tendency of gravity to pull galaxies together and so causes the universe's expansion to speed up.

The nature of dark energy (what precisely it is, and why it exists) is one of the biggest puzzles of modern science.

This is arguably the clearest detection to date of dark energy's stretching effect on vast cosmic structures: there is only a one in 200,000 chance that the detection would occur by chance.

"We were able to image dark energy in action, as it stretches huge supervoids and superclusters of galaxies," Szapudi said. Superclusters are vast regions of space, half a billion light-years across, that contain an unusually high concentration of galaxies, while supervoids are similarly sized regions with a below-average number of galaxies.

They are the largest structures known in the universe. The team made the discovery by measuring the subtle imprints that superclusters and supervoids leave in microwaves that pass through them.

"When a microwave enters a supercluster, it gains some gravitational energy, and therefore vibrates slightly faster," explained Szapudi. "Later, as it leaves the supercluster, it should lose exactly the same amount of energy.

But if dark energy causes the universe to stretch out at a faster rate, the supercluster flattens out in the half-billion years it takes the microwave to cross it. Thus, the wave gets to keep some of the energy it gained as it entered the supercluster."

"Dark energy sort of gives microwaves a memory of where they've been recently," postdoctoral scientist Mark Neyrinck said. The team also includes graduate student Benjamin Granett, the first author on the paper, which will be published in the Astrophysical Journal Letters in August or September.

The team compared an existing database of galaxies with a map of the cosmic microwave background radiation (CMB), the faint hiss of microwaves left over from the Big Bang. As predicted, they found that the microwaves were a bit stronger if they had passed through a supercluster, and a bit weaker if they had passed through a supervoid.

“With this method, for the first time we can actually see what supervoids and superclusters do to microwaves passing through them,” Granett said.

The signal is difficult to detect, since ripples in the primordial CMB are larger than the imprints of individual superclusters and supervoids. To extract a signal, the team averaged together patches of the CMB map around the 50 largest supervoids and the 50 largest superclusters that they detected in extremely bright galaxies drawn from the Sloan Digital Sky Survey, a project that mapped the distribution of galaxies over a quarter of the sky.

This website is about our discovery that vast superclusters and supervoids of galaxies, half a billion light-years across, in the Universe cause faint glows and shadows (respectively) in microwaves that pass through them. We interpret this as an effect from dark energy stretching and flattening these structures, working against the usual gravitational tendency for galaxies to pull on each other. For more information, please click on the links at left.

Supervoids and superclusters

To find the largest structures we could in the Universe, we used a catalog of extremely bright galaxies from the Sloan Digital Sky Survey. They occupy a vast volume; the most distant galaxies in the sample are about 10 billion light-years away, and the survey occupies about 1/4 of the sky. Using Voronoi tessellation-based algorithms called VOBOZ and ZOBOV, we found gentle supervoids, with relatively few galaxies in them, and superclusters, places slightly over-populated with galaxies over these distances. Conventional clusters of galaxies are about 10 times smaller and differ in that they are held together by gravity, while galaxies in supervoids and superclusters are riding large density waves, and are more affected by dark energy than gravity.

The supervoids and superclusters we found are likely similar to those in the picture at left (red=cluster, blue=void). This shows what the distribution of galaxies on very large scales might be like, using data from the Millennium simulation. This picture measures some seven billion light-years across. On the scale of this picture, our whole Milky Way galaxy, already unimaginably big on a human scale, would span less than a hundredth of a pixel! There is lots of smaller-scale structure in the simulation, but there really aren’t structures identifiable as voids and clusters much larger than the superstructures we’ve circled.

The movie at left (click on it for a larger version) starts at a representation of the Milky Way (it’s not to scale; really the Milky Way is much smaller), and zooms out, and then flies through the superclusters and supervoids. Each dot in the superstructures is a whole galaxy. We’ve left out the galaxies inbetween the superstructures for clarity.

Arramon wrote:Would be cool if they create a pullout animation as far away as they can, to show overall shape and structure of our entire known universe.