It’s ten billion light years across and almost as far away but nobody had spotted it…until now
What’s the largest structure in the Universe? That’s a question that has intrigued scientists for centuries. Today, they get an answer thanks to astronomers who say they’ve discovered the largest structure ever observed and one that dwarfs the previous record-holder by billions of light years.
Astronomer’s ideas about the universe’s largest structures have changed dramatically in the last 100 years. At the beginning of the 20th century, they began to suspect that stars were clustered together to form “island universes” or galaxies which themselves were separated by vast distances.
The question was eventually settled in the 1920s by Edwin Hubble and others who measured the distance to different galaxies, thereby proving that they were much further away than stars . These galaxies, they thought, were the largest structures in the universe and distributed more or less uniformly throughout space.
It wasn’t until 1989 that astronomers found something even bigger. In the 1970s and 80s, they had begun to systematically measure the distances to large numbers of galaxies and this eventually allowed them to produce a 3D map of them.
To their surprise, the galaxies were not distributed evenly but instead formed filamentary structures with walls and voids. They called the largest of these “the Great Wall”, a structure that is 200 million light years away and some 500 million light years long.
That was puzzling at the time because these walls and voids were too large to have formed through gravitational interactions in the time since the birth of the universe. Of course, astronomers now know that this structure comes from variations in the density of the early universe soon after the Big Bang, caused by quantum fluctuations.
Since then, astronomers have found even larger structures as the technology to look further into the universe improved. In 2003, they discovered the Sloan Great Wall, another wall of galaxies some 1.4 billion light years long and about a billion light years from Earth.
Earlier this year, they spotted a larger structure in the constellation of Leo called the Huge-LQG (Large Quasar Group) . This consists of 73 quasars stretching over a distance of 4 billion light years.
Now I Horvath at the National University of Public Service in Budapest, Hungary, and a couple of pals say they’ve identified something even bigger. Their data is based on gamma-ray bursts, the most energetic events in the universe.
Astronomers think these bursts are emitted by stars as they collapse to form neutron stars or black holes. These bursts are incredibly bright—a typical burst releases about the same energy in a fraction of a second as the Sun will during its entire life time.
Astronomers measure the distance of gamma ray bursts by looking for the optical afterglow of the explosion when it is detected and measuring its redshift. Since 1997, when this technique was first used, they’ve measured the distance to 283 of these bursts and mapped their position within the universe.
Astronomers have always assumed that these explosions are distributed evenly throughout the universe. And for the most part, this looks to be the case.
But Horvath and co say they’ve found a significant irregularity. They say there are far more gamma ray bursts at a distance of about ten billion light years than would be expected if the distribution was uniform.
These gamma ray bursts form a structure that is some ten billion light years in size, significantly larger than even the Huge-LQG. So this thing, presumably another wall of even more distant galaxies, is the new largest structure in the universe.
Of course, there are caveats associated with this new work. The first is the small sample size of just 283 gamma ray bursts of which only 31 make up this new giant structure. Horvath and co say that, statistically, this number of gamma ray bursts should not be grouped together in this way if they are evenly distributed.
That’s a decent pointer that something interesting might be going on here but it is by no means an astronomical slamdunk; more data is desperately needed. “One or two years more of gamma burst observations will hopefully provide the statistics to confirm or disprove this discovery,” they say.
If this tale seems a little familiar, that’s because astronomers have regularly assumed that the objects they can see must be distributed uniformly around the universe. As history has shown time and again, this usually turns out to be wrong. The universe always seems to have structure at every scale.
That may have even more significance. One of the fundamental tenets of cosmology is the Cosmological principle—this holds that the distribution of matter in the universe will appear uniform if viewed from a large enough scale.
This is equivalent to the idea that the universe appears the same to all observers, wherever in the cosmos they may be.
But the evidence, as far as astronomers can gather it, does not back up this idea. At whatever scale they look, large scale structures always seem to emerge.
That doesn’t disprove the Cosmological principle. Indeed, cosmologists are quick to say that the key idea is that the laws of physics must be the same for all observers, not necessarily the large scale structure.
Nevertheless, the possibility that the Cosmological principle may sit on shaky ground will provide many theorists with some interesting food for thought.
Ref: http://arxiv.org/abs/1311.1104 : The Largest Structure Of The Universe, Defined By Gamma-Ray Bursts