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. The new structure is ten billion light years across and almost as far away but nobody had spotted it…until now.
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.
To give some sense of scale, our galaxy, the Milky Way, is separated from its nearest neighbour, the Andromeda Galaxy, by about 0.75 Megaparsecs (Mpc) or 2.5 million light-years.
Whole clusters of galaxies can be 2-3 Mpc across but LQGs can be 200 Mpc or more across. Based on the Cosmological Principle and the modern theory of cosmology, calculations suggest that astrophysicists should not be able to find a structure larger than 370 Mpc.
Dr Clowes' newly discovered LQG however has a typical dimension of 500 Mpc. But because it is elongated, its longest dimension is 1200 Mpc (or 4 billion light years) - some 1600 times larger than the distance from the Milky Way to Andromeda.
Dr Clowes said: "While it is difficult to fathom the scale of this LQG, we can say quite definitely it is the largest structure ever seen in the entire universe. This is hugely exciting – not least because it runs counter to our current understanding of the scale of the universe.
'Even travelling at the speed of light, it would take 4 billion light years to cross. This is significant not just because of its size but also because it challenges the Cosmological Principle, which has been widely accepted since Einstein. Our team has been looking at similar cases which add further weight to this challenge and we will be continuing to investigate these fascinating phenomena."