The Boötes void: Why the Universe has a mysterious hole 330 million light-years across
Space is full of holes.
In 1981, Robert Kirshner was working with other astronomers at the University of Michigan to calculate the redshifts – a measure of how fast something is moving away from Earth – of a great number of galaxies. Due to the way our Universe is expanding, the farther away a galaxy is the faster it moves, meaning redshift can be used to measure distance. Kirshner and his team were taking advantage of this to create a 3D map of the Universe.
As the map became fleshed out, something strange appeared. At 700 million light-years from Earth was a blank void. In a roughly spherical region around 330 million light-years wide – a region the Milky Way could fit into billions of times over – there were barely any galaxies.
Initially, the region was referred to as The Great Nothing, but later came to be known as the Boötes Void, as it appears to lie in the constellation of Boötes, the herdsman who drives the Plough around the North Pole.
Since then, surveys have managed to create more detailed maps of our Universe. We now know that galaxies are arranged like a giant web. The majority of galaxies in our Universe are found in long structures, known as filaments, that wind through the cosmos. When these meet, they create regions with a high concentration of galaxies, known as clusters.
Between these threads, however, are huge empty voids with hardly any galaxies at all. The voids make up around 80 per cent of the observable Universe, and most are around 30 to 300 million light-years across. Boötes is one of the largest, earning it the title of ‘supervoid’. It’s thought that Boötes is the product of smaller voids merging together.
The cause of these voids is thought to lie in the origin of the Universe. In the early days of the cosmos, all the Universe’s matter was tightly packed together. Initially, this is thought to have been a uniform soup, but random quantum fluctuations soon created small differences in the distribution of matter.
Some areas were now slightly more dense, meaning their gravitational pull was greater so that they pulled matter away from the less dense areas. This made them even more dense, increasing their gravitational pull again so they attracted more matter and so on. At the same time, the Universe was expanding greatly and so these fluctuations that started on a quantum level eventually spanned hundreds of millions of light-years. Meanwhile smaller clumps of matter began to organise themselves into galaxies.
By studying these large structures, astronomers can gain a window into what the Universe looked like in its earliest moments. Today, advancements in both telescope and imaging technology mean they are able to create more detailed versions of Kirshner’s maps, such as the Dark Energy Survey which has mapped out a quarter of the southern sky, examining around 300 million galaxies.
Meanwhile, supercomputers can now create detailed simulations of how the Universe has grown from those first moments after the Big Bang to the cosmos we see today. By comparing these maps with the simulations, astronomers can begin to understand how our Universe came to look the way it does today.
- This article first appeared in issue 371 of BBC Science Focus Magazine – find out how to subscribe here
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