Cosmologists normally assume we live in a pretty average part of the universe, but there is growing evidence that this might not be the case. Some think that the Milky Way sits – like some kind of giant, space hillbilly – in a void, an immense region of near emptiness.
If true, this might help explain some gaps in our understanding of the cosmos.
The universe that emerged from the big bang was exceptionally smooth, but there were tiny ripples in the matter, imprinted by the rapid expansion known as inflation. Over billions of years, gravity drew matter out of the more tenuous regions, pooling it into the seeds of stars and galaxies. This has left the universe richly structured, with dense clusters of matter joined by a network of less dense sheets and filaments. Between these sit the cosmic voids.
Advertisement
While cosmologists know this structure exists, they generally ignore it in their calculations, assuming matter is smooth, rather than having to worry about clusters and voids. But is this assumption leading us astray?
Over the past few years, there has been growing observational evidence that our neck of the universe is not typical, and that our galaxy is in a large void, several hundred million light years across. What would be the effect of living in a whole lot of nothing? Benjamin Hoscheit and at the University of Wisconsin are the latest to try to answer this.
Our peculiar universe
What did they do? The fiendish mathematics of Einstein’s general relativity, the framework of modern cosmology, means that accurately calculating the passage of a light ray through a lumpy universe – for example, one we spy with our telescopes coming from a supernova – isn’t simple. So Hoscheit and Barger rely on a well-worn approximation for a void, known as the Lemaitre-Tolman-Bondi space-time. Incorporating new data from the likes of the South Pole Telescope, they showed that several of the peculiarities of our view of the universe .
However, there are bigger cosmological issues at stake, as a tension has emerged in measuring the rate at which the universe is expanding. This is usually encoded in the Hubble constant, a number that tells us how fast a galaxy at a certain distance should be moving away from us.
There are different ways of measuring this universal expansion, with some using distant cosmological observations, such as the cosmic microwave background, to gauge a global measurement of expansion. Others rely on supernovae, exploding stars in relatively nearby galaxies, to chart more local expansion.
But these two groups of measures, while giving similar values, still disagree by a few per cent. Cosmologists have scratched their heads over this, and have tried unsuccessfully to identify the many potential sources of error that could account for this difference.
Living in a cosmic void might account for the difference, with our local observations revealing a slightly faster expansion due to us sitting in relative emptiness, in line with the work of Hoscheit and Barger. This is because more dense regions on the edge of the void exert an extra pull.
If true, cosmologists will no longer be able to ignore our seemingly peculiar position in the universe. They will have to account for our living within an immense local void if they are ever to make sense of our growing wealth of precise cosmological observations.
