NASA, ESA, CSA, and STScI
In a sense, we are at the centre of the universe – but only because we can see the same distance in every direction, giving us the perfectly spherical observable universe. The speed limit of light combined with the inexorable expansion of the cosmos means that we can see about 46 billion light years in every direction. What lies beyond this horizon? That is a mystery we may never solve.
But there are clues. Two competing effects govern the overall size of the universe: gravity and dark energy. All matter has mass, which causes gravitational forces that pull everything towards everything else. To their surprise, however, cosmologists in the early 20th century found that distant galaxies seem to be hurtling away from us. The mysterious force causing this strange expansion of space was dubbed dark energy, and its nature remains elusive to this day.
“Up until the discovery of dark energy and the acceleration of expansion, the universe was simpler,” says cosmologist Wendy Freedman at the University of Chicago. Without dark energy, the universe would be much smaller and its size easier to predict.
Even with dark energy, the universe may only be slightly larger than what is observable. In March, Jean-Luc Lehners, then at the Max Planck Institute for Gravitational Physics in Germany, and Jerome Quintin at the University of Waterloo in Canada published a model that suggested the period of rapid expansion right after the big bang, called inflation, could have been even shorter than we thought. This would leave the universe smaller…