Evrim Yazgin
Cosmos science journalist
A new theory suggests that Earth and the entire Milky Way galaxy might be inside a giant void, possibly providing a surprising solution to an unsolved mystery, known as the “Hubble tension”, about the expansion of the universe.
The universe has been expanding since it came into existence due to the “Big Bang” 13.8 billion years ago.
Cosmologists have been unable to pin down how fast the universe is expanding, however. Different observational methods have provided different answers to the rate of expansion of the universe, known as the Hubble constant.
This disagreement is known as the “Hubble tension”, and it has plagued cosmology for nearly 100 years.
Physicists have sought new theories to reconcile the different values for the Hubble constant. Earlier this year, for example, a team of researchers created a model of a spinning universe that could explain the tension.
Now, researchers attending the 2025 National Astronomy Meeting (7–11 July) of the Royal Astronomical Society at the University of Durham in the UK have shared a study that provides a new way of resolving the tension: we live in a void.
“A potential solution to this inconsistency is that our galaxy is close to the centre of a large, local void,” explains Indranil Banik from the University of Portsmouth, UK. “It would cause matter to be pulled by gravity toward the higher-density exterior of the void, leading to the void becoming emptier with time.”
“As the void is emptying out, the velocity of objects away from us would be larger than if the void were not there. This therefore gives the appearance of a faster local expansion rate.”
Banik says that a local solution to the Hubble tension fits better with cosmologists’ understanding of the history of the universe.
“The Hubble tension is largely a local phenomenon, with little evidence that the expansion rate disagrees with expectations in the standard cosmology further back in time … So a local solution like a local void is a promising way to go about solving the problem,” he says.
If the theory is correct, the Milky Way galaxy should be near the centre of a void about 2 billion light-years across. Such a bubble in space would have to have a density roughly 20% less than the average for the whole observable universe.
Counting galaxies supports this theory. The number of galaxies in our neighbourhood of the universe is fewer than other regions. But the standard model of cosmology – the overarching theory of the universe – suggests that matter should be more or less uniformly spread out through the universe on large scales.
How could such a “void” be created?
Banik says baryonic acoustic oscillations (BAOs) – what he calls the “sound of the Big Bang” – could be the answer.
“These sound waves travelled for only a short while before becoming frozen in place once the universe cooled enough for neutral atoms to form. They act as a standard ruler, whose angular size we can use to chart the cosmic expansion history,” Banik explains.
“By considering all available BAO measurements over the last 20 years, we showed that a void model is about 100 million times more likely than a void-free model.”
The researchers will further test their theory by comparing it with other models to map out the history of the universe’s expansion.