How a "supervoid" and modified gravity could solve a cosmic conundrum
One of the biggest cosmological mysteries centers on a discrepancy in how fast the universe is expanding. A new study comes to an intriguing solution by applying a modified theory of gravity and an unsettling “supervoid” that our galaxy resides in.
From our vantage point here on Earth, it appears that galaxies are all rushing away from us, and that’s thanks to the expansion of the universe. But it’s not happening at a flat speed – the Hubble-Lemaître law describes how more distant galaxies are moving away from Earth much faster than those closer to us.
Over the past few decades, astrophysicists have attempted to work out the equation that describes this, using a value called the Hubble constant. This gives the speed in kilometers per second per megaparsec (km/s/Mpc) – so essentially, a galaxy 2 Mpc from Earth will be rushing away twice as fast as a galaxy 1 Mpc away.
Some astronomers measure the Hubble constant in the relatively nearby universe using predictable supernovae, which gives them a value of around 73 km/s/Mpc. Others have measured it in the distant universe by studying background radiation from the Big Bang – and assigned the Hubble constant a value of around 67.5 km/s/Mpc. The problem is, as technology improves, uncertainty is consistently reduced in both techniques, yet they disagree with no room for overlap, even after accounting for the known acceleration of expansion. This leads to a problem known as the Hubble tension.
But a new study proposes a solution to the Hubble tension. According to researchers from the University of Bonn and the University of St. Andrews, we may need to take into account our place in the cosmos and challenge some preconceived notions.
About a decade ago, a team of astronomers discovered that our home galaxy, the Milky Way, appears to be located in a gigantic void, where there’s significantly less matter than elsewhere in the universe. That’s because matter isn’t spread evenly throughout the cosmos – it tends to be distributed in clumps and empty patches, like a colossal sponge. We just happen to live in an air pocket in that sponge.
This could have the side effect that matter within this supervoid is being attracted to the more densely packed matter that surrounds the bubble. As such, nearby matter (i.e. galaxies) would be moving faster than more distant matter, accounting for the Hubble tension.
However, it’s still not quite that simple – for this explanation to work, astronomers also need to tinker with the laws of gravity. When the team applied an alternative theory of gravity called modified Newtonian dynamics (MOND), the Hubble tension completely disappeared, with the observed discrepancy explained entirely by irregularly distributed matter.
This isn’t just a convenient mathematical trick, though. MOND has some precedent as a legitimate theory, with evidence for it seen in over 150 galaxies, certain star clusters, and even the planets in our own solar system. It would also explain the oddities attributed to dark matter, that mysterious substance that’s a constant no-show in experiments designed to detect it. In fact, the supervoid itself doesn’t really make sense according to the Standard Model – but it works under MOND.
Although a growing body of evidence supports MOND, it’s still not a widely accepted theory. More work will need to be done to test the idea, and whether it can solve some of the biggest mysteries in the cosmos.
The research was published in the journal Monthly Notices of the Royal Astronomical Society.
Source: University of Bonn