Physics

How a "supervoid" and modified gravity could solve a cosmic conundrum

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A simulated segment of the large-scale structure of the universe – the Milky Way is located in a "supervoid" containing relatively little matter, and this could explain a major cosmic conundrum
NASA, ESA, and E. Hallman (University of Colorado, Boulder)
A simulated segment of the large-scale structure of the universe – the Milky Way is located in a "supervoid" containing relatively little matter, and this could explain a major cosmic conundrum
NASA, ESA, and E. Hallman (University of Colorado, Boulder)
This blue web represents the distribution of matter in the universe, with individual yellow dots indicating galaxies. The green dot represents the position of the Milky Way within a "bubble" of relatively empty space, where other nearby galaxies could be pulled towards the more dense matter around the edge of the bubble (via the red arrows)
AG Kroupa/University of Bonn

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.

This blue web represents the distribution of matter in the universe, with individual yellow dots indicating galaxies. The green dot represents the position of the Milky Way within a "bubble" of relatively empty space, where other nearby galaxies could be pulled towards the more dense matter around the edge of the bubble (via the red arrows)
AG Kroupa/University of Bonn

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

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4 comments
Nobody
Grasping at straws again??? They would have a lot more credibility if they would stop claiming that hypotheses have enough evidence to be theories. Tinkering with models isn't proof.
Karmudjun
Ah, Astrophysicists at work trying to reconcile their findings to older physics equations. The Hubble Tension exists because the older equations took certain constants for granted? Maybe - I'm neither an Astrophysicist or a Physicist. I enjoyed toying with relativity in college and always ended up loosing out to the Physics majors who would pose word questions that went beyond my understanding of Einstein's theories. We all know Einstein went beyond Newton's simplistic explanations of observations while confirming other aspects. These hypothesis definitely solve some of the conundrums are clearly are the best theories of our age. So Thanks Michael for the article. These brilliant scientists are incrementally making the MOND as feasible as the Standard Theory, and explaining themselves well!
Ranscapture
It’s not expanding at ever increasing speeds, things are moving in all directions, some towards the “center” some away. Light is scattered and bent and shifted by relative speed. It’s much more simple than what they’re saying.
Luca Ben
What I don't understand, reading the article, is how does this theory explain what's going on at larger distances. The behaviour described by the Hubble-Lemaître law doesn't just apply to the nearby universe, but to the entire universe. The more distant the object is, the faster it's moving away from us. The local anomaly doesn't seem to apply to the whole model.