The difference in the expansion rate of the universe is one of the greatest cosmological mysteries. A new study has come up with an interesting solution by applying modified gravity theory and a disturbing "supervoid" in which the Milky Way resides. From our vantage point on Earth, galaxies appear to be moving rapidly away from us, due to the expansion of the universe. However, the rate at which galaxies expand is not static - the Hubble-Lemaître law describes that galaxies further from Earth are moving away from Earth much faster than galaxies that are closer to us.
For the past few decades, astrophysicists have been trying to work out an equation that describes this using a value called the Hubble constant. It gives the velocity in kilometers per million parsecs (km/s/Mpc) - so essentially, a galaxy 2Mpc from Earth is traveling twice as fast as a galaxy 1Mpc from Earth.
Some astronomers have used predictable supernovae to measure the Hubble constant in the relatively nearby universe, and have come up with a value of about 73km/s/Mpc. Other astronomers measured the Hubble constant in the distant universe by studying the background radiation produced by the Big Bang, and obtained a value of approximately 67.5km/s/Mpc. The problem is that as technology advances, the uncertainties in both technologies continue to decrease, but there is no room for overlap in their divergence, even taking into account the known acceleration of expansion. This leads to a problem known as Hubble tension.
But a new study suggests a way to resolve the Hubble tension. We may need to consider our place in the universe and challenge some preconceived notions, according to researchers from the Universities of Bonn and St. Andrews.
About a decade ago, a team of astronomers discovered that our galaxy appears to be located in a vast void, where there is far less matter than elsewhere in the universe. This is because matter is not evenly distributed throughout the universe - it tends to be spread out in clumps, like a giant sponge. We happen to live in an air pocket of this sponge.
A possible side effect of this is that the material in the super-large bubble is attracted to the denser material surrounding the bubble. Therefore, nearby matter (i.e. galaxies) will move faster than more distant matter, which is the cause of the Hubble tension.
However, it's not that simple - for this explanation to work, astronomers also need to think about the law of gravity. When the team applied another theory of gravity called Modified Newtonian Dynamics (MOND), the Hubble tension completely disappeared, and the observed differences could be explained entirely by irregularly distributed matter.
This isn't just a convenient math trick, though. MOND has precedent as a legitimate theory, with evidence of it seen in more than 150 galaxies, certain star clusters, and even planets in our own solar system. It could also explain the oddities of dark matter, a mysterious substance that has eluded experiments designed to detect it. In fact, according to the Standard Model, super-oid itself doesn't really make sense - but it is feasible under MOND.
Although there is growing evidence supporting MOND, it is still not a widely accepted theory. More work needs to be done to test this idea and whether it could solve some of the biggest mysteries in the universe.
The research was published in the journal Monthly Notices of the Royal Astronomical Society.