A major discrepancy between different measurements of our universe's expansion rate could be explained if our galaxy, the Milky Way, sits in a two-billion-light-year-wide void. Such is the conclusion of scientists who argue that a modified theory of gravity can replace the standard model of cosmology. However, this hypothesis is strongly disputed by many astronomers.
The standard model of cosmology describes how we live in a universe dominated by dark energy and dark matter. Dark energy is a mysterious force that is seemingly causing the expansion of the universe to accelerate, while dark matter provides most of the gravity in the universe and is thought to surround galaxies in halo-like shapes while preventing them from sort of falling apart. Together, these elusive phenomena describe how matter is distributed across the cosmos and how galaxies move with respect to one another.
One of the biggest challenges for the standard model of cosmology to overcome, however, is known as the "Hubble Tension." This concept isn't named for the space telescope like you might imagine, but rather its namesake, astronomer Edwin Hubble. In 1929, Edwin Hubble discovered that the more distant a galaxy is, the faster it seems to be moving away from us. He was able to derive a relationship to describe this connection, which subsequently became known as the Hubble–Lemaître Law (after the Belgian theoretical physicist and priest, Georges Lemaître, who independently discovered it too). It says that the velocity with which a galaxy is moving away from us is a product of its distance multiplied by the expansion rate of the universe, which is given by a parameter called the Hubble constant.
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Since Edwin Hubble’s day, astronomers have strived to measure the Hubble constant with ever greater accuracy. By knowing the Hubble constant, and therefore precisely how fast the universe is expanding, we can calculate how old the universe must be for it to have reached its present size. Our current best measurements put the universe's age at 13.8 billion years.
However, there is a problem.
Measurements of the universe's expansion created by measuring the redshifted light of type Ia supernovas have resulted in a Hubble constant value of 73.2 kilometers per second per megaparsec. In other words, it says that every volume of space a megaparsec across (a parsec is 3.26 light years, and a megaparsec is a million parsecs, so 3.26 million light years) is expanding by 73.2 kilometers (45.5 miles) every second.
Yet, the expansion rate of the universe is also baked into the physics of the cosmic microwave background (CMB) radiation. Measurements of the CMB by the European Space Agency's Planck mission give a value of the Hubble constant of 67.4 kilometers per second per megaparsec. Both measurements have been made to high accuracy, but they can’t both be correct.
This strange dichotomy, which has become known as the Hubble Tension, is now arguably the most vexing problem in cosmology. Whereas some astronomers suspect it is the product of a measuring error somewhere along the line, others think it could be hinting at new physics.
That’s exactly what a new paper, from scientists in Germany, Scotland and the Czech Republic, is proposing.
"The universe … appears to be expanding faster in our vicinity — that is, up to a distance of around three billion light years — than in its entirety," says one of the paper's authors, Pavel Kroupa of the University of Bonn in Germany, in a press statement. "And that shouldn’t really be the case."
Their hypothesis centers on an astrophysical oddity called the Keenan–Barger–Cowie supervoid, named after the trio of astronomers who studied it. The supervoid is a so-called "under-density" of matter in the universe, a region where statistically there are fewer galaxies on average — and our Milky Way galaxy just happens to be sitting right in the middle of it, the scientists say.
Outside of this supervoid, galaxies are packed a little more densely on average, resulting in more gravity that can pull objects within the supervoid towards them. This could give the impression that space is expanding faster in our vicinity, the team suggests, as galaxies are dragged along by the gravity of matter beyond the supervoid.
"That's why they are moving away from us faster than would actually be expected," said co-author Indranil Banik of the University of St Andrews in Scotland.
The standard model of cosmology says that matter should be spread fairly evenly across the universe, and that any voids shouldn't grow beyond a certain size. Therefore, it has some difficulty explaining a supervoid as large as the Keenan–Barger–Cowie void. Some astronomers, including Kroupa and Banik, believe that the standard model cannot explain it, while others such as Martin Sahlén, Iñigo Zubeldía and Joseph Silk of the University of Oxford have gone on record saying that it can.
In Kroupa, Banik and their co-author’s (Sergij Mazurenko of Universität Bonn and Moritz Haslbauer of Charles University in the Czech Republic) hypothesis, our current theory of gravity, and therefore dark matter, is replaced by a new theory called Modified Newtonian Dynamics, or MOND for short. This posits that at low accelerations, gravity behaves differently to how it is described by Einstein and Newton, and that the extra gravity can replace the need for dark matter. In the MOND paradigm, the universe could more easily create large voids like the Keenan–Barger–Cowie supervoid.
However, the idea that the presence of the void can affect measurements of the expansion rate of the universe have been hotly contested in the past. Nobel laureate Adam Riess of Johns Hopkins University in Baltimore, who is leading efforts to measure the Hubble constant with type Ia supernovae, along with W. D'Arcy Kenworthy of Johns Hopkin and Dan Scolnic of Duke University in the United States, showed that type Ia supernovae observed beyond the boundary of the supervoid had the same expansion rate as those inside the void. In response, Kroupa, Banik, Mazurenko and Haslbauer argue that the effect of the supervoid would be felt far beyond the void itself, and so one would expect to measure a higher expansion rate in supernovae beyond the void's confines.
Other methods to measure the Hubble constant, which are independent of the supervoid and the standard model of cosmology, also maintain that the Hubble Tension cannot be explained away. By tracking the angular distance on the sky that water masers in molecular clouds orbiting supermassive black holes in distant galaxies make, and from that deriving their physical distance from geometry, has produced a value of the Hubble constant of 73.9 kilometers per second per megaparsec, which is close to the type Ia supernova measurements, given the uncertainty in the maser measurements. There's also the H0LiCOW (H0 refers to the Hubble constant) project, which studies how light from quasars in the early universe can take different paths of different lengths through foreground gravitational lenses. Quasars often have fluctuations in their brightness; while traversing the different paths through the gravitational lens, the universe is still expanding and the rate of this expansion is imprinted on the different lensed images of the quasar brightness variations. This project finds the expansion rate to be 73.3 kilometers per second per megaparsec, almost identical to the type Ia supernova value.
These measurements are in conflict with the CMB measurement, and are independent of the hypothesis that the supervoid can create the Hubble tension. So ultimately, if that hypothesis is to have legs, it seems that Kroupa, Banik, Mazurenko and Haslbauer will have to convince a lot more people.
The hypothesis was published in November in the journal Monthly Notices of the Royal Astronomical Society.
For any readers interested in further reading, a list of papers on the subject of the Hubble Tension and their measured values of the Hubble constant can be found here.
