Quirk in Our Galactic Neighborhood May Explain Cosmological Mystery

The Milky Way galaxy in which our solar system lies may exist in a "giant void", where space is about a fifth as dense as the universal average.

This is the conclusion of a team of researchers from the U.K. and Germany, who argue that such may explain a puzzle relating to how we measure the rate of the universe's expansion in the wake of the Big Bang.

One can think of the universe as a balloon—with all the galaxies, for example, like little dots drawn on the outside with a marker pen.

As the balloon is inflated, all the dots get further apart from each other, as do the galaxies as the space between them enlarges.

The Doppler Effect

We know that the cosmos is expanding because of the so-called Doppler effect, a phenomenon you also experience every time an ambulance passes you on the street, blaring its siren.

As the emergency vehicle races by, you may notice that the pitch of its siren appears to change—getting higher as it comes towards you, and lower as it moves away.

This is because the relative movement of the siren causes the sound waves to first appear bunched up as the ambulance draws near, and then stretched out as the vehicle recedes.

The same phenomenon happens in space, with the light from other galaxies stretched out as these bodies move away from us.

Physicists call this "redshift", in a nod to how the light is moved towards the red end of the electromagnetic spectrum

Given this, by measuring the motions of nearby galaxies and their bright supernovae relative to us, we should be able to estimate how fast the universe is expanding.

(The relationship between a galaxy's speed and distance is defined by "Hubble's constant"—named for the U.S. astronomer Edwin Hubble—with galaxies moving 50,000 miles per hour faster, relative to us, for every million light years further they are distant.)

But there are other ways to come up with a figure for universal expansion, such as by studying the evolution of the remnants of the first light that permeated the universe, the so-called "cosmic microwave background" (CMB).

And, confusingly, there is a discrepancy of about 10 percent between these results—a problem that physicists have dubbed "the Hubble tension".

The higher figure is an issue because it would also make the universe about 10 percent younger, and older than the earliest-observed stars.

Livin' La Void Local

Astrophysicist Indranil Banik of the University of Saint Andrews, Scotland, and his colleagues believe that they may have a solution to the Hubble tension.

"In our new paper, we present one possible explanation: that we live in a giant void in space: an area with below-average density," explained Banik in an essay in The Conversation.

This situation, he explains, could "inflate" local measurements of universal expansion because matter is "outflowing" from the void into the denser space around it.

"Outflows would arise when denser regions surrounding the void pull it apart—they'd exert a bigger gravitational pull than the lower density matter inside the void."

Despite its ominous name, our "void" would not be completely empty, the researchers say, but have an average density some 20 percent lower than the universe as a whole.

It would have arisen, they argue, as a result of small density fluctuations in the infant universe that grew with time.

The team has estimated that the void surrounding us would be vast—a whopping two billion light-years in diameter. (In contrast, the Milky Way is only about 100,000 light-years across.)

"Such a large and deep void is unexpected in the standard model [of cosmology]—and therefore controversial. The CMB gives a snapshot of structure in the infant universe, suggesting that matter today should be rather uniformly spread out," noted Banik.

However, he continued, "directly counting the number of galaxies in different regions does indeed suggest that we are in a local void."

Don't Tell Newton

For the "giant void" hypothesis to work, the team has called upon an alternate theory of gravity known as Modified Newtonian Dynamics, or "MOND" for short.

The standard cosmological model invokes invisible mass—so-called "dark matter"—to explain why galaxies appear to spin faster than they should based on what they can be seen to contain.

In MOND, meanwhile, this discrepancy is explained away by suggesting that Newton's law of gravity breaks down in situations where the pull of gravity is very weak—such as, for example, in the outer regions of galaxies.

Models suggest that in a MOND universe, cosmic structures like galactic clusters would grow faster.

This could explain oddities like "El Gordo" (literally: "The Fat One"), a massive galaxy cluster more than seven billion light-years distant that appears to be too massive, moving too fast and might have formed far too early to be compatible with the standard model of cosmology.

And, according to Banik and his colleagues' calculations, such a cosmos would experience density differences from place to place that would affect local measurements of the universe's expansion rate.

The team say that recent measurement of "bulk flow"—the average velocity of matter in a given part of space—supports their model.

"The bulk flow of galaxies on this scale has quadruple the speed expected in the standard model," Banik explained.

He added: "It also seems to increase with the size of the region considered—opposite to what the standard model predicts. The likelihood of this being consistent with the standard model is below one-in-a-million."

In contrast, the researchers said, the MOND-based model is a "quite good match" for the bulk flow measurements, assuming that the void is mostly empty at its core and that we, and by extension the Milky Way, are not too far from this center.

Banik concluded: "We could well be witnessing the first reliable evidence for more than a century that we need to change our theory of gravity."

Extraordinary Proof Needed

Professor Licia Verde is a cosmologist at the University of Barcelona, Spain, who was not involved in the present study.

She told Newsweek: "Since Copernicus, astronomy, cosmology and in general physics are based on the fundamental assumption that we are not special, and thus we are not in a special location of the Universe."

This, Verde explains, is what scientists call the "cosmological principle"—and it includes the tenet that the universe is statistically uniform and the same when measured in different directions.

So, she continues, "postulating the existence of a large void and postulating that we are precisely located at the center of it would go against such principle, twice.

"Of course, scientific revolutions happen exactly when the assumed principles, considered the uncontested truth, turn out to be in fact incorrect. However this would be quite an extraordinary claim and as such—copying the words of Carl Sagan—needs extraordinary proof."

While the standard model of cosmology is far from complete, Verde concludes, "a lot of work remains to be done" before Banik and colleagues MOND-based model can be regarded as superior.

The full findings of the study were published in the journal Monthly Notices of the Royal Astronomical Society.