Are we living in a giant void? This could solve the puzzle of the expansion of the universe, research suggests

One of the greatest mysteries of cosmology is how fast the universe is expanding. This can be predicted using the Standard Model of Cosmology, also known as Lambda Cold Dark Matter (ΛCDM). This model is based on detailed observations of the light left behind by the Big Bang, the so-called cosmic microwave background (CMB).

The expansion of the universe pushes galaxies away from each other. The farther away they are from us, the faster they move. The relationship between the speed and distance of a galaxy is governed by the Hubble constant, which is approximately 70 km per second per megaparsec (a unit of length in astronomy). This means that a galaxy gains about 50,000 miles per hour for every million light years that moves away from us.

But unfortunately for the Standard Model, this value has recently been challenged, leading to what scientists call the Hubble tension. When we measure the expansion rate using nearby galaxies and supernovas (exploding stars), it is 10% higher than when we predict it based on the CMB.

In our new article published in the Monthly Notices of the Royal Astronomical Society, we present a possible explanation: that we live in a giant void in space (an area with a lower than average density). We show that this could inflate local measurements due to material outflows from the vacuum. Flows would occur when denser regions surrounding a void separate it: they would exert a greater gravitational pull than the lower density matter inside the void.

In this scenario, we would need to be near the center of a vacuum with a radius of about a billion light years and a density about 20% lower than the average of the universe in its whole, therefore not completely empty.

Such a vast and deep void is unexpected in the standard model and therefore controversial. The CMB provides insight into the structure of the nascent universe, suggesting that matter today should be fairly evenly distributed. However, directly counting the number of galaxies in different regions does indeed suggest that we are in a local vacuum.

Change the laws of gravity

We wanted to test this idea further by matching many different cosmological observations under the assumption that we live in a large vacuum arising from a small density fluctuation at the beginning.

To do this, our model does not incorporate the ΛCDM but an alternative theory called Modified Newtonian Dynamics (MOND).

MOND was initially proposed to explain anomalies in the rotational speeds of galaxies, leading to the suggestion of an invisible substance called “dark matter”. MOND suggests instead that the anomalies can be explained by the breakdown of Newton’s law of gravity when the gravitational pull is very weak, as is the case in the outer regions of galaxies.

The overall history of cosmic expansion in MOND would be similar to the Standard Model, but structure (such as galaxy clusters) would grow more rapidly in MOND. Our model captures what the local universe might look like in a MOND universe. And we found that this would allow local measures of the current expansion rate to fluctuate depending on our location.

Recent galaxy observations have provided a crucial new test of our model based on the speed it predicts at different locations. This can be done by measuring what is called overall flow rate, which is the average velocity of matter in a given sphere, dense or not. This varies depending on the radius of the sphere, with recent observations showing this extends out to a billion light years.

Interestingly, the massive flow of galaxies at this scale quadrupled the speed expected in the Standard Model. It also seems to increase with the size of the region considered, contrary to what the standard model predicts. The chance that this conforms to the standard model is less than one in a million.

This prompted us to see what our study predicted for the massive flow. We found that this matched the observations quite well. This requires that we be quite close to the center of the void, and that the void be emptiest at its center.

Case closed?

Our results come at a time when popular solutions to the Hubble tension are in trouble. Some people think we just need more precise measurements. Others think the problem can be solved by assuming that the high expansion rate we measure locally is actually the right one. But this requires a slight modification of the expansion history in the early universe to make the CMB still look correct.

Unfortunately, an influential study highlights seven problems with this approach. If the universe had expanded 10% faster over the vast majority of cosmic history, it would also be about 10% younger, which would contradict the ages of the oldest stars.

The existence of a deep and extensive local void in the number of galaxies and the observed rapid massive flows strongly suggest that the structure is growing faster than expected in ΛCDM on scales of tens to hundreds of millions of light years.

Interestingly, we know that the massive El Gordo galaxy cluster formed too early in cosmic history and that its mass and collision speed are too high to be compatible with the Standard Model. This proves once again that structure forms too slowly in this model.

Since gravity is the dominant force on such large scales, we will likely need to extend Einstein’s theory of gravity, general relativity, but only to scales larger than a million light years.

However, we have no effective way to measure the behavior of gravity on much larger scales: no gravitationally huge objects exist. We can assume that General Relativity remains valid and compare with observations, but it is precisely this approach that leads to the very serious tensions currently facing our best model of cosmology.

Einstein is believed to have said that we cannot solve problems with the same thinking that led to the problems in the first place. Even if the changes required are not drastic, we may well see the first reliable evidence in over a century of the need to modify our theory of gravity.

This article is republished from The Conversation under a Creative Commons license. Read the original article.