Do we live in a giant void? That could solve the puzzle of the universe's expansion, research suggests

The pace of expansion of the cosmos is one of the great mysteries of cosmology. The standard model of cosmology, or Lambda-cold dark matter (λCDM), may be used to predict this. This hypothesis is based on in-depth measurements of the so-called cosmic microwave background (CMB), which is light that remains after the Big Bang.

As the cosmos expands, galaxies separate from one another. They travel faster the further away they are from us. Hubble's constant, which is approximately 43 miles (70 km) per second per megaparsec (an astronomical unit of length), controls the connection between a galaxy's speed and distance. This indicates that for every million light years that a galaxy is separated from us, it acquires around 50,000 miles per hour.

However, this number has lately been contested, which is bad news for the mainstream model and has resulted in what scientists refer to as the Hubble tension. The supernovae (exploding stars) and adjacent galaxies allow us to measure the expansion rate, which is 10% higher than what the CMB predicts.

We provide one theory in our recent publication, which was published in the Monthly Notices of the Royal Astronomical Society: that we are inhabitants of a massive emptiness in space, or a region with lower than normal density. We demonstrate how outflows of matter from the vacuum might lead to an inflation of local measurements. Denser areas around a void would exert a stronger gravitational attraction than the lower density matter inside the void, leading to the formation of outflows.

In this case, we would have to be close to the center of a vacuum that is around a billion light years in radius and with a density that is approximately 20% lower than the norm for the whole universe—that is, not quite empty.

The traditional model does not account for such a vast and profound emptiness, which makes it contentious. According to the CMB, which provides an image of the structure of the early cosmos, stuff should be rather evenly distributed today. Nevertheless, it is true that a straight count of the number of galaxies in various places indicates that we are in a local vacuum.

Modifying the gravitational laws

By assuming that we exist in a vast emptiness that originated from a little density fluctuation in the early universe, we hoped to test this theory further by matching a wide range of cosmological evidence.

In order to do this, our model used Modified Newtonian Dynamics (MOND), an alternative theory, instead of ΓCDM.

The idea of an invisible material known as "dark matter" was first put up in order to explain abnormalities in the rotation speeds of galaxies, an idea that was inspired by the MOND proposal. Instead, MOND proposes that the anomalies may be explained by the breakdown of Newton's law of gravity in situations when the gravitational pull is very weak, such as in galaxies' outer regions.

In MOND, structure (such galaxy clusters) would grow more quickly than in the standard model, but overall cosmic expansion history would be similar. Our model represents the possible appearance of the local universe in a MOND world. Furthermore, we discovered that depending on where we are, local estimations of the expansion rate today will vary.

A critical new test of our hypothesis based on the predicted velocities at various sites has been made possible by recent galaxy sightings. This may be achieved by taking a measurement of the average velocity of matter, whether dense or not, in a certain sphere, known as the bulk flow. This changes with the sphere's radius; new measurements indicate that it extends up to a billion light-years.

Fascinatingly, on this scale, the mass flow of galaxies is traveling at a pace that is triple that predicted by the standard model. In contrast to what the usual model predicts, it also appears to rise with the size of the region under consideration. There is less than a one in a million chance that this fits the usual model.

This made us check the bulk flow predictions made by our study. We discovered that it produces a rather accurate match to the data. That implies that the emptiness is most empty at its center and that we are somewhat near to it.

Case closed?

Our findings coincide with the deterioration of widely accepted solutions to the Hubble tension. Some people think we just need more accurate measurements. Some believe issue can be resolved by assuming that the high local growth rate we measure is the true one. To maintain the CMB's correct appearance, however, a little adjustment to the early universe's expansion history is needed.

Regretfully, a well-known assessment identifies seven issues with this strategy. The ages of the oldest stars would be in conflict if the universe grew 10% quicker throughout the great bulk of cosmic history. It would also be around 10% younger.

The rapid observed bulk flows and the presence of a deep and wide local vacuum in the galaxy number counts strongly imply that structure grows quicker than predicted in ΓCDM on tens to hundreds of millions of light years scales.

Fascinatingly, we are aware that the huge galaxy cluster El Gordo is not consistent with the mainstream model because it originated too early in cosmic history and has too high of a mass and collision speed. This provides more proof that the structure in this model forms too slowly.

We probably need to extend Einstein's theory of gravity, general relativity, since gravity is the dominating force on such enormous distances—but only on scales greater than a million light years.

Nevertheless, there are no massive gravitationally bound objects, thus we have no reliable means to determine how gravity works on much bigger scales. We can compare with data and presume that General Relativity is still true, but it is precisely this method that causes the extreme tensions that our best model of cosmology is presently facing.

It is said that Einstein once stated that we cannot address issues by using the same methods of thinking that caused them in the first place. Even if the necessary adjustments are not significant, this might be the first trustworthy indication in almost a century that our theory of gravity needs to be altered.

Provided by The Conversation