Do We Exist in a Vast Vacuum? It May Offer a Solution to the Riddle of the Universe’s Expansion

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), can be used to predict this. This hypothesis is based on in-depth measurements of the cosmic microwave background (CMB), or 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. A galaxy’s velocity and distance are determined by “Hubble’s constant,” or around 43 miles per second (70 km/s) every Megaparsec (an astronomical unit of length).This indicates that for every million light years that a galaxy is separated from us, it acquires roughly 50,000 miles per hour.

Unfortunately for the mainstream model, however, there has been some recent disagreement over this value, giving rise to the so-called “Hubble tension” among scientists. The supernovae (exploding stars) and adjacent galaxies allow us to measure the expansion rate, which is 10% higher than what the CMB predicts.

We offer one theory in our most recent paper: that we inhabit a vast empty space (a region with lower than average density). We demonstrate how outflows of matter from the vacuum could lead to an inflation of local measurements. Denser regions surrounding a void would exert a stronger gravitational attraction than the lower density matter inside the void, resulting in outflows.

In this case, we would have to be close to the center of a void that is not entirely empty, but has a radius of roughly a billion light years with a density that is 20% lower than the norm for the universe.

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 void 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 observations.

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

Originally, MOND was put up to explain irregularities in the rotational velocities of galaxies, which gave rise to the notion of an unseen material known as “dark matter.” As an alternative, MOND proposes that the anomalies may be explained by the breakdown of Newton’s law of gravity in situations when the gravitational pull is extremely weak, such as in the outer regions of galaxies.

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 measurements of the expansion rate today would 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 can 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 observations 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 quadruple 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 close to it.

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 expansion 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 review identifies seven issues with this strategy. The ages of the oldest stars would be in conflict if the universe had expanded 10% quicker during the great majority of cosmic history. It would also have been around 10% younger.

The fast observed bulk flows and the presence of a deep and wide local void in the galaxy number counts strongly imply that structure grows quicker than expected 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 additional proof that the structure in this model forms too slowly.

On such huge scales, gravity is the dominant force, thus we probably need to extend General Relativity, Einstein’s theory of gravity, but only on greater scales than a million light years.

Nevertheless, since there are no enormous gravitationally bound objects, we are unable to accurately quantify how gravity behaves on much bigger scales. We can make the assumption that General Relativity is still true and compare it with data, but this very approach is what causes the extreme tensions that our best model of cosmology is currently facing.

It is believed 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 could be the first trustworthy indication in almost a century that our theory of gravity needs to be altered.

Topics #universe expantion #vast vacuum