Do astronomical voids exist?

One of the biggest mysteries in cosmology is the rate of expansion of the universe. This can be predicted using the Standard Model of Cosmology, also known as Lambda Cold Dark Matter (ΛCDM). The model is based on detailed observations of the light left over from the Big Bang, the so-called cosmic microwave background (CMB). As the universe expands, galaxies move away from each other. The further away they are from us, the faster they move. The relationship between a galaxy’s speed and distance is determined by the Hubble constant.

The Hubble constant is approximately 43 miles (70 km) per megaparsec (a unit of length in astronomy) per second. This means that for every million light-years a galaxy moves from us, its speed increases by about 50,000 miles per hour. Unfortunately, however, this value of the Standard Model has recently been debated, resulting in what scientists call the Hubble stress. When we measure the expansion rate from nearby galaxies and supernovae (exploding stars), we find it is 10% larger than when predicted from the CMB. A new paper published in Monthly Notices of the Royal Astronomical Society presents a possible explanation: We live in a vast expanse of outer space – a region of below-average density.

This indicates that local measurements can be inflated due to the outflow of material from the cavity. Outflow is formed when the dense area around the cavity pulls the cavity apart. The outflow will exert a greater gravitational force than the less dense material within the cavity. In this scenario, we would have to be near the center of a not-completely-empty cavity with a radius of about 1 billion light-years and a density about 20% lower than the average for the entire universe. Such large and deep gaps are controversial because they are unexpected in the standard model. The CMB provides a snapshot of the structure of the early Universe, suggesting that matter should be more evenly distributed today. But directly counting the number of galaxies in different regions suggests that we are indeed in a local vacuum.

Modifying gravity’s laws

We wanted to further test this idea by comparing many different cosmological observations and assuming that we live within a large void resulting from small initial density fluctuations. . For this purpose, our model did not include ΛCDM, but an alternative theory called Modified Newtonian Dynamics (MOND). MOND was originally proposed to explain anomalies in the rotational speed of galaxies, and has come to suggest the existence of invisible matter called “dark matter.” Instead, MOND suggests that the anomaly can be explained by a breakdown of Newton’s law of gravity when gravity is very weak, as is the case in the outer regions of galaxies.

The overall universe expansion history in MOND is similar to the standard model, but structures (such as galaxy clusters) grow faster in his MOND. Our model captures what the local universe looks like in the MOND universe. And we know that today’s local measurements of expansion rates can vary by location. Recent observations of galaxies have provided important new tests of the model based on predicted velocities at different locations. This can be achieved by measuring the so-called mass flow rate.

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This is the average velocity of matter within a given sphere, whether dense or not. This depends on the radius of the sphere, with recent observations showing that the sphere can extend up to 1 billion light years. Interestingly, the mass flow rate for galaxies of this size is four times faster than expected in the Standard Model. It also appears to increase with the size of the region under consideration, contrary to the predictions of the standard model. The probability that this matches the standard model is less than 1 in a million.

This made us check the results of our study’s bulk flow prediction. It produces a fairly good match to the observations, as we discovered. That means the void is at its most empty at its center, and we must be relatively close to it.

Closed case?

Our results come at a time when common solutions to the Hubble voltage are facing difficulties. Some think we just need more precise measurements. Others believe that the problem can be solved by assuming that the high expansion rate that we are measuring locally is actually true. However, for the CMB to look right, the expansion history of the early universe must be slightly adjusted. Unfortunately, an influential review points out that there are seven problems with this approach. If the universe expanded 10% faster for most of its history, the universe would be about 10% younger.

This contradicts the age of the oldest stars. The presence of deep and widespread local voids in the galaxy diagram and the observed rapid mass flow strongly suggest that the structure of the ΛCDM is growing faster than expected on scales of tens to hundreds of millions of light years. doing. Interestingly, large Elgordic galaxy clusters were found to have formed too early in the history of the universe and had masses and collision velocities too high to be compatible with the Standard Model.

This is further evidence that structure formation is too slow in this model. Since gravity is the dominant force at such large scales, we will probably need to extend Einstein’s theory of gravity and general relativity, but only to scales beyond a million light years. However, there is no good way to measure the behavior of gravity on larger scales. No object exists that is affected by such a large force of gravity. We can assume that general relativity is still valid and compare it to observations, but it is precisely this approach that makes the very difficult situation that our best cosmological models currently face. This leads to strong tension. Einstein is said to have said that he cannot solve problems with the same thinking that created them in the first place. For the first time in more than a century, scientists may have reliable evidence that gravity theory needs to change, even if the changes needed are not radical.