Cosmologists propose a giant vacuum of space as a solution to “Hubble tension”, challenging traditional models and suggesting a revision of Einstein’s theory of gravity.
One of the biggest mysteries in cosmology is the expansion rate of the universe. 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 residual light the great explosion – The so-called cosmic microwave background (CMB).
The expansion of the universe causes galaxies to move away from each other. The farther away they are from us, the faster they move. The relationship between galactic speed and distance is governed by the “Hubble constant,” which is about 43 miles (70 km) per second per megaparsec (unit of length in astronomy). This means that the galaxy You gain about 50,000 miles per hour For every million light-years away from us.
Unfortunately for the Standard Model, this value has recently been disputed, leading to what scientists call the “Hubble tension.” When we measure the expansion rate using nearby galaxies and supernovas (exploding stars), it is 10% greater than when we predicted it based on the CMB.
In our New paperWe offer one possible explanation: that we live in a giant vacuum of space (a region of lower-than-average density). We have shown that this can lead to local measurements being amplified by matter fluxes from the void. Outflows may arise when denser regions surrounding a vacuum pull it apart, exerting a greater attractive force than the lower-density matter within the vacuum.
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 universe as a whole, i.e. not completely empty.
Such a large and deep void is unexpected in the Standard Model – and therefore controversial. The CMB gives a snapshot of the structure of the nascent universe, suggesting that matter today must be fairly uniformly spread out. However, the number of galaxies in different regions is calculated directly It is already suggested We are in a local vacuum.
Modifying the laws of gravity
We wanted to test this idea further by matching several different cosmological observations by assuming that we live in a large vacuum that arose from small density fluctuations at early epochs.
MOND was originally proposed to explain anomalies in the rotation velocities of galaxies, which led to the suggestion of the existence of an invisible substance called “dark matter.” MOND suggests instead that these anomalies can be explained by Newton’s law of gravitation, which breaks down when the force of gravity is too weak – such as in the outer regions of galaxies.
The overall cosmic expansion history in MOND will be similar to the Standard Model, but structure (such as galaxy clusters) will grow faster in MOND. Our model captures what the local universe in the MOND universe might look like. We found that this would allow local measurements of today’s expansion rate to fluctuate depending on our location.
Recent galaxy observations have allowed 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 bulk flow, which is the average velocity of the material in a given ball, whether it is dense or not. This varies with the radius of the ball, with Final notes an offer It continues To a billion light years.
Interestingly, the massive flow of galaxies on this scale quadrupled the speed expected in the Standard Model. They also appear to increase with the size of the region under consideration, contrary to what the Standard Model predicts. The probability that this is consistent with the Standard Model is less than one in a million.
This prompted us to see what our study of bulk flow predicted. We have found that it produces very good match To the notes. This requires that we be fairly close to the center of the vacuum, and that the vacuum be more empty at its center.
Our results come at a time when common solutions to Hubble tensor are running into trouble. Some think we just need more precise measurements. Others believe it can be solved by assuming the high expansion rate we measure locally as well Actually correct. But this requires a slight adjustment in the expansion history of the early universe for the CMB to still look correct.
Unfortunately, one influential review highlights seven problems With this approach. If the universe expanded 10% faster during the vast majority of cosmic history, it would also be about 10% younger – which contradicts prevailing theory. Ages One of the oldest stars.
The presence of a deep, extended local void in galaxy populations and the observed fast large outflows strongly suggest that structure is growing faster than expected in the ΛCDM on scales between tens and hundreds of millions of light-years.
Interestingly, we know that the El Gordo supercluster (see image above) has formed Too early In cosmic history, it has a mass and collision speed so high that it does not fit the Standard Model. This is further evidence that the structure forms very slowly in this model.
Since gravity is the dominant force on such large scales, we likely need to extend Einstein’s theory of gravity, general relativity – but only on scales. Larger than a million light years.
However, we have no good way to measure how gravity behaves on much larger scales, as there are no gravitationally bound objects that large. We can assume that general relativity remains valid and compare it to observations, but it is precisely this approach that leads to the extreme tensions that our best model of cosmology currently faces.
It is believed that Einstein 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 radical, we could be seeing the first reliable evidence in more than a century that we need to change our theory of gravity.
Written by Indranil Panik, Postdoctoral Research Fellow in Astrophysics, University of St Andrews.
Adapted from an article originally published in Conversation.
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