by This article was originally published in The conversation. The publication contributed the article to Space.com. Expert voices: opinion articles and opinions.
Indranil Banik He is a postdoctoral researcher in Astrophysics at the University of St. Andrews.
One of the greatest mysteries of cosmology is the rate at which 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 over from the Big Bang, the so-called cosmic microwave background (WBC).
The expansion of the universe causes galaxies to 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 governed by the “Hubble constant”, which is approximately 43 miles (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 it is far from us.
But unfortunately for the Standard Model, this value has recently been questioned, leading to what scientists call the “Hubble tension”. When we measure the expansion rate using nearby galaxies and supernovas (exploding stars), is 10% larger than when we predicted it based on the CMB.
Related: The expansion of the universe could be a mirage, a new theoretical study suggests
In our new paperWe present a possible explanation: that we live in a giant void in space (an area with below average density). We show that this could inflate local measurements through vacuum matter outflows. Outflows would arise when the denser regions surrounding a vacuum pull it apart: they would exert a greater gravitational attraction than the lower-density matter within the vacuum.
In this scenario, we would need to be near the center of a void about a billion light years in radius and with a density about 20% lower than average to the universe as a whole, so it is not completely empty.
Such a large and deep void is unexpected in the standard model and therefore controversial. The CMB offers a snapshot of the structure of the infant universe, suggesting that matter today should be fairly evenly distributed. However, directly counting the number of galaxies in different regions in fact it suggests We are in a local vacuum.
Modifying the laws of gravity
We wanted to test this idea further by comparing many different cosmological observations, assuming that we live in a large vacuum that arose from a small density fluctuation in early times.
To do this, our model did not incorporate ΛCDM but an alternative theory called Modified Newtonian Dynamics (MONDAY).
MOND was originally proposed to explain anomalies in the rotation speeds of galaxies, leading to the suggestion of an invisible substance called “dark matter”. MOND, instead, suggests that the anomalies can be explained by Newton’s law of gravity. breaking when the gravitational attraction 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 faster in MOND. Our model captures what the local universe would look like in a MOND universe. And we found that it would allow local measurements of the current expansion rate to fluctuate depending on our location.
Recent observations of galaxies have allowed a crucial new test of our model based on the velocity it predicts at different locations. This can be done by measuring something called mass flow, which is the average speed of matter in a given sphere, dense or not. This varies with the radius of the sphere, with recent observations demonstration Keep going up to a billion light years.
Interestingly, the mass flow of galaxies at this scale has quadrupled the speed expected in the standard model. It also appears to increase with the size of the region considered, contrary to what the standard model predicts. The chance of this being consistent with the standard model is less than one in a million.
This led us to see what our study predicted for mass flow. We found that it matches the observations quite well. That requires that we be fairly close to the center of the void, and that the void be emptier at its center.
Case closed?
Our results come in a time when popular solutions to the Hubble strain are in trouble. Some believe that we only need more accurate measurements. Others think it can be solved by assuming that the high expansion rate we measure locally is actually the right one. But that requires a slight adjustment to the expansion history in the early universe so that the CMB still looks correct.
Unfortunately, one influential review highlights seven issues with this approach. If the universe were expanding 10% faster for the vast majority of cosmic history, it would also be about 10% younger, which contradicts the theory. centuries of the oldest stars.
The existence of a deep and widespread local void in the number of galaxies and the fast mass flows observed strongly suggest that structure grows faster than expected in ΛCDM on scales of tens to hundreds of millions of light years.
Interestingly, we know that the enormous galaxy cluster Fat formed too early in cosmic history and has too high a mass and collision velocity to be compatible with the standard model. This is further evidence that the structure forms too slowly in this model.
Related stories:
— There is a mystery in the expansion rate of our universe and the Hubble Space Telescope is aware of the case.
—At what speed is the universe expanding? New data on supernovae could help make it happen
— The ‘Hubble problem’ could be deepened with a new measurement of the expansion of the universe
From gravity is the dominant force at such large scales, we will most likely need to scale up EinsteinThe theory of gravity General relativity – but only in scales bigger than a million light years.
However, we don’t have a good way to measure how gravity behaves at much larger scales: there are no objects that are gravitationally that large. We can assume that General Relativity is still valid and compare it with observations, but it is precisely this approach that leads to the very severe strains currently facing our best model of relativity. 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 could well be witnessing the first reliable evidence in more than a century that we need to change our theory of gravity.
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