by This article was originally published by The conversation. The publication contributed the article to Space.com’s Expert Voices: Tutorial & Insights.
Indranil Banik is a Postdoctoral Research Fellow in Astrophysics at the University of St Andrews.
One of the greatest mysteries in cosmology is the rate at which the Universe is expanding. This can be predicted using the standard model of cosmology, also called Lambda cold dark matter (ΛCDM). This model is based on detailed observations of the evidence left over from the so-called Big Bang cosmic microwave background (CMB).
The expansion of the universe causes galaxies to move apart. The further they are from us, the faster they move. The relationship between the speed and distance of a galaxy is governed by the “Hubble constant”, which is about 43 miles (70 km) per second per Megaparsec (a unit of distance in astronomy). This means that galaxy gains about 50,000 miles per hour for every million light years it is away from us.
But unfortunately for the standard model, there has recently been a dispute over this value, what scientists call it “Hubble tension”. When we measure the expansion rate using nearby galaxies and supernova (exploding stars), it 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 one possible explanation: that we live in a vast vacuum space (area with below average density). We show that this can induce local measurements by the outflow of matter from the vacuum. An outflow would occur when denser regions surrounding a void pull it apart – exerting a greater gravitational pull than the lower-density material within the void.
In this case, we would be close to the center of the void about a billion light-years in radius and with a density about 20% below the average for the universe as a whole – so not completely empty.
Such a large and deep gap is unexpected in the standard model – and therefore controversial. The CMB provides insight into the structure of the infant universe, suggesting that today’s matter should be uniformly distributed. However, exactly count the the number of galaxies in different regions indeed recommend we are in a local vacuum.
Breaking the laws of gravity
We wanted to test this idea further by matching many different cosmological observations by assuming that we live in a large vacuum that grew out of a small early density fluctuation.
To do this, our Miniature ΛCDM only incorporated 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 known as “dark dark“. Instead, MOND suggests that the anomalies can be explained by Newton’s law of gravity falling apart when the gravitational pull is very weak – as in the outer regions of galaxies.
The entire cosmic expansion history in MOND would be similar to the standard model, but structure (such as galaxy clusters) would grow faster in MOND. Our model represents what the local universe might look like in the MOND Universe. And we found that it would allow local measurements of the expansion rate today to fluctuate depending on our location.
Recent galaxy observations have allowed a crucial new test of our model based on the velocity predicted at different locations. This can be done by measuring something called the bulk flow, which is the average velocity of matter in a given sphere, compact or not. This varies with the radius of the sphere, with recent observations showing it continues away by a billion light years.
Interestingly, the mass outflow of galaxies on this scale is four times the speed expected in the standard model. It also appears to increase with the size of the observed region – contrary to what the standard model predicts. The probability of this being consistent with the standard model is under one in a million.
This prompted us to see what our study predicted for the bulk flow. We found that it has a good result on the observations. That requires us to be quite close to the void center, with the emptiest void in the middle.
Case Closed?
Our results come at a time when large solutions to the Hubble tension are in trouble. Some believe that we just have to more accurate measurements. Others think it can be solved by accepting the high rate of expansion we measure locally really the right one. But that requires a slight tweak to the early expansion history of the universe so the CMB is still correct.
Unfortunately, an influential review highlights seven problems with this approach. If the universe expanded 10% faster over the vast majority of cosmic history, it would also be about 10% younger—contradictory. ages of the oldest stars.
The presence of a deep and extended local void in the number of galaxies and the observed fast volumetric flows strongly suggest that structure grows faster than expected in ΛCDM on scales of tens to hundreds of millions of light years .
Interestingly enough, we know that the galaxy cluster is massive El Gordo done too soon in cosmic history and has a mass and collision speed too high to fit the standard model. This is further evidence that structure emerges too slowly in this model.
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— The ‘Hubble problem’ could deepen with a new measurement of the universe’s expansion
Since gravity It is the dominant force on such large scales, we probably need to expand Einsteintheory of gravity, General Relativity – but only on scales more than a million light years.
However, we have no good way to measure how gravity behaves on much larger scales – there are no gravitating objects that are massive. We can assume that General Relativity is still valid and compare it to observations, but it is precisely this approach that causes the severe tension facing our best model of cosmology.
Einstein is thought to have said that we cannot solve problems with the same thinking that caused the problems in the first place. Even if the necessary changes are not drastic, we may see the first reliable evidence in over a century that we need to change the theory of gravity.
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