This article was originally published on The conversation. The publication contributed the article to Space.com Expert voices: opinion pieces and insights.
Indranil Banik is a postdoctoral researcher 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 known as Lambda cold dark matter (ΛCDM). This model is based on detailed observations of the light left after the Big Bang – the so-called cosmic microwave background (CMB).
Due to the expansion of the universe, galaxies are moving 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”, which is about 70 km per second per Megaparsec (a unit of length in astronomy). This means a galaxy gains about 50,000 miles per hour for every million light years it is away from us.
But unfortunately for the Standard Model, this value has recently been challenged, leading to what scientists call the “Hubble Tension”. When we measure the expansion rate using nearby galaxies and supernovas (exploding stars), it is 10% larger than when we predict it based on the CMB.
Related: The expansion of the universe could be a mirage, new theoretical research suggests
In our new paperwe present one possible explanation: that we live in a gigantic void room (an area with a below average density). We show that this could inflate local measurements due to the outflow of matter from the void. Outflows would form when denser regions around a void pull it apart – they would exert a greater gravitational pull than the lower-density matter in the void.
In this scenario we should be close to the center of a void with a radius of about a billion light years and a density about 20% below average. the universe as a whole – so not completely empty.
Such a large and deep void is unexpected in the standard model – and therefore controversial. The CMB provides a snapshot of the structure in the early universe, suggesting that matter should be fairly uniformly distributed today. However, directly counting the number of galaxies in different regions indeed suggests we are in a local void.
Adjusting the laws of gravity
We wanted to further test this idea by comparing many different cosmological observations by assuming that we live in a large void created by a small density fluctuation in early times.
To do this, our fashion model did not contain ΛCDM, but an alternative theory called Modified Newtonian Dynamics (MOUTH).
MOND was originally proposed to explain anomalies in the rotation rates of galaxies, leading to the suggestion of an invisible substance called “dark matter”. MOND suggests instead that the anomalies can be explained by Newton’s law of gravity breaking down when gravity is very weak – as is the case in the outer regions of galaxies.
The overall cosmic expansion history in MOND would be similar to the Standard Model, but the structure (such as galaxy clusters) would grow faster in MOND. Our model represents what the local universe might look like in a MOND universe. And we found that local measurements of today’s expansion rate could fluctuate depending on our location.
Recent observations of galaxies have allowed a crucial new test of our model based on the speed it predicts 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 regardless of its density. This varies with the radius of the sphere recent observations to show It goes on up to a billion light years.
Interestingly, the bulk flow of galaxies at this scale has a fourfold increase in speed expected in the Standard Model. It also appears to increase as the size of the region under consideration increases – contrary to what the standard model predicts. The chance that this is consistent with the standard model is less than one in a million.
This prompted us to see what our research predicted for the bulk flow. We found that this provides a reasonably good match with the observations. That requires that we be fairly close to the center of the void, and that the void is the emptiest in the center.
Case closed?
Our results come to a time when popular solutions to the Hubble strain run into trouble. Some believe we just need it more accurate measurements. Others think this can be solved by assuming that the expansion rate we measure locally is high actually the right one. But that requires a little adjustment to the expansion history in the early universe so that the CMB still looks good.
Unfortunately, one influential review highlights seven issues with this approach. If the universe expanded 10% faster for the vast majority of cosmic history, it would also be about 10% younger – which contradicts the ages of the eldest stars.
The existence of a deep and extensive local void in the galaxy numbers and the fast observed bulk flows strongly suggest that the structure is growing faster than expected in ΛCDM on a scale of tens to hundreds of millions of light years.
Interestingly enough, we know it is a huge cluster of galaxies El Gordo formed too early in cosmic history and has too high a mass and impact velocity to be compatible with the Standard Model. This is further evidence that the structure forms too slowly in this model.
Related stories:
– There’s a mystery to the expansion rate of our universe and the Hubble Space Telescope is on the know
– How fast is the universe expanding? New supernova data can help determine this
– ‘Hubble problems’ could deepen with new measurements of the universe’s expansion
Since gravity the dominant force is on such a large scale, we should probably expand Einstein‘s theory of gravity, General relativity – but only on scales larger than a million light years.
However, we don’t have a good way to measure how gravity behaves on a much larger scale; 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 very serious tensions that our best model of research currently faces. cosmology.
Einstein is thought 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 be witnessing the first reliable evidence in more than a century that we need to change our theory of gravity.
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