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 over from 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 that 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 supernovae (exploding stars), it is 10% greater than when we predict it from the CMB.
In our new paper, we present one possible explanation: that we live in a gigantic void in space (an area of 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 would be near the center of a void with a radius of about a billion light-years and a density about 20% below the average of 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 does indeed suggest that 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 model did not use ΛCDM, but an alternative theory called Modified Newtonian Dynamics (MOND).
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 that gravity decays 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, with recent observations showing this continues for 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 at a time when popular solutions to the Hubble strain are in trouble. Some believe we simply need more accurate measurements. Others think this can be solved by assuming that the high expansion rate we measure locally is actually the correct one.
But that requires a little adjustment to the expansion history in the early universe so that the CMB still looks good.
Unfortunately, an influential review points out seven problems 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 age of the oldest 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, we know that the massive cluster of galaxies El Gordo formed too early in cosmic history and has too high a mass and collision rate to be compatible with the Standard Model. This is further evidence that the structure forms too slowly in this model.
Since gravity is the dominant force on such large scales, we most likely need to extend Einstein’s theory of gravity, general relativity – but only on scales greater 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 with observations, but it is precisely this approach that leads to the very serious tensions that our best cosmological model currently faces.
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.
Indranil Banik, Postdoctoral Fellow in Astrophysics, University of St Andrews
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