Considered one of the largest mysteries in cosmology is the speed at which the universe is expanding. This will be predicted using the standard model of cosmology, also often known as Lambda-cold dark matter (ΛCDM). This model relies on detailed observations of the sunshine left over from the Big Bang – the so-called cosmic microwave background (CMB).
The universe’s expansion makes galaxies move away from one another. The further away they’re from us, the more quickly they move. The connection between a galaxy’s speed and distance is governed by “Hubble’s constant”, which is about 43 miles (70 km) per second per Megaparsec (a unit of length in astronomy). Because of this a galaxy gains about 50,000 miles per hour for each million light years it’s away from us.
But unfortunately for the usual model, this value has recently been disputed, resulting in what scientists call the “Hubble tension”. After we measure the expansion rate using nearby galaxies and supernovas (exploding stars), it’s 10% larger than once we predict it based on the CMB.
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In our recent paper, we present one possible explanation: that we live in an enormous void in space (an area with below average density). We show that this might inflate local measurements through outflows of matter from the void. Outflows would arise when denser regions surrounding a void pull it apart – they’d exert an even bigger gravitational pull than the lower density matter contained in the void.
On this scenario, we’d have to be near the centre of a void a couple of billion light years in radius and with density about 20% below the common for the universe as an entire – so not completely empty.
Such a big and deep void is unexpected in the usual model – and subsequently controversial. The CMB gives a snapshot of structure within the infant universe, suggesting that matter today needs to be reasonably uniformly opened up. Nevertheless, directly counting the variety of galaxies in several regions does indeed suggest we’re in a neighborhood void.
Tweaking the laws of gravity
We desired to test this concept further by matching many various cosmological observations by assuming that we live in a big void that grew from a small density fluctuation at early times.
To do that, our model didn’t incorporate ΛCDM but another theory called Modified Newtonian Dynamics (MOND).
MOND was originally proposed to elucidate anomalies within the rotation speeds of galaxies, which is what led to the suggestion of an invisible substance called “dark matter”. MOND as a substitute suggests that the anomalies will be explained by Newton’s law of gravity breaking down when the gravitational pull may be very weak – as is the case within the outer regions of galaxies.
The general cosmic expansion history in MOND could be much like the usual model, but structure (comparable to galaxy clusters) would grow faster in MOND. Our model captures what the local universe might appear to be in a MOND universe. And we found it could allow local measurements of the expansion rate today to fluctuate depending on our location.
Recent galaxy observations have allowed an important recent test of our model based on the speed it predicts at different locations. This will be done by measuring something called the majority flow, which is the common velocity of matter in a given sphere, dense or not. This varies with the radius of the sphere, with recent observations showing it continues out to a billion light years.
Interestingly, the majority flow of galaxies on this scale has quadruple the speed expected in the usual model. It also seems to extend with the dimensions of the region considered – opposite to what the usual model predicts. The likelihood of this being consistent with the usual model is below one in one million.
This prompted us to see what our study predicted for the majority flow. We found it yields a quite good match to the observations. That requires that we’re fairly near the void centre, and the void being most empty at its centre.
Case closed?
Our results come at a time when popular solutions to the Hubble tension are in trouble. Some imagine we just need more precise measurements. Others think it may well be solved by assuming the high expansion rate we measure locally is actually the proper one. But that requires a slight tweak to the expansion history within the early universe so the CMB still looks right.
Unfortunately, an influential review highlights seven problems with this approach. If the universe expanded 10% faster over the overwhelming majority of cosmic history, it could even be about 10% younger – contradicting the ages of the oldest stars.
The existence of a deep and prolonged local void within the galaxy number counts and the fast observed bulk flows strongly suggest that structure grows faster than expected in ΛCDM on scales of tens to a whole bunch of hundreds of thousands of sunshine years.
Interestingly, we all know that the large galaxy cluster El Gordo formed too early in cosmic history and has too high a mass and collision speed to be compatible with the usual model. That is yet more evidence that structure forms too slowly on this model.
Since gravity is the dominant force on such large scales, we almost certainly need to increase Einstein’s theory of gravity, General Relativity – but only on scales larger than one million light years.
Nevertheless, we have now no good solution to measure how gravity behaves on much larger scales – there are not any gravitationally sure objects that massive. We are able to assume General Relativity stays valid and compare with observations, but it surely is precisely this approach which results in the very severe tensions currently faced by our greatest model of cosmology.
Einstein is assumed to have said that we cannot solve problems with the identical considering that led to the issues in the primary place. Even when the required changes should not drastic, we could well be witnessing the primary reliable evidence for greater than a century that we want to alter our theory of gravity.