The development of cosmic structure – the huge strands of galaxies and caverns of emptiness that make up our universe – seems to be slowing down more than expected. That could mean there is something wrong with our understanding of the universe.
There may be something wrong with our models of the cosmos. The matter in the universe is getting clumpier and clumpier over time but it appears to be happening more slowly than expected. This slow-down in the evolution of the large-scale structure of the universe could point to new particles or fields, or it could indicate that our understanding of dark matter isn’t quite right.
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| The universe is evolving more slowly than expected Javier Zayas Photography via Getty Images |
The evolution of the universe is largely governed by the push and pull of two competing forces – dark energy forces the entire cosmos to expand, while gravity pulls matter together. According to the most widely accepted model of cosmology, called lambda-CDM, this should mean that over time the cosmic web of galaxies gets denser and denser while cosmic voids grow and empty of matter.
But when Nhat-Minh Nguyen at the University of Michigan and his colleagues put together data on the structure of the early universe and its more recent state from a variety of sources – including galaxy surveys and measurements of relic light from just after the big bang – they found this process of structure growth has been slowing down at an unexpected rate. The cosmic web is getting denser and the voids are getting bigger and emptier, but not as fast as lambda-CDM would predict.
“Many different probes that are probing the universe at different times and different scales each suggest the same thing, which is that the growth of structure at recent time has been much more suppressed than we expect from the standard model,” says Nguyen. “We know that we should expect some slow-down, but we find that the slow-down is more prominent than we expected."
Their measurements are in tension with the standard model at a level of 3.7-sigma, meaning that there is a probability of about 1 in 4600 that a pattern of data like this would show up as a statistical fluke if the standard model correctly described the cosmos. A suppression in structure growth could also explain other tensions that have shown up in past cosmic observations.
“They didn’t use all of the data sets that are out there, but looking at some of that updated data I think it might have made their conclusions even stronger,” says Mike Hudson at the University of Waterloo in Canada. “There are many suggestions in the literature that lambda-CDM is wrong in some way, but most of them are not widely accepted in the community – this one has earned a little more attention.”
Where this slow-down could have come from – and therefore how the standard model would have to be modified to account for it – is a mystery. “Many of the models of modified gravity that we know would produce a structural enhancement rather than a suppression,” says Jiamin Hou at the Max Planck Institute for Extraterrestrial Physics in Germany. “Seeing suppression instead is a very interesting hint to what might be going on.”
Simply adding more dark energy won’t do the trick. “What could be causing this could be something that requires new fields, which means new particles or new interactions, which means new forces, possibly between dark matter particles,” says Nguyen. If dark matter interacts or decays differently than we predicted, it might account for these measurements – but we will need more observations to figure it out for sure.
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