$VWURQRPHUV DUH WU\LQJ WR UHVROYH WKH FRQ LFW between local and pan-Universal observations
Sky at Night presenter
What do you do when your observations don’t agree? Chris delves into one of the enduring challenges of particle physics.
Modern cosmologists have a difficult time of it. Thanks to 30 years or more of hard observational work, their freedom to create new Universes, each one more complex and interesting than the last, is severely curtailed. Any model they build has to match observations on a wide range of scales, producing sensible galaxies and explaining the whole Universe, from the near-beginning of its story to the present day.
It’s a complex jigsaw, so it’s little wonder that the community jumps on any sign that the picture doesn’t quite fit together. There don’t seem to be any obviously missing pieces just yet, but there are a few places where things aren’t as simple as they could be. For example, those who study galaxies and those who use data from ESA’s Planck satellite to study the cosmic microwave background disagree on the value of Hubble’s constant, which measures the rate at which the Universe is expanding.
Another source of ‘tension’ (the word physicists use for differences that aren’t stark, but which won’t go away) is examined in a new paper by Ian McCarthy of Liverpool John Moores University and colleagues. They compare observations of the cosmic microwave background with those of the cosmic web of galaxies in the local Universe. Each is sensitive to both the total amount of matter in the Universe, a quantity known as ‘Omega_M’ and how clumpy the arrangement of that matter is, captured in a measurement known as ‘sigma_8’.
There are lots of different observations that can produce measurements of these numbers, which are related to each other. A Universe with more matter is one in which gravity plays a more important role, and therefore one which will be lumpier. Measurements taken by looking at local galaxies seem to favour lower values of both than measurements that look at the cosmic microwave background, whereas if all is well they should agree.
The authors are the first to point out that estimating the potential measures in each of the measurements is difficult, and it’s certainly possible that the ‘tension’ could be resolved by correcting either set slightly. The approach here is different,
though; using the wonderfully named BAHAMAS simulations, they investigate how to change the Universe to better agree with the observations.
The results are surprising. The team found that fixing the details of the recipe that controls the simulation doesn’t help, but allowing neutrinos to have a more substantial mass than normally assumed really does. The spread out mass of neutrinos in simulations of the Universe normally helps to smooth
out clumpy structures. But adding mass makes them more significant. We know that neutrinos can’t be completely massless, but the simulations suggest they’re about three times heavier than otherwise assumed.
This is an exciting result. The kinds of masses needed are allowed by our current understanding of particle physics, and it may be that the subtle differences in cosmological observations are telling us something profound about these tiny particles. Sometimes, if you want to understand the very small, you have to think big.
CHRIS LINTOTT was reading… The BAHAMAS project: the CMB – large-scale structure tension and the roles of massive neutrinos and galaxy formation by Ian G McCarthy et al Read it online at https://arxiv.org/abs/1712.02411
“There don’t seem to be any obviously missing pieces yet, but there are a few places where things aren’t as simple as they could be”
The cosmic microwave background is an imprint of radiation; the ‘afterglow’ of the Big Bang
CHRIS LINTOTT is an astrophysicist and co-presenter of The Skyat Night on BBC TV. He is also the director of the Zooniverse project.