A fundamental tenet of the modern model of cosmology is coming into question. A survey of聽more than 1 million galaxies across the cosmos has shown that the distribution of matter may not be the same in every direction, which could upend much of what we understand about the universe.
The cosmological principle posits that, viewed on large enough scales, the distribution of聽matter should be smooth and regular in every direction. This assumption is used in many cosmological calculations, most notably in the standard model of cosmology, called lambda-CDM.
Nathan Secrest at the US Naval Observatory in Washington DC and his colleagues set out to test聽the principle using more than聽1.3聽million quasars, which are聽supermassive black holes surrounded by bright matter that聽are found at the centre of some galaxies.
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They expected to see a slight unbalance, or lack of symmetry, due to the movement of our solar system and galaxy through space. 鈥淚n our direction of motion, we should see more objects, and in the opposite direction, we should see fewer objects,鈥� says Secrest.
This cosmic Doppler effect has聽been measured in the past using the cosmic microwave background, a sea of radiation left over from the big bang. But the unbalance in quasar distribution was more than twice as big as expected.
The disagreement between the聽quasar distribution and the cosmic microwave background may point to a fundamental error in the standard cosmological model. If it remains, it could change what we think we know about the big bang and the moments following it, which affected the shape and expansion of the universe.
鈥淎ccording to lambda-CDM, the聽universe is really lumpy on聽the聽scales we live on, galaxies and clusters of galaxies, and then it becomes smoother as you get bigger,鈥� says Michael Turner at the University of Chicago. 鈥淎ssuming this analysis is correct, it doesn鈥檛 get as smooth as quickly as you expect 鈥� all this means is the simplest model of lambda-CDM is聽not right.鈥�
Several physical mechanisms could resolve the discrepancy. The聽most elementary explanation is that we are moving much faster through the universe than we thought. But there are more complex possibilities as well, such聽as unexpected curvatures in聽space-time or strange properties of dark energy.
鈥淟ambda-CDM is not the whole story and we鈥檙e getting closer to聽the point where we can get beyond it to something deeper,鈥� says Secrest. 鈥淲e are starting to see the intrinsic structure of the universe better than ever before.鈥�
Figuring out what that structure is and how it differs from our current understanding will require many more observations of the large-scale distribution of matter, using not only quasars but many other types of cosmological objects as well.
The Astrophysical Journal Letters
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