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New flowchart to eliminate Universe models

Cosmologists have many possible models for the Universe, of which only one can be true. A new flowchart will eliminate some of them when two specific Universe features are accurately measured. Publication in Physical Review D on 7 November.

Cosmologists try to understand how the entire Universe formed and evolves. In short, cosmology is the science of everything, except for that pale blue dot floating around in the vastness of space. By aiming their telescopes at distant galaxies and the afterglow of the Big Bang, cosmologists look back in time and pick up pieces of the puzzle. They put these pieces as parameters in the many possible models they have created for our Universe. The more parameters are precisely measured, the more models can be excluded.


Lately, astrophysicists have done many observations to measure two specific parameters. These are called μ and Σ. They represent how fast galaxies formed from the irregularities in the Universe just after the Big Bang and how much distant light is bent by gravitational lensing. Recently measured values of μ and Σ generally show a mild tension with the leading cosmological model called ΛCDM. More and better observations will zone in on an accurate value of the two parameters. Things would get very interesting if the observed μ and Σ indeed did not agree with the values expected in the popular ΛCDM model. So then what?


Leiden University physicist Alessandra Silvestri and Levon Pogosian from the Simon Fraser University publish a paper on in Physical Review D with an overview of all the models we can rule out in case of each value that will be measured. In a flowchart they answer questions with ‘yes’ and ‘no’, leading to the subsequent conclusion. For example, if μ is greater than one and Σ is less, then a large collection of models is ruled out; the so-called Horndeski class. With their paper, they add more significance to future cosmological observations, as it puts concrete meaning in the measurement of otherwise abstract values.


Levon Pogosian and Alessandra Silvestri, ‘What can cosmology tell us about gravity? Constraining Horndeski gravity with Σ and μ’, Physical Review D


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