Beautiful model to explain the universe to physicists

An international team of cosmologists has made the first step towards creating the most accurate ever model of the universe by simulating Einstein’s field equations, according to recent research published in Physical Review Letters.

Computer simulations are vital in cosmology and allow scientists to study and test theories about the evolution of the universe. In order to begin the task of modelling the universe, cosmologists make simple assumptions that the universe is homogenous and isotropic - that it looks the same in every direction regardless of the viewer’s position.

Two teams worked on the project. One was from the US, working at Case Western Reserve University and Kenyon College; the other was from Europe, drawing people from the University of Portsmouth and the University of Catania, Italy. Both teams have independently written software that models the various lumps and bumps of matter in the otherwise smooth fabric of space-time.

Previous models coded clumps of matter in the universe as large particles which grew and interacted using standard Newtonian gravity and uses assumptions that are too simplistic, Dr Marco Bruni, a Reader in Cosmology and Gravitation at the University of Portsmouth, told The Register.

The European team incorporated Einstein’s field equations and fluid dynamics equations with the Einstein Toolkit, a free open-source code that models general relativity. In his new model, however, Bruni models the universe in a box as an “inhomogeneous pressureless fluid” that evolves according to Einstein’s field equations - a set of equations that explain how spacetime and gravity interact. The equations are notoriously hard to solve but provide a more accurate model of how the universe works.

Cosmologists believe that Einstein’s cosmological constant could be dark energy and accelerate the expansion of the universe. It is not the only driver, however, and scientists believe that “backreaction” could also be important.

Backreaction is used to describe the pushing and pulling that large structures such as clusters of galaxies can assert on the fabric of spacetime. By taking the cosmological constant to be zero, Bruni’s model focuses only on the acceleration caused by backreaction and has found that its effects are negligible.

“This leaves the question open to what is driving the acceleration of the universe. The simplest explanation would be the cosmological constant but we cannot confirm this with this model,” said Bruni.

Bruni’s new model shows that the universe is inhomogeneous with clumps of matter and voids and space. The expansion of the universe is not simple; it accelerates at different rates at different locations. He compares the universe to the sea. At far distances it looks flat and smooth, but up close there are waves at different sizes and locations, he says.

“At small scales the universe has curvature, but overall it's pretty flat,” said Bruni. ®