Models of the universe as Einstein saw it

Einstein’s full theory of general relativity has been used to precisely model the universe’s evolution for the first time – and to expose its lumpy early years.

Independent teams from the US and Europe found regions of matter “clumped” together to start forming the first galaxies much earlier than thought. This could help explain the origin of the large-scale structure of the universe. 

“By assuming less, we’re seeing something new,” says John Giblin, a physicist at Kenyon College, Ohio and part of the US team.

The cosmic microwave background (CMB) radiation is a snapshot of the universe as it was just a few hundred thousand years after the Big Bang. It shows an incredibly homogeneous tapestry – as if the universe were one indistinct cloud of gas.

Yet now, 13.7 billion years later, the universe is composed of an intricate architecture: millions of stars congregate in galaxies, galaxies gang together in clusters and these clusters form superclusters that stretch across billions of light-years in filaments and sheets.

How the universe evolved from a homogeneous fog is one of the great unsolved mysteries of cosmology.

To tackle the problem, physicists use Einstein’s theory of general relativity which describes how mass warps space and time. Some 100 years after it was first written, it’s still our best theory of gravity.

Physicists routinely use computers to apply Einstein’s equations to individual objects such as black holes and neutron stars.

For instance, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves last September, computer models worked out the signal came from two colliding black holes 36 and 29 times the mass of the sun.

But physicists have struggled to apply the same computational techniques to the evolution of the universe as a whole. The problem is the mathematics of relativity are notoriously difficult to solve.

So they simplify their models using approximations. This way, physicists can see the general picture but lose the nuances – a bit like watching a low-resolution video online because your internet connection can’t handle full high-definition.

Now two teams, one in the US and one in Europe, separately wrote software to solve Einstein’s field equations – the nuts and bolts of general relativity – and model the universe in all its relativistic glory.

In their software, the teams created a homogeneous baby universe, then ran the clock forward, watching it evolve over billions of simulated years.

They saw regions of mass clumping together through gravity – a bit like droplets of water condensing out from a cloud of steam. But both teams found the first clumps in their precise models emerged “much earlier” than those predicted by approximate models – although they don’t specify exactly how much earlier.

The universe’s rate of expansion did not change as some astrophysicists had thought it might, so does not dispel dark energy as an essential component of our universe.

Stuart Shapiro, a physicist at the University of Illinois at Urbana-Champaign who was not involved in the work, says these works are important, not only for their results but also for being “forerunners” in applying this kind of computational modelling to explain the universe.

Want more? Both teams will make their software open source for other researchers to freely use and to improve.

The European and US teams published their work in Physical Review Letters.The US team also published a general synopsis of their work in in Physical Review D.