Space is kind of lumpy!
The observable universe (everything we can see) stretches roughly 92 billion light-years across, and it has likely been expanding for more than 13.8 billion years. Astronomers map the universe using the equations from Albert Einstein’s General Theory of Relativity, published 100 years ago. It remains the best theory we have for explaining how gravity emerges from the curvature of space and time. It has even successfully predicted the phenomenon of gravitational waves.
However, as you can imagine, the equations of general relativity are difficult to solve. That’s why physicists have been using simplified equations in order to apply Einstein’s theory of the universe. But now, thanks to research teams from the United States and Europe, Einstein’s complete, unsimplified theory could be used to model the vastness of space in its entirety for the first time.
Turns out, it’s lumpy.
Two separate research teams created computer models to build the most detailed map of the universe to provide insight into gravity and its effects and to get a better understanding of the evolution of the universe. The new codes are "the first" to use the complete theory of relativity, and "account for the effects of the clumping of matter in some regions [...] and the lack of matter in others," as explained in a press release from Case Western Reserve University.
“Both we and the other group examine the universe using the full theory of general relativity, and have therefore been able to create more accurate models of physical processes than have been done before,” said James Mertens, a doctoral student in physics at Case Western who took the lead in developing and implementing the numerical techniques for the US team.
The groups created software to solve Einstein’s equations at billions of places and times throughout the history of the universe. Comparing the outcomes of these new numerical simulations to the outcomes of traditional simplified models, the researchers discovered that approximations break down in the simplified versions. However, more work is needed to fully understand these differences.
“By assuming less, we’re seeing something new,” said John T. Giblin Jr., an adjunct associate professor of physics at Case Western.
Slice of a galaxy profile representing the space-time background shaped by the distribution of matter. Regions of blue color contain more matter, while regions lacking matter are darker. Photo credit: James Mertens
This new approach should also provide insight into things such as gravitational lensing. "This is a really exciting development that will help cosmologists create the most accurate possible model of the universe," said Marco Bruni, an astrophysicist who works at the Institute of Cosmology and Gravitation and co-author of the European paper, to Wired.