Searching the skies for holes in Einstein’s theory of general relativity
Einstein’s general theory of relativity describes the nature of gravity, perfectly describing the movement of astronomical bodies in our solar system. And yet it fails to explain galaxies unless they are held together by invisible mass, called dark matter. It also cannot account for the apparent accelerating expansion of the universe without invoking the presence of a dark source of energy. The origin of both dark entities in the universe remains unknown. “Fundamental physics can only explain a tiny part of the universe,” remarks project coordinator Catherine Heymans. The EU-funded GLOBE project: “Focused on using world-class data to explore whether going beyond Einstein with a new model of gravity could provide a compelling alternative explanation to the dark universe.”
An incomplete theory
Previously this would have been considered ‘heresy’, says Heymans. Yet despite decades of searching, nobody has yet found a dark matter particle, either in supercollider experiments or liquid xenon traps. The origin of dark energy remains even more elusive, driving astrophysicists like Heymans to question whether Einstein’s theory of gravity may be incomplete. “It’s the best-tested theory on Earth – satellite navigation wouldn’t work if it was wrong,” adds Heymans. “But Earth is a pretty strange space. Most of the universe is empty, there are huge voids between galaxies, and maybe gravity works differently there.” Heymans and her team at the University of Edinburgh took a two-pronged approach to challenge the prevailing theory of gravity. The main data was generated by the European Southern Observatory using the VST survey telescope in Paranal, Chile. This Kilo-Degree Survey (KiDS) was considered so important that its measurements were prioritised whenever the observatory experienced perfect weather conditions. KiDS mapped one fifth of the sky in the visible spectrum, from blue light to near infrared, probing galaxies as distant as 10 billion light years away. The GLOBE team then searched for any evidence of light being distorted by gravitational anomalies on its path toward Earth. “We looked at 30 million galaxies and used that data to map out where dark matter is in our universe,” says Heymans. “We can use that data to directly test Einstein’s theory of how mass curves space and time.” This data was combined with a second data set that mapped out the movement of galaxies in regions filled with dark matter.
The stars align
The team found that their data fit with Einstein’s predictions. “We’ve come to the conclusion that it’s very hard to break Einstein’s theory. If dark matter and dark energy exist, general relativity works really well across the whole universe,” notes Heymans. Yet she says that it’s important to keep interrogating it. ”You can never prove a theory, only disprove it!” A new telescope currently under construction in Chile, the 8.4 m Vera Rubin Observatory, will help to advance these efforts. In a matter of weeks, it will be able to collect the same data which took the GLOBE project 7 years to amass. All the methodology and techniques developed by the GLOBE project are being used for this much more powerful telescope. “The fact our current understanding of physics can’t explain dark energy or dark matter means it’s missing something quite big,” concludes Heymans. “A lot of people think to finally understand this, we need a revolution in physics.”
Keywords
GLOBE, relativity, Einstein, gravity, energy, dark matter, galaxies
