Gravitational distortion of time helps tell modified gravity apart from a dark force
Einstein’s equations described how stars, galaxies and all other matter curve or warp space and time.
- Einstein’s equations described how stars, galaxies and all other matter curve or warp space and time.
- The galaxies and the light rays then travel in this distorted space-time according to the equation provided by the 18th-century Swiss mathematician Leonhard Euler.
- But can we differentiate a universe where Einstein’s equations were violated from a universe where Euler’s equation were modified?
A universe filled with unknowns
- The reason to put those to the test comes from the fact that our universe is filled with unknowns.
- In the 1930s, the Swiss-American astrophysicist Fritz Zwicky observed that there was five times more matter in the universe than we can detect with our telescopes.
- Then, in 1998, two groups of astrophysicists observed that the expansion of our universe is accelerating, contrary to the deceleration expected because of the gravitational attraction between galaxies.
Vast distances of the universe
- Checking if Einstein’s gravity works over the vast distances of the universe has become an active field of research.
- Theoreticians propose new ideas for how gravity could work differently, while astronomers use increasingly advanced facilities to provide the data needed to test them.
- The latter would be different if, for example, the dark matter in the galaxies were subject to a new force.
Gravitational slips
- To our surprise, while both of us came into this thinking that the answer would be obvious, our initial answers were opposite to each other.
- Our conclusion was that, despite the common expectation, measuring the gravitational slip would not allow one to distinguish a modification of Einstein’s laws from a modification of Euler’s equation.
- So, if a galaxy were to fall into a gravitational potential in a way that was not consistent with our expectations, we would be unable to tell if it’s because the dark matter is doing something, or because gravity was modified.
Light and gravity
- These differences are extremely tiny but are, in fact, very important in the design of satellite navigation systems.
- When the light from a galaxy escapes the gravitational potential it is falling into, its colour shifts closer to red.
- In time, new technologies will be able to detect the tiny effects of gravitational redshift, and consequently distinguish a modification of Euler’s equation for dark matter from a modification of gravity.