Sagittarius A

• Einstein’s theory of gravity — general relativity — has been very successful for more than a century.
• This is not surprising: the theory predicts its own failure at spacetime singularities inside black holes — and the Big Bang itself.

Deviations and quantum mechanics

• These signal that Einstein’s theory is failing there and must be replaced with a more fundamental one.
• Naively, spacetime singularities should be resolved by quantum mechanics, which apply at very small scales.
• This is enough to understand that a theory that embraces both general relativity and quantum physics should be free of such pathologies.
• However, all attempts to blend general relativity and quantum physics necessarily introduce deviations from Einstein’s theory.

Cosmology matters

• A century of research has given us the standard model of cosmology known as the Λ-Cold Dark Matter (ΛCDM) model.
• Here, Λ stands for either Einstein’s famous cosmological constant or a mysterious dark energy with similar properties.
• Dark energy was introduced ad hoc by astronomers to explain the acceleration of the cosmic expansion.
• Despite fitting cosmological data extremely well until recently, the ΛCDM model is spectacularly incomplete and unsatisfactory from the theoretical point of view.

Alternatives to Einstein’s theory

• That we are observing the first deviations from general relativity while the mysterious dark energy simply does not exist?
• This idea, first proposed by researchers at the University of Naples, has gained tremendous popularity while the contending dark energy camp remains vigorous.
• A very popular class of alternatives is the so-called scalar-tensor gravity.

The current situation

• Theorists have spent the last decade extracting physical consequences from these theories.
• The recent detections of gravitational waves have provided a way to constrain the physical class of modifications of Einstein gravity allowed.