Probing gravity and dark energy in the era of multi-messenger cosmology.

 It is now an exceptional time for modern cosmology, when we can observe the universe with high precision and connect cosmological measurements with theory. The excitement about the advances of observational cosmology is accompanied by the awareness that we face a mix of long standing challenges (nature of dark matter, dark energy and the physics of inflation) with deep roots in particle theory and gravity, as well as new tantalizing "curiosities". I will focus on the challenge posed by dark energy and the quest of testing gravity on cosmological scales. I will discuss recent progress in identifying the optimal theoretical framework to tackle these in the context of large scale structure missions as well as direct detection of gravitational waves. I will also address the so-called Hubble tension, and present some recent work where we have explored it "from first principles".