Particle Physics

The initial conditions of our Universe can be summarized on a single sheet of paper. Yet the Universe is full of complex structures today, such as stars, galaxies and groups of galaxies. I will describe how complexity emerged in the form of the first stars out of the simple initial state of the Universe at early cosmic times. The future of the Universe is even more surprising. Over the past decade it was realized that the cosmic expansion has been accelerating. If this accelerated expansion will continue into the future, then within a hundred billion years there will be no galaxies left for us to observe within the cosmic horizon except one: the merger product between our own Milky Way galaxy and its nearest neighbor, the Andromeda galaxy.

The relic neutrino background contains a gapless, spin-2 sound mode, as well as a spin-1 mode if there is a neutrino-antineutrino asymmetry. The self-coupling of the spin-2 mode is given by Z boson exchange in the Standard Model and is parametrically similar to Newton's constant given the expected density of relic neutrinos. I will describe this emergent gravity theory and also describe how emergent theories avoid the Weinberg-Witten theorem, when the constituent degrees of freedom live in a flat Lorentz invariant space.