It is an exciting time for fundamental physics. The last few decades have treated us to scintillating glimpses into the underlying fabric of our universe.
The discovery of the Higgs Boson at the Large Hadron Collider in 2012 rounded out the Standard Model of Particle Physics. The Higgs Boson gives all the fundamental particles that we know of their masses (except neutrinos). With the Higgs Boson discovered, we now have fixed predictions for what comes out on the other side when we smash all known particles into each other. New colliders are being designed and constructed to find even heavier or more weakly interacting particles. Lighter particles are being pursued with lasers, force sensors, cooled gasses, and other precision equipment. It will be most interesting if these particle observations demand an extension of the Standard Model — this would begin the process of revising the rulebook by which reality operates.
Meanwhile, our understanding of the universe has undergone a complete revolution in the last 60 years. In 1964 radio astronomers discovered a primordial bath of microwave photons, which exists everywhere in our universe. This cosmic microwave background was emitted 14 billion years ago, when primordial protons cooled down and captured electrons, forming the first hydrogen atoms in a terrific flash of light. Looking closely at these microwaves has allowed us to precisely reconstruct the first few seconds of existence in our universe. Very detailed sky maps of the cosmic microwave photons have been obtained using the COBE, WMAP, and PLANCK satellites in the last two decades. By looking at fluctuations in these photons across the sky, we now know the proportions of known matter, dark matter, and dark energy in our universe. With these fluctuations, along with maps of galaxy locations, networks of quasars, and other astronomical tracers of matter, we have assembled a concordance history of our universe, the “LambdaCDM” cosmological model, where CDM stands for collisionless dark matter, and Lambda is dark energy.
Even with all this information, the first few seconds of our universe remain a mystery. A discovery of dark matter’s non-gravitational interactions in the coming decade would not only revolutionize our understanding of the present day universe, but also illuminate the earliest moments of our universe. Dark matter was likely produced before big bang nucleosynthesis of Helium and other well known heavy elements, which is the last moment in time for which we have ample data.
In addition, we are at the advent of understanding the overarching structure of the Standard Model of particle physics. Some basic questions remain unanswered. Do all the forces unify into one force at high energies? What does gravity do at high energies? How exactly does quantum chromodynamics keep quarks inside nucleons? Can a neutrino annihilate itself and what gives it a mass?
- The identity of dark matter. We have discovered dark matter through its gravitational interactions in galaxies, clusters of galaxies, and its imprint in the cosmic microwave background, but the exact nature of dark matter remains a mystery.
- Matter and antimatter. We do not know what physics in the early universe caused the observed asymmetry in matter versus antimatter.
- Neutrino masses. There are myriad mechanisms to explain the observed tiny masses for neutrinos; these all require physics beyond the Standard Model.
- Physics underlying dark energy. Our simplest calculations for the amount of dark energy in the universe predict so much dark energy that our universe should not exist. We may need new theoretical tools to address this.
- Non-locality and quantum gravity. While we have remarkably good descriptions of quantum fields and gravity, it is increasingly clear that these theories do not always play nicely with each other near black hole horizons.