The remarkable precision with which we know the history of our Universe is one of the unabashed successes of science. Decades of experimental, observational, and theoretical work have revealed a fascinating picture of the origin of matter. As we now understand it, about 13.7 billion years ago, a fraction of a second after the Big Bang, a brief and sudden period of rapid expansion, called inflation, froze small quantum mechanical fluctuations into a nearly homogeneous state of spacetime and inflated them to cosmological sizes. These frozen fluctuations acted as small seeds of attraction for all the surrounding matter that slowly clustered around them. Some 300,000 years later the background radiation encoded these small variations in tiny local changes in temperature, now measurable through its afterglow as the Cosmic Microwave Background (CMB).
For the next 7 billion years the matter itself continued to cluster to form stars and galaxies, following the very pattern laid down at the beginning. All this time the matter and radiation slowly tempered the expansion of the Universe, up to about 6.5 billion years ago. Around this time the slowing expansion surprisingly changed course and started accelerating again to give us the constellations on the sky we observe today.
This detailed picture is a comprehensive but phenomenological description of the full range of cosmological observations. The very detail of the experimental data has brought into sharp focus the fundamental gaps in our theoretical knowledge. The unity of physics demands that we integrate the cosmological description at the largest scales with the four fundamental forces (notably gravity) and the laws of quantum mechanics acting at the smallest scales. Among the fundamental questions, including how structure evolved in the universe and how matter came to dominate over antimatter, the D-ITP efforts focus on the following three challenges:
- What is the nature of the Big Bang?
- What is the nature of Dark Matter?
- What drives the expansion of the Universe?