The Quantum Universe Chapter 1 Introduction

What is the nature of the universe and what is it made of? What is matter, energy, space and time? How did we get here and where are we going?

The quest to answer the most fundamental questions about the universe has reached a singular moment. With the advent of the 21st century, physicists have gained a full understanding of the particles and forces that characterise the ordinary matter around us. At the same time, space observations in astrophysics and cosmology have shown that this picture of the universe is incomplete - 95% of the universe is made up not of ordinary matter, but of a mysterious something else: dark matter and dark energy. We already know that we don't actually know what most of the universe is made of.

Understanding this unknown "new" universe requires the discovery of the particle physics that determines its fundamental nature. There are powerful tools available to bring physics within reach. Through astrophysical observations, we can explore the parameters of the universe; through accelerator experiments, we can search for its quantum interpretation. The energy of particle accelerators is now close to the original conditions after the Big Bang, giving us the means to discover what dark matter and dark energy are - creating a revolution in our understanding of particle physics and the universe.

To answer fundamental questions about the nature of the universe, astrophysical observations of the remains of the Big Bang must be reconciled with experimental physics data that reproduce the particles and forces of the early universe. The two ends of the exploration must meet. We will answer these challenging questions by combining what we have learned from the most powerful and insightful observations and experiments of these approaches.

The following chapters delve into the nine key questions that define the field in succession. Chapter 2 clarifies the implications of these questions and their impact on particle physics in the 21st century. Chapter 3 defines the scientific plan for addressing and answering these questions. The table in Chapter 5 summarises the US facilities where the major physics programmes provide the most direct response to these questions.