equivalence principle:

a fundamental principle of general relativity, one of whose consequences is that all objects (and light) fall in a gravitational field in the same way independent of their internal structure or other properties. This universality of free fall is one of the most accurately verified principles in physics.

eV:

an electron-volt, a measure of energy equal to that gained by an electron passing through a potential difference of 1 volt; also a unit of particle mass when divided by the speed of light (c) squared. Electrons have a mass of about 0.511 MeV/c² (million electron-volts); protons have a mass of about 938 GeV/c² (billion electron-volts).

event horizon:

the surface of a black hole. It is a one-way membrane, allowing matter or signals to flow in but not out.

Fermilab:

The U.S. Department of Energy’s Fermi National Accelerator Laboratory, located in Batavia, Illinois.

fermion:

particles with the property that only one can occupy a quantum state (the Pauli exclusion principle). Such particles have half-integer values of spin.

flat universe:

a universe where space is uncurved and described by the geometry of Euclid.

forces of nature:

the four basic forces of physics: gravity, electromagnetism, and the weak and strong interactions.

freeze-out:

the disequilibrium by which relics are formed in the universe.

galaxy:

a large assemblage of stars. Our own galaxy, the Milky Way, contains 10¹¹ stars.

gamma ray:

electromagnetic radiation more energetic than x rays.

gamma-ray bursts:

bursts of gamma rays from cosmic sources observed by detectors on satellites. Several hundred are detected per year, and they range in duration from fractions of a second to several seconds. Most gamma ray bursts come from objects at cosmological distances.

general relativity:

Einstein’s theory of gravity in which the gravity is the curved geometry of space and time.

gluon:

a massless particle that carries the strong force.

grand unification era:

the era when the universe cooled sufficiently for gravity to be described by Einstein’s general relativity theory, but where the temperature was still sufficiently high that the other remaining three forces of nature remained unified.

grand unified theories:

theories that combine the strong, electromagnetic, and weak interactions into one unified theory.

gravitational collapse instability:

the process whereby a small lump in an expanding universe can grow under gravity, pulling in surrounding

Front Matter (R1-R16)

Executive Summary (1-8)

1 Introduction: Where We Are and Where We Can Be (9-14)

2 Foundations: Matter, Space, and Time (15-42)

3 How Are Matter, Space, and Time Unified? (43-59)

4 How Did the Universe Get Going? (60-77)

5 What Is the Nature of Dark Matter and Dark Energy (78-104)

6 What Are the Limits of Physical Law? (105-131)

7 Realizing the Opportunities (132-172)

Appendix A: Meeting Agendas (173-184)

Appendix B: Call for Community Input (185-186)

Appendix C: Project Proposals Received (187-190)

Appendix D: Glossary and Acronyms (191-206)