matter and ultimately collapsing to form an object like a galaxy or cluster of galaxies.

gravitational lens:

an object in which rays of light from a distant astronomical source are deflected by the gravitational pull of an intermediate mass that may be a galaxy or a cluster of galaxies. The deflection causes a distortion in the image of the distant source and sometimes also leads to multiple images.

gravitational lensing:

a consequence of Einstein’s general relativity theory is that the path of light rays can be bent by the presence of matter. Astronomers have observed that the light from a distant galaxy or quasar can be “lensed” by the matter in an intervening galaxy to form multiple and often distorted images of the background object.

gravitational wave:

according to general relativity, a ripple in the geometry of space-time propagating as a wave.

gravitational wave background:

gravitational waves arriving from so many sources that the individual signals are indistinguishable.

graviton:

an as-yet-undetected massless particle that carries the gravitational force.

Great Attractor:

a region or structure of huge mass (equivalent to tens of thousands of galaxies) exerting a gravitational pull on the surrounding galaxies, including the Milky Way, proposed to explain the observed movement of these galaxies toward the Hydra-Centaurus superclusters in the southern sky with velocities significantly different from those predicted by Hubble law expansion.

hadron:

a strongly interacting particle such as a proton or neutron.

halo:

the matter surrounding a galaxy.

Hawking radiation:

when the effects of quantum mechanics are included in the analysis of black holes, it turns out that they are not, strictly speaking, black but rather radiate energy. This phenomenon is called Hawking radiation.

horizon:

edge of the portion of the universe visible to us. Light signals from beyond this point have not had time to reach Earth yet.

HST:

NASA’s Hubble Space Telescope, an optical/infrared telescope launched in 1990.

Hubble’s law:

the principle that any two distant celestial objects (e.g., galaxies) move away from each other at a speed that is proportional to the distance between them, due to the homogenous expansion of space.

hyperon:

a subatomic particle that is a quasi-stable member of the class of particles known as baryons and that is more massive than the nucleons (protons and neutrons).

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)