Introduction

Advances made by physicists in understanding matter, space, and time and by astronomers in understanding the universe as a whole have closely intertwined the question being asked about the universe at its two extremes—the very large and the very small. This report identifies 11 key questions that have a good chance to be answered in the next decade. It urges that a new research strategy be created that brings to bear the techniques of both astronomy and sub-atomic physics in a cross-disciplinary way to address these questions. The report presents seven recommendations to facilitate the necessary research and development coordination. These recommendations identify key priorities for future scientific projects critical for realizing these scientific opportunities.

Defining 11 Questions that Plague Physics at the 21st Century

Both disciplines—physics and astronomy—have seen stunning progress within their own realms of study in the past two decades. The advances made by physicists in understanding the deepest inner workings of matter, space, and time and by astronomers in understanding the universe as a whole as well as the objects within it have brought these scientists together in new ways. The questions now being asked about the universe at its two extremes—the very large and the very small—are inextricably intertwined, both in the asking and in the answering, and astronomers and physicists have been brought together to address questions that capture everyone's imagination.

 The answers to these questions strain the limits of human ingenuity, but the questions themselves are crystalline in their clarity and simplicity. In framing this report, the committee has seized on 11 particularly direct questions that encapsulate most of the physics and astrophysics discussed here. They do not cover all of these fields but focus instead on the interface between them. They are also questions that we have a good chance of answering in the next decade, or should be thinking about answering in following decades. Among them are the most profound questions that human beings have ever posed about the cosmos. The fact that they are ripe now, or soon will be, further highlights how exciting the possibilities of this moment are. The 11 questions are these:

1.  What Is Dark Matter?

 Astronomers have shown that the objects in the universe, from galaxies a million times smaller than ours to the largest clusters of galaxies, are held together by a form of matter different from what we are made of and that gives off no light. This matter probably consists of one or more as-yet-undiscovered elementary particles, and aggregations of it produce the gravitational pull leading to the formation of galaxies and large-scale structures in the universe. At the same time these particles may be streaming through our Earth-bound laboratories.

2.  What Is the Nature of Dark Energy?

Recent measurements indicate that the expansion of the universe is speeding up rather than slowing down. This discovery contradicts the fundamental idea that gravity is always attractive. It calls for the presence of a form of energy, dubbed “dark energy,” whose gravity is repulsive and whose nature determines the destiny of our universe.

3.  How Did the Universe Begin?

There is evidence that during its earliest moments the universe underwent a tremendous burst of expansion, known as inflation, so that the largest objects in the universe had their origins in subatomic quantum fuzz. The underlying physical cause of this inflation is a mystery.

4.  Did Einstein Have the Last Word on Gravity?

Black holes are ubiquitous in the universe, and their intense gravity can be explored. The effects of strong gravity in the early universe have observable consequences. Einstein's theory should work as well in these situations as it does in the solar system. A complete theory of gravity should incorporate quantum effects - Einstein's theory of gravity does not, or explain why they are not relevant.

5. What Are the Masses of the Neutrinos, and How Have They Shaped the Evolution of the Universe?

Cosmology tells us that neutrinos must be abundantly present in the universe today. Physicists have found evidence that they have a small mass, which implies that cosmic neutrinos account for as much mass as do stars. The pattern of neutrino masses can reveal much about how nature's forces are unified, how the elements in the periodic table were made, and possibly even the origin of ordinary matter.

6. How Do Cosmic Accelerators Work and What Are They Accelerating? 

Physicists have detected an amazing variety of energetic phenomena in the universe, including beams of particles of unexpectedly high energy but of unknown origin. In laboratory accelerators, we can produce beams of energetic particles, but the energy of these cosmic beams far exceeds any energies produced on Earth.

7. Are Protons Unstable?

The matter of which we are made is the tiny residue of the annihilation of matter and antimatter that emerged from the earliest universe in not-quite-equal amounts. The existence of this tiny imbalance may be tied to a hypothesized instability of protons, the simplest form of matter, and to a slight preference for the formation of matter over antimatter built into the laws of physics.

8.  What Are the New States of Matter at Exceedingly High Density and  Temperature?

The theory of how protons and neutrons form the atomic nuclei of the chemical elements is well developed. At higher densities, neutrons and protons may dissolve into an undifferentiated soup of quarks and gluons, which can be probed in heavy-ion accelerators. Densities beyond nuclear densities occur and can be probed in neutron stars, and still higher densities and temperatures existed in the early universe.

 9.  Are There Additional Space-Time Dimensions?

In trying to extend Einstein's theory and to understand the quantum nature of gravity, particle physicists have posited the existence of space- time dimensions beyond those that we know. Their existence could have implications for the birth and evolution of the universe, could affect the interactions of the fundamental particles, and could alter the force of gravity at short distances.

10.  How Were the Elements from Iron to Uranium Made?

Scientists' understanding of the production of elements up to iron in stars and supernovae is fairly complete. Important details concerning the production of the elements from iron to uranium remain puzzling.

11.  Is a New Theory of Matter and Light Needed at the Highest Energies? 

Matter and radiation in the laboratory appear to be extraordinarily well described by the laws of quantum mechanics, electromagnetism, and their unification as quantum electrodynamics. The universe presents us with places and objects, such as neutron stars and the sources of gamma ray bursts, where the conditions are far more extreme than anything we can reproduce on Earth that can be used to test these basic theories.

Concluding Remarks

Each question reveals the interdependence between discovering the physical laws that govern the universe and understanding its birth and evolution and the objects within it. The whole of each question is greater than the sum of the astronomy part and the physics part of which it is made. Viewed from a perspective that includes both astronomy and physics, these questions take on a greater urgency and importance.

Taken as a whole, the questions address an emerging model of the universe that connects physics at the most microscopic scales to the properties of the universe and its contents on the largest physical scales. This bold construction relies on extrapolating physics tested today in the laboratory and within the solar system to the most exotic astronomical objects and to the first moments of the universe. Is this ambitious extrapolation correct? Do we have a coherent model? Is it consistent? By measuring the basic properties of the universe, of black holes, and of elementary particles in very different ways, we can either falsify this ambitious vision of the universe or establish it as a central part of our scientific view.

Science, remarkable in its richness, cuts across the traditional boundaries of astronomy and physics. It brings together the frontier in the quest for an understanding of the very nature of space and time with the frontier in the quest for an understanding of the origin and earliest evolution of the universe and of the most exotic objects within it.

Realizing the extraordinary opportunities at hand will require a new, crosscutting approach that goes beyond viewing this science as astronomy or physics and that brings to bear the techniques of  both, astronomy and physics, telescopes and accelerators, and ground- and space-based instruments. The goal then is to create a new strategy.

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