Understanding the Universe

An artist's impression of Supernova SN 2006gy, the brightest supernova recorded to date.

Some of humankind's greatest intellectual challenges have to do with understanding how the universe began, how it works, and how it will end. A study by the National Research Council, Connecting Quarks to the Cosmos, produced a list of eleven questions that are crucial to advancing this understanding. Research at the National Ignition Facility could help answer five of these questions:

What is the nature of dark energy?

Scientists now believe that more than two-thirds of the universe consists of dark energy, a mysterious force that may be causing the universe to expand at an ever-faster rate. Attempts to deal with this question are in their infancy, but are currently more directed toward how dark energy acts than what it is. As a result, it will continue to be studied through one of the means by which it was discovered: using "Type Ia" supernovae—exploding stars—as the "standard candles" or measuring sticks with which cosmological distances are determined.

But confidence in these measurements requires a detailed understanding of how these supernovae explode. Remarkably, computer simulations have shown that they are subject to the same hydrodynamic instabilities that affect inertial confinement fusion, the process at the heart of much of NIF's research (see How to Make a Star). NIF, then, will provide a unique way to study and understand these instabilities in the laboratory.

These same hydrodynamic instabilities also affect core-collapse supernovae, a different class of exploding stars that is also of great interest to astrophysicists. Understanding the explosion mechanism of these "Type II" supernovae has been a persistent problem in astrophysics for several decades. Current efforts appear to be focusing on instabilities as at least a factor in these huge explosions.

“The next two decades could see a significant transformation of our understanding of the origin and fate of the universe, of the laws that govern it, and even of our place within it.”
—Committee on the Physics of the Universe,
National Research Council

Did Einstein have the last word on gravity?

According to Albert Einstein's General Theory of Relativity, gravity results when any object with mass, such as our sun, warps the space and time around it. Determining if Einstein was right necessitates studying the objects in the universe that provide the most extreme gravitational effects—the supermassive objects in the center of galaxies called black holes, whose gravity is so strong that not even light can escape. Of course, black holes cannot be studied in the way that astronomers study most objects, by observing the light that they emit. The matter that swirls into a black hole, however, will be subject to an extraordinarily hostile environment. This means that this matter will be in a highly ionized state—the atoms will have been stripped of nearly all of their electrons. The radiation that results will be characteristic of those ions, so understanding them will provide a vast amount of information. Such ions are difficult to study in the laboratory, however, because the conditions that produce them are so extreme. NIF will be able to produce such ions in its intense x-ray environment and will provide data that will directly affect our ability to understand what is going on in the matter that surrounds a black hole.

Ions in very high ionization states also affect other astrophysical environments, so the studies that will further our understanding of black holes will also provide insights into other astrophysical sites, such as the accreting neutron stars that produce x-ray bursts, and the regions around active galactic nuclei. The detailed knowledge of highly ionized ions will provide essential information about temperatures and densities in such extreme environments.

How do cosmic accelerators work and what are they accelerating?

The highest-energy cosmic rays in the universe have energies of around 10²⁰ (100 quintillion) electron volts, many orders of magnitude greater than the highest energies that can be achieved in modern particle accelerators. How cosmic rays achieve those energies is not known, however; this is one of the questions posed by the NRC. NIF will produce extraordinary electric and magnetic fields in its most energetic shots, and thus should provide an environment in which particles acting in such extreme fields can be studied.

What are the new states of matter at exceedingly high density and temperature?

NIF will provide the highest temperatures and densities that have ever been created in a laboratory environment, enabling experiments to produce states of matter unlike any previously achieved. These studies clearly will improve our understanding of materials in extreme conditions, and may also further our knowledge of stellar evolution, as well as of the inner structure of the largest planets such as Jupiter and Saturn.

How were the elements from iron to uranium made?

The stellar processes that synthesize, or create, the different isotopes of the heavy elements have been studied for several decades, yet science has not identified the exact location where the process that creates half of the heaviest elements, known as the r-process (for "rapid"), occurs. The properties of this process are well established by the nuclear physics of the nuclei synthesized in it—but whether it occurs in core collapse supernovae, in colliding neutron stars, or in some other site is not yet known. NIF will be able to shed light on this question in several ways. It may provide an environment in which some of the reactions that affect the r-process can be studied—primarily because of the high energy density that NIF will achieve. It also may, because of its very high neutron density (as high as 10³³ neutrons per cubic centimeter per second), even be able to create some of the neutron-rich nuclides that will help scientists better understand the properties of those nuclei as they are synthesized during the r-process.

NIF will enable other studies in nuclear astrophysics besides those associated with the r-process. NIF's extreme conditions will make it possible to study nuclear reactions at energies that would be difficult to achieve in experiments with a beam from a particle accelerator and a conventional target. This is primarily a result of NIF's extremely high density, but it also depends on some other special features of the NIF environment.

These studies are discussed in more detail in Science at the Extremes.

More Information

Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century, Committee on the Physics of the Universe, National Research Council, 2003