Fermilab A Plan for Discovery

Chapter 5
The Cosmic Frontier

The Cosmic Frontier

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SDSS image of Galaxy n4753

In the universe's vast web of galaxies, black holes grab matter at the speed of light into cataclysmic space-time vortices and fling off particles at energies millions of times greater than any laboratory accelerator. Particles of invisible dark matter are a critical element in the formation of galaxies. A mysterious dark energy accelerates the expansion of space between galaxies. Such extraordinary things never happen in a laboratory, but they can reveal deeply hidden details of fundamental physics beyond the Standard Model.

At the Cosmic Frontier, scientists investigate the relationship of matter with gravity and spacetime and search for new physics beyond the reach of particle accelerators. Fermilab pioneered particle physics at the Cosmic Frontier and has continued to develop the connection between the very large and the very small for almost 30 years. Cosmic Frontier experiments make use of Fermilab's world-leading technical capabilities, such as high-precision, low-noise silicon detector technology, large and ultra-clean vacuum and cryogenic systems, fast electronics, and large-scale data management, analysis and simulation tools. Fermilab serves as the host institution for global Cosmic Frontier experiments, providing scientific, technical and administrative support for widely distributed international scientific collaborations.

Over the next two decades, Fermilab will lead the Dark Energy Survey and participate in the Large Synoptic Survey Telescope as they explore the nature of dark energy; support the search to directly detect dark matter particles by playing a major role in four experiments using different technologies; study the highest-energy cosmic rays with the Pierre Auger Observatory; and take experimental physics to the Planck scale with the Fermilab Holometer.

Theoretical Physics at the Cosmic Frontier

Fermilab theorists explore the connections between new ideas in fundamental physics and experiments at the Cosmic Frontier. They compute the interactions of particle dark matter in accelerators and experiments; estimate the measurable effects of various possible forms of dark energy in Dark Energy Survey data; and calculate how the properties of the modern universe, such as cosmic background radiation, depend on theories of the inflationary beginnings of the Big Bang. Close collaboration with theorists is important in all stages of experiments, from first conceptualization to the final analysis and interpretation of data. The synergy between Cosmic Frontier and the Energy and Intensity Frontiers, and the various theorists who work in these areas, has never been stronger at Fermilab than it is today.

Dark Energy

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Dark Energy Camera telescope simulator

Fermilab leads the international Dark Energy Survey collaboration, which is deploying the 570-megapixel Dark Energy Camera on the 4-meter telescope at the Cerro Tololo Interamerican Observatory in Chile.

Measurements of the distant cosmos reveal a new force that appears to accelerate the expansion of the universe and makes up most of its total energy content. This mysterious dark energy can be studied through large-scale precision measurements of the expansion and structure of the universe using massive surveys—maps of the cosmos extending far into space and back in time.

Fermilab's connection to dark-energy research began with its role as the anchor laboratory for the Sloan Digital Sky Survey. The SDSS, the highest-impact astronomy facility of the 2000s, was the world's first large, deep digital survey of the universe. The SDSS pioneered precision cosmology and made many important contributions to dark energy measurements. Fermilab now leads a survey with SDSS that is setting new standards for precision and error calibration in the use of supernovae to measure very large cosmic distances.

In this decade, Fermilab's dark-energy research program is focused on the Dark Energy Survey, a deeper, more precise successor to the SDSS. Fermilab leads the international DES collaboration, which is deploying a powerful new instrument called the Dark Energy Camera on the 4-meter telescope at the Cerro Tololo Interamerican Observatory in the Chilean Andes. Starting in 2012, and over the course of 525 nights through 2017, the DES will map about 5,000 square degrees of the sky. The final petabyte-scale survey database will comprise approximately 300 million galaxies and will look back in time over more than half of the age of the universe.

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Measurement uncertainties in dark energy behavior

Sensitivity of the Dark Energy Survey to equation-of-state parameters of dark energy: w denotes the ratio of mean dark energy pressure to density of the cosmic vaccuum state and wa is the derivative of w with respect to cosmic scale factor. DES will use a variety of complementary techniques to study dark energy, including baryon acoustic oscillations (BAO), galaxy clustering, weak gravitational lensing (WL) and supernovae (SN).

The DES will probe dark energy using a variety of complementary techniques: galaxy clustering, weak gravitational lensing, baryon acoustic oscillations and supernovae. Together they will provide unprecedented probes of the dark-energy equation of state and its effects on the expansion of the universe and on the growth of cosmic structure. These probes will allow scientists to distinguish the effects of various possible forms of dark energy from each other. For example, they will test whether cosmic acceleration is due to the energy of the vacuum or a more exotic form of dark energy, or if it is due to a new theory of gravity.

The deep DES sky survey will be even more powerful if it obtains spectra of many of its galaxies, providing velocity and 3D position information. Fermilab and its collaborators are developing the technology to create a catalog of 10 to 100 million spectra that will provide an exquisitely sensitive probe of dark energy's effects on structure growth over time.

The next step in cosmic surveys after the DES is the Large Synoptic Survey Telescope, which should start its survey in the early 2020s. Now in the final stages of planning and design, this project proposes to create a dedicated telescope and camera system that will survey the universe significantly wider, deeper and faster than the DES. LSST will combine a larger mirror (8 meters), a bigger field of view (3 gigapixels over 10 square degrees) and a faster cadence to make a dedicated survey of the universe. Fermilab brings its decades of experience in digital surveys to the large consortium of institutions developing this project.


Dark Matter

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Cryogenic Dark Matter Search

Assembly of the CDMS detector in its cryostat in Minnesota's Soudan Mine. The next-generation SuperCDMS experiment will be deployed in Canada's SNOLAB.

Since the 1930s astronomers have accumulated evidence that the mass that holds galaxies and clusters together with its gravity far exceeds the amount of mass in normal atoms in any detectable form. Scientists suspect that this gravity comes from an invisible "dark matter" made of some new kind of elementary particle left over from the early universe. A leading postulate, based on Big Bang cosmology and natural extensions of Standard Model physics, is that the dark matter is made of Weakly Interacting Massive Particles. WIMPs would weigh more than atomic nuclei but would interact only by the weak force and gravity. There is hope that some of them might one day be produced in accelerators, but they have never been directly detected.

Fermilab is a leader in the worldwide hunt for dark matter particles. In collaboration with international partners, the laboratory plays a major role in four kinds of experimental searches. These experiments, which are based on a variety of technologies, all seek to detect the very rare interactions of dark-matter particles from the Galaxy's halo with nuclei of ordinary matter using very low-background detectors placed deep underground.

Fermilab is a lead laboratory in the Cryogenic Dark Matter Search, which uses ultra-cold silicon and germanium crystals to achieve a high level of control over unwanted backgrounds. The experiment is entering a new phase with the deployment of 10 kilograms of advanced detectors in its existing facility at the Soudan Underground Laboratory in Minnesota. The 10-kg SuperCDMS experiment will take data until about 2013. A next-generation 100-kg experiment is planned for Canada's SNOLAB, with the goal of improving sensitivity to dark-matter particles by a factor of 100 without any backgrounds generated by normal-matter particles.

The Chicagoland Observatory for Underground Particle Physics uses bubble-chamber technology, allowing for relatively inexpensive scaling to larger volumes and flexibility in target material. A 4-kg COUPP chamber is operating at SNOLAB, and a 60-kg chamber being tested underground at Fermilab will be deployed in Canada in 2012. Designs for a 500-kg system are underway.

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Pierre Auger Observatory

Seventeen countries contributed to the construction of Auger, which uses 1,600 water tanks and 24 fluorescence telescopes to study the highest-energy cosmic rays.

Fermilab contributes key technology for the DarkSide project that is based on a liquid-argon detector similar to that being developed for Fermilab's long-baseline neutrino experiments. A 50-kg chamber is now under construction that will start operations in 2013 at the Gran Sasso National Laboratory in Italy. Larger follow-up experiments are planned. A new Fermilab project called DAMIC is based on the same CCD detectors used to build the Dark Energy Camera. In the DAMIC technology, very fine-grain imaging of particle tracks allows eYcient diagnosis of unwanted particle backgrounds and an extended sensitivity to low WIMP masses. An experimental system is currently under development underground at Fermilab, and will be deployed to the much deeper SNOLAB site in 2012.

Over the next few years, the next generation of experiments will be able to either detect these particles for the first time—revealing a new kind of matter and opening a new way to study the universe—or to exclude many of the theoretical ideas that lead to the WIMP hypothesis. In the next decade, based on the results of those experiments, some of these technologies will be deployed with a ton or more of detector mass, leading to a conclusive test.

High-Energy Cosmic Particles

The universe accelerates particles to energies far greater than human-made accelerators can, reaching collision energies nearly 100 times greater than those achieved at the Large Hadron Collider. Although the highest-energy particles are extremely rare, they can be studied by large arrays of ground-based detectors. Their composition, interactions and natural history provide a unique window into high-energy particle interactions and the extreme sources in the universe that accelerate them.

Fermilab is the lead laboratory supporting the Pierre Auger Observatory in Argentina, the world's premier facility for studying the highest-energy cosmic rays. Auger's international collaboration has published many groundbreaking results on the composition, interactions, anisotropy and spectrum of ultra-high-energy cosmic rays. The highest-energy events can be statistically traced back to their sources in the sky, the nuclei of distant galaxies.

Quantum Spacetime

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The Planck scale

Experiments beyond the Planck scale probe a deeper level of quantum spacetime. In this diagram, log of length is plotted against log of mass-energy. At upper left, the equation relating wavelength and energy for a quantum particle, Einstein's photoelectric formula. At upper right, the equation relating the radius and mass of a black hole, predicted by Einstein's theory of spacetime. These two realms cross at the Planck scale.

A new experiment called the Fermilab Holometer will study physics at the Planck scale, the deepest layer of reality we can explore with the tools of conventional physics. The Planck scale defines the intersection of two realms of physics: the quantum world, which describes the fundamental particle/wave behavior of all forms of mass and energy; and spacetime physics or relativity, which describes the large-scale space and time within which the mass and energy move and transform.

Planck-energy particles are impossible to study at particle accelerators because they have such high energies—about 1016 TeV. The Fermilab Holometer will study Planck-scale physics indirectly by detecting directionally coherent quantum noise in spacetime position. With collaborators from the gravitational-wave community, Fermilab is developing this new experimental capability using intense, ultra-stable laser cavities and interferometers. The Holometer will start taking data in late 2012 and will run for several years.

The target precision of the Holometer is set by the Planckian amplitude of predicted "holographic noise" in spacetime. If holographic noise exists, it causes the difference of position compared in two different directions to wander randomly, by about a Planck length every Planck time. Although the Planck length itself is far too small to see directly, the effect of the accumulated wandering over the light crossing time of the 40-meter apparatus—a few attometers in a fraction of a microsecond—can be detected. The holometer will find out whether or not such noise exists in nature. If it does, it will be our first experimental glimpse beyond spacetime as we know it.