Experiments at the Cosmic Frontier take advantage of naturally occurring events to observe fundamental physical processes that are impossible to investigate in terrestrial laboratories. Researchers use detectors to examine high-energy particles that approach Earth from space such as gamma and cosmic rays. They also search for weakly interacting massive particles, or WIMPS, which are hypothetical particles that may constitute dark matter.
Gamma rays are particles of light that are millions to hundreds of billions times more energetic than light in the visible spectrum. They typically come from powerful astrophysical phenomena such as supermassive black hole systems and rapidly spinning neutron stars.
Cosmic rays are fast-moving particles, mostly the nuclei of atoms or electrons, that constantly bombard the Earth from all directions. The particles can enter Earth's atmosphere with many millions of times more energy than any particle scientists are able to create. With each passing decade, scientists have discovered higher-energy and increasingly more rare cosmic rays. No one knows the origin of the high-energy cosmic rays that regularly bombard the Earth.
To learn about the nature of these cosmic particles, scientists measure their energy and their direction as they arrive from space. They can measure particles directly by sending detectors above most of Earth's atmosphere, using high-flying balloons and satellites. But in some cases, researchers hope to observe particles that rarely appear, some only once a year in each kilogram of space. Then it is more efficient to build large, earthbound detectors that can observe rare particles as they interact with the atmosphere.
When fast-moving particles strike air molecules in the Earth's atmosphere, debris flies from the collision in what is called an air shower. Fragments hit other air molecules in a cascade that continues until the energy of the original particle is spread among millions or possibly billions of particles raining down on Earth. By studying these air showers, physicists can measure properties of the original particles.
For detectors in search of slow-moving rare particles, such as WIMPs, the key to observation is the ability to filter out distracting signals from other particles raining down from space.
In 1993, Fermilab physicists proposed the construction of the world's largest cosmic ray detector, the Pierre Auger Observatory, to address the question of the origin of high-energy cosmic rays. Researchers previously had assumed that cosmic rays approach the Earth uniformly from random directions. However, in 2007, researchers at Pierre Auger announced that the most energetic cosmic rays to impact the Earth generally come from the direction of active galactic nuclei.
Many large galaxies, including our own Milky Way, have a supermassive black hole in their centers. While most black holes will sit quietly in the nucleus of a galaxy for billions of years, if a galaxy's black hole happens to be surrounded and fed by a steady stream of gas and stars, it creates what is called an active galactic nucleus. An active galactic nucleus releases high-energy radiation observable by radio, x-ray and gamma ray telescopes on Earth.
Outer space is not empty. For example, on average, one electron floats in every cubic centimeter of space between the planets and stars in our solar system. Between the galaxies, one can find a diffuse sea of photons and other particles. When cosmic rays travel through this medium, they sometimes interact with these particles.
When cosmic rays with high energies collide into other particles, they recoil and lose a fraction of their energy. Lower-energy cosmic rays can pass through these particles without interacting. The farther a cosmic ray travels through space, the greater chance it has of running into one of these particles.
Based on this information, scientists hypothesized that they would observe more low-energy cosmic rays than expected and fewer high-energy cosmic rays, since the high-energy cosmic rays would break down into lower-energy cosmic rays as they moved through space.
Detectors at the Pierre Auger Observatory confirmed this theoretical prediction. Because researchers know that high-energy cosmic rays cannot travel too far through space before colliding with something, they can set limits on how far away the source of a high-energy cosmic ray that makes it to the Earth can be. This helps astrophysicists decide where to look in space for the source of high-energy cosmic rays.
Scientists found the first evidence of dark energy in 1998 when they discovered through the observation of distant supernovae that the universe was expanding at an increasing rate. They had expected to find a slowing the rate of expansion due to the force of gravity. The observation to the contrary led them to theorize that another force was pushing the universe apart.
Fermilab researchers found a way to test the finding with the Sloan Digital Sky Survey. Through SDSS, scientists could observe large clusters of galaxies and connect them to fluctuations in the cosmic microwave background, mapped by a separate satellite called WMAP. The cosmic microwave background gives astrophysicists a picture of the universe as it was about 300,000 years after the big bang. Objects in space have left imprints on the cosmic microwave background in the form of areas of concentrated particles and energy. Imagine those hot spots as the light areas left on an old carpet when a sofa that has sat in one place for years is put in another room. SDSS measured the deflection light from the background hotspots, or the light spots on the carpet, as it passed by the foreground galaxy clusters, or furniture. From this, researchers deduced that the accelerating expansion of the universe was real.
Scientists compared the intensities of the hot spots left in the cosmic microwave background to the locations and sizes of clusters of galaxies they observed with the SDSS to determine how the universe has expanded since a time shortly after the big bang until the present epoch. The experiment confirmed that the universe has expanded at an increasing rate. This offered independent confirmation of the study that suggested the existence of dark energy.