Underground Universe: Physics and Astrophysics
Some of the 10,000 light-sensing photomultipliers used to detect neutrino interactions in a water vessel under 2 km of rock. The small white objects are plastic mounting points spaced 1 m apart and attached to an acrylic vessel so transparent it cannot be seen.
Source: Lawrence Berkeley Laboratory
- What is the universe made of?
- What is dark matter?
- What are neutrinos telling us?
- What happened to the antimatter?
- Are protons unstable?
- How did the universe evolve?
The last decade in physics and astrophysics has defined fundamental questions about the universe, with its elementary particles and forces and its mysteries of neutrinos, dark matter and dark energy. Underground experiments will play a unique part in addressing these questions. The answers will revolutionize the human understanding of the cosmos. Why go below the surface to probe the universe? Some signals of the revolutionary new physics will only reveal themselves in experiments in the shelter of the deep underground.
WHAT IS THE UNIVERSE MADE OF?
(IT'S NOT WHAT WE THOUGHT.)
In recent years, astronomical observations have revealed that most of the universe is not made from ordinary matter. Scientists have made the startling discovery that the atoms that make up the stars, the planets and people are in the minority in the universe. Photons (particles of light) and ghost-like neutrinos outnumber everyday atoms by a factor of about a billion to one. In terms of mass, the mystery substance called dark matter outweighs ordinary matter five to one. If that were not bizarre enough, the empty space of the universe is filled by a strange force, termed "dark energy," that pushes the universe apart at ever- accelerating speed. Even the existence of matter itself is a puzzle. Strictly speaking, in the inferno of the Big Bang, antimatter should have annihilated the matter, leaving only energy in the form of photons and neutrinos, yet matter indubitably exists. However, the world of ordinary matter makes up only four percent of a universe so mysterious that it will take a revolution in physics to explain it. Underground experiments, together with observations in space and experiments at particle accelerators, will play a key role in this revolutionary physics.
WHAT IS DARK MATTER?
Astrophysical observations, including studies of the behavior of stars and galaxies, have over the past decade established that 73 percent of the mass and energy of the universe is dark energy, and 23 percent is dark matter, called "dark" because it is invisible. Without it, galaxies would not have formed, the stars would not shine, and life would not exist.
Source: NASA/ESO
What is this dark matter that binds the galaxies? Although physicists have studied ordinary matter–atoms–in detail, nothing they have seen so far has the right qualities for dark matter. Discovering what dark matter really is stands as one of the major challenges in science today. Intriguing new theories of elementary particles suggest that dark matter might consist of undiscovered neutral particles, either much heavier than the proton or much lighter even than neutrinos. Discovering such particles would not only shed light on dark matter but solve other longstanding problems in elementary particle physics.
If the dark matter all around us is indeed an unknown heavy particle, scientists believe that all it should take is an ultrasensitive device to see the signal produced when a dark matter particle hits an atom in a detector–in a place that is quiet enough for the tiny signal to be picked up. The challenge with direct detection of dark matter is that environmental noise from cosmic rays can mimic its feeble signal. To avoid the noise, experiments must go deep underground. The deeper the experiment, the more protected it is from cosmic noise. Physicists can only claim that they have detected dark matter when they are completely certain that the signal is real, not merely noise. Large detectors at great depth have the best chance of yielding an unmistakable signal of dark matter particles.
Scientists also plan to produce dark matter particles in the laboratory, using high-energy particle colliders to recreate the conditions of the early universe when today's dark matter particles were born. Collider experiments will attempt to produce dark matter particles and measure their properties in detail. Although these experiments are also expected to shed considerable light on dark matter, one key element will be missing. Accelerator experiments will not tell us if the collider-produced particles are the same as those that make up the actual dark matter of the universe. For that, direct detection of cosmological dark matter particles is required.
Should dark matter particles both be detected directly in deep underground experiments, and produced at an accelerator such as the Large Hadron Collider, it will be an extraordinary achievement for physics. The properties of the particles will be known and their place in the universe understood. It will mark a giant step towards the "theory of everything."
WHAT ARE NEUTRINOS TELLING US?
Although physicists first detected neutrinos in 1956, these elusive particles remain almost as enigmatic as dark matter. For many years, physicists believed that neutrinos had zero mass and always moved at the speed of light. Underground experiments of the past decade, though, have shown that in fact neutrinos do have a mass, and that they will eventually come to rest as the universe expands and cools. The mass is at least 200,000 times smaller than that of any other matter particle. Moreover, physicists learned that neutrinos have mixed identities; one type of neutrino morphs into another and back. These discoveries represent great advances in solving the mysteries of neutrinos. Yet physicists know the masses of the neutrinos only within a broad range. Collectively, the neutrinos made in the Big Bang assuredly outweigh the luminous stars. And exactly when they slow down and come to rest has critical implications for the formation of superclusters of galaxies, the largest structures in the universe.
Underground experiments would allow physicists to zero in on the exact mass of neutrinos by looking for an extremely rare nuclear transformation called neutrinoless double beta decay. A quiet environment underground, sheltered from the noise of cosmic rays, is crucial to detecting this extraordinarily rare event, if it occurs. Another underground neutrino experiment would use beams of neutrinos from a distant particle accelerator aimed at an underground detector to decode which of the masses of the three different types of neutrino is the heaviest and which the lightest. Combining these two techniques would reveal the ghostly hand of neutrinos in shaping the universe.
WHAT HAPPENED TO THE ANTIMATTER?
It's a good thing antimatter does not exist in today's universe. When matter particles meet antimatter particles, they annihilate into pure energy. Conversely, Einstein's E=mc2 shows that with a high enough energy, pairs of matter and antimatter particles are created. At the super-high energy of the Big Bang, antimatter particles must have been created, presumably in equal amounts with matter particles. And yet no detectable signs of antimatter particles survive in the universe today. Where did the antimatter go? If the amounts of matter and antimatter were the same at the Big Bang, they should have annihilated each other, leaving the universe empty of matter–an outcome that clearly did not happen. A yet-to-be-found process must have reshuffled the matter-antimatter balance, transforming one part in a billion of antimatter to matter, with the result that the universe–and we–survived. So far, although scientists have caught glimpses of matter-antimatter asymmetry, they have not seen anything that could account for the dominance of matter over antimatter.
What exactly do scientists look for? The goal is to find evidence that antimatter is not just some sort of mirror image of the matter in the universe. They look for differences in the behaviors of matter and antimatter. Neutrinos produced by an accelerator can morph, or oscillate, into a different type of neutrino on their way to a detector many hundreds of kilometers away. Scientists measure the oscillation rate for these neutrinos and compare it to the oscillation rate for antineutrinos produced at the same accelerator. A difference in these rates shows that there are neutrino processes in nature that distinguish antimatter from matter. A second key ingredient is to show that nature actually permits changes in the relative amounts of matter and antimatter. A direct way to find that out would be the discovery of neutrinoless double beta decay, in which two new matter particles (electrons) are created from the energy available in a nucleus. Discovering the asymmetry in the accelerator test, along with the observation of neutrinoless double beta decay, would provide the data to show how the universe survived the Big Bang.
Source: Hitoshi Murayama
ARE PROTONS UNSTABLE?
Another possibility to explain the existence of matter is the decay of a proton (matter) into a positron (antimatter). Indeed, unified field theories, the kind of theories Einstein dreamed of, predict that such a process does happen. However, current data have shown that it happens extremely seldom–less than once in 1034 years for a given proton. To have a chance of spotting proton decay, researchers need to collect more than 1036 protons (for example in a million tons of water) and watch them carefully over many years in a quiet underground location. The huge detector needed in this quest is also one that can detect neutrinos beamed toward it from an accelerator thousands of kilometers away. The discovery of proton decay would shed light not only on how matter prevailed over antimatter, but also on the nature of matter and forces at the most fundamental level.
HOW DID THE UNIVERSE EVOLVE?
Since stars are made of conventional atoms, perhaps they present no scientific mysteries? Not quite. Using light, astrophysicists see only the bright surfaces of stars, but neutrinos offer a view directly to their cores. Observations of a tiny fraction of the neutrinos from the sun have revealed the temperature at its center, 15 million degrees, to the amazing precision of only two percent. But the sun's true nature remains imperfectly understood. Precisely how does the sun generate life-giving energy? Does its energy output vary slightly over thousands of years? To address these questions, scientists need to take a direct look into the center of the sun, impossible with light but possible with neutrinos. Neutrinos are very difficult to detect, and only the most energetic neutrinos from the sun have been studied extensively. To see the majority of neutrinos from the sun, again scientists need a very quiet underground location to discover how much energy the sun is generating now.
The intensity of muons created by cosmic rays as a function of the depth in feet of standard rock (density 2.65) or in meters water equivalent, m.w.e. (density 1.00). The world's underground laboratories are shown as dots, and the area of the dot is proportional to the area available for science at each lab.
The universe sometimes experiences cataclysmic events, for example the merging of two black holes. Such events may be impossible to see with telescopes, but they have such a huge impact on space and time that ripples of bending spacetime spread out from the massive event. Protected from the disturbance of human and seismic noise, underground gravitational wave detectors can see the resulting small bends in spacetime.
"We are made of star stuff," as the late astrophysicist Carl Sagan once put it. Each atom of our bodies was processed through many generations of stars before condensing to make our solar system and earth–and to make us. Understanding the birth and death of stars is an important part of our quest to understand the origin of life in the universe. To address the questions of how the atomic nuclei that form us were made, scientists use small underground particle-accelerator laboratories well shielded from cosmic rays.
Particle astrophysicists have detected a handful of neutrinos from a supernova, a dying massive star in a nearby galaxy. Neutrinos must also exist from past supernovae in galaxies near and far. If scientists can detect them, they will learn how many supernova explosions have happened in the past and hence how galaxies have evolved over billions of years. By detecting neutrinos from past supernovae, underground experiments may even shed light on the expansion history of the universe and hence on the nature of dark energy.
