The next frontier for cosmologists: dark energy

May 5, 2001 -- The recent announcement of the discovery of a distant supernova - the catastrophic explosion of an ancient star, caught in the act by the Hubble Space Telescope - has provided strong evidence to confirm one of the most unexpected scientific discoveries in recent years. The expansion of the universe is accelerating, and the cause is thought to be due to a new form of energy with unusual anti-gravitational properties called "dark energy." Robert Caldwell, Assistant Professor of Physics and Astronomy at Dartmouth, is one of a few cosmologists trying to develop and test theories to better understand this mysterious dark energy.

It was long thought the cosmic expansion was slowing down, as if the Big Bang was running out of gas. Now it appears the universe has recently shifted into overdrive, and in the process overturned long-held ideas in cosmology, the study of the origin, evolution and composition of the universe.

"Dark energy is a mystery," said Caldwell. "One of the most basic questions we want to answer is whether the dark energy density is constant in time and uniformly distributed throughout space, or is it dynamic?" There is a widespread suspicion among physicists that identifying the properties of dark energy will help unlock mysteries about the ultimate properties of matter, and therefore help us learn about the earliest moments of the universe.

"It's a great time to be working in cosmology. We're living through a time of breakthrough observations and experiments, prompting fundamental questions and startling theories," said Caldwell.

This turn of events has great implications for physics of the very large (cosmology) and the physics of the very small (high energy particle physics). As an example, the synergy between these disparate fields can be seen in the marriage of nuclear physics and general relativity to describe the conditions in the universe in the first seconds of the Big Bang. Ramping up the energy of colliding protons or electrons in particle accelerators (such as at Fermilab or CERN), physicists have been able to explore new phenomena that must have been prevalent at even earlier times in the universe.

"It was thought that you could only make new discoveries exclusively at high energies," Caldwell said. "But considering the cold, rarified and accelerating universe today, there may be ultra-low energy phenomena yet to be discovered which would open a door to a new world of fundamental physics."

The most direct way to search for clues to the nature of dark energy sounds like a page from a football playbook: go deep and go wide. Dark energy has its most direct effect on the cosmos through the rate of expansion, so measuring astrophysical phenomena like supernovae or the abundance of galaxies at different points along deep probes of space should reveal information about the evolution of the universe. Unless the dark energy really is constant, and Caldwell doesn't think so, theorists expect to find wiggles and lumps in the distribution of the dark energy, which in turn influence both the distribution of galaxies and the temperature patterns of the cosmic microwave background (CMB). So wide angle maps of galaxies and the CMB may carry tell-tale signs of a dynamical dark energy. Satellites, present and future, should provide the raw data to help with this research.

Dark energy theories can be tested in other ways, too. The fundamental constants, like the strength of gravity or electromagnetism, may not be fundamental after all. There might be a slight time- or spatially-dependent drift in the force of attraction between two masses or two charges, which may be connected to the rate of change in the dark energy density. And if that's not exciting enough, there may be more than four dimensions to spacetime - the dark energy may be lurking in extra spatial dimensions, ubiquitous and yet hidden from grasp except by its anti-gravitational push. The implications are mind-boggling, but if any turn out to be true, they would reveal worlds of information about fundamental physics and the nature of the universe.