Freese's work is in theoretical cosmology, at the interface of astrophysics and particle physics. This field has seen remarkable successes in the past decade, yet many questions remain, including: What is the universe made of? What is the dark matter? What is the dark energy? What makes the universe accelerate, both now and during an early period of inflation? Freese's research seeks to address these questions.
- DARK MATTER: Freese is investigating the nature of the dark matter, the primary (unknown) constituent of the mass in galaxies. She studied two types of explanations for this dark matter: faint stars and new exotic particles. Ten years ago, there were two camps of scientists with different beliefs about dark matter. Some believed that this dark matter could be faint stars, planetary objects, or dead stars. DEATH OF STELLAR MATTER: In a series of papers, Freese's theoretical work, combined with examination of various date sets, showed that these objects cannot explain the dark matter. In fact she argued that stellar remnants in the form of white dwarfs can constitute at most 20% of the dark matter in galaxies; a contribution of this magnitude is quite interesting.
Currently the favored dark matter candidates for the bulk of the mass of the Galaxy include supersymmetric (SUSY) particles. Freese was one of the founders of the field of calculations for SUSY dark matter detection experiments: she performed the original calculations of rates for WIMP (Weakly Interacting Massive Particle) particles in both direct and indirect detection experiments. Experiments all over the world use the ideas she first proposed. DIRECT DETECTION: Among other contributions, she first proposed the idea of using annual modulation of the signal in cryogenic detectors, a technique routinely used today and the basis of the claimed dark matter detection by the DAMA experiment (which, however, is almost entirely ruled out). INDIRECT DETECTION: Freese was the first to suggest WIMP annihilation in the Earth as a means to detect particles. STREAMS OF DARK MATTER: Galaxies form due to accumulation of smaller objects; hence they are far more lumpy than previously imagined. Freese investigated consequences in detectors of the Sagittarius stream, a stream of matter headed for the Solar System which was torn off of a small galaxy being shredded apart by the Milky Way.
DARK STARS: Freese recently discovered a new early phase of stellar evolution. Dark matter annihilation in the first stars gives rise to a heat source that overpowers all cooling mechanisms, preventing the further collapse of the stars. "Dark stars" result, which are giant stars powered by dark matter annihilation rather than by fusion. She is currently working on evolution and observable consequences of these objects.
- DARK ENERGY AND THE ACCELERATING UNIVERSE: Recent data indicate that the expansion of the universe is accelerating. A new paradigm is emerging in which 70% of the mass density of the universe is some kind of "dark energy.” One possibility for this dark energy is the cosmological constant that Einstein called his biggest blunder. Prior to this data, Freese in 1986 examined the possibility of a time-dependent vacuum energy; a variant known as quintessence is now a favored explanation of the dark energy. Recently Freese proposed Cardassian expansion, in which a modification of the Friedmann equation (Einstein's equations as applied to the universe) replaces any need for vacuum energy.
Freese also investigates what one can learn about dark energy from supernovae surveys. She suggested an approach to determine directly the time dependence of the dark energy density (rather than via the equation of state). While current data cannot yet ascertain the time-dependence, Freese studied what can be learned from future surveys about the evolution of the dark energy density.
- INFLATION: Freese proposed mechanisms for inflation, an early rapid growth phase of the universe that explains its large scale homogeneity. In rolling models of inflation, Freese quantified the required flatness and hence fine-tuning of the potentials. She proposed NATURAL INFLATION in 1990, which requires no fine-tuning, as shift symmetries can naturally provide the required flat potentials. This idea is seeing a resurgence of interest in the past few years in the context of string theory. In the past year Freese showed that the model successfully matches current measurements and makes testable predictions for tensor modes.
Recently Freese proposed a new approach to inflationary cosmology: CHAIN INFLATION. It consists of a series of tunneling events which each provide a fraction of the required amount of inflation, totaling at least 60 efolds at the end. Chain inflation can take place in the string theory landscape, and also with the QCD axion. Using the QCD axion to inflate is particularly exciting as this particle has been proposed for particle physics reasons and detectors are running to search for it.
- THE FUTURE OF THE UNIVERSE: Freese has investigated the fate of future life in an accelerating universe. Whereas life cannot persist if the dark energy is constant in time, as long as it decreases in time, life should be able to go on (but certainly not as we know it!).
- MAGNETIC MONOPOLES: Freese found the most stringent bounds on the flux of magnetic monopoles in the universe. Monopoles in neutron stars or white dwarfs catalyze nucleon decay, and would destroy these stars. In addition, she has the strongest bounds due to eating the magnetic fields in early phases of the Galaxy (these bounds are in the Particle Data Book). These bounds are important guideposts for experimental monopole hunters.
- DEVALUATION: A DYNAMICAL MECHANISM TO REDUCE THE VALUE OF THE COSMOLOGICAL CONSTANT: We proposed a natural solution to the cosmological constant problem consistent with the standard cosmology and successful over a broad range of energies. This solution is based on the existence of a new field, the devaluton, with its potential modeled on a tilted cosine. After inflation, the universe reheats and populates the devaluton's many minima. As the universe cools, domain walls form between different regions. The domain wall network then evolves and sweeps away regions of higher vacuum energy in favor of lower energy ones. Gravitation itself provides a cutoff at a minimum vacuum energy, thus leaving the universe with a small cosmological constant comparable in magnitude to the present day dark energy density.