ASTROPHYSICS


[ Astrophysics Experiment ]    [ Astrophysics Theory ]

ASTROPHYSICS EXPERIMENT
Gravitational Waves
Space tells matter how to move, matter tells space how to curve. This is the main conclusion that can be drawn from Einstein's theory of general relativity. Any change in a distribution of masses will change how space curves. These changes in curvature will send ripples in the form of gravitational waves through space time, much like water waves are generated when two boats are circling each other.

The motion of distributions of mass which can generate potentially measurable amounts of gravitational waves are all galactic or cosmic in nature. These sources include binary systems, ranging from neutron star binaries to mergers between super-massive black holes, supernovae, spinning neutron stars (pulsars), and the echo of the big bang. A Detection of gravitational waves will a) confirm their existence and b) open a new observational window to the universe. The University of Florida is involved in two large projects dedicated to the detection of gravitational waves:

LIGO: The University of Florida is a member of the Laser Interferometer Gravitational-wave Observatory (LIGO) project. The LIGO detector consists of separate laser interferometers in Washington State and Louisiana. These interferometers make extremely precise measurements of the distance between mirrors (called “test masses”) located 2.5 miles (4 kilometers) apart. If the test masses are subjected to a gravitational wave, their positions will change in a small, but well-defined way; it is this resulting motion that LIGO is designed to detect. Known sources of gravitational waves in the LIGO observational window include neutron star mergers, supernovae, and pulsars.

The initial LIGO detectors reached displacement sensitivity on the scale of atometers and put important upper limits on several potential gravitational wave sources. The interferometers are currently undergoing a minor upgrade before the next measurement campaign, with improved sensitivity, will start. In 2011, all components of the interferometer will be replaced to improve the sensitivity of LIGO 10-fold.

Florida’s role in LIGO is to build, deliver, and maintain the so-called "input optics." This is one of the most complex parts of the detector, being all of the components between the laser source and the main interferometer. We are also working on analysis of the data from initial LIGO, on sensing and control aspects, on the design of the upcoming LIGO upgrades, on methods for cooling that could lead to lower nose backgrounds, and are involved in measurements of gravity gradient noise in the new underground national laboratory DUSEL. Gravity gradient noise is one of the limiting noise sources in ground-based gravitational wave detectors and is expected to be significantly lower underground than above ground, suggesting an underground detector, perhaps operating at cryogenic temperatures, could be the ultimate ground-based instrument.

LISA: Gravity gradient noise and seismic noise will limit the performance of ground-based gravitational wave detectors at very low frequencies (even underground). The only known solution to this problem is to go to space. The Lisa Interferometer Space Antenna (LISA) is a joint NASA/ESA mission aimed at detecting low-frequency gravitational waves in space. LISA will fly 3 spacecraft in a triangular formation with 5 million km arm length. The formation will be in an earth-like, helio-centric orbit that trails earth by 20 degrees. LISA will measure gravitational waves from mergers between super-massive black holes, the burst of gravitational waves when small black holes are swallowed by massive black holes, and the waves from galactic white dwarf and neutron star binaries long before they merge (and become a LIGO signal). LISA is expected to be launched in 2018.

Our group has developed and is using an experimental testbed to test LISA interferometric techniques, data correlation techniques, and data reduction schemes. We are also characterizing the stability of various ultra-low-expansion materials, testing potential telescope designs, and studying laser communication techniques for LISA.

Dark Matter
Many observations imply the existence of large halos of non-luminous matter surrounding galaxies. It seems probable that much of the dark matter is non-baryonic; leading candidates are finite-mass neutrinos, weakly interacting massive particles, and axions.
ADMX: The University of Florida is a member of a team conducting a search for axions. The discovery of the axion, or placing strong limits on its existence, would have profound implications for two of the most important problems in contemporary physics: (i) the origin of CP symmetry in the strong interactions, and (ii) the composition of the dark matter that makes up approximately 90% of the mass of the universe. The Axion Dark-Matter eXperiment (ADMX) searches for axions as a component of the dark-matter halo of our galaxy. The axion is special in the sense that a laboratory experiment can be carried out with current technology that can detect the particle at the expected level of abundance. The experiment exploits the fact that axions may be stimulated to convert into microwave photons in a microwave cavity threaded by a large magnetic field. This detection method was proposed by Prof. Sikivie of our department and was developed during pilot experiments at Brookhaven National Laboratory and at the University of Florida. The detector, built and operated by a collaboration of Lawrence Livermore National Laboratory, the University of Washington, the University of California at Berkeley, and Florida, consists of a large superconducting magnet containing one or more microwave cavities; axions that enter the high-field region will be stimulated to decay into microwave photons when the resonant frequency of the cavity equals the mass of the axion. The detector is tuned by changing its resonant frequency; it is planned to cover about a decade in mass range over the next few years.

The Axion group and the LIGO/LISA group are currently teaming with a group a Fermi National Laboratory to build a new axion detector probing a different mass scale. This experiment will use laser fields and optical cavities in strong magnetic fields to generate and then detect axions.

CDMS: WIMPS have a natural explanation in the supersymmetric extension to the Standard Model of particle physics, and are believed to represent the lightest stable supersymmetric particle. The Cryogenic Dark Matter Search Experiment (CDMS) aims to directly detect the interactions of WIMPS (in the galactic halo) with nuclei (in our particle detectors). Such interactions will deposit a small amount of energy, ~ 10 keV, in the detector, characteristic of an elastic scattering between the Weakly Interacting Massive Particle and the nucleus. The two main challenges in detecting WIMPs are a) to accurately measure the small energy deposition, and b) to distinguish “genuine” WIMP interaction from the vastly larger rate of undesired backgrounds. Both issues are addressed by using superconducting thermometers. Fig A shows a photograph of one of the dark matter detectors. The detectors can distinguish between an interaction with a nucleus (due to a WIMP) and an interaction with the atomic electrons (due to background events) thus allowing the search for the rare dark matter interactions (see Fig B).

The University of Florida is involved several aspects of the CDMS experiment including detector development and data analysis, and Monte-Carlo simulations. The detector development requires a study of condensed matter phenomena (at temperatures of ~10 mK) to understand the behavior of the phonons, electron-holes, and quasiparticles that contribute to the signal. The Monte-Carlo simulation and data analysis use typical particle physics techniques to extract and measure the properties of a potential dark matter signal from the collected data.

To date, the CDMS experiment has not detected any events that are consistent with dark matter interactions. With this "negative result" we are able to rule out a large number of possible theories, which aim to predict the properties of dark matter. CDMS is currently in the process of increasing the total number to detectors in order to extend the sensitivity of the experiment and it is quite possible that a dark matter signal will be detected within the next 5 years, helping shed some light on the particle that makes up 25% of the Universe.

Neutrino and X-ray Astrophysics:
Micro-X and MARE-II: The university of Florida is involved in the development of microcalorimeters. These devices use a superconducting thermometer to measure an energy deposition of ~ 1 keV in an absorber with an energy resolution of better than 1 eV. These devices can be used as spectrometers for X-ray telescopes such the Micro-X mission. In conjunction with MIT, NASA, and several other institutions, the University of Florida is involved in the construction of the micro-X mission which aims to launch, by 2012, an X-ray telescope on a sounding rocket to observe the Puppis-A supernova remnant. The same devices can also be used to determine to mass of the electron neutrino, which is the goal of the MARE-II experiment. By accurately measuring the endpoint of the Re beta decay the mass of the neutrino can be determined. By fabricating the absorber from Re, a radioactive material, the spectrum of the Re beta decay, with an endpoint of ~2 keV, can be measured. The excellent energy resolution of the miocrocalorimeters will allow a mass sensitivity of 1 eV or better.




ASTROPHYSICS THEORY
Stellar variability has been found to be the norm rather than the exception and practically all stars undergo some phase of pulsationally unstable behavior during their evolution. Our study of the nonlinear pulsational behavior of stellar models uses a mixture of tools ranging from brute force numerical hydrodynamics to sophisticated dimensional reduction techniques. Recently, numerical hydrodynamic remodeling has been able to uncover a period doubling bifurcation sequence followed by chaotic behavior in population II Cepheid model sequences. The implication is that irregular variability is an intrinsic property of the dynamic of these stars and that we now have begun to understand the so-called Irregular Variables. A concomitant effort in nonlinear signal processing analyzes observational data for low dimensional dynamical structure.

One of the grand outstanding problems in stellar structure and evolution concerns convection. We have embarked on 2 and 3 dimensional modeling of turbulent convection in stellar envelopes. This is expected to lead to a better understanding of the structure and pulsations of Cepheids and RR Lyrae stars.

Exciting observational and theoretical developments in astronomy and astrophysics have focused attention on the probability that astronomical systems with strong gravitational fields play important roles in the astrophysical universe. Locally, considerable effort is currently devoted to deciphering the physical properties of collapsed stars, dense star clusters, and black holes; and to understanding better the interaction of such objects with their environment. Topics of specific current interest include accretion onto collapsed objects, the dynamical and secular stability of general relativistic objects and the effects of rotation on the properties of systems in general relativity and in Newtonian gravity. Such compact objects are governed by general relativity, but that theory has been well tested only in the solar system and in some binary pulsars. A current interest is to study possible tests of the theory in the strong field regime near the massive black hole that resides at the center of our galaxy.

A new window on the universe is expected to open during the next decade. NSF’s LIGO project the proposed LISA mission in space have the capability to detect gravitational waves from some of the most unusual objects in the universe. Local theoretical interest focuses on binary black hole systems, a leading possible source of detectable waves. One important example is that where one hole is much more massive than its companion. Such extreme-mass-ratio binaries permit clean theoretical modeling via perturbation theory and allow for detailed understanding of the possible astrophysical sources. For more general systems with comparable masses, an approximation known as post-Newtonian theory is used to analyze the orbit and gravitational wave signal, both within general relativity and alternative theories of gravity.

The distribution of matter on the largest observable scales and the large-scale cosmological structure itself are now subject to many observational probes on different scales and at different times, including primordial and induced anisotropies of the cosmic microwave background radiation, the angular and spatial distributions of optical- and infrared-selected galaxies, absorption along the line of sight of light from distant quasars, and weak and strong gravitational lensing.

There is a growing consensus that the universe on large scales is homogeneous and isotropic, spatially flat, with significant amounts of cold dark matter and dark energy, quintessence, or cosmological constant. On more moderate scales, fluctuations become important. Local efforts at expanding our understanding include studying statistical correlations of the mass and of galaxy number distributions and the effects of foreground scattering on the microwave background, including nonlinear evolution in the halo model. An area of increasing activity has been the common ground at which particle physics and astrophysics meet in cosmology. Local efforts in cosmology focus in part on higher-dimensional theories of gravity, the gravitational effects of structures arising from phase transitions in the early universe, baryon generation, and the origin and development of the structure in the universe.

Current efforts in the Physics Department also focus on studying the environment of Solar and Extra solar system planets with special interests toward the atmospheres of gas giants. Recent projects involve modeling of propagation and dissipation of atmospheric waves and their impact on the energy and momentum budget of Jupiter and Saturn. We study the response of the ionospheric plasma as a wave propagates through the upper atmosphere and we are developing techniques that would allow us to detect the presence of waves on other planets and study their spectral characteristics. To do this we analyze data from ground-based stellar occultations by planets, spacecraft-based radio occultations, in-situ observation from atmospheric probes as well as observations of the planetary thermal emission. We are also involved in studying the structure and the properties of the cloud/haze layers of the atmospheres of Jupiter and Saturn based on infrared observation by NASA planetary missions (Cassini and Voyager).