This project focuses on searches for the stochastic gravitational-wave background (SGWB). In particular, searches will be conducted by cross-correlating the data acquired by the LIGO observatories during the latest LIGO science run (S5), and during the upcoming science run (S6) which will take advantage of the better sensitivity of LIGO detectors. These searches will establish the world's most sensitive measurements of the energy density of gravitational waves. Moreover, a study will be performed to assess the gravity gradient noise for Advanced LIGO: the gravity gradient noise is expected to limit the sensitivity of Advanced LIGO detectors at lowest frequencies, which are most sensitive to the SGWB.
The SGWB is expected to be produced in the earliest stages in the history of the Universe. Hence, direct observation of the SGWB would provide a glimpse of a very young Universe, much younger than what is possible using other astrophysical techniques. It would also represent a very rare probe of the particle physics of the highest energies, currently not accessible to accelerator-based particle physics experiments. The SGWB could also be produced by a large number of violent astrophysical events, such as gamma ray bursts or supernovae. Hence, direct observation of the SGWB would provide information about these exotic events as well. This project will allow undergraduate and graduate students, and postdoctoral scholars to be directly involved in this very exciting field of cosmology and astrophysics, and to make significant contributions to them. The project also includes building a web-accessible archive of models and experiments in the field of SGWB, hence providing a broader educational impact on the cosmology and astrophysics communities, as well as on non-scientific audience.
This project has produced the most sensitive upper limit on the energy density of the stochastic gravitational wave background (SGWB) in the frequency band between 40 and 170 Hz. The SGWB is expected to be produced in a variety of cosmological and astrophysical processes. For example, inflation is expected to generate the SGWB in the earliest moments after the Big Bang, implying that the observation of this background would provide unique information about the beginning of the universe, and about the physical laws that apply at highest energy scales. Similarly, the SGWB is expected to arise as a superposition of signals from many astrophysical objects, such as compact binary neutron stars and/or black holes, magnetars and others. The upper limit produced by this project for the first time surpasses the indirect bounds on SGWB due to the Big-Bang nucleosynthesis and Cosmic Microwave Background observations (in the LIGO frequency band), and it begins to directly constrain some of the cosmological and astrophysical models. These results were described in a paper published in the Nature journal. The project has also produced a new method for measuring spatial anisotropies in the SGWB - that is, for measuring fluctuations in the SGWB energy density in different directions on the sky. This method provides a more sophisticated way of searching for the SGWB and for understanding its properties. The method was applied to recent data acquired by LIGO, producing upper limit maps of the gravitational-wave sky, and producing the first spherical harmonic decomposition of the SGWB. These results are published in a Physical Review Letter. The project has also developed a new search pipeline targeting transient gravitational-wave signals on time scales of minutes, hours, or longer. While the gravitational-wave community (and the LIGO Scientific Collaboration) conducts many searches for transients, they typically target time scales of 1 second or shorter. The new pipeline takes the approach of cross-correlating two LIGO detectors on longer time-scales, producing time-frequency maps of the cross-correlation. The pipeline then applies different pattern recognition algorithms to find structures in these maps. Two papers have been published describing the capabilities of this pipeline. The pipeline is currently being applied in multiple astrophysical searches with LIGO data - for example, looking for gravitational waves in coincidence with electromagnetic observations of Gamma Ray Bursts, or with neutrino observations by detectors such as IceCube or Antares. Finally, the project produced a new technique for estimating multiple parameters in SGWB models (published in Physical Review Letters), and it conducted systematic studies of the accessibility of some selected SGWB models, such as the model due to compact binary coalescences (published in Physical Review D). These studies have demonstrated the substantial science potential of the SGWB searches to be conducted with the next generation of gravitational-wave detectors, and have identified promissing data analysis methods for realizing this science potential. This project supported the work of a postdoctoral scholar and three graduate students, and it enabled research of several other graduate and undergraduate students (supported by other sources). These students and postdoc had opportunities to conduct frontier research in the fields of gravitational waves and astrophysics, as well as to learn a number of data analysis techniques, including data processing, statistical methods, pattern recognition algorithms, various software platforms, and working with large computing clusters. They played leading roles in writing of several papers mentioned above. The project also introduced the Interactions in Understanding the Universe (I2U2) program to the Twin Cities area, by holding a workshop for the local highschool teachers to educate them about the program, and by interacting with the teachers as they introduced the program in their classrooms. This program enables highschool students to access some of the environmental data acquired by LIGO, and to perform various studies with it.
