**Searches for stochastic gravitational waves.**
Stochastic gravitational-wave backgrounds can be created in the early universe from amplification of vacuum fluctuations following inflation, phase transitions in the early universe, cosmic strings and pre-Big Bang models.
Stochastic gravitational-wave foregrounds, meanwhile, can be created from the superposition of astrophysical sources such as core-collapse supernovae, protoneutron star excitations, binary mergers and the persistent emission from neutron stars.
We use data from the LIGO and Virgo interferometers in order to search for stochastic gravitational waves.
Recent upper limits on an isotropic stochastic gravitational-wave background are the first to improve on indirect limits inferred from Big Bang nucleosynthesis and measurements of the cosmic microwave background.
We have also published results using the most sensitive initial LIGO and Virgo data and using co-located LIGO interferometers.

**Searches for long gravitational-wave transients.**
Gravitational-wave transients lasting from seconds to weeks may be associated with sources such as young neutron stars following core-collapse supernovae, flares associated with isolated neutron stars and binary systems.
We study the properties of such sources of long transients and look for their signatures in data from the LIGO and Virgo interferometers.
We have run a search for long transients associated with GRBs in LIGO S5 data and an all-sky search for long transients in S5 and S6 data.

**Studying gravity-gradient noise at Homestake.**
Seismic noise and fluctuations in the local gravitational field (called Newtonian, or gravity gradient noise) are large enough to limit the sensitivity of gravitational-wave detectors that operate on the surface. For this reason, it is likely that the next generation of detectors will be built underground. We are conducting a detailed study of seismic and Newtonian noise using an array of seismometers deployed at the Homestake Mine in Lead, SD. The results of this study will be folded into the design of the next generation of gravitational-wave detectors. Studies on Wiener subtraction are ongoing. We have also published a study characterizing the Homestake seismic environment. A link to our wiki page is here.

**Instrumental development.**
The sensitivity of (terrestrial) gravitational-wave interferometers at low frequencies (below ~10 Hz) are limited in part by seismic noise.
However, it is desirable to extend the operating band of detectors to lower frequencies (0.1-10 Hz), where a large number of gravitational-wave sources is expected to be.
We are studying the feasibility of new techniques to limit seismic noise at low frequencies.

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**93**, 042005 (2016), arXiv: 1511.04398. - "New technologies in gravitational-wave detection," S. Ballmer and V. Mandic, Ann. Rev. Nucl. Part. Sci.
**65**, 555 (2015). - "Gravitational waves from Scorpius X-1: a comparison of search methods and prospects for detection with advanced detectors," C. Messenger, H. J. Bulten, S. G. Crowder,
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**726**, 66 (2013), arXiv: 1212.4165. - "Search for long-lived gravitational-wave transients coincident with long gamma-ray bursts," LIGO Scientific Collaboration and Virgo Collaboration, Phys. Rev. D
**88**, 122004 (2013), arXiv: 1309.6160. - "Accessibility of the stochastic gravitational wave background from magnetars to the interferometric gravitational wave detectors," C. Wu, V. Mandic, and T. Regimbau, Phys. Rev. D
**87**, 042002 (2013). - "Parameter estimation in searches for the stochastic gravitational-wave background," V. Mandic, E. Thrane, S. Giampanis, and T. Regimbau, Phys. Rev. Lett.
**109**, 171102 (2012), arXiv: 1209.3847. - "Accessibility of the gravitational-wave background due to binary coalescences to second and third generation gravitational-wave detectors," C. Wu, V. Mandic, and T. Regimbau, Phys. Rev. D
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**107**, 271102 (2011), arXiv: 1109.1809. - "Gravitational-Wave Stochastic Background from Kinks and Cusps on Cosmic Strings," S. Olmez, V. Mandic, and X. Siemens, Phys. Rev. D.
**81**, 104028 (2010), arXiv: 1004.0890. - "Characterization of the seismic environment at the Sanford Underground Laboratory, South Dakota," J. Harms
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