We propose an upgrade of Advanced LIGO (aLIGO), named LIGO-LF, that focuseson improving the sensitivity in the 5-30 Hz low-frequency band, and we explorethe upgrade's astrophysical applications. We present a comprehensive study ofthe detector's technical noises, and show that with the new technologies suchas interferometrically-sensed seismometers and balanced-homodyne readout,LIGO-LF can reach the fundamental limits set by quantum and thermal noises downto 5 Hz. These technologies are also directly applicable to the futuregeneration of detectors. LIGO-LF can observe a rich array of astrophysicalsources such as binary black holes with total mass up to 2000 M_\odot. Thehorizon distance of a single LIGO-LF detector will be z ~ 6, greatly exceedingaLIGO's reach. Additionally, for a given source the chirp mass and total masscan be constrained 2 times better, and the effective spin 3-5 times better,than aLIGO. The total number of detected merging black holes will increase by afactor of 16 compared with aLIGO. Meanwhile, LIGO-LF will also significantlyenhance the probability of detecting other astrophysical phenomena includingthe gravitational memory effects and the neutron star r-mode resonances.
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