Gravitational wave detector with cosmological reach

Twenty years ago, construction began on the Laser Interferometer Gravitational-wave Observatory (LIGO). Advanced LIGO, with a factor of 10 better design sensitivity than Initial LIGO, will begin taking data this year, and should soon make detections a monthly occurrence. While Advanced LIGO promises to make first detections of gravitational waves from the nearby universe, an additional factor of 10 increase in sensitivity would put exciting science targets within reach by providing observations of binary black hole inspirals throughout most of the history of star formation, and high signal to noise observations of nearby events. Design studies for future detectors to date rely on significant technological advances that are futuristic and risky. In this paper we propose a different direction. We resurrect the idea of using longer arm lengths coupled with largely proven technologies. Since the major noise sources that limit gravitational wave detectors do not scale trivially with the length of the detector, we study their impact and find that 40 km arm lengths are nearly optimal, and can incorporate currently available technologies to detect gravitational wave sources at cosmological distances $(z\gtrsim 7)$.

Figure 1

Projected sensitivity of a 40 km long interferometer based on Advanced LIGO. The only major added technology with respect to the existing interferometers is the use of a squeezed light source for reducing quantum noise.

Figure 2

Astrophysical reach for compact binary inspiral systems. The horizontal axis in this plot represents the intrinsic chirp mass of a symmetric binary for the solid lines, and the observed chip mass for the dashed lines. Blue lines represent the maximum observable distance for Advanced LIGO, whereas red lines represent the reach of Advanced LIGO with extended arms, based on the sensitivity shown in Fig. 1. A Hubble constant of $67.9\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$ was assumed [11].

Figure 3

The design noise budget of Advanced LIGO. All dominant noise sources below about 100 Hz are displacement noise, and therefore benefit from longer arms.