Though the recent discovery of GW150914 is a thrilling success in the field of gravitational-wave astronomy, LIGO is only one tool the scientific community is using to hunt for these elusive signals. After 10 years of unsuccessful searching, how likely is it that pulsar-timing-array projects will make their own first detection soon?
Frequency ranges for gravitational waves produced by different astrophysical sources. Pulsar timing arrays such as the EPTA and IPTA are used to detect low-frequency gravitational waves generated by the stochastic background and supermassive black hole binaries. [Christopher Moore, Robert Cole and Christopher Berry]
Supermassive Background
Ground-based laser interferometers like LIGO are ideal for probing ripples in space-time caused by the merger of stellar-mass black holes; these mergers cause chirps in the frequency range of tens to thousands of hertz. But how do we pick up the extremely low-frequency, nanohertz background signal caused by the orbits of pairs of supermassive black holes? For that, we need pulsar timing arrays.
Pulsar timing arrays are sets of pulsars whose signals are analyzed to look for correlations in the pulse arrival time. As the space-time between us and a pulsar is stretched and then compressed by a passing gravitational wave, the pulsar’s pulses should arrive a little late and then a little early. Comparing these timing residuals in an array of pulsars could theoretically allow for the detection of the gravitational waves causing them.
Globally, there are currently four pulsar timing array projects actively searching for this signal, with a fifth planned for the future. Now a team of scientists led by Stephen Taylor (NASA-JPL/Caltech) has estimated the likelihood that these projects will successfully detect gravitational waves in the future.
Probability for Success
Expected detection probability of the gravitational-wave background as a function of observing time, for five different pulsar timing arrays. Optimistic and conservative assumptions are made for merger rates (blue and red lines, respectively) and environmental conditions (solid and dashed lines, respectively). [Taylor et al. 2016]
First the bad news: based on the authors’ estimates, small arrays — which contain only a few pulsars that each have minimal timing noise — will not be likely to detect gravitational waves within the next two decades. These arrays are more useful for setting upper limits on the amplitude of the gravitational-wave background.
On the other hand, large pulsar timing arrays have far more promising detection probabilities. These include the Parkes Pulsar Timing Array, the European Pulsar Timing Array, and NANOGrav — which each target tens of pulsars, with the intent to add more in the future — as well as the International Pulsar Timing Array, which combines the efforts of all three of these projects. There is an 80% chance that, within the next decade, these projects will successfully detect the gravitational-wave background created by orbiting supermassive black holes.
Based on this study, the outlook for these large arrays remains optimistic even in non-ideal conditions (such as if supermassive-black-hole merger rates are lower than we thought). So, though we may still have to wait a few years, the possibility of probing an otherwise inaccessible range of frequencies continues to make pulsar timing arrays a promising avenue of study for gravitational waves.
Citation
S. R. Taylor et al 2016 ApJ 819 L6. doi:10.3847/2041-8205/819/1/L6