Yesterday was an incredibly exciting day for everyone in the gravitational physics community. After months of rumours, the LIGO collaboration made the spectacular announcement that they have achieved the first ever direct detection of gravitational waves. It’s difficult to put into words the enormity of their accomplishment. It’s a shoo-in for a Nobel prize (probably for LIGO’s three founders Kip Thorne, Ronald Drever and Rai Weiss) and has been widely heralded as one of the biggest scientific breakthroughs of our time. As if that wasn’t enough, the gravitational waves – which were detected by both LIGO detectors on September 14, 2015 at 09:51 UTC –  were generated over a billion years ago by the merger of a  binary black hole system. So in one go the discovery has made two huge scientific breakthroughs: it has confirmed Einstein’s prediction that gravitational waves exist and can be detected on Earth; and it has confirmed that black holes exist and can appear in binary pairs.

It was particularly impressive that the announcement came along with the simultaneous publication of a peer-reviewed paper in Physical Review Letters (also on arXiv), along with 12 other papers giving all of the nitty–gritty technical details and a full release of the data from the detection, which has been given the name GW150914. The real excitement now is what’s next. Detecting gravitational waves was just the start; as LIGO continues to collect data and its sensitivity is improved we can expect  to see gravitational waves from a whole host of other sources including binary neutron stars, supernovae and relic gravitational waves from the first instants after the big bang.

Looking further into the future, the next grand challenge will be to take gravitational-wave detection into space, where it is unaffected by the low-frequency noise encountered on the Earth. We have already made the first steps towards this challenging goal: the LISA pathfinder technology demonstrator mission was recently launched into space and is set to begin tests within the next few weeks. This will be followed up by the European Space Agency’s eLISA mission, which will take three LIGO-like interferometers, scale the arm lengths up from 4km to several million kilometres, and launch them into space.  The mission will open gravitational wave astronomy up to a whole host of new sources including super-massive black hole binaries and extreme-mass-ratio inspirals.

For more details on yesterday’s announcement, you can watch the full thing, listen to the signal they found, and view some
movies produced by the SXS collaboration using numerical relativity simulations.

The preprint of my latest paper with Antoine Klein, Enrico Barausse, Alberto Sesana, Antoine Petiteau, Emanuele Berti, Stanislav Babak, Jonathan Gair, Sofiane Aoudia, Ian Hinder and Frank Ohme is now available on the arXiv as 1511.05581, and has also been published as Phys. Rev. D 93, 024003 (2016). The abstract for the article is below:

We compare the science capabilities of different eLISA mission designs, including four-link (two-arm) and six-link (three-arm) configurations with different arm lengths, low-frequency noise sensitivities and mission durations. For each of these configurations we consider a few representative massive black hole formation scenarios. These scenarios are chosen to explore two physical mechanisms that greatly affect eLISA rates, namely (i) black hole seeding, and (ii) the delays between the merger of two galaxies and the merger of the black holes hosted by those galaxies. We assess the eLISA parameter estimation accuracy using a Fisher matrix analysis with spin-precessing, inspiral-only waveforms. We quantify the information present in the merger and ringdown by rescaling the inspiral-only Fisher matrix estimates using the signal-to-noise ratio from non-precessing inspiral-merger-ringdown phenomenological waveforms, and from a reduced set of precessing numerical relativity/post-Newtonian hybrid waveforms. We find that all of the eLISA configurations considered in our study should detect some massive black hole binaries. However, configurations with six links and better low-frequency noise will provide much more information on the origin of black holes at high redshifts and on their accretion history, and they may allow the identification of electromagnetic counterparts to massive black hole mergers.