On September 14, 2015 at 09:50:45 UTC the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration detected a gravitational wave (GW) signal from the binary black hole merger GW150914. This discovery confirms a 100-year-old prediction Einstein made based on his theory of General Relativity. Gravitational waves are ripples in the fabric of spacetime created by moving masses, much as electromagnetic waves are created by moving charges. But because gravity is the weakest of the fundamental forces, GWs are exceedingly elusive. The effect of GWs is incredibly minuscule. For example, even the strongest GWs from astrophysical events change the distance between our solar system and the nearest star by the width of a human hair.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment designed to directly detect GWs. The LIGO detectors, at the LIGO Hanford Observatory (LHO) in Washington State and the LIGO Livingston Observatory (LLO) in Louisiana are looking for GW signals. Coalescence of compact binary systems containing neutron stars and/or black holes is the most promising source of GW for LIGO observatories. Compact binary systems and their corresponding GW strain are well-modeled, which allow powerful search techniques to be used.
General Relativity predicts the shape of the GW signal produced by a compact binary coalescence which we call a “template waveform”. The details of the template waveform depend on the parameters of the compact binary system. Using the template waveform, we can perform a modeled search using the matched filtering method as the optimal search method. Matched filtering is a data analysis technique that efficiently searches for a signal of known shape buried in noisy data. In compact binary coalescence modeled search, the interferometer output is cross-correlated with the template waveform in order to find which part of the data has a similar morphology to the template waveform and potentially might be a GW signal.
Since the parameters of signals are not known in advance, each detector’s output is filtered against a discrete bank of templates that span the search target space. The search parameter space is defined by the limits placed on the compact objects’ masses and spins. The minimum component masses of the search are determined by the lowest expected neutron star mass, which we assume to be 1 solar mass. There is no known maximum black hole mass, however we limit this search to binaries with a total mass less than 100 solar masses. However, the LIGO detectors are sensitive to higher mass binaries. The details on the spin limits can be found in the paper.
|Left: Search results from the PyCBC analysis. The histogram shows the number of candidate events (orange) and the number of background events due to noise in the search class where GW150914 was found (black) as a function of the search detection-statistic and with a bin width of ∆ρˆc = 0.2. The significance of GW150914 is greater than 5.1 σ. The scales immediately above the histogram give the significance of an event measured against the noise backgrounds in units of Gaussian standard deviations as a function of the detection-statistic. The black background histogram shows the result of the time-shift method to estimate the noise background in the observation period. The tail in the black-line background of the binary coalescence search are due to random coincidences of GW150914 in one detector with noise in the other detector. The significance of GW150914 is measured against the upper gray scale. The purple background histogram is the background excluding coincidences involving GW150914 and it is the background to be used to assess the significance of the second loudest event; the significance of this event is measured against the upper purple scale. Right: Search results from the GstLAL analysis. The histogram shows the observed candidate events (orange) as a function of the detection statistic ln L . The black line indicates the expected background from noise where zero lag events have been included in the noise background probability density function. The purple line indicates the expected background from noise where zero lag events have not been included in the noise background probability density function. The independently-implemented search method and different background estimation method confirms the discovery of GW150914.|
The LIGO collaboration performed its compact binary coalescence modeled search using two independently implemented analyses, referred to as PyCBC and GstLAL. These analyses use a common set of template waveforms, but differ in their implementations of matched filtering, their use of detector data-quality information, the techniques used to mitigate the effect of non-Gaussian noise transients in the detector, and the methods for estimating the noise background of the search.
GW150914 was observed in both LIGO detectors within the 10 ms inter-site propagation time, with a combined matched-filter signal to noise ratio (SNR) of 24. The search reported a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1 sigma. In other words, both analyses have shown that the probability that GW150914 was formed by random coincidence of detector noise is extremely small. Therefore GW150914 is a significant GW signal. The basic features of the GW150914 signal point to it being produced by the coalescence of two black holes. The best-fit template parameters from the search are consistent with detailed parameter estimation that identifies GW150914 as a near-equal mass black hole binary system with source-frame masses 36+5-4 solar masses and 29+4-4 solar masses at the 90% credible level. Detailed parameter estimation for GW150914 is reported in Ref  of the paper.