The Shadow of the Black Hole by John W. Moffat

Author:John W. Moffat
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2020-06-15T00:00:00+00:00

Figure 7.1. Gravitational wave template obtained from numerical relativity. The template fits the first event, GW150914; the data can be seen as a shadow underneath the bold line of the template. Credit: LIGO

The answer to the second question is that a fair amount of information can be obtained from the data after software has been used to clean the data of noise. The details of the technology are complicated; but, simply put, the cleansing of the data pulls out a signal-to-noise ratio. If this ratio is sufficiently large, then it can be claimed that a real gravitational wave signal has been detected. Such an analysis of the first event, GW150914, produced a large enough signal compared to the background noise that the advanced LIGO scientists could claim with certainty that gravitational waves had been detected.

The library of templates deduced from numerical calculations of Einstein’s field equations contains several hundred thousand templates, which describe every conceivable combination of mass and spin of the merging black holes that could produce the observed waveforms. For GW150914, the first event, the best matching template picked out the two masses of the black holes at 36 and 29 solar masses.

Determining the spins of the black holes is a greater challenge. These are indicated by a special spin parameter that can determine the alignment of the spins to the binary orbital angular momentum plane, or the magnitudes of the spins of the two binary compact objects—in this case, black holes (Figure 7.2). A spin alignment vector is combined with the orbital angular momentum vector to produce an effective measurement of the spins of the black holes. The measurements showed there were two possibilities for the spins of the black holes. Because the effective measurements of the spin parameter were clustered around zero or were negative for the gravitational wave events detected, this meant either that the spins of the black holes were small (i.e., they were not rotating much at all) or that the spins were misaligned with respect to the orbital angular momentum vector. Currently, with the available gravitational wave events, we are not able to decide which is the correct interpretation.

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