11.6 Detection of Gravitational Waves
181
The first signal seen by LIGO, in September 2015, was a pulse that began as a
sine wave, then increased in frequency to become a chirp and ended with further
decreasing oscillations. It fits very well the scenario we discussed in Sect. 11.5 and
Example 11.3, as illustrated in Fig. 11.4; the signal is interpreted as coming from a
pair of black holes in close orbit, losing energy by gravitational radiation, decreasing
their orbital period as they move closer, and merging to form a larger black hole,
which undergoes “ringdown” oscillates. Its name is GW 150914 and a sketch of its
waveform is shown in Fig. 11.6a. A matching theoretical template obtained using
numerical methods is shown in Fig. 11.6b. The early part of the waveform is indeed
consistent with Fig. 11.4. Some parameters of the event are shown in Table 11.1
(LIGO).
It is notable that the masses of the black holes in GW150914 were rather larger
than expected, about 30 solar masses, and also notable that the velocity near merger
Fig. 11.6 Waveform sketch of GW150914 is shown in (a). Theoretical matching template sketch
is shown in (b); the merger and ringdown are included. The vertical scale is in 10 −21 units and the
duration is about 0.5 s
Table 11.1 Parameters associated with GW150914
Distance
0.75–1.9 Gly
Peak GW strain
10 −21
Redshift
0.054–0.136
Radiated GW energy
2.5–3.5 m
Signal to noise
24
Peak speed of BHs
0.6 c
Total mass (m )
60–70
Duration of event
∼ 1 s .
Primary BH
32–41
Frequency
Ballpark of 50 Hz
Secondary BH
25–33
Remnant BH
58–67
181
The first signal seen by LIGO, in September 2015, was a pulse that began as a
sine wave, then increased in frequency to become a chirp and ended with further
decreasing oscillations. It fits very well the scenario we discussed in Sect. 11.5 and
Example 11.3, as illustrated in Fig. 11.4; the signal is interpreted as coming from a
pair of black holes in close orbit, losing energy by gravitational radiation, decreasing
their orbital period as they move closer, and merging to form a larger black hole,
which undergoes “ringdown” oscillates. Its name is GW 150914 and a sketch of its
waveform is shown in Fig. 11.6a. A matching theoretical template obtained using
numerical methods is shown in Fig. 11.6b. The early part of the waveform is indeed
consistent with Fig. 11.4. Some parameters of the event are shown in Table 11.1
(LIGO).
It is notable that the masses of the black holes in GW150914 were rather larger
than expected, about 30 solar masses, and also notable that the velocity near merger
Fig. 11.6 Waveform sketch of GW150914 is shown in (a). Theoretical matching template sketch
is shown in (b); the merger and ringdown are included. The vertical scale is in 10 −21 units and the
duration is about 0.5 s
Table 11.1 Parameters associated with GW150914
Distance
0.75–1.9 Gly
Peak GW strain
10 −21
Redshift
0.054–0.136
Radiated GW energy
2.5–3.5 m
Signal to noise
24
Peak speed of BHs
0.6 c
Total mass (m )
60–70
Duration of event
∼ 1 s .
Primary BH
32–41
Frequency
Ballpark of 50 Hz
Secondary BH
25–33
Remnant BH
58–67