Sky and Telescope - September 2017 - 28

Gravitational-Wave Astronomy

Strain (10 -21 )

1.0

LIGO Hanford Data
Predicted

0.5
0.0
-0.5
-1.0

Strain (10 -21 )

1.0

LIGO Livingston Data
Predicted

0.5
0.0
-0.5
-1.0

Strain (10 -21 )

1.0

LIGO Livingston Data

0.5
0.0
-0.5
-1.0

LIGO Hanford Data (Shifted)
0.25

0.30

0.35

0.40

0.45

Time (seconds)
p FIRST SIGNAL On September 14, 2015, a set of gravitational waves
passed through the LIGO detector in Livingston, Louisiana, and 7 milliseconds later, through the detector in Hanford, Washington. The data
matched perfectly the brief signal expected from two black holes spiraling
toward each other before coalescing into one.

is basically the mass-induced warp in the fabric of spacetime
(S&T: Dec. 2015, p. 18). This curvature increases rapidly as
we get closer to massive, highly compact objects such as black
holes. In these regions of strong gravity, the motion a particle
experiences as it slides along spacetime's curvature approaches
the speed of light. The black holes LIGO detected were zipping
around each other at about 50% the speed of light before they
merged - about 500 times faster than the speeds relevant
to the best observational test before now. This is the regime
where, if general relativity were to break down, we might begin
to ﬁnd unexpected effects in the gravitational-wave signal.
Einstein's fans will be relieved to know that, at present,
the LIGO data do not support any measurable deviations
from general relativity. While some have recently claimed
potential evidence for the presence of post-merger gravitational-wave echoes, only possible via quantum modiﬁcations
to general relativity, members of the LIGO scientiﬁc collaboration debate the validity of these claims.
For theoretical astrophysicists, LIGO's discoveries provide
the ﬁrst direct conﬁrmation that close black hole binaries
can form in nature and merge within the age of the universe.
Mergers are one of two ways in which black holes should
grow (the other is by accreting gas), yet we've never actually
seen two black holes merge before. Now, we've "heard" it happen multiple times.
The discoveries also reveal the existence of surprisingly
heavy stellar-made black holes. Although supermassive black
holes of millions or billions of solar masses exist, they didn't
form from a single star (S&T: Jan. 2017, p. 24). The black
holes that LIGO has detected, on the other hand, are probably
the remains of individual, albeit massive, stars.
Before LIGO, astronomers could only see stellar-mass
black holes when they siphoned off gas from a companion
star, heating the plasma to X-ray-emitting temperatures
before devouring it. Among these systems, the most massive
black hole contained about 20 times the mass of the Sun. The
vast majority of theoretical predictions also homed in on this
mass as an upper limit. In fact, only since about 2010 have
some theorists suggested that stars of low metallicity might
produce black holes with masses several tens of times that of
the Sun. But these predictions weren't widely recognized - as

Event

Date Detected

Mass of Black Holes

Final Mass

Distance (light-years)

GW150914

Sept. 14, 2015 (O1)

29 MSun, 36 MSun

62 MSun

1.2 billion

LVT151012*

Oct. 12, 2015 (O1)

13 MSun, 23 MSun

35 MSun

2.5 billion

GW151226

Dec. 26, 2015 (O1)

7.5 MSun, 14 MSun

20.8 MSun

1.2 billion

GW170104

Jan. 1, 2017 (O2)

20 MSun, 30 MSun

48.7 MSun

2.2 billion

O1 and O2 refer to the first and second observing runs, respectively. The masses of merging black holes do not add up to the final mass because some of the
mass is radiated away as energy, in the form of gravitational waves. Error bars excluded for simplicity. *Candidate detection.

S E P T E M B E R 2 0 1 7 * SK Y & TELESCOPE

LE AH TISCIONE / S&T, SOURCE: LIGO

Gravitational Waves Detected

Sky and Telescope - September 2017

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