Scientific American - USA (2019-12)

2 ,^000 M

pc^ (^6.^5 billion^ light-years)

4 ,^000

megapa

rsecs^ (Mpc

)

from^ E

arth

0 Days 50 100 150 200

Jan. 2016

Aug. 2017

Sept. 2015

Nov. 2016

Apr. 2019 Oct. 2019

Gaps represent breaks of a week or longer when detectors were not operational.

*

* SOURCES: GRACEDB—LIGO GRAVITATIONAL-WAVE CANDIDATE EVENT DATABASE; “GWTC-1: A GRAVITATIONAL-WAVE TRANSIENT CATALOG OF COMPACT BINARY MERGERS OBSERVED BY LIGO AND VIRGO DURING THE FIRST AND SECOND OBSERVING RUNS,” BY B. P. ABBOTT ET AL. (LIGO SCIENTIFIC COLLABORATION AND VIRGO COLLABORATION), IN

PHYSICAL REVIEW X,

VOL. 9, ARTICLE 031040;

J U LY

2 0 1 9

Vast Reaches
Events occurred
astoundingly far
from Earth; our
Milky Way galaxy
is only 0.03
Mpc across.

Event Zero

The first detected

event—on September

14, 2015—was created

by the merger of

two black holes.

O1: 3 detections

O2: 8 detections

O3: 33 detections

(as of October 1, 2019)

GRAPHIC SCIENCE

Text and Graphic by Katie Peek

Earth and

Milky Way

( center )

Quickening Pace of Discovery

LIGO detected three confirmed events during

its first run—dubbed “O1”—over four months

in 2015. By run O3, detections were piling up

five times as fast (although some may not with­

stand scientific scrutiny). Improvements in

LIGO’s physical setup have led to the increase.

Key

Symbols mark LIGO’s

44 gravitational­wave

detections as of

October 1, 2019.

Approximate directions

and distances are relative

to the Milky Way, a tiny

spiral at the center.

Observation run

O1 (2015)

O2 (2017)

O3 (2019,

unverified)

3 10 20 80

Mass

Mass of merging

objects (approximate

solar masses)

For O3, masses are

not yet calculated,

so symbols are

the same size.

Source of

gravitational

wave

Record Holder

Two black holes

merged 5,200 Mpc

away, the farthest

event recorded by

LIGO, detected on

July 6, 2019.

Lopsided Pairs

In 2019 LIGO picked

up the first signal

likely created by the

merger of a neutron

star with a black

hole. By October 1,

five of these had

been found.

Closer Encounters

Neutron stars—less

massive than black

holes—make weaker

signatures when they

merge, so LIGO sees

only events relatively

near to Earth.

Ballpark Locations

Many LIGO stations are needed

to pinpoint an event, so the ones

depicted could have happened

anywhere within a local region,

such as this one in pink. As more

detectors are built, positions

will become more precise.

82 Scientific American, December 2019

Event type

Merger of

two black holes

Merger of

black hole and

neutron star

Merger of

two neutron stars

Odd Disturbances

Pierce the Universe

Detectors reveal the origins of gravitational waves

On September 14, 2015, lasers underground in Louisiana and Washington State wavered together in re ­

sponse to a disturbance in spacetime, and a new window opened onto the cosmos. The two sites are part

of the LIGO gravitational­wave detector—sensitive, powerful lasers so carefully isolated from Earth’s

motion that they can pick up incredibly minuscule vibrations. The signal was a gravitational wave, a rip­

ple in spacetime created by two black holes merging 1.4 billion light­years away, far beyond our Milky

Way galaxy. The event—the first detection of gravitational waves—also proved that black holes can

orbit each other and merge. Since then, sensors have detected 43 more events, making them seem

almost commonplace, says Christopher Berry, a member of the LIGO team. The accumulating data

are helping astronomers better understand the menagerie of objects that populate the universe.

© 2019 Scientific American