62 ASTRONOMY • JUNE 2022
ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions.
QI
WHAT WOULD HAPPEN IF TWO
SUPERMASSIVE BLACK HOLES
MERGED?
Aaron James Oommen
Doha, QatarAI
Supermassive black holes that are millions to
billions of times more massive than the Sun are
commonly found at the hearts of galaxies. When galaxies
merge, their central black holes should also combine.
And when they do, these collisions are so powerful
that they create distortions in space-time, known as
gravitational waves. These ripples were first theorized
by Albert Einstein in his theory of general relativity in- Astronomers believe that the power output from
the merger of two supermassive black holes would be
brief ly larger than the combined luminosity of all the
stars in all the galaxies in the entire observable universe.
The direct detection of the mergers of two stellar-mass
black holes — about 3 to 10 solar masses — by the Laser
Interferometer Gravitational-wave Observatory (LIGO)
has ushered in the exciting new era of gravitational-wave
astronomy. However, the frequency (or wavelength) of
the gravitational waves produced by supermassive black
When black
holes collide
hole mergers is significantly lower (longer) than those
produc ed by stel la r-ma s s black hole mergers. T h i s ma ke s
them undetectable by LIGO, and so no direct detection
of two supermassive black holes merging has been made
yet. To make a direct observation, we will need instru-
ments sensitive to the frequency of the gravitational
waves produced in supermassive black hole mergers.
Such instruments could include ground-based pulsar
timing arrays — which use the stable and precise periods
of neutron stars to search for gravitational waves — and
space-based gravitational-wave observatories such as the
Laser Interferometer Space Antenna (LISA), scheduled
for launch in the next decade.
When we are able to detect the gravitational waves
from a supermassive black hole merger, it will be unlike
any other black hole merger we’ve seen before. Most, if
not all, stellar-mass black hole mergers detected by LIGO
seem to have no accompanying electromagnetic coun-
terparts (that is, events that give off light). But unlike
their smaller counterparts, coalescing supermassive
black holes are expected to occur in gas-rich environ-
ments. This means that bright electromagnetic signals
would likely accompany the gravitational waves, making
for a truly spectacular show.
Xin Liu
Associate Professor, University of Illinois at Urbana-ChampaignQI
HOW IS IT POSSIBLE, WITH ALL
THE MATTER IN THE UNIVERSE,
THAT THE BIG BANG STARTED FROM
THE SIZE OF A PINHEAD?
Pete Neiland
Calgary, AlbertaAI
Our universe is and has been expanding
throughout its history, and this means that it
was hotter and denser in the past than it is today. Any
given piece of space was smaller in the past than it is now.
The fact that nothing can travel faster than the speed of
light limits how much of our universe we can observe;
the currently observable part of our universe is about
46.5 billion light-years in radius.
When the first stars were forming in our universe — a
couple of hundred million years after the Big Bang — the
r ad iu s of what i s now ou r obser vable u n iverse wa s about
20 times smaller than it is today. When the first atoms
were forming — a few hundred thousand years after the
Big Bang — it was about 1,000 times smaller. The particle
accelerator known as the Large Hadron Collider near
Geneva, Switzerland, enables physicists to study the
cond it ions t hat we t h i n k were i n plac e when ou r u n iverse
was only a trillionth of a second old. At that time, the
region of space that is now our observable universe was
only a couple of hundred million kilometers in extent.Two supermassive
black holes spiral
toward one another in
this simulation. NASA’S
GODDARD SPACE FLIGHT CENTER