Sky & Telescope - February 2021 - 9

ASTRONOMY & SOCIETY

Black Hole Scientists Win
Nobel Prize in Physics
A PHYSICIST AND TWO ASTRONOMERS have bagged 2020 Nobels for

SIM UL ATION: ESO / L. CA LÇA DA / SPACEENGINE.ORG;
HUBBLE ULTR A DEEP FIELD: B. SA X TON (NR AO / AUI / NSF ) /
A LM A (ESO / N AOJ / NR AO) / N ASA / ESA HUBBLE

helping clinch the case for black holes'
existence: physicist Roger Penrose and
astronomers Reinhard Genzel and
Andrea Ghez. Penrose will receive
half of the 10 million Swedish kronor
($1.1 million) prize; Genzel and Ghez
will share the other half equally.
We know today that black holes
are an unavoidable consequence of
Einstein's general theory of relativity,
published in 1915. In this framework,
gravity is geometry: Mass tells spacetime
how to curve, and spacetime's curves
tell mass how to move. Black holes are
where mass curves spacetime so much
that nothing, not even light, can escape.
But black holes were not scientiﬁcally palatable for several decades. Many
researchers helped change that, but it
was Penrose (University of Oxford, UK)
who, in the mid-1960s, showed that
collapse to a singularity always happens when there's enough mass/energy
packed together. He introduced a new
mathematical concept to make this
leap, called trapped surfaces, from which
the path inward is inexorable - oneway trafﬁc only.

Fabian Walter (Max Planck Institute for
Astronomy, Germany) and colleagues
used the Atacama Large Millimeter/submillimeter Array (ALMA) to survey the
Hubble Ultra Deep Field, publishing the
results in the October 20th Astrophysical Journal.
ALMA's radio dishes spy cool gas
and dust in galaxies whose light has
been traveling for up to 12 billion years,
enabling Walter and colleagues to
understand the ﬂow of gas throughout
cosmic history.
The gas that falls into galaxies is ionized and has to cool before it can form
stars; ﬁrst ions become neutral atoms,
then they pair up into molecules. Walter and colleagues traced both atomic
(H) and molecular (H2) gas to follow

S Simulation of known stars orbiting the Milky Way center

Penrose also discovered that it's possible to extract energy from a spinning
black hole, which helps explain how
distant supermassive black holes, known
as quasars, can power jets that extend
thousands, even millions of light-years
long. Still, astronomers needed more
evidence to show that supermassive
black holes sit at the center of nearly
every major galaxy.
Work by Genzel (Max Planck Institute for Extraterrestrial Physics, Germany) and Ghez (University of California, Los Angeles) was pivotal in this
effort. Thanks to a series of technical
feats, the two competing astronomers
and their teams have spent nearly three
decades using ground-based near-infrared telescopes to watch dozens of stars

the ﬂow from galaxies' outer reaches to
their star-forming hearts.
The new radio and millimeter-wavelength observations show that galaxies
never, at any one point in time, hold
all the gas they need to make all their
stars. The gas has to come from outside,
ﬂowing inward in a continual process.
Gas inﬂow will keep declining, the
researchers write, " and the cosmic star
formation rate density will continue its
steady descent to the inﬁnitesimal. " But
even as the inﬂux of star-making material decreases by half over the next 5
billion years, galaxies will nevertheless
continue to make new stars with what
they still receive. We're hardly at the
end of times just yet.
MONICA YOUNG

looping pell-mell around an invisible
object at the heart of the Milky Way. By
tracing stellar motions, the astronomers
revealed that the stars revolve around
an unseen, unmoving, and compact
something-or-other with the mass of
4 million Suns. Astronomers call this
black hole Sagittarius A*.
The teams' observations have also
enabled them to conﬁrm two predictions of general relativity (S&T: Sept.
2018, p. 22): the gravitational redshift of
light as one of the stars dipped into the
spacetime valley the black hole makes
around itself, and the slight shift in
that star's path as it traveled through
the warped spacetime, a process known
as orbital precession.
CAMILLE M. CARLISLE

S Within the Hubble Ultra Deep Field, galaxies
rich in molecular hydrogen, as seen by ALMA,
are highlighted in gold.
sk yandtelescope.org * FEBRUARY 2 021

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Sky & Telescope - February 2021

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