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http://www.facebook.com/AstronomyMagazine6 ASTRONOMY • JUNE 2019BY DAVID J. EICHERFROM THE EDITOR
T
he famous Hubble
constant has been a
source of heated con-
troversy since its earli-
est days in the 1920s.
The number that defines the
expansion rate of the uni-
verse, the Hubble constant
(or H 0 ) is estimated primarily
by measuring redshifts:
changes in a galaxy’s spec-
trum attributed to the fast
motion of the galaxy away
from an observer. Spectral
lines are shifted toward
the red end of the
spectrum by a small
amount, giving away
a galaxy’s distance.
This may all sound
complex, but it’s not
the end of the story.
For decades, astrono-
mers have argued
about the value of the Hubble
constant and faced off in
different camps, depending
on the technique used to find
the magic number. The clas-
sic battle raged from the
1960s through the 1980s and
saw teams led by Allan
Sandage on one side and
Gérard de Vaucouleurs on
the other. The Sandage team
argued for 50 to 55 kilometers
per second per megaparsec,
and the de Vaucouleurs team
fixed upon 100 kilometers per
second per megaparsec. (Aparsec, one of the fundamen-
tal units of distance in the
cosmos, is 3.26 light-years.)
This month’s cover story
by science writer Robert
Naeye examines the history
of decoding the expansion
rate of the universe and all
of its consequences. It trans-
lates into big implications
for the cosmos, including its
age, fate, and size, among
other things.
Naeye describes howcontroversy stalks the
Hubble constant even now.
For some years, astronomers
held the magic number at
73, after extensive research
by Wendy Freedman and the
Hubble Space Telescope’s
Key Project. Many scientists
think the controversy
over the number will
disappear, and is brought
about by uncertainties
in measurements.
But recent measurements
of the cosmic microwave
background, the afterglow ofradiation left from the Big
Bang, suggest a somewhat
smaller number. They are
not direct measurements of
the Hubble constant, but
imply a number more like 67.
Data from the Planck satel-
lite, with the known compo-
sition of the universe — dark
energy, dark matter, and
normal matter — yield this
smaller result.
The implications are big.
Right now, astronomers
believe our universe is
13.8 billion years old,
based on a Hubble
constant of 67 from
Planck. But if the
Hubble constant is
73, it would mean a
younger universe by
some several hundred
million years.
As methods converge and
theorists sharpen their mod-
els, any remaining tension
among scientists over this
magic number might signal
a new understanding of the
standard model of physics.
Exciting times we live in, no?Yo u r s t r u l y,David J. Eicher
EditorThe
expanding
cosmos
Editor David J. Eicher
Art Director LuAnn Williams Belter
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http://www.Astronomy.com/davesuniverse
Follow Dave Eicher on Twitter: @deicherstarFor decades, astronomers
have argued about the
value of the Hubble
constant and faced off
in different camps.