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The Constant Fight
Behind the astronomical dispute that’s splitting apart the cosmos.
BY COREY S. POWELLREIDAR HAHN/FERMILAB→
Back in the 1980s, astronomers
were caught up in a debate
so huge, you could drive a universe
through it. The point of contention
was a number called the Hubble
constant, which describes the rate at
which the cosmos is expanding and, by
extension, how much time has passed
since the Big Bang: the slower the
expansion rate, the older the universe.
On one side was Allan Sandage, the
towering successor to Edwin Hubble
at Mount Wilson Observatory, who
calculated the age of the universe was
roughly 20 billion years. On the other
side was a group of apostates whose
observations showed it was only about
half that old. Tempers ran hot.
The Hubble constant is measured
in arcane units (kilometers per secondper megaparsec), so for brevity,
the researchers would use just the
number itself. “We measure the
Hubble constant to be 100,” one of
the upstarts would announce in a
talk. “It’s 50,” Sandage would roar in
response, ridiculing his colleagues for
their awed measurements. For me,a young reporter covering the ield at
the time, it was a revelatory display of
scientiic passion. The entire history
of the universe was at stake!
Three decades later, a revived
debate on the Hubble constant has
our understanding of the universe
hanging in the balance all over again.
At least the old shouting matches are
long settled. Vastly improved data
from the Hubble Space Telescope
and the Planck satellite showed that
Sandage and his rivals were both
wrong; the true age of the universe lies
in between, at 13.8 billion years. With
this new clarity, though, a new conict
has popped into view.
Measurements of the Hubble
constant made by directly observing
stars yield a cosmic expansion
rate of 73. Measurements made
by studying the distant microwave
glow of the early universe give a
value of about 68. Although the gap
is small, it has proven persistent.
Resolving the earlier debate led to the
startling realization that we live in an
accelerating universe being pushed
apart by a mysterious “dark energy.”
The implications of this latest debate
could be equally momentous.
Adam Riess at the Space Telescope
Science Institute, who shared the
2011 Nobel Prize in physics for the
discovery of dark energy, is alternately
amazed and amused by the latest turn
of events. “Who would have thought
we’d be measuring the Hubble
constant and seeing differences at
just the 10 percent level and saying,
‘Hmm, there’s something wrong
here?’ ” he asks. “But here we are.”CLIMBING THE LADDER
Riess and his team specialize in the
star-oriented measurements of the
Hubble constant — what’s known
as the distance ladder approach.
The researchers begin with relatively
nearby objects whose distance they
can measure accurately, then use thosehree decades on, a
revived debate on the
Hubble constant has
our understanding of
the universe hanging
in the balance all
over again.
Observational
data has led
astronomers
to differing
conclusions
about how fast
the cosmos is
expanding.Out
here