http://www.skyandtelescope.com.au 15NASA/ESA/M.J. JEE & H. FORD (JHU)
Astronomers and cosmologists can’t agree on the va bble constant —
the number that describes the current expansion rate of th rse. A solution to
the problem is nowhere in sight.
IN THE 1980S, when people in East and West Berlin still
lived in two very different universes, politically speaking,
Checkpoint Charlie was an intimidating and heavily guarded
crossing point between communist oppression and liberal
democracy. Today, it is one of the most popular tourist
attractions in the capital of united Germany. But 29 years
after the Berlin Wall opened in 1989, another insuperable
barrier, this time scientific in nature, manifested itself
just 600 metres eastward of Checkpoint Charlie, in the
Auditorium Friedrichstrasse. On a drizzly Saturday in
November 2018, this unadorned Soviet-style building served
as the intellectual battleground for a cosmological Cold War.
Some 130 scientists flocked to a one-day symposium here
to discuss an unnerving crisis in our understanding of the
universe. It was a diverse bunch from all over the world:
astrophysicists and cosmologists, observers and theorists,
young postdocs and eminent professors. Some of them had
spent more time in the air on their way there than they
would in the lecture room. Their mutual worry: The universe
appears to be expanding too fast, and no one knows why. At
the end of the meeting, Brian Schmidt (Australian National
University, ANU), co-recipient of the 2011 Nobel Prize in
Physics, said: “I’m even more puzzled after today.”
Here’s what astronomers and physicists alike scratch
their heads about: Detailed observations of the cosmic
microwave background (CMB, the cooled-down ‘afterglow’
of the Big Bang) yield a very precise value for the current
expansion rate of the universe, with an error margin of just
1%. However, measurements of objects in the ‘local’ universe
arrive at a number that is also fairly precise, but a whopping
9% higher. “And neither side has obvious weak points,” says
Matthew Colless (ANU), one of the organisers of the Berlin
symposium.
According to co-organiser Matthias Steinmetz (Leibniz
Institute for Astrophysics Potsdam, Germany), the
determination of the universe’s current expansion rate “has
a history of crisis and controversy”. Indeed, the earliest
guesstimates for the Hubble constant (H 0 , a measure of
the present-day expansion rate) seemed to indicate that
the universe was much younger than Earth. And 30 years
ago, you’d be offered values that differed by a factor of two,
depending on whom you asked.
“The good news is that the controversy was much larger
when I started to work in cosmology” than it is now, quips
theorist Abraham Loeb (Harvard University). “So in a sense,there’s progress in the
But Loeb is worried,too.Cos as become a hig
precision science, and never bef he gap between
two different estimates of the nstant been so
statistically significant.Climbing up the distance lad
In March 1929, American cosmologist Edwin Hubble
published observations that, for the first time, revealed
that our universe is expanding. According to Hubble’s
measurements, distant galaxies appear to be receding from
us at a higher velocity than nearby galaxies — something
that (unknown to Hubble at the time) had been predicted to
hold in an expanding universe by Belgian cosmologist and
Jesuit priest Georges Lemaître two years earlier. The Hubble-
Lemaître Law describes this linear relationship between
distance and ‘recession’ velocity; the proportionality constant
became known as the Hubble constant, or, more precisely, the
Hubble parameter, because its value slowly changes with time.
However, the true value of the Hubble constant, measured
in kilometres per second per megaparsec (km/s/Mpc, see
box ‘How fast does the universe expand?’), turned out to
be elusive. To determine it, you need to know both the
cosmological ‘recession’ velocity of a galaxy and its distance.
In principle, the recession velocity (the rate at which a
galaxy’s distance is increasing due to the expansion of the
universe) can be found by measuring the redshift: The
more time the galaxy’s light waves spend travelling through
expanding space on their way to Earth, the more they are
stretched to longer (redder) wavelengths. But for a nearby
galaxy — one for which it’s relatively easy to measure the
distance — the redshift measurement is compromised by the
galaxy’s real motion through space. These spatial velocities
can be as high as a few hundred kilometres per second. And
for remote galaxies — the ones for which any spatial motion
is negligibly small compared to the cosmological recession
velocity — it’s frustratingly hard to measure their distances.
Over the decades, astronomers have set up an elaborate
distance ladder to establish distances to other galaxies.
Cepheid variables — luminous pulsating stars — are a key
ingredient of this technique. The more luminous a Cepheid
is, the slower it pulsates. Henrietta Swan Leavitt at Harvard
College Observatory discovered this period-luminosity
relationship in the early 1900s, and it’s now known as the
Leavitt Law. So if you find a Cepheid in a remote galaxy, its