Cosmic Expansion Puzzle
24 JUNE 2019 • SKY & TELESCOPE
I
n the 1980s, when people in
East and West Berlin still
lived in two very different
universes, politically speak-
ing, Checkpoint Charlie was
an intimidating and heavily
guarded crossing point between
communist oppression and lib-
eral 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 scientifi c in nature, mani-
fested itself just 600 meters commie-
ward 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 fl ocked 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: astrophysi-
cists and cosmologists, observers and
theorists, young postdocs and emi-
nent professors. Some of them had
spent more time on the plane 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 uni-
verse, 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
organizers of the Berlin symposium.
According to co-organizer Matthias
Steinmetz (Leibniz Institute for Astro-physics 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 indi-
cate that the universe was much
younger than Earth. And 30 years
ago, you’d be offered values that dif-
fered by a factor of two, depending on
whom you asked.
“The good news is that the con-
troversy 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 fi eld.”
But Loeb is worried, too. Cosmol-
ogy has become a high-precision
science, and never before has the gap
between two different estimates of
the Hubble constant been so statisti-
cally signifi cant.Climbing up the
Distance Ladder
In March 1929, American cosmologist
Edwin Hubble published observations
that, for the fi rst 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 galax-How Fast Does the
Universe Expand?
The expansion of the universe cannot be
expressed as a simple velocity. Galaxies
are not receding from each other through
some imaginary static space. Instead,
space itself is expanding, thereby pushing
the galaxies away from one another. As a
result, the distance between two galax-
ies increases over time. The more space
there is between the two galaxies, the
faster their mutual distance is growing,
in kilometers per second. In other words,
there’s no single “velocity value” for the
expansion of the whole universe – it all
depends on the scale you consider. It’s
like monetary infl ation: The infl ation rate
for 2018 cannot be expressed in dollars
(that would only work for a particular sum
of money), but must always be given as a
percentage, or a proportionality constant.
Not that the universe is expanding that
fast. In fact, cosmic distances increase
by only some 0.01% in 1.4 million years.
In other words: If the current distance
to a remote galaxy is 100 million light-
years, it will increase by one light-year
every 140 years or so. A “recession”
velocity of one light-year per 140 years
corresponds to about 2,150 kilometers
per second (4.8 million mph). But a galaxy
at a distance of 200 million light-years
appears to “recede” twice as fast, at
some 4,300 kilometers per second. Thus,
the “recession” velocity is growing by
21.5 kilometers per second for every ad-
ditional million light-years, or by some 70
kilometers per second for every addition-
al megaparsec (Mpc; 1 parsec equals 3.26
light-years). There’s your proportionality
constant, H 0 : 70 km/s/Mpc.Distance: Astronomers spot a standard
candle star or supernova in a distant
galaxy. They calculate how far away the
galaxy has to be in order for the star to
look as faint as it does.How Astronomers Calculate
H 0 from SupernovaeH^0
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