skyandtelescope.com • JUNE 2019 27the cosmological distance ladder, and at a correspondingly
much more precise value for the Hubble constant. The result:
73.5 km/s/Mpc, with an uncertainty of just 2.2%. “The value
hasn’t changed very much,” he says, “but the uncertainty has
come down signifi cantly.”
To achieve this high level of precision, Riess’s “Super-
nova, H 0 , for Equation of State of Dark Energy” (SH0ES)
team determined the parallax, and thus the distance, of 50
Cepheid variables in our own Milky Way Galaxy — a neces-
sary step in accurately calibrating the Leavitt Law. Subse-
quently, they studied Cepheids in 37 galaxies in which Type
Ia supernovae had also been observed. Using the Cepheid
distances of these 37 galaxies, the team then calibrated the
standard candle properties of Type Ia’s. Finally, they derived
the Hubble constant from observations of some 300 super-
novae in more distant galaxies, for which the redshift is a
reliable measure of the cosmological recession velocity.
“Our data set has been re-analyzed by many, many
independent groups,” Riess says, “and they all arrive at the
same value” of 73.5 km/s/Mpc. That’s about 9% higher than
the value obtained by Planck. Given the precision of both
estimates, the statistical signifi cance of this discrepancy is 3.8
sigma, according to Riess. That means the chance of the mis-
match being some statistical fl uke is about 1 in 7,000. Clearly,
there’s something amiss.
Huterer agrees. “The Hubble tension is real,” he says.
Now What?
While some scientists still think there may be an undiscov-
ered error in either one of the two approaches (or maybe in
both!), most believe that the results are solid. But that doesn’t
Keep Your Distance
There are various ways to determine the distance to
another galaxy:Variable stars The pulsation period of Cepheids – a
particular and easy-to-recognize type of variable
star – is related to their peak luminosity. Measur-
ing a Cepheid’s period reveals its luminosity;
comparing that to its apparent brightness reveals
its distance. Similar period-luminosity relation-
ships hold for other types of variable stars, most
notably RR Lyrae stars. The relationships need to
be calibrated by measuring distances to Milky Way
variables by means of the parallax method.
Eclipsing binaries By spectroscopically measuring
the orbital speeds of the two components of an
eclipsing binary star (a close binary in which the
stars mutually eclipse each other), and combining
this information with the observed light curve, it
is possible to geometrically calculate the physical
dimensions of the two stars. Comparing this with
their observed temperature and apparent bright-
ness yields a distance. Eclipsing binaries were
used in 2013 to determine the distance to the
Large Magellanic Cloud to a precision of 2%.Red giant stars At the end of their lives, Sun-like
stars turn into bloated red giants, slowly in-
creasing in size and luminosity. Just before they
experience the so-called helium fl ash, they all
produce the same amount of energy. By observing
a large number of red giants in another galaxy, it
is possible to derive the apparent brightness of
this so-called tip of the red giant branch (TRGB).
Comparing that to the known absolute brightness
yields the distance. The TRGB method has been
calibrated by observing red giants in the Large
Magellanic Cloud, which is at a precisely known
distance.
Megamasers Excited by X-rays from a supermassive
black hole, water molecules in orbiting gas clouds
can be stimulated to emit maser light (just like
laser, but at microwave frequencies). These maser
regions can be seen orbiting the black hole as
a disk through very long baseline interferometry
(VLBI). Combining the apparent size of the mega-
maser disk with the actual size that would match
the clouds’ velocities, derived from spectroscopic
observations, yields a precise distance, to within
some 3% in the case of M106. Unfortunately, water
megamasers are not extremely common, but the
technique has proved to be useful in calibrating
other “standard candles.”Step 3: Astronomers then deduce the properties the primordial
plasma had to have in order to produce this spectrum, including the
speed of sound in the plasma and the density of diff erent types of
matter. From those, researchers work out what the CMB patterns’
true physical size must be. Since they know the apparent size, the
comparison provides the distance, which depends on the history of
cosmic expansion.
EarthDistanceH 0 = 67.4 km/s/Mpc