skyandtelescope.com • JUNE 2019 25ies — 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 ear-
lier. The Hubble-Lemaître Law describes this linear relation-
ship between distance and “recession” velocity; the propor-
tionality 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 kilometers 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 cosmo-
logical “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 traveling 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 kilometers 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 relation-
ship in the early 1900s, and it’s now known as the Leavitt
Law. So if you fi nd a Cepheid in a remote galaxy, its observed
period tells you how luminous it is, and the star’s apparent
brightness then reveals the galaxy’s distance.
Laboratory reference Reds
hift
Distant galaxy
400 450 500 550 600 650 700 750 800
Distance (megaparsecs)H 0 = 73.5 km/s/MpcVeloci
ty(km/second)Rate: A plot of the galaxies’ distances versus
redshifts shows that farther galaxies recede faster.
(Simulated data shown.) The slope of the line is the
universe’s expansion rate.Velocity:The expansion of the universe shifts the
standard candle’s light to longer, redder wavelengths.
The amount of redshift tells astronomers the galaxy’s
apparent recession velocity.
Using the eagle-eyed vision of the Hubble Space Telescope,
which was designed in part for this work, a team led by
Wendy Freedman (now at the University of Chicago) suc-
ceeded in identifying Cepheid variables in spiral galaxies at
distances of hundreds of millions of light-years. “The fi nal
results of our Key Project, published in 2001, yielded a Hubble
constant of 72 km/s/Mpc,” she says, “but the uncertainty in
that value was some 10%.” Still, this was a huge achievement:
Before the launch of Hubble in April 1990, the best estimates
for H 0 ranged from 50 to 100 km/s/Mpc. Moreover, the Hub-
ble results enabled astronomers to calibrate other distance
indicators that could be used farther out, where individual
Cepheids can’t be seen anymore.
One of those standard candles are Type Ia supernovae —
the catastrophic detonations of white dwarf stars that become
too massive to resist their own gravity, either by accreting
matter from a companion star or by merging with another
white dwarf (see page 14). Since the temperature and density
at which a white dwarf succumbs to gravitational collapse
is usually the same, Type Ia supernovae explode and fade in
a standard pattern, and from these light curves it’s pretty
straightforward to deduce their true luminosity. Once cali-
brated, a comparison with the observed apparent brightness
yields a distance estimate. (Some other ways of determining
cosmic distances are described in the box on page 27.)
Using Type Ia supernovae as standard candles, two inde-
pendent teams of astronomers made a startling discovery
in 1998: Even though the value of H 0 was not yet known to
a high level of precision, the observations of really remote
galaxies revealed that the cosmic expansion rate isn’t slowing
down, as had always been assumed, but is actually speeding
up, despite the mutual gravitational attraction of all matter
in the universe. This momentous discovery, for which Saul
Perlmutter (University of California, Berkeley), Adam Riess
(Johns Hopkins University), and Brian Schmidt received the
2011 Nobel Prize in Physics, is now seen as evidence for a=