March 2020, ScientificAmerican.com 35Baseline
Perceived wavelength
of light emitted
TelescopeRetreating objectRapidly retreating objectAdvancing objectCosmic Microwave BackgroundBig BangTodayCepheid in Milky WayCepheid and type Ia
supernova in nearby galaxyType Ia supernova
in distant galaxySunEarth2000 2004 2008 2012 2016 20206065707580Publication Year of the EstimateHubble Constant Estimate(kilometers per second
per 3.26 million light-years)^677074Error bars span values within one
standard deviation of the meanDistance ladder method
using CepheidsDistance ladder method
using flaring red giant starsCMB measurementsIncreased precision of Hubble constant calculations over timeDistance ladder method:
Largely from increase
in number of discrete
measurementsCMB method:
Largely from increase
in precision of
measurementsVelocity DistanceBest fit (H 0 )
Error rangeWMAP telescope Planck telescopeCLIMBING
THE DISTANCE LADDER
Reckoning distances to remote galaxies is
far harder than measuring their velocities
via redshift. Astronomers seeking the late
universe value of H 0 do this by ascending
what is known as the cosmic distance
ladder, in which different measurement
methods are successively stacked to gauge
vast dis tances. Cepheids—variable stars
with known intrinsic brightness—
typically constitute the ladder’s first
“rung” and can establish distances
to nearby galaxies. More distant
galaxies, however, require
different, brighter objects with
known intrinsic brightness—
usually certain types of
ex ploding stars called type Ia
super novae. As tronomers
cali brate be tween these
two distinct techniques
using nearby galaxies
harboring both Cepheids
and type Ia supernovae.TOWARD A MORE PERFECT
UNION—OR NEW PHYSICS
Astronomers and cosmologists alike are
working to increase the precision of their
respective estimates of H 0 , progressively
reducing uncertainties and possible errors
in hopes their results may eventually
overlap. Larger telescopes are gazing deeper
into the cosmos, measuring Cepheids ever
farther from Earth, and the CMB-mapping
Planck satellite has dramatically improved
on the measurements of its predecessor,
the Wilkinson Microwave Anisotropy Probe
(WMAP). If, however, the discrepancy
endures, profound revisions to our
cosmological models may be required.DIVERGING RESULTS
The CMB-based, early universe value
for H 0 is 67 (in units of kilometers per
second per 3.26 million light-years). The
Cepheid-based, late universe value is 74.
A new alternative to Cepheids—red giant
stars that flare with a known intrinsic
brightness—only complicated the tension.
They indicated an H 0 of about 70—a value
that is midway between the other two, with
no overlap of error ranges.First rung in distance ladder: Nearby
Cepheid position (in Milky Way) is
calculated based on triangulation using
more than one telescope viewing.
Apparent brightness is noted.Second rung: Position of Cepheid in
a nearby galaxy is determined based
on first-rung calculations and is
calibrated against apparent brightness
of a nearby supernova.Third rung: Position of supernova
in distant galaxy is determined based
on second-rung calculations.ESA AND PLANCK COLLABORATION (Planck CMB); NASA AND WMAP SCIENCE TEAM (WMAP CMB detail); SOURCE: “THE CARNEGIE-CHICAGO HUBBLE PROGRAM. VIII. AN INDEPENDENT DETERMINATIONOF THE HUBBLE CONSTANT BASED ON THE TIP OF THE RED GIANT BRANCH,” BY WENDY L. FREEDMAN ET AL., INASTROPHYSICAL JOURNAL,VOL. 882, NO. 1; AUGUST 29, 2019 (diverging results chart)© 2020 Scientific American © 2020 Scientific American