34 Scientific American, March 2020 Illustration by George Retseck (expansion cone) and Jen Christiansen (schematics)
Baseline
Perceived wavelength
of light emittedTelescopeRetreating 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 telescopeA Conflict at the Core of Cosmology
The value of the Hubble constant (H 0 ), which measures the universe’s current rate of expansion, is both an essential and
a controversial number for all of cosmology. In defiance of all expectations, estimates of H 0 from the “early” universe shortly
after the big bang and from the “late” universe closer to the present day do not agree. The discrepancy may be the result
of errors in either set of estimates, or it could reflect fundamental gaps in our current understanding of the universe.VIEW FROM THE
EARLY UNIVERSE
H 0 can also be measured using the cosmic
microwave background (CMB), the big bang’s
all-sky afterglow from when the universe was
just a 379,000-year-old expanse of dense,
seething plasma. Sound waves reverberating
through this plasma created overdensities
and underdensities of matter that are im -
printed on the CMB as minor temperature
variations. By studying the size and other
properties of these variations, cosmologists
can use them as a “standard ruler” against
which to chart the universe’s subsequent
growth and evolution. These studies, in turn,
yield an estimate of H 0.CLOCKING SPEEDING GALAXIES
The classic approach to calculating H 0 in the late
universe requires measuring both the velocities
and distances of far-off galaxies. Getting a velocity
relies on a phenomenon called cosmological
redshift—the stretching out, or reddening,
of light from objects receding from us as
the universe expands. The greater
the redshift, the faster an
object is receding.© 2020 Scientific American © 2020 Scientific American