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y^(km/s)Distance (megaparsecs)05001,000012WWW.ASTRONOMY.COM 27
But that’s not the end of the story.
Cosmologists also can infer the value
of the Hubble constant by studying the
primordial light released when the first
atoms formed some 380,000 years after
the Big Bang. The detailed temperature
patterns of this light — known as the
cosmic microwave background — serve
as a map that shows how matter was
distributed throughout the universe
at that time.
When scrutinized, this map reveals
many details about our young universe,
including how much matter and other
forms of energy were present, as well as
how fast space was expanding. It also
tells us that the Hubble constant is about
67 km/s/Mpc — a significantly smaller
value than cosmologists have found
through more direct measurements.
What does this mismatch mean for
our universe? Assuming that these studies
have correctly accounted for all the sys-
tematic uncertainties inherent in the
observations, these two ways of determin-
ing the Hubble constant appear to be
incompatible — at least within the con-
text of the standard cosmological model.
To make these discrepant results mutually
consistent, astronomers would be forced
to change how we think the cosmos
expanded and evolved, or to reconsider
the forms of matter and energy in the
universe during the first few hundred
thousand years following the Big Bang.
According to Einstein’s general theory
of relativity, the rate at which space
expands depends on the density of
Edwin Hubble discovered the expanding
universe in 1929, when he first plotted the
velocities of individual galaxies relative to
their distances. (The line gives the best fit to
the data.) Although his estimated distances
were a factor of nearly 10 too small, he got
the basic relationship right. ASTRONOMY: ROEN KELLY,
AFTER EDWIN HUBBLE/PNAS (MARCH 15, 1929)GALAXIES IN MOTION
The Antennae Galaxies (NGC 4038 and NGC 4039) are two interacting spirals located 65 million light-years
from Earth. Host to many Cepheid variable stars as well as a type Ia supernova, the pair is one of a handful of
systems that possess both types of standard candles and thus forge a link between the two. ESA/HUBBLE & NASASpiral galaxy MCG+01-38-005 (the lower member
of this galaxy pair) harbors a water megamaser —
amplified microwave emission from water molecules
— orbiting the supermassive black hole at its center.
The microwave emission provides an independent
way to measure the galaxy’s distance and thus helps
astronomers refine values for the Hubble constant.
ESA/HUBBLE & NASASpiral galaxy NGC 3972 forms a key link in the cosmic
distance ladder. The galaxy contains dozens of
Cepheid variables, which astronomers use to gauge
distances to relatively nearby galaxies, and in 2011
hosted a type Ia supernova, an exploding white dwarf
that serves as a vital link to finding distances to more
remote galaxies. Scientists need both markers to pin
down the Hubble constant. NASA/ESA/A. RIESS (STSCI/JHU)