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called the “K-factor.” The gradient
is described mathematically by a
value now known as the Hubble
Constant (H 0 ). This important
number determines not only the
size of the observable universe
but also its age. The Hubble
Constant allowed astronomers
to work backward and calculate
the moment in time of the Big
Bang itself, when the radius of
the universe was zero.
The initial calculation of H 0
was 300 miles (500 km) per second
per megaparsec (one megaparsec
is approximately 3.26 million light-
years). This presented a problem,
since it gave a figure of 2 billion
years for the age of the universe,
less than half the accepted age
of Earth. The discrepancy was
found to have been caused by
systematic errors in Hubble’s
distance measurements. Many
were out by a factor of seven
due to his method of taking the
brightest star in any galaxy—or
even the luminosity of the galaxy
itself—and assuming it to be a
Cepheid variable star. Luckily for
Hubble, the inaccuracies were
fairly consistent throughout the
dataset, allowing him to plot
the trend in spite of them.Hubble Key Project
Calculating the rate of expansion
of the cosmos drove the decision
to develop the Hubble Space
Telescope from its inception
in the 1970s to its 1990 launch.
NASA made one of the telescope’s
“Key Projects” determining the
Hubble Constant to within 10
percent. As a result, the instrument
spent years measuring CepheidATOMS, STARS, AND GALAXIES
light curves. The final result,
delivered in 2001, gave an age for
the universe of 13.7 billion years.
This figure was fine-tuned to 13.799
billion years (with an error of 21
million years either way) by data
from the Planck Space Observatory
in 2015. The most dramatic revision
to Hubble’s law, however, came in
1998 when astronomers discovered
that the universe’s expansion is
accelerating due to a mysterious
and unknown agent known as
dark energy, which has led to a
renewal of interest in Einstein’s
so-called blunder, the cosmological
constant (pp.298–303). ■The ESA’s Planck Observatory
operated between 2009 and 2013. It
produced data that helped to measure
many cosmological parameters,
including the Hubble Constant.