the 1920s astronomer Edwin Hubble discovered that galaxies
are all moving away from us at a rate proportional to their
distance. The farther a galaxy is, the faster it appears to
be moving away. This relationship, called the Hubble constant,
establishes an expansion rate that is critical for estimating
the age and size of the universe. The universe is expanding
because it was born in a fiery explosion, called the Big Bang,
many billions of years ago.
quest to precisely determine the Hubble constant was headed
by the Key Project team, a group of astronomers who used NASA's
Hubble Space Telescope to look far away for accurate "milepost
markers," a special class of stars called Cepheid variables.
The rhythmic pulsation of these aged, bright stars yields
their intrinsic brightness, which in turn is needed to measure
precise distances. Hubble's clear vision has allowed astronomers
to accomplish in a few years Cepheid observations that previously
required decades of painstaking work with ground-based telescopes.
used Hubble to systematically search for Cepheids in our galactic
neighborhood out to the distance of the Virgo cluster of galaxies.
Cepheids were used to calibrate even more remote markers such
as supernovae, which are so bright they can be seen at far
greater distances, billions of light-years.
team found that the universe is between 12 and 14 billion
years old, depending on the density of matter in space.
played an important role in the discovery that the expanding
universe may actually be accelerating. In other words, the
galaxies are rushing away from each other ever faster as time
observations of distant supernovae were used to measure how
quickly the universe was expanding long ago. Astronomers were
surprised to discover that the universe was actually expanding
at a slower rate billions of years ago. This implies that
there may be a mysterious force, an "anti-gravity" that pushes
galaxies apart at an increasingly faster rate the farther
away they are from each other. This means the universe may
continue expanding forever and never collapse in a fiery "Big
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