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Optical
image of Supernova 1994d exploding in the outer parts of the
spiral galaxy NGC 4526.
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Astronomers
and engineers carefully design giant visible-light and infrared
telescopes, including the NGST and the GSMT, to make detailed observations
of the individual objects they have found, such as galaxies, quasars,
or protoplanetary disks. In addition, however, we also require telescopes
capable of surveying the entire sky, providing us with the numbers
and distribution of different species of cosmic objects. Astronomy
has continually benefited from such surveys, each of them made with
greater accuracy than its predecessors, probing deeper into space
by recording objects still fainter than previous surveys could within
a particular domain of the spectrum. These survey efforts might
seem relatively straightforward exercises in improving our catalogs
of the cosmic zoo, but each significant improvement in survey techniques
has opened the door to new discoveries, and often to entire new
realms of astronomical objects.
In
addition to future discoveries, which we cannot predict with specificity,
surveys offer the opportunity to resolve long-standing astronomical
mysteries. One such mystery deals with the recent discovery that
the universe teems with dark energy, energy that lurks in empty
space and accelerates the universe’s expansion. Surveys of galaxies
can test this discovery by determining whether the observed distribution
of galaxies matches the prediction of a model in which the universal
expansion indeed has entered an accelerating phase.
Until
now, surveys of the cosmos have proceeded, by necessity, on the
supposition that the universe remains the same on time scales measured
in a few years, the time needed to complete a single survey. This
supposition has governed surveys of galaxies, stars, or asteroids
(the small objects that orbit our Sun by the hundreds of thousands).
During the next decade, it will become possible to make a complete
survey of the sky a hundred times more rapidly than before. Such
a survey will add the time dimension to the three dimensions of
space, revealing objects that vary on time scales measured in days.
Thus, for example, the new surveys will reveal a host of exploding
stars, which typically remain visible only for a few weeks or months.
The new survey techniques will replace still photographs of the
cosmos with an ongoing movie — a movie that will show more detail
in each frame than any of the still frames of its predecessors.
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