The universe is big in both space and time and, for much of
humankind’s history, was beyond the reach of our instruments and our
minds. That changed dramatically in the 20th century. The advances were
driven equally by powerful ideas—from Einstein’s general relativity
to modern theories of the elementary particles—and powerful
instruments—from the 100- and 200-inch reflectors that George Ellery
Hale built, which took us beyond our Milky Way galaxy, to the Hubble
Space Telescope, which has taken us back to the birth of galaxies. Over
the past 20 years the pace of progress has accelerated with the
realization that dark matter is not made of ordinary atoms, the discovery of dark energy, and the dawning of bold ideas such as cosmic inflation and the multiverse.
The universe of 100 years ago was simple: eternal, unchanging,
consisting of a single galaxy, containing a few million visible stars.
The picture today is more complete and much richer. The cosmos began
13.7 billion years ago with the big bang. A fraction of a second after
the beginning, the universe was a hot, formless soup of the most
elementary particles, quarks and leptons. As it expanded and cooled,
layer on layer of structure developed: neutrons and protons, atomic
nuclei, atoms, stars, galaxies, clusters of galaxies, and finally
superclusters. The observable part of the universe is now inhabited by
100 billion galaxies, each containing 100 billion stars and probably a
similar number of planets. Galaxies themselves are held together by the
gravity of the mysterious dark matter. The universe continues to expand
and indeed does so at an accelerating pace, driven by dark energy, an
even more mysterious form of energy whose gravitational force repels
rather than attracts.
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