1998

Dark Energy

Albert Einstein (1879–1955), Edwin Hubble (1889–1953)

When physicist Albert Einstein developed his theory of general relativity early in the twentieth century to explain the relationship between space, time, and matter in the presence of gravitational fields, it was in the context of what astronomers believed at the time to be a static, or unchanging, universe. To make his theory work, Einstein had to invent an as-yet-unseen force, which he called the cosmological constant, to counteract the attractive force of gravity and enable the universe to remain static. When Edwin Hubble discovered in 1929 that space is actually expanding, Einstein figured that his cosmological constant wasn’t needed after all. Everything seemed to fit nicely.

But astronomers’ detailed study of spiral galaxies and galactic cluster motions over subsequent decades led to the surprising discovery of an unseen but gravitationally attractive material known as dark matter, and the humbling realization that most of the universe consists of matter that we cannot observe directly. Even more surprising was the discovery by astronomers in 1998 that the expansion of the universe appears to be accelerating with time. That is, galaxies nearer to us, which we observe in the relatively “modern” era of the universe, are moving away from each other faster than extremely distant galaxies, which we observe from much earlier in the universe’s history. One possible explanation is that there is some sort of unseen energy force or pressure that permeates the vacuum of space and acts opposite to gravity, helping to accelerate the normal expansion of space from the Big Bang. Cosmologists call this hypothesized force dark energy. Maybe Einstein was right about his cosmological constant after all.

The true nature (or existence) of dark energy is impossible to study directly by traditional telescopic observations. As with dark matter, its presence can only be inferred indirectly, by studying its gravitational effects on normal matter. If dark energy proves to be real, however, it would lead to an even more stunning and humbling realization about our universe: dark energy and dark matter, things we currently have no way to measure or characterize, comprise 96 percent of the energy in our universe, while ordinary matter—galaxies, stars, planets, us—accounts for only 4 percent of the cosmos!

SEE ALSO Big Bang (c. 13.7 Billion BCE), Recombination Era (c. 13.7 Billion BCE), Einstein’s “Miracle Year” (1905), Hubble’s Law (1929), Dark Matter (1933), Spiral Galaxies (1959), Gravitational Lensing (1979), Walls of Galaxies (1989), Hubble Space Telescope (1990), Mapping the Cosmic Microwave Background (1992).

Clusters of galaxies, as in this 2002 Hubble Space Telescope photo of Abell 1689, represent concentrations of normal matter (galaxies), dark matter (colored blue based on astronomers’ inferences from gravitational lensing), and the hypothetical force known as dark energy.