Age of the Universe

The Universe is approximately 14 billion (14,000,000,000) years old. Its age is measured from the event known as the big bang—an explosion filling all space and generating all of the matter and energy that exist today.

Although only in the last 50 years have astronomers been able to estimate the age of the Universe, they have long argued that the Universe must be of finite age, finite size, or both. This conclusion follows from the fact that the night sky is mostly dark. German astronomer Wilhelm Olber (1758–1840) noted in 1823 that if the Universe consisted of identical stars sprinkled through infinite space, and if it had existed for an infinitely long time, starlight would have had time to illuminate every point in the universe from every possible direction; in other words, no matter where you were and what direction you looked in, the sky would be a solid mass of light as bright as the surface of a star. Because the night sky is, in fact, dark, either the Universe does not contain an infinitely extensive population of stars, has not existed for an infinite time, or both.

Twentieth-century cosmologists have proved that "both" are true. Although space has no edges or boundaries, it does contain a finite number of cubic miles. Furthermore, time did have a beginning, some 1.4 × 10¹⁰ years ago. This figure is determined primarily by using the Doppler shift of light from distant galaxies. Doppler shift is the apparent change in frequency of a wave emitted by a source that is approaching or receding from an observer. If a wave source is receding from an observer, the waves detected by that observer are compressed—that is, their peaks and troughs arrive at longer intervals than they would if the source were stationary (or approaching). More widely spaced peaks and troughs correspond to lower frequency. Therefore, light from celestial sources that are receding from Earth is redshifted (shifted to lower frequencies, in the direction of the red end of the visible spectrum), while light from sources that are approaching is blueshifted (shifted to higher frequencies, in the direction of the blue end of the visible spectrum). In the 1920s, U.S. astronomer Edwin Hubble (1889–1953) observed that every distant galaxy, regardless of its position in the sky, is, as judged by redshift, receding rapidly from Earth; furthermore, more-distant galaxies are receding more rapidly than closer galaxies, and the speed of a galaxy's recession is approximately proportional to its distance (i.e., if galaxy B is twice as far from Earth as Galaxy A, it is receding about twice as fast).

Astronomers did not seriously consider the possibility that the whole Universe was expanding outward from a central point, with Earth located, by chance, at that central point, even though this would have explained Hubble's data. Even if the Universe had a central point (which seemed unlikely), the chances against finding Earth there by luck seemed large. Rather, Hubble's observations were interpreted as proving that space itself was expanding. However, if space was expanding at a constant rate—a concept which many scientists, including Hubble himself, resisted for years as too fantastic—it could not have been expanding forever. To know age of the Universe, one needed only to measure its present-day rate of expansion and calculate how long such an expansion could have been going on. If the expanding Universe were played backward like a film, how long before all the galaxies came together again?

This calculation turned out to be more difficult than it sounds, due to difficulties in measuring the rate of expansion precisely. It is easy to measure the Doppler shift of light from a star in a distant galaxy, but how does one know how far away that star is. All stars in distant galaxies are so far away as to appear as points of light without width, so their size and intrinsic (true) brightness cannot be directly measured. This problem was solved by discovering a class of stars, Cepheid variables, whose absolute brightness can be determined from the rapidity of their brightness variations. Since the absolute brightness of a Cepheid variable is known, its absolute distance can be calculated by measuring how dim it is. A Cepheid variable in a distant galaxy thus reveals that galaxy's distance from the Earth. By observing as many Cepheid variables as possible, astronomers have continually refined their estimate of the Hubble constant and thus of the age of the Universe.

Because of various uncertainties in measuring the characteristics of Cepheid variables, there is still some observational doubt about the Universe's rate of expansion. An independent method of calculating the age of the Universe relies on observing the types of stars making up globular clusters (relatively small, sphericalshaped groups of stars found in the vicinity of galaxies). By comparing the characteristics of clusters to knowledge about the evolution of individual stars, the age of the Universe can be estimated. The value estimated from globular cluster data—14 to 18 billion years—agrees fairly well with that estimated from the Hubble constant.

Except for hydrogen, all the elements of which we and Earth are composed were formed by nuclear reactions in the cores of stars billions of years after the big bang. About 4.5 billion years ago (i.e., when the Universe was about two-thirds its present age) the solar system condensed from the debris of exploded older stars containing such heavy elements.

Starting in the late 1990s, data have indicated that the expansion of the Universe initiated by the big bang is, contrary to what cosmologists long thought, accelerating. Several observational tests since the late 1990s have confirmed this result. If it continues to hold up, we can predict that, barring some bizarre quantum-mechanical reversal of cosmic history (as speculated by some physicists), the Universe will continue to expand forever. As it does so, its protons and neutrons will slowly break down into radiation, until, eventually, the entire Universe consists of a dilute, ever-expanding, ever-cooler gas of photons, neutrinos, and other fundamental particles.

It should be noted that all references to a "beginning of time" or a "zero moment" for the Universe—and thus of the "age" of the Universe itself—are a simplification. The young Universe cannot be meaningfully described in terms of "space" and "time" until its density drops below the Planck density, approximately 10⁹⁴ gm/cm³; below this threshold, our commonsense concept of "time" does not apply. Therefore, if one could watch time run backwards toward the big bang one would not encounter a zero-time moment—a "beginning" of time—but rather a set of conditions under which the notion of "time" itself loses its meaning.