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Age of the Universe

Age of the Universe

The idea that the universe had a beginning is common to various religions and mythologies. However, astronomical evidence that the universe truly has a finite age did not appear until early in the twentieth century. The first clue that the universe has a finite age came at the end of World War I, when astronomer Vesto Slipher noted that a mysterious class of objects, collectively called spiral nebula , were all receding from Earth. He discovered that their light was stretched or reddened by their apparent motion away from Earththe same way an ambulance siren's pitch drops when it speeds away from a stationary observer.

Hubble's Contribution

In the early 1920s American astronomer Edwin P. Hubble was able to measure the distances to these receding objects by using a special class of mile-post marker stars called Cepheid variables. Hubble realized that these spiral nebulae were so far away they were actually galaxiesseparate cities of starsfar beyond our own Milky Way.

By 1929, Hubble had made the momentous discovery that the farther away a galaxy is, the faster it is receding from Earth. This led him to conclude that galaxies are apparently moving away because space itself is expanding uniformly in all directions. Hubble reasoned that the galaxies must inevitably have been closer to each other in the distant past. Indeed, at some point they all must have occupied the same space. This idea led theoreticians to conceive of the notion of the Big Bang, the theory that the universe ballooned from an initially hot and dense state.

Hubble realized that if he could measure the universe's speed of expansion, he could easily calculate the universe's true age. Assuming the universe's expansion rate has not changed much over time, he calculated an age of about 2 billion years. One problem with this estimate, however, was that it was younger than geologists' best estimate for the age of Earth at the time.

Astronomers since then have sought to refine the expansion rateand the estimate for the universe's ageby more precisely measuring distances to galaxies. Based on uncertainties over the true distances of galaxies, estimates for the universe's age have varied from 10 billion to 20 billion years old.

More Recent Estimates

A primary task of the Hubble Space Telescope (HST), launched in 1990, was to break this impasse by observing Cepheid variable stars in galaxies much farther away than can be seen from ground-based telescopes. The HST allowed astronomers to measure precisely the universe's expansion rate and calculate an age of approximately 11 to 12 billion years.

Estimating the age is now complicated, however, by recent observations that show the universe expanded at a slower rate in the past. This is due to some mysterious repulsive force, first envisioned by physicist Albert Einstein as part of his so-called fudge factor in keeping the universe balanced. The presence of such a repulsive force pushing galaxies apart means that the universe is more likely to be 13 to 15 billion years old.

Using Stars to Estimate Age.

The universe's age can also be estimated independently by observing the oldest stars. Astronomers know that stars must have started forming quickly after the universe expanded and cooled enough for gas to coagulate into stars. So the oldest star must be close to the true age of the universe itself. The oldest stars, which lie inside globular clusters that orbit our galaxy, are estimated to be at least 12 billion years old. These estimates are difficult because they rely on complex models and calculations about how a star burns its nuclear fuel and ages.

A simpler cosmic clock is a class of star called white dwarfs, which are the burned-out remnants of Sun-like stars. Like dying cinders, it takes a long time for dwarfs to cool to absolute zerolonger than the present age of the universe itself. So the coolest, dimmest dwarfs represent the remnants of the oldest stars. Because they are so dim, these dwarfs are hard to find. Astronomers are using the HST to pinpoint the very oldest white dwarfs in globular clusters.

The HST has uncovered the very faintest and coolest dwarfs in the Milky Way galaxy, with ages of 12.6 billion years, thus giving an age estimate for the universe of 13 to 14 billion years. This is a very successful and entirely independent confirmation of previous age estimates of the universe.

Astronomers now know the age of the universe to within a good degree of accuracy. This is quite an achievement considering that less than a century ago, astronomers did not even realize the universe had a beginning.

see also Cosmology (volume 2); Hubble, Edwin P. (volume 2); Stars (volume 2).

Ray Villard

Bibliography

Guth, Alan H., and Alan P. Lightman. The Inflationary Universe: The Quest for a New Theory of Cosmic Origins. Reading, MA: Addison-Wesley Publishing, 1998.

Hogan, Craig J., and Martin Rees. The Little Book of the Big Bang: A Cosmic Primer. New York: Copernicus Books, 1998.

Livio, Mario, and Allan Sandage. The Accelerating Universe: Infinite Expansion, the Cosmological Constant, and the Beauty of the Cosmos. New York: John Wiley & Sons, 2000.

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Age of the Universe

Age of the Universe


In contemporary scientific cosmology, the age of the universe is the time that has elapsed since the Big Bang, which in standard cosmological models is the past limit to the hotter, denser phases that are encountered as one goes farther and farther back into the past. In these models the Big Bang is a singularity, a region characterized by infinite density, temperature, and curvature. Quantum gravitational and quantum cosmological treatments of the Big Bang, using concepts like superstrings, are beginning to provide a more adequate description of this primordial cosmological epoch, which is often referred to as the Planck era, during which the temperature of the universe was above 1032 K (kelvin). Here, classical relativistic gravitational theory (Albert Einstein's General Relativity) breaks down. It is from this extremely hot Planck era that the universe emerges with its three spatial dimensions, its one time dimension, its four basic physical interactions, and its matter and radiation. Before that emergence they were all unified in ways that are not yet completely understood.

A rough upper limit on the age of the universe, t h, is given by the reciprocal of the Hubble parameter now, H 0, which gives the rate of expansion of the universe per unit distance. Thus, t h = 1/H 0. Using the currently measured range of values of H 0, t h is between twelve to sixteen billion years. Compare this to the very reliable age of the Earth and the sun, which is about 4.8 billion years. These ages have been confirmed by a variety of astronomical and isotopic techniques, including the measurement of the ages of stars in globular clusters (which are very old), and the estimation of how much uranium has decayed to lead and how much rubidium has decayed to strontium.

From the point of view of prescientific cultural and religious traditions, the age of the universe is the time that has elapsed since the world or the universe was created. In many traditions the creation is also taken to be the "event" in which time itself began. Some of those who interpret the Genesis creation and pre-Abraham historical accounts literallyas scientifically and historically reliable documents describing the formation of the universe and of the world, and earliest human historyhave calculated the age of the world and of created reality (the universe) to be about 6,000 years, having begun in 4004 b.c.e. This has been done by counting the generations listed in Genesis from Adam and Eve to Abraham, and then estimating the number of years from Abraham to Moses, both of which are fairly well known, to the present. Experts have disputed this literal approach, of course, particularly because it is strongly contradicted by independent bodies of evidence from both the natural and the human sciences. It also fails to recognize the mythological and legendary character of the relevant Genesis sources. This does not mean that the Genesis sources are not revealing and expressive of important truths, but it does mean that those truths are neither scientific nor directly historical, but rather religious and theological truths.

The cosmological age of the universe since the Big Bang, although it certainly has important theological significance, cannot be interpreted as the time since the creation of the universe, if universe is understood to mean all that exists and not God. There could have been and there could be many other regions of reality, either completely separate from or linked with ours only at the Big Bang itself, which preceded or are older than our observable universe. Furthermore, it is unclear whether "creation" or "the first moment of creation" took place at any definite time. However, it does make some sense to date the beginning of the observable universe at the Big Bang, even though the coordinated manifold of primordial quantum events is not adequately understood.


see also big bang theory; cosmology, physical aspects; singularity; string theory


Bibliography

börner, gerhard. the early universe: facts and fiction, 3rd edition. berlin, heidelberg, and new york: springer-verlag, 1993.

coles, peter, and lucchin, francesco. cosmology: the origin and evolution of cosmic structure. new york: wiley, 1995.

kolb, edward w., and turner, michael s. the early universe. reading, mass.: addison-wesley, 1990.

william r. stoeger

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