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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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Age of the universe

Age of the universe

Resources

According to NASAs Wilkinson Microwave Anisotropy Probe project, the age of the universe is estimated to be 13.7 billion (13,700,000,000) years oldplus or minus 200 million years. (The NASA satellite, launched in 2001, measures the temperature of radiant heat remaining from the big bang.) In a study published January 2003 in the journal Science, the age of the universe is estimated to be between 11.2 and 20 billion years old. Other recent estimates give its age to be somewhere between 10 and 15 billion years old. The universes age is measured from the event known as the big bangan 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 (17581840) 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 a person stood and what direction that person looked in, the sky would be a solid mass of light as bright as the surface of a star. (Such an argument would state that the universe has no age because it always existed. Such scientists believe in the steady state theory, or static universe theory. However, most observational and theoretical cosmologists support the big bang theory of the universe.) 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 proved that both are true and, today, in the twenty-first century, cosmologists continue to use these two ideas as the basis of their research. Although space has no edges or boundaries, it does contain a finite number of cubic miles. Furthermore, time did have a beginning, some 1.37 × 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 compressedthat 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 is 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 (18891953) 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 galaxys 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 Hubbles data. Even if the universe had a central point (which seemed unlikely), the chances against finding Earth there by luck seemed large. Rather, Hubbles observations were interpreted as proving that space itself was expanding. However, if space was expanding at a constant ratea concept which many scientists, including Hubble himself, resisted for years as too fantasticit could not have been expanding forever. To know the 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 galaxys distance from 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 universes 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, spherical-shaped groups of stars found near 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 data14 to 18 billion years agrees fairly well with that estimated from the Hubble constant.

Except for hydrogen, all the elements of which humans 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 iscontrary to what cosmologists long thought accelerating. Several observational tests since the late 1990s have confirmed this result. If it continues to hold up, scientists 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 universeand

KEY TERMS

Cepheid variable star A class of young stars that cyclically brighten and dim. From the period of its brightness variation, the absolute brightness of a Cepheid variable can be determined. Cepheid variables in distant galaxies give a measure of the absolute distance to those galaxies.

thus of the age of the universe itselfare 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 grams/centimeter3; below this threshold, the 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 momenta beginning of timebut rather a set of conditions under which the notion of time itself loses its meaning.

Resources

BOOKS

Harland, David Michael. The Big Bang: A View from the 21st Century. New York: Springer, 2003.

Hawking, Stephen. Universe in a Nutshell/Illustrated Brief History of Time. Random House, 2002.

Lemonick, Michael D. Echo of the Big Bang. Princeton, NJ: Princeton University Press, 2005.

Livio, Mario. The Accelerating Universe. New York: John Wiley & Sons, 2000.

Mallary, Michael. Our Improbable Universe: A Physicist Considers How we Got Here. New York: Thunders Mouth Press, 2004.

Singh, Simon. Big Bang: The Origins of the Universe. New York: Harper Perennial, 2005.

OTHER

Goddard Space Flight Center, National Aeronautics and Space Administration. Home page of the Wilkinson Microwave Anisotropy Probe project. <http://map.gsfc.nasa.gov/> (accessed September 28, 2006).

Larry Gilman

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Age of the Universe

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.

Resources

books

Hawking, Stephen. A Brief History of Time: From the Big Bang to Black Holes. New York: Bantam, 1988.

Hawking, Stephen. Universe in a Nutshell/Illustrated BriefHistory of Time. Random House, 2002.

Livio, Mario. The Accelerating Universe. New York: John Wiley & Sons, 2000.


Other

University of Cambridge. "Our Own Galaxy: The Milky Way." Cambridge Cosmology May 16, 2000 [cited February 3, 2003]. <http://www.damtp.cam.ac.uk/user/gr/public/gal_milky.htm>.


Larry Gilman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cepheid variable star

—A class of young stars that cyclically brighten and dim. From the period of its brightness variation, the absolute brightness of a Cepheid variable can be determined. Cepheid variables in distant galaxies give a measure of the absolute distance to those galaxies.

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