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The Age of the Universe — Measuring Cosmic Time
By Joel R. Primack and Nancy Ellen Abrams
The earth is about 4.5 billion years old, slightly younger than the oldest objects in the solar system whose ages we have been able to measure.1 But the age of the earth is conceptually a much simpler idea than the age of the universe. We’re all riding the earth together. This means we have simply ignored the fact that the earth has been spinning around its axis, rotating around the sun, spiraling around the Galaxy as our sun orbits the Galactic center roughly every 220 million years, and moving with the Galaxy itself in both the local flow of galaxies and the overall expansion of the universe. What does all this motion have to do with age? Age is measured by clocks, and in our relativistic universe motion affects the rate at which clocks run. When we talk about the age of the universe, we can no longer ignore this.
People often make three fundamental assumptions about time: that it is universal; that if clocks were absolutely accurate, they would all measure time passing at the same rate; and that time can be divided basically into past and future, with the past over and gone. These are useful approximations for getting along on Earth, but with regard to the universe as a whole, all three are wrong.
Weird effects described by Einstein’s relativity happen to time as soon as people or things move with respect to each other at speeds comparable to the speed of light; one of those effects is that each one observes the other one’s clock as running slow. There’s nothing wrong with the clocks – time itself slows down for the fast traveler, compared to the rate at which it runs for those left behind. The effect is real: if one twin could travel at a speed approaching that of light, from the viewpoint of Earth her time would slow down so much that she would come back younger than her twin sister. Clocks also run slow when gravity is strong. That moving clocks run slow is constantly demonstrated at particle accelerator laboratories, at which the lifetimes of unstable elementary particles are sometimes lengthened by huge factors when they are moving close to the speed of light. Both this effect and the effect of gravity on the speed of clocks described by Einstein’s general theory of relativity must be taken into account in the operation of the Global Positioning System (GPS). If they were neglected, the GPS errors would amount to many kilometers per day!
For the universe as a whole, in which everything is moving with respect to everything else and some of the light that we see comes from near black holes where the gravity is very strong indeed, what clock can we possibly use to describe a meaningful age? What all cosmologists do is to imagine that there are clocks everywhere that started running at the Big Bang and that move with the expansion of the universe along with the nearby galaxies. This gives a definite cosmic time that all observers can agree on.2
The age of the universe can’t be measured directly, but it can be calculated several different ways based on different sorts of observations. One method is to figure out how long it would have taken for the universe to expand to its present size. To do that, we need to know the expansion rate now and how much matter and dark energy there are; fortunately, we now know all three numbers fairly accurately.3 According to general relativity, the higher the expansion rate, the younger the universe. Independently of the expansion rate, there is another factor that depends only on how much matter and dark energy there is: for the same expansion rate, the universe is younger the more matter there is, and older the more dark energy. Consequently, until we knew what the universe was made of, no one could tell for sure the age of the universe. There was even a period in the 1990’s when the universe seemed to be younger than some of its stars.4 At that time, no one knew whether the universe just contained matter or whether dark energy might actually exist. The age of the universe is the linchpin of our understanding, because interpretation of many aspects of the universe depends on its age, and those aspects all remained uncertain until we pinned the age down. In 1998, the discovery of convincing evidence for dark energy ended the uncertainty. Dark energy exists. All discrepancies have disappeared. The universe is about fourteen billion years old. We finally understand how our cosmos fits together.Today every measurement of the age of the universe, made in every way cosmologists have dreamed up, agrees.5 The age will be refined, but it won’t change. Like the age of the earth, we now know the age of the universe (about 13.7 billion years). Knowing how much time there has been since the beginning, we humans can locate ourselves in the immensity of it. Earth is almost incomprehensibly ancient, yet the universe was twice as ancient before Earth had even formed. The value of knowing the age of the universe is not just to toss around a big number but to start putting together a scientifically verifiable story of the evolution of the universe, the earth, and life, painting a picture on a scale no one could visualize before. As recently as the seventeenth and eighteenth centuries, the general view was that the earth and universe were the same age – about 6000 years – which was only five days older than the first humans, who were created on the sixth day according to Genesis. Even today on opinion polls almost half of U.S. respondents typically say that they believe that the earth and the entire universe are less than 10,000 years old.6 But the evidence of the enormity of time fills space and is written in fossils, rocks, and meteorites here on Earth, and in the rocks that the astronauts brought back from the moon. The universe is so old that the people who wrote the Bible could not even have conceived of the number, let alone the reality of such a universe.
1.^ This article was adapted by Joel R. Primack and Nancy Ellen Abrams from Chapter 5 of their book The View from the Center of the Universe: Discovering our Extraordinary Place in the Cosmos (Riverhead, 2006). An explanation of how the age of the earth was determined can be found earlier in the same chapter. See also the article "How Old Is The Earth" in this series, by Steven Newton.
2.^ There is a preferred reference frame (the cosmic clock reference frame) at every point in space and time, in which the cosmic background radiation has the same temperature in all directions (except for the tiny fluctuations discussed later in this chapter). In special relativity, which applies to motion in straight lines at constant speed in empty space, there cannot be any special reference frame. But our universe isn’t empty space – it is filled with the cosmic background radiation and the light from all the galaxies. If one is moving with respect to the reference frame of the cosmic background radiation, the radiation will appear blueshifted in the spaceship’s front window and redshifted looking out the rear. Measurements of the cosmic background radiation using the COBE satellite showed just these sorts of redshifts and blueshifts as the earth moved around the sun. But this was not the first proof that Copernicus was right that the earth orbits the sun: a slight seasonal change in the apparent direction of stars (“stellar aberration”) caused by the earth’s motion was discovered in 1726 by astronomer James Bradley.
3.^ The composition of the universe is discussed in Chapter 4 of Primack and Abrams, The View from the Center of the Universe: Discovering our Extraordinary Place in the Cosmos (Riverhead, 2006). The latest measurements show that the main components of the cosmic density are dark energy (about 72%) and non-atomic dark matter (about 23%). Atoms make up about 5%, but only about 1/2% is in the visible parts of galaxies. Almost all the atomic matter is made of the two lightest elements, hydrogen and helium. The heavier atoms that the earth and people are mostly made of make up only about 0.01% of the cosmic density.
4.^ The age that astronomers had deduced for these stars was based on the now-standard theory of how stars shine and evolve as they fuse hydrogen to helium, and also on the very reasonable assumption that all the stars in dense star clusters called “globular clusters” were born at about the same time. In a given cluster, stars that are more massive than the sun are brighter and burn the hydrogen in their centers faster, and thus have shorter lives than the sun; less massive stars are dimmer and take longer to use up the hydrogen in their centers. The age of the star cluster can thus be determined by measuring the brightness of the brightest stars that are still burning hydrogen in their centers – the ones that are just about to run out. The disagreement between the age of the oldest star clusters and the age of the universe was partly resolved when better data showed that the distances to the clusters with the oldest stars had been underestimated, so these stars were actually brighter and therefore younger – about 13 billion years old, as expected in a 14 billion year old universe. The revised distances and ages were first published in 1997 from analysis of data from the European Space Agency’s Hipparcos satellite, which was launched in 1989. Hipparcos was the first astrometric satellite, designed to measure the positions of stars with unprecedented accuracy. It determined the distances to more than 100,000 stars using stellar parallax, and thereby allowed astronomers to calibrate methods that could be used to determine the distance to the oldest globular star clusters in the Milky Way whose ages were in question.
5.^ The age of the universe has been confirmed recently with radioactivity. Thorium and uranium, some of the same elements used to date the formation of the earth, have now been measured in some of the oldest stars in our galaxy, revealing that they are about 13 billion years old. Thus three independent methods of measuring the age agree: the expansion of the universe, the evolution of stars, and radioactive dating.
6.^ In the May 2008 Gallup poll on evolution, creationism, and intelligent design, 44% of respondents chose "God created human beings pretty much in their present form at one time within the last 10,000 years or so," 36% selected "Human beings have developed over billions of years from less advanced forms of life, but God guided this process," and 14% chose "Human beings have developed over millions of years from less advanced forms of life, but God had no part in this process." The American public has not significantly changed its opinion on this question since Gallup started asking it in 1982 (see www.gallup.com/poll/21814/Evolution-Creationism-Intelligent-Design.aspx.)