on the age of the universe
How did the world begin? How old is it? Do mysterious and invisible forces determine its fate? Surprisingly enough, such questions are now at the forefront of scientific research.
Over the past century, old ideas about the cosmos and our place in it have been dramatically overturned. We now know that the Sun does not occupy the center of the universe, and that in addition to our own Milky Way, space is filled with hundreds of billions of other galaxies. Even more astonishingly, we know that the universe itself is expanding everywhere, and that as space expands, galaxies are being swept apart from each other at colossal speeds.
In the last few years, tantalizing hints have begun to appear that the expansion of the universe is even accelerating. These results imply the existence of a mysterious force able to counter the attraction of gravity. The origin and nature of this force currently defy explanation. But astronomers have reason to hope that ongoing research will soon resolve some of the deepest riddles of nature.
It was Edwin Hubble, a Carnegie Astronomer based in Pasadena, California, who first learned that the universe was expanding; in 1929, he discovered that the farther away from our Milky Way galaxies are, the faster they are moving apart. A few years before, Albert Einstein in his general theory of relativity had published a mathematical formula for the evolution of the universe. Einstein’s equations, like Hubble’s observations, implied that the universe must once have been much denser and hotter. These results suggested that the universe began with an intense explosion, a ‘big bang.’
The big bang model has produced a number of testable predictions. For example, as the universe expands, the hot radiation produced by the big bang will cool and pervade the universe – thus we should see heat in every direction we look. Big bang theory predicts that by today the remnant radiation should have cooled to a temperature of only 3 degrees above absolute zero (corresponding to a temperature of -270 degrees Celsius). Remarkably, this radiation has been detected. In 1965, two radio astronomers, Arnold Penzias and Robert Wilson, discovered this relic radiation during a routine test of communications dishes, a discovery for which they were awarded the Nobel Prize.
The current expansion rate of the universe, known as the Hubble constant, determines the size of the observable universe and provides constraints on competing models of the evolution of the universe. For decades, an uncertainty of a factor of two in measurements of the Hubble constant existed. (Indeed, determining an accurate value for the Hubble constant was one of the main reasons for building the Hubble Space Telescope.) However, rapid progress has been made recently in resolving the differences. New, sensitive instruments on telescopes, some flying aboard the Hubble Space Telescope, have led to great strides in the measurement of distances to galaxies beyond our own.
In theory, determining the Hubble constant is simple: one need only measure distance and velocity. But in practice, making such measurements is difficult. It is hard to devise a means to measure distances over cosmological scales accurately. And measuring velocity is complicated by the fact that neighboring galaxies tend to interact gravitationally, thereby perturbing their motions. Uncertainties in distances and in velocities then lead to uncertainties in their ratio, the Hubble constant.
Velocities of galaxies can be calculated from the observed shift of lines (due to the presence of chemical elements such as hydrogen, iron, oxygen) in the spectra of galaxies. There is a familiar analogous phenomenon for sound known as the Doppler effect, which explains, for instance, why the pitch of an oncoming train changes as the train approaches and then recedes from us. As galaxies move away from us, their light is similarly shifted and stretched to longer (redder) wavelengths, a phenomenon referred to as redshift. This shift in wavelength is proportional to velocity.
Measuring distances presents a greater challenge, which has taken the better part of a century to resolve. Most distances in astronomy cannot be measured directly because the size scales are simply too vast. For the very nearest stars, distances can be measured using a method called parallax. This uses the baseline of the Earth’s orbit, permitting the distance to be calculated using simple, high-school trigonometry. However, this technique currently can be applied reliably only for relatively nearby stars within our own galaxy.
In order to measure the distance of more remote stars and galaxies, astronomers identify objects that exhibit a constant, known brightness, or a brightness that is related to another measurable quantity. The distance is then calculated using the inverse square law of radiation, which states that the apparent brightness of an object falls off in proportion to the square of its distance from us. The effects of the inverse square law are easy to see in everyday life – say if we compare the faint light of a train in the distance with the brilliant light as the train bears down close to us.
To get a sense of the (astronomical) scales we are talking about, the nearest star to us is about 4 light-years away. One light-year is the distance that light can travel within a year moving at the enormous speed of 186,000 miles per second. At this speed, light circles the Earth more than 7 times in 1 second. For comparison, the ‘nearby’ Andromeda galaxy lies at a distance of about 2 million light-years. And the most distant galaxies visible to us currently are about 13 billion light-years away. That is to say, the light that left them 13 billion years ago is just now reaching us, and we are seeing them as they were 13 billion years ago, long before the Sun and Earth had even formed (4.6 billion years ago).
Until recently, one of the greatest challenges to measuring accurate distances was a complication caused by the presence of dust grains manufactured by stars and scattered throughout interstellar space. This dust, located in the regions between stars, absorbs and scatters light. If no correction is made for its effects, objects appear fainter and therefore apparently, but erroneously, farther away than they actually are. Fortunately, dust makes objects appear not only fainter, but also redder. By making measurements at more than one wavelength, this color dependence provides a powerful means of correcting for the presence of dust and allowing correct distances to be derived.
Currently, the most precise method for measuring distances is based on the observations of stars named Cepheid variables. The atmospheres of these stars pulsate in a very regular cycle, on timescales ranging from 2 days to a few months. The brighter the Cepheid, the more slowly it pulsates, a property discovered by astronomer Henrietta Leavitt in 1908. This unique relation allows the distance to be obtained, again using the inverse square law of radiation – that is, it allows the intrinsic brightness of the Cepheid to be predicted from its observed period, and its distance from Earth to be calculated from its observed, apparent brightness.
High resolution is vital for discovering Cepheids in other galaxies. In other words, a telescope must have sufficient resolving power to distinguish individual Cepheids from all the other stars in the galaxy. The resolution of the Hubble Space Telescope is about ten times better than can be generally obtained through Earth’s turbulent atmosphere. Therefore galaxies within a volume about a thousand times greater than accessible to telescopes from Earth could be measured for the first time with Hubble. With it, distances to galaxies with Cepheids can be measured relatively simply out to the nearest massive clusters of galaxies some 50 to 70 million light-years away. (For comparison, the light from these galaxies began its journey about the time of the extinction of the dinosaurs on Earth.)
Beyond this distance, other methods – for example, bright supernovae or the luminosities of entire galaxies – are employed to extend the extragalactic distance scale and measure the Hubble constant. Supernovae are cataclysmic explosions of stars near the end of their lives. The intrinsic luminosities of these objects are so great that for brief periods, they may shine as bright as an entire galaxy. Hence, they may be seen to enormous distances, as they have been discerned out to about half the radius of the observable universe. Unfortunately, for any given method of measuring distances, there may be uncertainties that are as yet unknown. However, by comparing several independent methods, a limit to the overall uncertainty of the Hubble constant can be obtained. This was one of the main aims of the Hubble Key Project.
This project was designed to use the excellent resolving power of the Hubble Space Telescope to discover and measure Cepheid distances to galaxies, and to determine the Hubble constant by applying the Cepheid calibration to several methods for measuring distances further out in the Hubble expansion. The Key Project was carried out by a group of about 30 astronomers, and the results were published in 2001. Distances measured using Cepheids were used to set the absolute distance scale for 5 different methods of measuring relative distances. The combined results yield a value of the Hubble constant of 72 (in units of kilometers per second per megaparsec, where 1 megaparsec corresponds to a distance of 3.26 million light-years), with an uncertainty of 10 percent. (The previous range of these measurements was 40 to 100 in these units.) Unlike the situation earlier, all of the different methods yield results in good agreement to within their respective measurement uncertainties.
The Hubble constant is the most important parameter in gauging the age of the universe. However, in order to determine a precise age, it is important to know how the current expansion rate differs from past rates. If the universe has slowed down or speeded up over time, then the total length of time over which it has been expanding will differ accordingly. Is the universe slowing down (as expected if the force of gravity has been retarding its expansion)? If so, the expansion would have been faster in the past before the effects of gravity slowed it down, and the age estimated for the universe would be younger than if it had always been expanding at a constant rate.
Indeed, this deceleration is what astronomers expected to find as they looked further back in time. The calculation for a Hubble constant of 72 and a universe with a slowing expansion rate yields an age for the universe of about 9 billion years. This would be fine, except for one not-so-small detail from other considerations: the measured ages of stars.
The best estimates of the oldest stars in the universe are obtained from studying globular clusters, systems of stars that formed early in the history of our galaxy. Stars spend most of their lifetimes undergoing the nuclear burning of hydrogen into helium in their central cores. Detailed computer models of the evolution of such stars compared with observations of them in globular clusters suggest they are about 12 or 13 billion years old – apparently older than the universe itself. Obviously, this is not possible.
The resolution of this paradox appears to rest in a newly discovered property of the universe itself. A wealth of new data over the past few years has begun to evolutionize cosmology. Probably the most surprising result is the increasing evidence that instead of decelerating as expected, the universe is accelerating! One implication is the existence of a form of energy that is repulsive, acting against the inward pull of gravity. Astronomers refer to this newly discovered universal property of the universe as ‘dark energy.’
Before the expansion of the universe was discovered, Einstein’s original mathematical equation describing the evolution of the universe in general relativity contained a term that he called the cosmological constant. He introduced this term to prevent any expansion (or contraction) of the universe, as it was thought that the universe was static. After Hubble discovered the expansion, Einstein referred to the cosmological constant as his greatest blunder. He had missed the opportunity to predict the expansion.
However, a recent discovery suggests that, although the universe is expanding, the term in Einstein’s equation may have been correct after all: it may represent the dark energy. In a universe with a Hubble constant of about 70, and with matter contributing one-third and dark energy providing approximately two-thirds of the overall mass plus energy density, the resulting estimated age for the universe is 13 billion years, in very good agreement with the ages derived from globular clusters.
It is too soon yet to know whether the existence of dark energy will be confirmed with future experiments. But to the surprise of an initially skeptical community of astronomers and physicists, several independent observations and experiments are consistent with this theory. Perhaps most exciting is the prospect of learning more about an entirely new form of mysterious energy, a property of the universe that to date has evaded all explanation.
The dark energy observed is smaller by at least 10 billion, billion, billion, billion, billion, billion times than the best theories of elementary particle physics would predict from first principles. Hence, by studying the behavior of the universe, astronomers are posing new challenges to fundamental physics. It is often the case in science that as old questions are resolved, novel, perhaps even more exciting, questions are uncovered. The next decade promises to be a fruitful one in addressing profound questions about the nature of the universe we live in.