How Old is the Universe?
Most astronomers would agree that the age of the Universe - the time elapsed since the "Big Bang" - is one of the "holy grails of cosmology". Despite great efforts during recent years, the various estimates of this basic number have resulted in rather diverse values. Current cosmological models which are used to derive the age of the Universe have to make a number of theoretical assumptions, and these assumptions are not very well supported by the incomplete available observational data. At present, a value in the range of 10-16 billion years is considered most likely. But now, an international team of astronomers has used the European Southern Observatory’s “Very Large Telescope” (VLT) and its efficient spectrograph to perform a unique measurement that paves the way for a new and more accurate determination of the age of the Universe. They measured for the first time the amount of the radioactive isotope Uranium-238 in a star that was born when the Milky Way was still forming. It is the first measurement ever of uranium outside the Solar System. This method works in a way similar to the well-known Carbon-14 dating in archaeology, but over much longer time scales. So how exactly does it work? Well, first we have to understand a little bit about how stars form and how they age.
While hydrogen, helium and lithium were produced during the Big Bang, all heavier elements result from nuclear reactions in the interiors of stars. For details of exactly how this happens, you can read the relevant sections on stellar evolution on this web site. The main message is that when stars die heavy-element enriched matter is dispersed into surrounding space and eventually becomes incorporated in the next generations of stars. In fact, if you are wearing a ring on your finger the gold was produced in an exploding star and deposited in the interstellar cloud from which the Sun and its planets were later formed. Thus, the older a star is the lower, generally, is its content of heavy elements like iron and other metals. Measurements have shown that very old stars that are members of large agglomerations known as globular clusters are normally quite "metal-poor"- their metal-content ranges down to about 1/200 of that of the Sun, in which these metals constitute only 2% of the total mass, the rest being still in the form of hydrogen and helium. This is because it is believed that the globular clusters formed early in the life of the Universe. Scientists have been searching for even older stars for a long time, and after decades of mostly fruitless efforts a large spectral survey by American astronomer Timothy C. Beers and his collaborators uncovered hundreds of stars with much lower metal content than even the globular clusters, in some cases only 1/10,000 of the solar value. It is evident that these extremely metal-poor stars must have formed during the very infancy of the Milky Way. These particular stars exhibit a great variety of element abundances that may ultimately throw more light on the processes at work during this early period. As a step in this direction an international team of astronomers decided to study these stars in much more detail. In 2000 and 2001 they were awarded observing time on the VLT and used its very efficient high-dispersion spectrograph to carry out their observations. Not unexpectedly a lot of new information has been obtained - which takes us back to the use of Uranium-238 as a clock.
Cosmic Time Clock
It is possible to make a fundamental determination of the age of a star, provided it contains a suitably long-lived radioactive isotope. The use of this procedure depends on the measurement of the abundance of the radioactive isotope, as compared to a stable one. As was said at the beginning, this technique is analogous to the Carbon-14 dating method that has been so successful in archaeology over time spans of up to a few tens of thousands of years. In astronomy, however, this technique must obviously be applied to vastly longer time scales. For the method to work well the right choice of radioactive isotope is very critical. Contrary to stable elements that formed at the same time, the abundance of a radioactive (unstable) isotope decreases all the time. The faster the decay the less there will be left of the radioactive isotope after a certain time, the greater will be the abundance difference when compared to a stable isotope, and the more accurate is the resulting age. Yet, for the clock to remain useful, the radioactive element must not decay too fast - there must still be enough left of it to allow an accurate measurement, even after several billion years. This leaves only two possible isotopes we can use for astronomical measurements, Thorium-232, with a half-life of 14.05 billion years and Uranium-238, with a half-life of 4.47 billion years. Several age determinations have been made by means of the Thorium-232 isotope. Its strongest spectral line is measurable with current telescopes in a handful of comparatively bright stars, including the Sun. However, the decay is really too slow to provide sufficiently accurate time measurements. It takes around 47 billion years for this isotope to decay by a factor of 10, and with a typical measuring uncertainty of 25%, the resulting age uncertainty is about 4-5 billion years, or approx. one third of the age of the Universe! This slow-moving clock runs forever, but is hard to read accurately! The faster decay of Uranium-238 would make it a much more precise cosmic clock. However, because uranium is the rarest of all normal elements, its spectral lines in stars are always very weak, and if visible at all they normally drown entirely in a vast ocean of stronger spectral lines from more abundant elements. Nevertheless, this is exactly where the low abundance of heavier elements in very old stars comes to the rescue. In the stars that were studied by the team using the VLT, which typically contain 1,000 times less of the common elements than the Sun, the predominance of the maze of atomic and molecular lines in the spectrum is greatly reduced. The lines of rare elements like uranium therefore stand a real chance of being measurable. So the astronomers using the VLT became very excited when they examined the spectrum of the 12th-magnitude star CS 31082-001. It showed what is probably the richest spectrum of rare, heavy elements ever seen. In particular the faint lines of these elements were unusually free of interference from the lines of the iron-group elements and from molecular lines (of CH and CN), which are often quite numerous, even in such low-metallicity stars. While only one or at most two thorium lines have ever been measured in any other stars, no less than 14 thorium lines are seen in the spectrum of CS 31082-001. Best of all, the long sought-after line of singly ionized uranium is clearly detected at its rest wavelength of 389.59 nm in the near-ultraviolet region of the spectrum. Not surprisingly the uranium line is still quite weak. After all, uranium is the rarest of elements to begin with and it has further decayed by a factor of eight since this star was born. But using the resolving power and light-gathering capability of the VLT, in combination with the UV spectrograph, the uranium line can be measured with very good accuracy despite its faintness. The results of this work have produced a most likely age for CS 31082-001 of 12.5 billion years. Estimates of the age of the Universe using different techniques range from 11 to 16 million years, the Universe is older than CS 31082-001, hence it must be older than 12.5 billion years. So how accurate is this measurement, and how much faith can we place in the numbers?
Limits of Error
Well, because of the faster decay rate of Uranium-238 and the highly accurate spectroscopic measurement which is now possible, the age uncertainty for the stellar observation is only ± 1.5 bn years. Of more concern is the lack of sufficiently detailed knowledge regarding the way Uranium-238 decays and also uncertainties in the available laboratory data for uranium by which chemical abundances are calculated using measured line strengths. The errors inherent in both these fundamental understandings of the way U-238 behaves are greater than those associated with the stellar measurement. Work is now underway at laboratories in France and Sweden to improve these basic data, so that the overall measurement error can be reduced. In the meantime the astronomical team is trying to find other stars like CS 31082-001. There may not be many, but if the uranium line can be seen and measured in more spectra, it will also become possible to judge whether those very old stars, as surmised, are all of about the same age, i.e. that of our Milky Way galaxy.