Universe, Age of

In 1926 the American astronomer Edwin P. Hubble discovered that all distant galaxies are rapidly moving away from us. This pioneering observation was (and still is) broadly interpreted as a global expansion of the universe. Hubble's discovery, together with the cosmological Friedmann-Lemaitre-standard model, led to the general insight that the universe should have a temporal origin, which, since the 1950s, has been called the big bang. The hot big bang picture received what is considered final confirmation in the mid-60s when the cosmic microwave background radiation (CMBR) was first observed. The CMBR is broadly considered as a leftover of an early hot and dense universe. We define the age of the universe by the time t0 that has elapsed since the big bang.

After his observations, Hubble estimated the universe to be expanding with a current rate of H0≈ 500 kilometers/second/Mpc (1 Mpc = 3.26 million light-years = 30,840,000,000,000,000,000 km). In cosmology, H0 is called the Hubble parameter. If we could assume the cosmic expansion to be constant over the whole history of the universe, we could simply determine its age as the inverse Hubble parameter VH0. According to Hubble's first estimate, t0 would obtain a value of 2 billion years, in contrast to what we know about the lifetimes of our earth and many other celestial objects. During the following decades, until the 1970s, the value of H0 was dropping to a range of 50 … 100 km/sec/ Mpc due to major advances in our knowledge of stellar populations and more sophisticated measuring methods. Accordingly the estimates of t0 were increasing to about 10–20 billion years.

Certainly the cosmic expansion has not proceeded at a constant rate throughout its history. Thus one has to keep in mind that the 1/H0 estimate can be only a very rough approximation for the age of the universe.

Other independent methods of age determination are thus in great demand. For a start, it is possible to extract a lower t0 limit from geo-cbronology, that is, from the determination of our planet's age. One can access reliable radiometric methods for this purpose. It has been known for a century that uranium-235 is decaying to the stable lead-207 with a half-life-period t½ ≈ 700 million years. For uranium-238, on the other hand, it takes 4.5 billion years to (finally) decay to lead-206 by 50%. In order to determine the age of rock, one has to measure the mass fractions of such long-living radioactive elements as well as their stable decay products. Once knowing the “mother”- and “daughter”-elements' mass ratio, it is simple to reliably calculate the ages of the earth's crust's oldest rocks, as well as those of meteorites and the moon. By this method one could assign the age of 4.6 billion years to the earth and the solar system.

from stellar evolution theories, anyway, it is known that there must have been existing earlier generations of stars before the sun. One can tell from the solar metal content. Heavy elements (or metals, as denoted by astronomers) are formed over millions and billions of years in the inner regions of massive stars and admixed to the interstellar medium during nova- and supernova explosions. If the sun were a first-generation star, it should contain heavy elements in much lower amounts than observed.

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