American astronomer Edwin Hubble in 1929 discovered that the recession rate of galaxies (v) was directly proportional to their distance (d). This is what we call the Hubble's Law which defines that v = H₀ x d, whereby H₀ stands for Hubble's constant. With Hubble's Law being put forward, the idea of the expansion of the Universe is gradually established. Astronomers later found out that Hubble's constant was something more than what Hubble conceived. H₀ is not only a parameter for measuring expansion of the Universe, it can also be used to determine the age and the size of the Universe, amount of dark matters, numbers of hadrons and abundance of light elements in the Universe and even the structure of the early Universe, and so forth. The search for the value of H₀ has become a major subject for contemporary astronomers to deal with. Actually, one of the missions of Hubble's telescope is to search for H₀ and hence the design and the size of lenses are tied in with such purpose.

From Hubble's Law, we know that measurement of recession rate of celestial objects (mostly clusters of galaxies) and distances are the pre-requisites for the search of H₀. Hubble himself in 1929 provided the first value: H₀ = 513km/s/Mpc, ie, for a celestial object 1,000,000 parsec away from us, its recession rate is 513km (1 parsec = 3.26 light years). Shortly after the death of Hubble in 1953, Allan Sandage revised the value into between 50 and 100. If we accept the Big Bang Theory, H₀ = 50 means the Universe is aged between 13 and 16.5 billion years old. If H₀ = 100, then it indicates the age is between 6.5 and 8.5 billion years old. The actual age is determined by the density of matter in the Universe. Until the 90s before the Hubble's telescope was launched into space, the value of H₀ was still between 50 and 100, measured by astronomers through observation. Why was there a twofold error? There are two major errors. The first one is the error occurred in measuring v. Although astronomers can locate v of individual galaxy accurately through the spectrum and the Doppler's Effect, owing to the gravitational pull exerted from neighbouring galaxies and clusters of galaxies, v is not wholly determined by the expansion of the Universe. The second error stems from the uncertain distance of galaxies.

Except those celestial objects of several hundred light years, astronomers rely primarily on cepheid variables for measuring distances of celestial objects. Using ground telescopes to gauge cepheid variables enable us to directly measure celestial objects within 20 to 30 millions light years. For more distant stars, other methods have to be used. Nevertheless, such methods require calibration using cepheid variables. One of the key project of Hubble's telescope is to gauge as many cepheid variables as possible in more distant galaxies (up to 65 millions light years). By doing so, measuring methods can be precisely calibrated and the measurement of distance objects can be expanded up to 320 million light years. Furthermore, this key project will also target on the galaxy clusters of Virgo and Fornax so as to obtain a more accurate value for H₀.

The 26-strong team, led by astronomer Wendy Freeman, is held in charge for this project. Though still underway, the project has so far achieved significant results. Early in 1994 when this key project was put into place, the group found out that H₀ = 80 ± 17 through observation of the spiral galaxy M100 and thereby deducted that the Universe was aged between 8 and 11 billions years old. Such a figure caused controversy. The stellar evolution theory states that the oldest globular cluster might have existed for 15 billion years. How could a star be older than the Universe? With the observations made in NGC925, NGC1023, NGC3351, M101, NGC7331, NGC4414 and NGC1365 in the subsequent years, the value of H₀ was later revised to 70 ± 10 in 1999, which means the Universe is aged between 9 and 12 billion years. Meanwhile, data from Hipparcos satellite shown that the globular cluster might be much further away from us. Added to that, a review of theoretical model on globular cluster also prompted astronomers to believe that the globular cluster was not as old as previously estimated. All these have narrowed the divergence of views over the age of the Universe.

Besides the Freeman-led team, Sandage used Ia supernova as a yardstick for measuring distance and deduced that H₀= 55 - 60. Moreover, Ellis and other astronomers drew from the statistical results that H₀ is between 66 and 82. There are also others new methods measuring the value of H₀, for examples, observation of the gravitational lens phenomenon and the scattering phenomenon produced by microwave background radiation and hot plasmas from galaxies clusters. Compared with the twofold error of H₀ found in the 50s, astronomers today have made a considerable progress.


Photo courtesy: NASA