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 ˇÓ 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 ˇÓ 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