When was the Big Bang again?
(May 1999)
The latest estimate for the age of the
universe, published in Science during May, sets the Big Bang at 13.4 billion
years ago. According to Sydney (University of NSW) astronomer, Charles
Lineweaver, the secret to calculating the date lies in last year's
revolutionary discovery that the cosmos appears destined to expand forever. The
reasons for this expansion are as yet unexplained, but they are believed to
involve some sort of "antigravity" effect called the "cosmological
constant"
The previous estimate for the Big Bang set it
at about 15 billion years ago, so the new estimate makes the universe younger
than before, but still a little older than the oldest known stars. Several
years ago, when some estimates set the Big Bang at 8 billion years ago, the
universe was dated younger than its contents, but this embarrassing discrepancy
is not a problem with the new estimate.
Lineweaver bases his calculation on the Hubble
constant and the cosmological constant. The Hubble constant, of course,
describes how fast the universe is expanding right now, while the cosmological
constant allows us to estimate the forces pushing the universe apart.
The embarrassingly young age for the universe
was based on a high estimate for the Hubble constant, published in 1994, but
Lineweaver's lower value for the Hubble constant was also confirmed by NASA
during May.
Since the realization in 1998 that the
universe will not stop expanding, and will not come together again in a
"Big Crunch," scientists have been looking for an explanation of
this. The most likely explanation is that there is some sort of "vacuum
energy" which somehow works against the force of gravity.
Lineweaver suggested last October that 70% of
the matter in the universe is in this form, and that has allowed him to
estimate a value for the cosmological constant. This leads him to his estimate,
but it is still subject to error; his published figure is actually 13.4 billion
years, plus or minus 1.6 billion years - and however you look at it, that error
bar is time for a great deal to happen.
Appropriately, the new NASA estimate for
Hubble's constant came from the Hubble Space telescope, and represents the end
of an eight-year effort to measure precise distances to galaxies, far, far
away, as they say in the movies.
Once this key measure is in place it becomes
much easier to determine the age, size and fate of the universe. In reality,
before Hubble, the best age estimate was no better than somewhere between 10
and 20 billion years ago. And with that degree of uncertainty, there was little
to be said, according to NASA, about "... most of the basic questions
about the origin and eventual fate of the cosmos."
Now the universe's origin, evolution, and
destiny can be argued about with a proper degree of scientific reliability.
Hubble's constant was first estimated by Edwin Hubble some 70 years ago, when
he realized that the galaxies were rushing away from each other at a rate
proportional to their distance. The further away two parts of the universe are,
the faster they separate, so Hubble's constant is stated in terms of the
relative velocities of two points a megaparsec (3.26 million light years).
Up until the launch of the Hubble telescope
the best estimates for the constant were in the range from 50 to 100 kilometers
per second per megaparsec. Now the estimate is set at 70 km/sec/mpc, plus or
minus 10%, and the race is on to tie the value down even more precisely.
Yet even moving from a factor of two to a
factor of 10% is a big step forward, achieved by using the Hubble Space
telescope to observe 18 galaxies which are at distances up to 20 mpc, and
almost 800 Cepheid variable stars were found.
Cepheid variables are very special objects,
because they are "standard candles," stars which vary at a rate
proportional to their absolute brightness. In other words, if an observer
measures the variation, then the real brightness of the star is known. When
this is compared with the star's relative brightness, its distance can be
estimated very reliably.
The next step is to use the Doppler shift of
the star's radiation to estimate its velocity relative to us, and then, with
the distance and the velocity in our notebooks, it is a simple matter to
calculate Hubble's constant.
Even though Cepheid variables are rare, the 18
galaxies gave enough of them for the NASA team to undertake many measurements,
and to calibrate many different methods for measuring distances.
Coincidentally, another Australian scientist, Jeremy Mould of the Australian National University, is a co-leader of the team, and he also announced an age for the universe - 12 billion years - which is in the range set down by Lineweaver, who is at the University of New South Wales.