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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.