Still elusive

Two varying estimates of the Hubble constant mean that we are no closer to establishing the age of the universe

 
Published: Tuesday 30 November 1999

 Calculating the change in dis ever since Edwin Hubble discovered in 1929 that the universe is expanding, scientists have been incessantly trying to establish its rate of expansion, known as the Hubble constant. This is crucial to our understanding of the past, present and future of the universe. Recently, two groups of astronomers have reported values of the Hubble constant that are very different from each other. If one group's observations are to be believed, then we live in a universe that is only 12 billion years old, while the other group claims that the universe is around 13-16 billion years old.

The Hubble constant is notoriously difficult to measure. We need to know the distance to a far-away galaxy and the speed at which it is moving away from us. Measuring the speed of recession of the galaxy is not very difficult. It is usually done with the help of measuring the redshift, the stretching of its light towards longer wavelengths. This is known as the Doppler Effect, and it causes the pitch of an approaching or receding ambulance siren to change. Astronomers can now measure the redshift to a fair amount of accuracy.

The difficulty is in reliably measuring the distance to the galaxy. What is needed is a standard ruler that allows one to fix the distance scale in the universe. The 'ruler' in use since 1912 is based on the waxing and waning of giant stars known as cepheids (the Pole Star is one of them). Cepheids fluctuate in brightness in an extremely regular fashion with periods ranging from 1 to 50 days. As the rate of pulsation of cepheids is related to their true brightness, knowing the period of a cepheid, one can find its true brightness. Comparing this with the brightness observed on the Earth, we can find the distance of the cepheid. For this, one needs to calibrate the brightness period relationship, which is done by using some nearby cepheids the distances of which can be measured by other methods.

The value of the Hubble constant observed by the astronomers has been changing since day one. In the time of Hubble, the value was around 500 km per second per megaparsec (1 megaparsec is equal to 3.258 million light-years). Since then, the value has been coming down. Since 1991, the Hubble Space Telescope's Key Project has studied more than 800 cepheids in 18 galaxies. The results suggest a value of 68 km per second per megaparsec.

A high value of the Hubble constant means the universe is relatively young. The recent controversy stems from a discrepancy in using the results of the Key Project. E Maoz and colleagues at the us National Aeronautics and Space Administration's Ames Research Center in Moffett Field, California, and the University of California, Berkeley, have used the Key Project's methods to estimate the distance to a galaxy called ngc 4258. They estimated a distance of around 8.1 megaparsecs. However, a team led by Jim Herrnstein of the National Radio Astronomy Observatory in Socorro, New Mexico, measured the same distance with a different method. Their estimate, around 7.2 megaparsecs, remains the most accurate and reliable measurement of the distance to any other extra-galactic object ( Nature , Vol 401, p351; and Vol 400, p539-541).

The exact age of the universe still eludes us.

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