12 billion years...

...is the age of the universe. And calculations based on data from the Hubble space telescope have established the elusive Hubble constant

Published: Sunday 15 August 1999

 The Hubble space telescope: t the Hubble Space Telescope Key Project team has announced that its efforts to measure the distances of far-away galaxies are now complete. To achieve this, the eight-year effort has been using the Hubble space telescope ( hst ). This, in turn, is an essential ingredient for determining the characteristics of the universe like its age, size and even its ultimate fate. The team estimates the universe to be approximately 12 billion years old.

One of the essential measurements in astronomy is the rate of expansion of our universe. According to the current theory, the universe started with a cataclysmic event known as the Big Bang. The universe has been expanding since. The objects in the universe are all moving away from each other. The rate of this expansion is known as the Hubble constant after Edwin Hubble, who in 1929 was the first to discover that the galaxies were receding from each other. The measurement of the Hubble constant can tell us how fast two galaxies are moving away from each other.

Only it is not as easy as that -- measuring the Hubble constant is extremely difficult because to do so, we need to know the distance and the speed of recession of galaxies. Measuring the speed of recession of objects in the universe is not too hard. Astronomers use the fact that light emitted by an object moving away from us is shifted towards the red end of the spectrum, an effect similar to the changing pitch of an approaching police siren. The faster the body is moving away, the more the light is stretched out. But measuring the distance accurately is very difficult. Astronomers use stars whose properties are well-known to act as milestones in determining the distances of galaxies. These are a special class of pulsating stars that are able to provide us intergalactic distances accurately. The basic problem is that there are not too many of these stars to give us a reliable measurement.

The value of the Hubble constant has changed drastically. In Edwin Hubble's time, it was thought to be 500 km per second per megaparsec (a megaparsec is about 3.26 million light years). This means that a galaxy which was one megaparsec away would appear to be moving away from us at a fantastic rate of 500 km per second. In the intervening seven decades, the determination of this expansion rate improved considerably. But it was not until hst that an accurate measurement could be done.

The essential element in the project was to measure the distances to far away galaxies accurately. Ground-based telescopes could only resolve the Cepheid variable stars in nearby galaxies. But to get an accurate measurement of the expansion rate, astronomers had to see much farther. This was one of the main aims of hst when it was launched in 1990. It was expected to be able to resolve Cepheids in galaxies about 10 times further than ground-based telescopes.

The team of 27 astronomers from 13 international institutions led by Wendy Freedman of the Carnegie Observatories in Pasadena, California, usa , used hst to get a cosmological distance ladder. For this, they used the Cepheids and another class of objects called supernovae. They discovered almost 800 Cepheid variable stars in 18 galaxies about 65 million light years from us. Using the Cepheids and the supernovae, they have pegged the value of the Hubble constant at 70 km per second per megaparsec (with an uncertainty of 10 per cent). If this sounds unimpressive, it should be noted that before hst , the same quantity was quoted with an uncertainty of about 100 per cent.

The value of the expansion rate of the universe is essential for understanding the fate and the size of the universe. A lower Hubble constant implies that the universe is expanding slowly and that it has taken a longer time to reach its present size. The team arrived at the conclusion that the universe is about 12 billion years old after calculating current estimates of the amount of matter in the universe. With the completion of this project, a major mystery about our universe has been solved. Hopefully.

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