Accurately measuring the Hubble constant (H0), the initial rate of expansion of the universe, from which one can determine the age of the universe is a major preoccupation of astronomers.
One of the key projects of the Hubble Space Telescope’s (HST) was the determination of H0 more accurately. In the 1970s, and up to the time the HST was launched, there were two groups that projected competing values for H0, one of which was almost twice the other.
The role of the Hubble Space Telescope was to resolve this difference.
The HST launch, in 1990, came as a breath of fresh air. In fact, the HST was instrumental in democratising cutting edge astronomy. “The HST allowed, and even encouraged younger scientists to lead interesting projects,” says Dr Arunav Kundu, Associate professor in Tata Institute of Fundamental Research, Mumbai.
When the HST started work, two groups, led by Allan Sandage and Gerard de Vaucouleurs, respectively, had calculated values for H0 that differed nearly by a factor of two.
The Sandage group said it was about 55 km per sec per million-parsecs and the de Vacouleurs group said it was 100 km per second per million-parsecs. (A parsec is 3.1 light years.) It was hard to resolve this difference due to unavailability of data.
“In this particular situation, the understanding of what data existed was incomplete and subject to interpretation about how to fill in the gaps. There were different routes to the answer but it was hard with limited data to show which methods had smaller uncertainties,” Abhijit Saha of Astronomer, National Optical Astronomy Observatory, Tucson, Arizona, explained in an email to this Correspondent. Dr. Saha has been involved in many key HST projects to measure H0.
To calculate H0, one needed to know the distances of faraway galaxies. Before the HST, the maximum distances that could be directly measured were about a few million light years.
From these, one had to use other yardsticks to infer the distances from earth to distant galaxies (known as secondary distance indicators). The answers that one got were heavily method-dependent.
Also within the measurable distance, there were only a few galaxies to compare. When the Hubble Space Telescope came into the picture, the distance to which one could directly measure increased 10 times. “A 1,000 times bigger volume for exploring suitable galaxies to calibrate secondary distance indicators,” says Dr Saha.
The Hubble Telescope has been able to narrow down the difference between the values put forth by the two groups to about 20 per cent.
A value of H0 =74 +/- 3 km per second per million parsec is most quoted today. But the Sandage and Tammann group still favour a lower value of H0 = 62 +/- 6 km per second per million parsec.
The HST was able to observe what are known as Cepheids (bright variable stars) in order to estimate the secondary distance indicators, mainly because it had higher resolution than much larger ground-based telescopes.
“The HST resolves out the background better than any other telescope. This is why you can study Cepheids in more distant galaxies using the HST, and why it has been crucial to establishing the Hubble constant,” says Dr Kundu.
