Constant defined

The Hubble constant, which helps determine the Universe's age, is calculated

Published: Wednesday 15 August 2001

after 11 years, the Hubble Space Telescope ( hst ) has completed its measurement of the Hubble constant, a fundamental number that determines how old and how big the Universe is. The value of the constant determined by the hst is close to its value expected by astronomers.

As noticed by the E Hubble in 1929, our universe is expanding and hence each galaxy in it is moving away from every other galaxy. The speed at which an object is receding from us depends on its distance. The larger the distance, the greater is its speed of recession. The proportionality constant in this relationship is the Hubble constant. Measuring the Hubble constant is a notoriously difficult task because one needs to measure the distance and speeds of a large number of galaxies. Measuring speeds is relatively easy by using the red shift of light emitted by the galaxy (light emitted by a moving source has a modified frequency, an effect called Doppler shift).

Accurate measurements of distances are difficult because of a variety of reasons. The objects are too far off and too dim and the intervening dust could absorb the light emitted by them. Before the hst started its measurements, the value of the Hubble constant was considered to be between 50-100 kilometres per second per megaparsec (a megaparsec is a unit of distance in astronomy). Now, after 11 years of observations and careful data analysis the Hubble Space Telescope team has announced the value for the Hubble constant to be 72 kilometers per second per megaparsec.

The value of the Hubble constant is extremely important for understanding the past, present and future of our universe. A lower value would result in the so-called age problem, in which the age of the universe calculated with a lower value of the Hubble constant turns out to be smaller than the age of certain objects called globular clusters in the universe.

A larger value of the Hubble constant would result in problems being consistent with other observations like the cosmic microwave background radiation. In a sense it is good that the value obtained by the hst is consistent with all other measurements in our universe ( Astrophysical Journal , 553, p47).

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