The great cosmic discovery - III

Prof H Nandakumar Sarma *

The discovery of microwave background radiation, neutron star, pulsar; trying to understand the cosmic forces, the discovery of anisotropy of cosmic microwave background radiation, the discovery of Higgs boson ( God’s particle ), the detection of gravitational wave and black hole etc are some of the exciting moments in the understanding of the universe.

In this series, we trace some of the great cosmic discovery made from the beginning of the 20th century to the present day.

In 1920s, Edwin Powell Hubble (1889 - 1953) working at the 100 inch reflecting telescope Mount Wilson observatory (at that time the largest telescope in the world) observed that galaxies he had discovered are moving further apart as time passes.

This was evidenced from the Doppler shift of the spectral lines emitted by the galaxies (Doppler effect is the change in frequency of the light / sound waves due to the relative motion of the source/observer or both).

As a child, Hubble wandered the Kentucky countryside of U.S.A., where he grew up observing the habits of birds and animals. In 1910, he received an undergraduate degree from the University of Chicago, where he also lettered in basketball and almost become a professional boxer.

He studied law under a Rhodes scholarship at Oxford and passed bar, and practiced law briefly. During the World War I, he enlisted in the infantry. He completed graduate studies at the Yerkes Observatory at the University of Chicago, where he began his examination of spiral nebulae and earned his doctorate in 1917.

Two years later he began working at the Mount Wilson Observatory. It was at Mount Wilson Observatory that he discovered the red shift in ·the spectral lines of the galaxies. Hubble measured the depths of space out to 500 million light years (a light year is the distance traverse by light in one year and is approximately equal to 3 x105 x 60 x 60 x 24 x 365 = 9.46 X 1012 km) distances farther than any previous surveys.

He concluded that galaxies were moving away from the earth at velocities proportional to their distances. This supported the concept that the universe was originated in a cosmic explosion (big bang). Hubble found that the ratio of the velocity of receding galaxies to their distances from the earth is constant (the Hubble constant).

Hubble constant is a significant astrophysical number still not calculated with certainty today. Unfortunately, as the distances of objects increase, astronomers are faced with uncertainties; distances are hard to measure accurately.

Current estimates of the Hubble constant, and thus the rate of expansion of the universe differ by a factor of two. Immediately after the big bang the Universe consisted only of radiation and plasma at unimaginable high temperatures. The expanding space stretches the wavelength of radiation making it appears as though it comes from a cooler body.

In 1940s George Gamow, Ralph Alpher and Robert Herman explored the Big Bang model cosmology. In 1948, they made a specific prediction that there should be a background cosmic radiation with an average temperature of about 5 degree kelvin (-268C) if the universe had a big bang origin.

Alpher and Herman asked several groups of radio astronomers whether it was possible to measure this radiation. Karl Jansky, the founder of radio astronomy, while working at Bell Telephone Laboratories observed noise picking up by his large radio antenna when it was turned towards stars and Milky Way.

In 1933, he had been able to show that most of the noise came from the centre of the Milky Way, our own galaxy. Thus, the science of radio astronomy was born. Radio astronomy, as the name suggests, is the study and detection of radio waves from astronomical sources.

In 1941, Grote Reber an enthusiastic radio amateur built in his garden, a thirty-foot paraboloidal bowl antenna (this is one of the familiar shapes of disc antenna of cable TV and radio telescope antenna we see these days). Using his antenna, Reber was able to confirm Jansky’s result and to make the first radio maps of the sky.