The Big Bang theory

The universe is believed to have originated some 10-20 billion years ago in an event known as the big bang. This event is regarded as the beginning of space-time - to talk about events 'before' the big bang is meaningless; although you can try! According to the big bang theory, the universe expanded tremendously rapidly from an initial, extremely compact, high density state and it has continued to expand ever since. In the expansion, the density and temperature of the universe has continually decreased, and radiation and matter have evolved into the forms in which we see them today. We will now see evidence there is for the big bang theory of the origin of the universe.

(a) Expansion of the Universe:

Measurements on the spectra from distant galaxies show that they are all red-shifted. This surprising result means that the galaxies are all moving away from us and therefore the universe must be expanding. Electromagnetic signals from distant galaxies take a long time to reach us and so tell us about the galaxy not as it is now, but as it was millions of years ago when the signals left them; they allow us to 'look back' in time. Hubble's observations show that more distant galaxies (those nearer the big bang in time) are moving faster than nearby galaxies; it seems then that the expansion of the universe is slowing down. Latest results from galaxies at the furthest reaches of observation have however raised some doubts about this conclusion. Using light from intensely bright supernovae from the edges of the observable universe it would appear that the universe is not only expanding, but accelerating. This result is shocking for most scientists and they have labelled the force causing this acceleration as dark energy. Still very little is known about this energy form, although it accounts for 73% of the universe.

(b) Cosmic microwave backround radiation:

The big bang produced radiation energy which still exists in the universe today in the form of cosmic microwave backround radiation, or '''CMB. '''It was first detected in 1964 by Arno Penzias and Robert Wilson as a backround hiss in the microwave signals detected by their radio antenna, and its discovery gave strong support to the big bang theory. CMB fills the whole universe with an intensity that is nearly the same in every direction; it has a peak wavelength of 1.1 mm corresponding to a black body temperature of 2.7 K. NASA's Cosmic Backround Explorer Satellite (COBE) has mapped the CMB in detail. Ripples in the 2.7 K value at 'hot; and 'cold' spots have been found, these may act as 'seeding' sites for galaxy formation.

(c) Light element abundance:

Further evidence for the big bang theory comes from the quantity of light nuclei such as the isotopes of hydrogen, helium and lithium present in the universe. These are thought to have formed a few minutes after the big bang when protons and neutrons combined to form nuclei in nucleosynthesis; for example

1H + 2H --> 3He + γ

The relative abundances of the light nuceli measured in the stars and interstellar gas clouds of today agree well with those predicted by the big bang theory.