The cosmological theory that all the matter and energy in the universe originated from a state of enormous density and temperature that exploded at a finite moment in the past.
BIG-BANG THEORY Newton's work gave a mathematical basis for the universe on a large scale. However, the data available at the time suggested a static unchanging universe. This could not easily be explained in the context of the law of gravitation, since all bodies in the universe attract all other bodies with the force of gravity. Newton realized that there was only one solution to this problem: in a static universe, matter had to be uniformly spread throughout an infinitely large space. In 1826, Heinrich Olbers published a paper containing what is known as Olbers' paradox; the universe had to be infinite and uniform to avoid gravitational collapse, but such a universe would lead to a perpetually bright sky on earth. Space–time Cosmologists now believe that Newton's model was based on incorrect assumptions about the structure of space, time, and matter. Both Newton and Olbers imagined space to be infinite and to be independent of the matter it contained. Einstein in his general theory of relativity (1915) proposed that the universe exists in four-dimensional space-time. This space-time is curved by the presence of matter, and the matter moves by following the resulting curves. The expanding universe The discovery by Hubble in 1929 that the universe is expanding provided a starting point for the ideas on which our present understanding of the universe is based. Hubble made his discovery by analysing the spectra of light from distant galaxies and noting a persistent redshift, which he explained in terms of the Doppler effect; an increase in observed wavelengths of light occurs because the light source is receding from the observer. The larger the speed of recession, the larger the red-shifts. Hubble discovered a pattern in his data: the further away the galaxy, the greater the speed of recession. Known as Hubble's law , this provided the evidence that the universe is expanding and a resolution to Olbers' paradox. If the galaxies and the earth are moving apart, the radiation falling on the earth from the galaxies is reduced and is also redshifted by an amount proportional to the distance. The further galaxies are away from the earth, the smaller their contribution to the radiation falling on the earth. The night sky is dark because the universe is expanding.
This model might seem to place the earth at the centre of the universe again. However, it is space itself that is expanding and the galaxies are embedded in this space. The ring (space) in the diagram has dots (the galaxies). The expansion of the ring means that the view from any one dot is that the other dots are receding at a speed proportional to their distance away. No single dot is at the centre of the system but all dots see the same thing. Hubble's discovery satisfies the cosmological principle that the universe should appear the same no matter where one is in the universe. Age of the universe Hubble's law may be stated in the form: H0 = v/d, where v is the speed of recession of the galaxy, d is the earth-galaxy distance, and H0 is called the Hubble constant . Assuming that the galaxies have always been moving apart at the speeds observed at present, the age of the universe (T) can be estimated, i.e. T = 1/H0. On this basis the age of the universe would be 15–18 billion years. This implies that there must have been a time when space and time were minutely small, with all the matter and energy of the universe concentrated into this small volume.
The expansion leads to the recession of B from A along the ring. The speed of recession will be directly proportional to the distance of B from A along the ring. The origin of the universe This view of the origin of the universe is called the big-bang theory . The theory suggests that the universe originated as a minute but very hot body and that the temperature has been falling as the expansion has continued. In 1945 George Gamow predicted that there should be a microwave background corresponding to a black-body temperature a few degrees above absolute zero. This microwave background was discovered 20 years later. Before the big-bang theory, cosmologists could only account for 2–3% of the helium in the universe by arguing that it had been formed by nuclear fusion of hydrogen in early stars. The big-bang theory actually required a period in the evolution of the universe when helium should have been produced to present levels (approximately 70% hydrogen and 27% helium).
In 1992, the COBE satellite discovered that there were very small variations in the microwave background. This discovery helped to explain why the universe formed into galaxies and stars. Had the universe been uniform in all directions, there would be no reason for gravitational collapse at any one point. The non-uniformities that began the nucleation of galactic matter in the early universe now appear as the small variations in the microwave background. Fundamental forces It is thought that the four fundamental interactions in the universe are all manifestations of the same force. This force existed when the big bang occurred at a temperature above 1015 K. As the universe cooled the forces separated as the original symmetries were broken. Gravity was the first to separate, followed by the strong nuclear force, and the weak and electromagnetic forces (see table).
| Time from big bang | Temperature (K) | State of the universe/forces |
| 0 second | infinite | The universe is infinitesimally small and infinitely dense (i.e. a mathematical singularity) |
| 10-12 second | 1015 | Weak and electromagnetic forces begin to separate |
| 10-6 second | 1014 | Quarks and leptons begin to form |
| 10-3 second | 1012 | Quarks form the hadrons; quark confinement begins |
| 102 seconds | 107 | Helium nuclei formed by fusion |
| 105 years | 104 | Atomic era; atoms form as protons combine with electrons |
| 106 years | 103 | Matter undergoes gravitational collapse |
| 0.5–1.8 × 1010 years | 2.7 | Present day: cosmic background corresponds to about 2.7 K |
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