Physics : The Universe


The Universe

• The Universe is the sum total of all that exists in space and time.

• The study of the Universe is known as ‘cosmology’, which has its roots at the beginning of the 20th century with the  introduction of Albert Einstein’s theories of relativity, which joined space and time into a single continuum.


A galaxy is a huge aggregate of stars, gas and dust held together by gravitational attraction, usually in the form of a flattened disc, and generally with the matter being contained in spiral arms radiating away from the central nucleus of the disc. The Milky Way  is our own galaxy, one of 100 billion to a trillion known galaxies. Stars are globes of gas in which nuclear fusion reactions at the

centre create vast quantities of energy which become radiated into space mainly in the form of light, heat and ultraviolet  radiation. The Sun is the nearest star, the star at the centre of our solar system. Planets are major-sized bodies directly orbiting a star. In the case of our own solar system there are eight planets. Now, Pluto is no longer considered to be a planet. Smaller bodies  orbiting the Sun are given special names such as ‘asteroids’. A satellite is any body which orbits an object which is directly orbiting a star, i.e. a planet or smaller body.


The stars are moving and they are not fixed, as a result, it is known that the Universe is expanding. One consequence of this discovery was the realization that reversing time led to a contraction of the Universe to a point-like source or singularity. The origin of the Universe was therefore a cataclysmic event known popularly as the ‘Big Bang’. Since space and time also began at this point, there is no ‘time’ before the Big Bang. The original Big Bang theory, explains both the large-scale smoothness of the Universe and the small-scale non-uniformities (that is, the clumping of matter into galaxies). This was done by suggesting that all parts of the Universe were in contact with each other during the critical period before 10-35 seconds, but that after this there was a 1050 expansion, possibly due to the separation of the strong force from the electroweak forces. Events in the Universe’s early history occurred very rapidly. All the light elements were initially formed within the first 15 minutes. Quarks, and leptons such as electrons and neutrinos, as well as an equal number of anti-particles, were formed after only 10-35 seconds but, by 10-32 seconds. After 100,000 years, when the temperature had fallen to 3700°C (6700°F), ions and electrons joined together to form atoms of these light elements. Proto-galaxies began to form when the temperature had reached 100°C (212°F) and the formation of coherent galaxies is calculated to have begun one billion (109) years after the Big Bang. Proof of the Big Bang is considered to be the detection of the cosmic background radiation by Arno Penzias and Robert W. Wilson in 1965. The Universe was filled with a ‘sea’ of cosmic radiation during its formation, and this has since cooled. The end of the Universe will be dire. Either it will expand for ever or reach an equilibrium size. In both cases all of the hydrogen will be used up and the Universe will die, or gravity will overcome the amount of mass in the Universe and it will collapse back to a point-like source.


Astronomical Unit (AU): The mean distance of the Earth from the Sun as defined in 1938. The current value is 149,597,871 km.

Light Year (ly): the distance travelled by light in vacuum in one tropical year of 365.24219878 mean solar days, that is  9,460,528,405,000 km.

Parsec (pc): The distance at which an angle of one second of arc will represent the distance from the Earth to the Sun, that is 206,264.806 astronomical units or 3.2616 light years or 30,856,776,000,000 km.

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