Big Bang Theory
The most widely accepted theory about how the universe came into being is the Big Bang Theory. About 14 billion years ago, the universe existed as a very small, hot and dense point. From this tiny point, the universe expanded outwards into the universe it is today.
Evidence supporting the Big Bang Theory are:
- red-shift
- cosmological microwave background radiation
Red shift
When we observe the emission spectrum from a star, some dark lines are present in the spectrum because elements in the star absorb some of the emitted radiation. If we compare the spectrum of light from the Sun and that from distant galaxies, we can observe that the light spectrum from distant galaxies is shifted towards the red end of the spectrum. This phenomenon is called the red-shift.
Red shift happens because the expansion of the universe stretches the light, thus elongating the wavelength, while the light is travelling in the universe. The more red-shifted a light spectrum is, the faster the galaxy is moving away from Earth, and the more distant the galaxy is.
Cosmological Microwave Background Radiation
One of the predictions made with the development of the Big Bang Theory was that there should be remnant heat from the Big Bang across the universe. This "heat" was discovered to be cosmological microwave background radiation. Cosmological microwave background radiation, which is around 5K in temperature, can be observed from all directions of the universe.