- Geocentric Model – The term “geocentric” means “earth–centered”. The geocentric model claimed that Earth was the center of the cosmos. The geocentric model was devised by Ptolemy, a Greek philosopher who compiled the historical astral observations of the ancient Greeks and Egyptians, which formed the framework for his theory. Ptolemy’s model illustrated that the Earth was stationary and that the Sun, Moon, planets and stars moved around it in perfect circles at constant speeds. Aristotle, one of the most prominent Greek philosophers, advocated the Ptolemaic geocentric model, which was widely accepted until the heliocentric model emerged in the 1500s.भारत का इतिहास CLICK HERE
- Heliocentric Model – The term “heliocentric” means “sun–centered”. The heliocentric model suggested that the sun was the cenetr of the universe. Nicolaus Copernicus was a Polish cleric who debunked the Ptolemaic model by proposing the heliocentric model. Copernicus argued that the Sun was motionless and that all planets, including Earth, revolved around it. He further suggested that the stars were stationary and that the Moon orbited Earth. Although initially rejected by the public and the majority of the scientific community, the Copernican theory garnered support from Galileo and was later proven by Johannes Kepler.
- The universe began with the Big Bang around 13.6 billion years ago. The Big Bang was a rapid expansion of the universe from a tiny single point. This hot expansion has continued since then, resulting in the expansion of the universe in every direction.
- At the time of the Big Bang, the universe and all of its matter was concentrated in a single point. As the Big Bang started, this matter was heated to very high temperatures and expanded rapidly.
- Several scientific observations have convinced astronomers that the Big Bang theory is correct. One of the strongest pieces of evidence is called the cosmic microwave background, or CMBR. This is the remaining heat from the Big Bang that can be seen in all directions when looking into the universe. As the universe has expanded, the initially hot material has cooled, so that it now emits light as microwaves.
- By looking at distant galaxies and stars, scientists have also discovered that everything seems to be moving away from the Earth. The further away a galaxy is, the faster it appears to be moving. This shows that the universe is still expanding, although there is debate as to whether this expansion will continue forever.
- The universe has been known to be expanding since 1929, when Edwin Hubble measured the speed at which distant galaxies appear to be receding from Earth’s position in space. In the early 21st century, astronomers measured the rate of expansion and discovered that it is increasing.
- An increasing rate of expansion means that, contrary to what might be expected from classical models, the universe contains insufficient matter to slow or reverse the expansion. In the present epoch, the rate of expansion has been measured at 42 miles per second per 1 million light–years. This means that distant galaxies are receding much faster than nearby galaxies, and that objects near the universal horizon, which is the farthest an Earthbound observer can theoretically observe, will eventually disappear over the edge and will no longer be visible from Earth.
- Astrophysicists have modelled the expansion using a concept called dark energy, which describes the poorly understood force that is overcoming gravity and driving the expansion ever faster. The nature of dark energy, and even whether it is a form of energy at all, is unknown. Given enough time, the universe is destined to expand until nothing remains in existence but a diffuse soup of radiation in a near–perfect vacuum.
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