The Universe

1) Theories on the Origin of the Universe

· Initial arguments for the Nebular Hypothesis were given by German philosopher Immanuel Kant in 1796.

· The hypothesis was later revised by mathematician Laplace.

· This theory proposed that the planets formed out of a cloud of material associated with a youthful sun, which was slowly rotating.

In 1900, Chamberlin and Moulton put forward the Planetesimal Hypothesis, suggesting a wandering star approached the sun.
Sir James Jeans and Sir Harold Jeffrey supported this argument.

Arguments at a later date considered a companion to the sun to have been coexisting, leading to the idea of binary theories.

In 1950, Otto Schmidt in Russia and Carl Weizscar in Germany somewhat revised the nebular hypothesis.

This theory was given by Edwin Hubble.
According to the 'Big Bang Theory', everything in the universe emerged from a point known as 'Singularity' approximately 15 billion years ago.
Later, this point expanded, and inside it, galaxies moved apart due to the space between them expanded.
All matter in the universe was created at one instant in a fixed moment of time.
A single fireball existed along with wispy clouds of matter.
When it exploded, it formed a cluster of galaxies which exploded to form stars and then stars exploded to form planets.

The solar system comprises the Sun and its eight planets which are believed to have been developed from the condensation of gases and other lesser bodies.
All the planets revolve around the Sun in elliptical orbits.
Alternatively, the first four are called 'Terrestrial', meaning earth-like as they are made up of rock and metals, and have relatively high densities.
The rest four are called 'Jovian' or Gas Giant planets.
Jovian planets, like Jupiter, are much larger than the terrestrial planets and have thick atmospheres, mostly of helium and hydrogen.
Till recently (August 2006), Pluto was also considered a planet.
However, in a meeting of the International Astronomical Union, a decision was taken that Pluto and other celestial objects (2003 UB313) discovered in recent past may be called 'dwarf planet'.
The eight bodies officially categorized as planets are often further classified in several ways.

Cosmic Microwave Background Radiation (CMBR): Discovered in 1964, CMBR is considered strong evidence for the Big Bang Theory, as it represents the residual heat from the early universe.
Dark Matter and Dark Energy: These mysterious components are believed to constitute about 95% of the universe's mass-energy content. Dark matter is thought to provide extra gravitational pull, while dark energy is accelerating the expansion of the universe.
Inflation Theory: Proposed by Alan Guth, this theory suggests a period of extremely rapid exponential expansion of the early universe, which explains its large-scale uniformity and flatness.

Stellar Life Cycle: Stars are born from nebulae (clouds of gas and dust), evolve through various stages (main sequence, red giant/supergiant), and eventually die as white dwarfs, neutron stars, or black holes, depending on their initial mass.
Types of Galaxies: Galaxies are vast systems of stars, gas, dust, and dark matter held together by gravity. Main types include Spiral (like our Milky Way), Elliptical, and Irregular galaxies.
Milky Way Galaxy: Our solar system is located in the Milky Way, a barred spiral galaxy approximately 100,000 light-years in diameter.

Asteroids: Rocky, airless remnants left over from the early formation of our solar system, mostly found in the asteroid belt between Mars and Jupiter.
Comets: Icy, dusty cosmic snowballs that orbit the Sun. When they get close to the Sun, they heat up and spew gases and dust, forming a glowing head and a tail.
Meteoroids, Meteors, and Meteorites: Meteoroids are small rocky or metallic bodies in outer space. When they enter Earth's atmosphere, they become meteors (shooting stars). If they survive the passage and hit the ground, they are called meteorites.
Dwarf Planets (beyond Pluto): Besides Pluto, other recognized dwarf planets include Eris, Haumea, Makemake, and Ceres.

Exoplanets: Planets orbiting stars other than our Sun. Thousands have been discovered, indicating that planetary systems are common.
Search for Extraterrestrial Life: Scientists are actively researching exoplanets located in the "habitable zone" (where liquid water could exist) to look for signs of life.