Exploring the Origins of the Universe and Our Solar System Theories and Discoveries: Physical Geography for UPSC

Introduction

The universe is almost 13 billion years old. Huge clusters of galaxies comprise the universe. There have been many theories proposed by various experts which attempt to explain the origin and evolution of the Universe. Different scientists and philosophers have put forward many hypotheses and theories regarding the evolution of the Earth and Universe. There have been many early theories propounded by philosophers, but they had lacked proper scientific backing. That led to the formation of new modern theories.

Early Theories: Origin of the Earth

Nebular Hypothesis

This theory is proposed by German philosopher Immanuel Kant and revised by mathematician Laplace in 1796. Kant argued that gaseous clouds—nebulae, which slowly rotate, gradually collapse and flatten due to gravity and eventually form stars and planets. As the nebula shrank, it flattened and shed eight angular momentum rings of material, which later collapsed into the planets. Sun was regarded as a young and slowly rotating body. Planets were considered to be formed out of a cloud-like substance associated with the Sun.

Binary Theories

In 1900, Chamberlain and Moulton refined the Nebular theory analysis. They considered that when a wandering star approached the Sun, a cigar-shaped extension came out of the solar surface. When the passing star moved away, the material separated from the solar surface (as the extension) continued to revolve around the Sun, and slowly those materials condensed into planets. Later, scientists Sir James Jeans and Harold Jaffrey also supported this theory’s arguments which were called Binary Theories.

Revised Nebular Hypothesis

Revised Nebular Hypothesis was given by Otto Schmidt of Russia and Carl Weizascar of Germany in 1950. They regarded that the Sun was encircled by solar nebula comprising mostly of Hydrogen and Helium along with dust particles. The friction and collision of particles helped in the creation of a disk-shaped cloud, and the planets formed by the process of accretion. (Note: In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter.) These theories only attempted to explain the formation of our solar system and did not quite explain the genesis or origin of the entire Universe.

Modern Theories: Evolution of the Universe

Big Bang Theory

This is the most popular theory of the origin of the Universe and it is called the Big Bang Theory or Expanding Universe Hypothesis. On witnessing that the galaxies move far away from each other and the distance between them increasing, universal expansion was proved by Sir Edwin Hubble in 1920.

Big Bang theory can be explained in three developmental stages:

1. At first, all matter forming the universe existed in one place in the form of a tiny ball (this tiny ball called a ‘Singularity’) with a small volume as an atom, having infinite mass (density) and temperature.
2. A violent explosion (bang) of the tiny ball happened which resulted in a huge expansion. As the ball continued to expand, there were changes of some particles into energy form. That means, some energy was converted into matter. There was a particularly rapid expansion within fractions of seconds after the bang. Thereafter the expansion slowed down. This happened about 13.7 billion years ago and continues till today.
3. Then the temperature dropped to 4,500K (Kelvin). After 300,000 years of explosion, atomic matter was formed, and the universe became transparent.

The expansion of the universe meant an increase in space between the galaxies. An alternative was provided to this theory by Hoyle’s concept of Steady State.

Steady State Theory

The standard model of cosmology is the Big Bang Theory of how things came to be; however, there have been a few different theories for the universe. The Steady State Hypothesis proposes the possibility that the universe looks similar regardless of the perspective and that the universe has consistently resembled this. Basically, the hypothesis expresses that the universe is uniform all throughout both existence. The favorable position of the Steady State hypothesis over some different theories is its basic and aesthetic clarifications of certain irksome subjects. For instance, since the universe is perpetual all through time, the universe needs no tangled clarification of its start. Likewise, to represent the decline in density that would result from expansion, Steady State Hypothesis asserts new matter continually should be made to keep a constant density (and hence a static appearance).

The Steady State Hypothesis offered straightforward answers for the manner in which the universe worked, but astronomers found that the universe actually develops over the long haul. The end of the Steady State Hypothesis came in the late 1960s with the discovery of the Cosmic Microwave Background (CMB).

Cosmic Microwave Background (CMB)

CMB is thought to be leftover electromagnetic radiation from the Big Bang, or the time when the universe began. These radiations are invisible. But using optical telescopes and radio telescopes, they can be detected.

Formation of Stars

The dissemination of matter and energy was lopsided in the universe. The density difference offered ascent to differences in gravitational forces. It made the matter get drawn together and lead to the formation of galaxies. A galaxy contains enormous numbers of stars. It begins shaping by the collection of Hydrogen gas as a cloud. The denser gases were consolidated into stars. The distance between the stars is measured with light-years.

Light-Year

One light year can be defined as the distance covered by light in one year (the speed of light is 3 lakh km per second). It should be remembered that it is a measure of distance and not of time. The average diameter of the stars is 80,000 to 1,50,000 light years.

A galaxy starts forming by the accumulation of Hydrogen gas in the form of a large cloud called Nebula. The Nebula develops localized clumps of gas. These clumps continue to grow into denser gaseous bodies. The denser gases were condensed into stars. The formation of stars is believed to have occurred 5-6 billion years ago. The mean distance from the Sun to Earth is 8.311 minutes in terms of light-year.

Formation of Planets

The formation of planets started after the formation of the stars. The stars are restricted pieces of gases found in the nebula. The gravitational force prompted the development of the core. A gigantic turning gas plate and dust developed around the gas core. In the following stage, the gas cloud started getting dense, and the matter around the core formed into small rounded objects.

The small rounded objects formed into planetesimals due to the procedure of cohesion. Larger bodies began forming due to impact and attraction, making the materials remain together. At the last stage, the small planetesimals accumulated to form large bodies in the form of planets.

Our Solar System

Our solar system consists of eight planets. The nebula of our solar system started collapsing about 5-5.6 billion years ago. The planets were formed around 4.6 billion years ago. Our solar system comprises eight planets, 146 moons, millions of asteroids, comets, and a huge quantity of gas and dust. There are, in general, two types of planets – inner planets and outer planets. Among the eight planets, the first four are inner planets, which are also called rocky or terrestrial planets. The rest of the planets are outer planets and are called gaseous or Jovian planets.

Planetesimal Hypothesis

A planetesimal is an object formed from dust, rock, and other materials. The word has its roots in the concept “infinitesimal,” which indicates an object too small to see or measure. Planetesimals can range in size from several meters to hundreds of kilometers. The term refers to small celestial bodies formed during the creation of planets.

According to the planetesimal hypothesis:

• A planetary system starts as a protoplanetary disk, a rotating disk of dense gas and dust surrounding a young, newly formed star.
• The material from the nebula gradually gets pulled together by gravity, forming small chunks.
• These chunks grow larger until they form planetesimals.

The rocky planets formed near the Sun’s vicinity, where it was too warm for gases to condense to solid particles. Gaseous or Jovian planets formed farther away from the Sun.

• The solar wind, being quite intense near the Sun, blew off gases and dust from the terrestrial planets.
• Rocky planets cannot hold gases due to weaker gravity, unlike Jovian planets that retained large amounts of gas, resulting in a larger radius and lower density.

Until 2006, Pluto was considered a planet. However, the International Astronomical Union reclassified Pluto as a dwarf planet.

The Moon

Formation

The Moon is Earth’s solitary natural satellite.

• In 1838, Sir George Darwin proposed that the Earth and Moon initially formed as a single, quickly rotating body. The mass took a dumbbell shape and eventually separated. This hypothesis suggested that the Moon formed from a portion of Earth and that the Pacific Ocean marks the site of separation.
• This theory is not accepted today. The current, widely accepted theory is the Giant Impact Hypothesis or Big Splat Hypothesis.
  • An enormous celestial body, 1 to 3 times the size of Mars, collided with the Earth shortly after its formation.
  • The impact ejected a large portion of Earth’s material into space.
  • This material coalesced to form the Moon, which started orbiting Earth.

The Moon is believed to have formed about 4.4 billion years ago.