The Earth’s Origin: Early Theories and the Modern Big Bang Theory

At a Glance

• Age of the universe About 13.8 billion years
• Age of the Earth About 4.6 billion years
• Early theories Nebular, planetesimal, tidal, binary star
• Modern theory Big Bang, expanding universe
• Key evidence Galaxies moving apart; cosmic microwave background
• Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This is the first topic of the first macro, which covers the origin, interior and motions of the Earth. It begins with the largest question in physical geography: how did the universe, and then the Earth, come into being? The topics that follow describe the stars, the planets and the Earth’s structure. Next topic: Star Formation and Formation of Planets.

What Were the Early Theories of the Earth’s Origin?

In short: early theories tried to explain how the solar system was born, not how the universe began.

The nebular hypothesis was the first scientific model. It explained why all planets orbit in the same direction and in nearly one plane. Its weakness was that it could not explain why the Sun, which holds most of the mass, has only a small share of the angular momentum. The tidal and planetesimal ideas assumed a close encounter between two stars, which is extremely rare in space.

What Is the Big Bang Theory?

In short: the Big Bang Theory says the whole universe began from one extremely hot, dense point and has been expanding ever since.

1. Proposal: the Belgian priest and astronomer Georges Lemaître proposed in 1927 that the universe began from a ‘primeval atom’. The idea is now called the Big Bang Theory.
2. Evidence of expansion: in 1929 Edwin Hubble showed that galaxies are moving away from us, and that the more distant a galaxy is, the faster it recedes. This shows that the universe is expanding.
3. Age: working back from the rate of expansion, scientists estimate that the Big Bang occurred about 13.8 billion years ago.
4. Afterglow: in 1965 Arno Penzias and Robert Wilson found a faint microwave radiation that fills the sky. It is the cooled remnant of the early hot universe, called the cosmic microwave background.
5. Composition: the most abundant elements formed in the first minutes were hydrogen and helium, and these are still the main elements of the stars.

A common mistake is to picture the Big Bang as an explosion inside empty space. It was the start of space, time and matter themselves. The Big Bang is a theory of the universe, while the formation of the solar system is a separate and later event.

How Did the Universe Develop After the Big Bang?

• The first fraction of a second: the universe expanded extremely fast and cooled. Matter and energy became separate, and the first particles formed.
• Light atoms: within a few minutes, protons and neutrons joined to form hydrogen and helium nuclei.
• Atoms: after about 380,000 years the universe had cooled enough for electrons to join nuclei, and light could travel freely. This light is the cosmic microwave background.
• Stars and galaxies: gravity pulled gas into dense clumps. The first stars lit up a few hundred million years after the Big Bang, and they gathered into galaxies.
• Our galaxy: the Milky Way is a spiral galaxy that contains the Sun. The solar system formed within it much later, about 4.6 billion years ago, as described in The Solar System and the Evolution of the Earth.

Three further ideas are often asked. The ‘steady state’ theory, proposed by Hoyle, held that the universe has no beginning and that new matter is created as it expands, and it lost support after the cosmic background was found. The ‘big crunch’ and ‘big freeze’ are possible futures: either the universe falls back on itself, or it keeps expanding and cools. Current observations favour continued expansion, driven by a mysterious dark energy.

How Does This Link to the Earth?

The Big Bang produced only hydrogen, helium and a trace of lithium. All heavier elements, such as carbon, oxygen, silicon and iron, were made inside stars and in stellar explosions called supernovae. The Earth and its living things are therefore made of material that was once inside stars. Star Formation and Formation of Planets describes how such material gathered into planets.

As an analytical point, the Earth’s origin is not a single event with a single date. It is the end of a long chain: the universe, the first stars, heavy elements, the Milky Way, the solar nebula and then the planets. UPSC questions test the sequence, the names of proponents and the evidence, so these should be learnt together.

Why Is This Topic Important?

• Names Proponents are often asked
• Evidence Hubble and microwave background
• Base Foundation for the next topics

What Came Next?

The next topic explains how stars and planets form from clouds of gas and dust, which is the second step in the story. Next topic: Star Formation and Formation of Planets.

Key People and Terms

• Nebula Cloud of gas and dust in space
• Planetesimal Small body that joins to form a planet
• Redshift Stretching of light from receding galaxies
• Cosmic microwave background Faint radiation left from the early universe
• Primeval atom Lemaître’s name for the starting point
• Dark energy Unknown force speeding up expansion

Exam Corner

Mains Angle

Questions ask for a comparison of theories and the evidence for the modern view. These points add depth.

• Shift From solar system theories to cosmic theories
• Evidence Observation and radiation
• Limits Dark matter and dark energy