Big Bang theory

Also called: Expanding universe hypothesis · Topic: Origin and Evolution of the Earth and the Solar System · NCERT: Class 11, Ch 2 "The Origin and Evolution of the Earth"

Meaning

The Big Bang theory, also called the expanding universe hypothesis, says the universe began as one tiny, extremely hot and dense point (the singular atom). This point expanded very fast, and space has kept growing ever since. Current data put the start at about 13.8 billion years ago, from Planck's final 2018 data [3]. NCERT gives 13.7 billion years.

It is the most widely accepted idea of how the universe began. It is also the first step in the Earth's story: universe → atoms → stars and galaxies → solar nebula → Earth (about 4.6 billion years ago).

Explanation

How it worked: the five stages (NCERT, Figure 2.1)

• Stage 1: Singular atom. All the matter in today's universe was packed into one tiny ball.
• Its volume was extremely small.

• Its temperature and density were infinite.

• Stage 2: "Explosion" and violent expansion. The ball "exploded" and expanded very fast.

• The fastest expansion came in the first fractions of a second.

• "Explosion" is a simplification. It was not a bomb going off inside space. Space itself began to expand.

• Stage 3: Slower expansion. The expansion went on but slowed down. Some energy turned into matter.

• Stage 4: First atoms, within 3 minutes. The first atoms began to form.

• These were really atomic nuclei (the centres of atoms), not full atoms.

• Stage 5: The universe becomes transparent.

• NCERT: within 300,000 years, when the temperature fell to 4,500 K.
• Current figures: about 380,000 years [2] and about 3,000 K.

• Kelvin (K) is a temperature scale that starts at absolute zero (0 K = −273 °C). So 3,000 K is about 2,727 °C.

• Why did it become "transparent"?

• Before this stage, the universe was a hot "fog" of loose electrons and nuclei. Light kept bouncing off them.
• As it cooled, electrons joined nuclei and made whole atoms.
• Light could now travel in straight lines, so the universe became see-through.

The evidence that the universe is expanding

• Expanding universe: galaxies move away from each other because the space between them is growing. They are not flying through space like pieces of a bomb.

• A galaxy is a huge group of stars, gas and dust held together by gravity. Our Milky Way is one.

• Georges Lemaitre (Belgian astronomer and priest), 1927:

• He showed that Einstein's equations of gravity support a universe that expands rather than one that stays still [5].
• He stated the link between how fast a galaxy moves away and how far away it is [5].

• He traced the universe back to a "primeval atom" (a first atom from which everything came).

• Edwin Hubble (USA):

• NCERT dates his evidence for galaxies moving apart to 1920.
• In 1929 he published the velocity–distance relation: the farther away a galaxy is, the faster it moves away from us [5].
• He used redshift in spiral galaxies [5]. Redshift is the stretching of light from an object that is moving away. It makes the light look redder.

• This rule is now officially called the Hubble–Lemaitre law [5].

• Hubble constant (today's rate of expansion): 67.4 (± 0.5) km/s/Mpc, from Planck 2018 [3]. An Mpc (megaparsec) is a unit for the huge distances between galaxies.

• Cosmic microwave background (CMB):
• It is faint leftover light from when the universe became transparent, and the oldest light in the universe [2].
• Arno Penzias and Robert Wilson (USA) found it by accident in 1965. They picked it up as a steady "noise" in a radio antenna.
• A hot, dense early universe predicts this glow, so the CMB strongly supports the Big Bang.

The balloon analogy: what it shows and where it fails

• NCERT activity: mark dots on a balloon to stand for galaxies. Blow it up, and every dot moves away from every other dot.
• What it gets right:
• The dots move apart as the balloon swells, just as galaxies move apart as space grows.

• No dot is the "centre". Every galaxy sees all the others moving away.

• What it gets wrong:

• The dots themselves get bigger, but galaxies do not expand.
• Gravity holds each galaxy together. Only the space between galaxies grows.

What we know today: faster expansion and the CMB map

• 1998: the expansion is speeding up.
• Two separate teams studied distant Type Ia supernovae [6]. These are exploding stars that always give off about the same amount of light, so their distance can be measured.
• Both teams found that the expansion is accelerating, which means it is getting faster [6].

• This was a surprise. Scientists had expected gravity to slow the expansion down [6].

• Dark energy is the unknown force that pushes things apart. It is blamed for the speeding up [5][7].

• What the universe is made of [7]:
• Dark energy: about 68%.
• Dark matter: about 27%. It has mass and gravity but gives off or reflects almost no light.

• Normal matter: about 5% (protons, neutrons, electrons, and so stars, planets and us).

• What the CMB map shows:

• The CMB is almost perfectly even across the whole sky. It has only tiny temperature differences [3][4].
• These differences are imprints of sound waves from moments after the birth of the universe [3].
• They were the "seeds" from which matter grew into stars, galaxies and clusters of galaxies [3].

Where it occurs

• The whole universe: the Big Bang is not an event at one place. Space expanded everywhere at once, so every galaxy, including our Milky Way, sees the others moving away.
• Lemaitre in Belgium (1927): he set out the theory and the "primeval atom" idea [5].
• USA:
• Edwin Hubble (1929): he observed redshift in spiral galaxies and published the velocity–distance law [5].

• Penzias and Wilson (1965): they detected the CMB with a radio antenna.

• Space missions that mapped the CMB, in order [2]:

• COBE (1989, NASA).
• WMAP (2001, NASA).

• Planck (2009, European Space Agency with NASA).

• Planck's results:

• In 2013, Planck's first full-sky map showed that the universe is about 100 million years older than earlier estimates [4].
• Its final data in 2018 gave the age as 13.8 billion years and the Hubble constant as 67.4 km/s/Mpc [3].

Don't confuse with

• Steady state theory: proposed by Fred Hoyle, and separately by Hermann Bondi and Thomas Gold, in 1948. It also accepted expansion. The difference is that it said the universe had no beginning and that new matter is created all the time to fill the gaps, so density stays the same. The CMB (1965) could not be explained this way, and the theory was rejected [8].
• Galaxies expanding vs space expanding: only the space between galaxies grows. Gravity holds galaxies together, so they do not swell. This is where the balloon analogy fails.
• First atoms (within 3 minutes) vs a transparent universe (within 300,000 years in NCERT; about 380,000 years today [2]): the first stage made only atomic nuclei. The later stage made whole atoms, which let light travel freely and produced the CMB.
• Dark energy vs dark matter [7]: dark energy (about 68%) pushes the universe apart and speeds up expansion. Dark matter (about 27%) has mass and gravity but gives off almost no light.

Prelims Hooks

• The Big Bang theory is also called the expanding universe hypothesis. The starting point is the singular atom, which had tiny volume and infinite temperature and density.
• Within 3 minutes: first atoms (nuclei) formed. Within 300,000 years at 4,500 K (NCERT): the universe became transparent. Current figures are about 380,000 years [2] and about 3,000 K.
• Age of the universe: NCERT says 13.7 billion years. Planck 2018 says 13.8 billion years [3]. The Earth is about 4.6 billion years old.
• Chronological order: Lemaitre's primeval atom (1927) → Hubble's velocity–distance law (1929) → steady state theory (1948) → Hoyle coins "Big Bang" (1949) → Penzias and Wilson find the CMB (1965) → accelerating expansion found (1998) [5][6].
• Trap: Hubble's law is now called the Hubble–Lemaitre law [5]. The name "Big Bang" came from Fred Hoyle, the theory's chief opponent, who used it as a jibe on radio in 1949.
• Make-up of the universe: dark energy about 68%, dark matter about 27%, normal matter about 5% [7]. So everything we can see is only about 5% of the universe.

Mains Points

• How evidence settled the debate (GS-I): the Big Bang beat steady state theory step by step, through the evidence each theory predicted.
• Redshift (1929) showed that galaxies are moving apart [5].
• The CMB (1965) showed that the early universe was hot and dense.

• COBE, WMAP and Planck then mapped the CMB precisely [2].

• Cosmology is still an open field: the faster expansion found in 1998 and dark energy (about 68% of the universe) show that cosmology still has big unanswered questions [6][7].

• Key figures keep being refined. The age has moved from 13.7 billion years (NCERT) to 13.8 billion years (Planck 2018), and the Hubble constant is now put at 67.4 km/s/Mpc [3].

• Link to the Earth and the limits of models: the Big Bang produced the first atoms (mostly hydrogen and helium). These atoms built stars, galaxies, the solar nebula and finally the Earth, so this is the first chapter in explaining the Earth's materials.

• A good answer also says where analogies fail. In the balloon model the galaxies swell, but real galaxies do not.

Related concepts

• Expanding universe
• Steady state theory

Read more

• The origin of the universe: Big Bang, expanding universe and steady state