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6.2.5Space Physics

The Big Bang Theory

5 min read 9 parts Lesson 10 of 10 2 quizzes

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Section 1 of 9The idea

Cosmology is the study of the origin, structure and evolution of the whole Universe.

Big Bang theory
The accepted theory for the origin of the Universe is the Big Bang theory.
It says that about 13.8 billion years ago all the matter and energy in the Universe was concentrated in an extremely small, extremely hot and dense state.
Expanding
Since then, space itself has been expanding, and as it expanded the Universe cooled and structure formed.

It is important to avoid two common misconceptions.

The idea
The Big Bang was not an explosion in space, like a bomb going off at a point in an empty void: it was the expansion of space everywhere at once.
And there is no centre of the Universe from which things are moving away; every observer sees the same pattern of galaxies receding.

Section 2 of 9A timeline of the early Universe

1
In the first tiny fractions of a second the Universe was an extremely hot, dense mixture of energy and fundamental particles,
  • and it expanded very rapidly.
2
After about a few minutes the temperature fell enough for protons and neutrons to combine,
  • forming the nuclei of hydrogen and helium.
    • (by mass, about 75% hydrogen and 25% helium)
3
After about 380 000 years the Universe had cooled to about 3000 K, so that electrons could join nuclei to form neutral atoms.
  • Light could then travel freely and it is this light that we still observe today as the cosmic microwave background.
4
After a few hundred million years gravity gathered the gas into the first stars and galaxies.
  • Later generations of stars made heavier elements, which formed planets, including Earth about 4.6 billion years ago.

Section 3 of 9Evidence 1: galactic red-shift

Light from distant galaxies is shifted towards longer wavelengths, towards the red end of the spectrum.

Red-shift
This red-shift shows that almost all galaxies are moving away from us, and the further away a galaxy is, the faster it moves.
This is exactly what is expected if the Universe is expanding.
Running the expansion backwards in time, all the galaxies were once close together,
  • which points to a very dense beginning.
    • (see the lessons on Doppler shift and galactic red-shift)

Section 4 of 9Evidence 2: cosmic microwave background radiation

Astronomers detect cosmic microwave background radiation (CMBR) coming from every direction in space.

It is the radiation released when the Universe first became transparent, about 380 000 years after the Big Bang.
At that time it was visible light and infrared at about 3000 K.
2.7 K
The expansion of the Universe has stretched the wavelengths of that radiation by a factor of about 1000,
  • so it is now in the microwave region and corresponds to a temperature of just 2.7 K.
The radiation is almost perfectly uniform in all directions, with tiny variations that correspond to the seeds of today’s galaxies.
No rival theory has explained the CMBR as well as the Big Bang.

Section 5 of 9Further evidence and limits

The Big Bang theory also correctly predicts the observed proportions of hydrogen and helium in old stars and gas clouds: about 3 hydrogen atoms for every helium atom by number (roughly 75% to 25% by mass). Alternative ideas, such as the old steady state theory, in which the Universe has always looked the same and new matter is created as it expands, could not explain the CMBR or the changes in galaxies over time, and are no longer accepted.

The theory does not explain what happened before the Big Bang, or what caused it, and many details, such as the nature of dark matter and dark energy, are still being studied. Scientific theories are tested and refined as new evidence arrives.

Section 6 of 9Expanding space, not moving galaxies

A good analogy for the expanding Universe is raisins in a rising loaf of dough.

1
As the dough expands, every raisin moves away from every other raisin, and the raisins that are farther apart separate faster.
2
No raisin is at the centre.
3
The raisins are like galaxies and the dough is like space.
4
Notice that the raisins themselves do not expand, just as galaxies and atoms are held together by forces and do not grow in size.
5
This model does have limits, because the dough has an edge and the Universe is not known to have one.

Section 7 of 9Worked examples

  1. State two pieces of evidence for the Big Bang theory.

  2. The CMBR was emitted at a temperature of about 3000 K and is now observed at 2.7 K. By what factor has the Universe expanded since then?

  3. The age of the Universe is estimated as 13.8 billion years. How far (in metres) has light travelled in that time? Use 1 light-year = 9.5 × 1015 m.

Section 8 of 9Key terms

TermMeaning
Big Bang theoryTheory that the Universe began from a very hot dense state about 13.8 billion years ago and has been expanding since.
CosmologyThe study of the origin and evolution of the Universe.
Red-shiftIncrease in the wavelength of light from a receding source.
CMBRCosmic microwave background radiation: uniform microwaves from all directions, a relic of the early Universe.
Expanding UniverseThe observation that space is stretching, carrying galaxies apart.

Section 9 of 9Exam technique

1
Describing the Big Bang as an explosion at one place in space. It was an expansion of space itself.
2
Saying galaxies are moving away from a central point. Every observer sees the same recession; there is no centre.
3
Saying the CMBR is visible light. Its wavelength has been stretched into the microwave region.
4
Thinking the Big Bang explains the origin of the planets directly. It describes the Universe, not the formation of Earth.
5
Saying red-shift proves the Universe is expanding without explaining why: further galaxies show greater red-shift.
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