5.5FieldsStretch

The Big Bang and the Expanding Universe

Point a sensitive enough radio antenna at absolutely any part of the sky, in any direction, and you’ll pick up a faint, perfectly uniform hiss of microwave radiation — the afterglow of the universe’s own birth, discovered almost by accident, and one of the most decisive pieces of evidence in the history of cosmology.

40 min Video by FuseSchool Module 5.5 Astrophysics and cosmology

The Big Bang — AstrophysicsWatch the full walkthrough before the notes below.

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What you'll be able to do

Describe the Big Bang model of the origin of the universe
Describe the Cosmic Microwave Background Radiation (CMBR) as evidence for the Big Bang
Explain why the CMBR favours the Big Bang model over the (historical) steady-state model
Describe critical density and its role in determining the fate of the universe

The Big Bang model

The $Big Bang model$ proposes that the universe began from an extremely hot, dense state and has been continuously expanding — and cooling — ever since. This model is supported by two major pieces of observational evidence: the universal redshift of distant galaxies (Hubble’s law, covered in the previous lesson) and the Cosmic Microwave Background Radiation.

The Cosmic Microwave Background Radiation

The $Cosmic Microwave Background Radiation (CMBR)$ is faint electromagnetic radiation detected uniformly from every direction in the sky, corresponding almost exactly to black body radiation at a temperature of about 2.7 K. It is interpreted as the highly redshifted remnant of radiation released when the early universe first became transparent, shortly after the Big Bang — originally far hotter, but stretched to microwave wavelengths by the enormous cosmological redshift accumulated over billions of years of universal expansion.

The CMBR’s remarkable uniformity in every direction is strong evidence that it originates from a genuinely early, universal event, rather than from any particular nearby source.

Tip — The CMBR was discovered somewhat by accident in 1965 by Penzias and Wilson, who initially thought the persistent background "noise" in their radio antenna was an equipment fault, before realising it was a genuine cosmic signal.

Big Bang versus the steady-state model

Before the CMBR’s discovery, an alternative $steady-state model$ proposed that the universe has always looked essentially the same, expanding but with new matter continuously created to keep its average density constant — with no beginning at all. The steady-state model predicts no leftover background radiation from any single early "beginning" event, whereas the Big Bang model specifically predicts it. The discovery of the CMBR, at almost exactly the predicted temperature, was decisive observational evidence in favour of the Big Bang model over the steady-state alternative.

Tip — If asked why the CMBR supports the Big Bang over the steady-state model, the key point is: the Big Bang PREDICTS a uniform relic radiation from an early hot, dense phase; the steady-state model has no such single origin event to produce one.

Critical density and the fate of the universe

The average density of matter (and energy) in the universe, compared with a theoretical $critical density$ , determines its ultimate fate: whether gravity will eventually halt and reverse the expansion, or whether the universe will continue expanding forever. A universe denser than critical density would eventually stop expanding and collapse; a universe less dense than critical would expand forever, at an ever-slowing rate; a universe at exactly critical density would expand forever, its rate of expansion approaching (but never quite reaching) zero.

ρ_{c} = \frac{3 H _{0}^{2}}{8 π G}

Critical density, in terms of the Hubble constant and the gravitational constant.

1Use

ρ_{c} = \frac{3 H _{0}^{2}}{8 π G}

ρ_{c} = \frac{3 \times ( 2.2 \times 1 0 ^{- 18} ) ^{2}}{8 π \times 6.67 \times 1 0 ^{- 11}}

3Numerator:

3 \times 4.84 \times 1 0^{- 36} = 1.452 \times 1 0^{- 35}

. Denominator:

8 π \times 6.67 \times 1 0^{- 11} \approx 1.676 \times 1 0^{- 9}

Answer

ρ_{c} \approx 8.66 \times 1 0^{- 27}

kg m⁻³ — an extraordinarily low density, roughly a few hydrogen atoms per cubic metre.

Tip — Current astronomical observations suggest the universe’s actual density is very close to critical density — one reason cosmologists take the concept of "flat" space (neither open nor closed) seriously.

Equation recap

ρ_{c} = \frac{3 H _{0}^{2}}{8 π G}

Critical density of the universe.

Common mistakes to avoid

Believing the CMBR is radiation from a specific object or point in space.

The CMBR arrives essentially uniformly from every direction — it is relic radiation from the early universe as a whole, not from any particular source.

Thinking the steady-state model also predicts a cosmic background radiation.

The Big Bang model specifically predicts a uniform relic radiation from an early hot phase; the (now largely abandoned) steady-state model has no such single origin event to produce one — this is exactly why the CMBR discovery favoured the Big Bang.

Assuming critical density is simply "the density of the universe" without a comparison.

Critical density is a specific THRESHOLD value — the universe’s actual density must be compared to it to determine whether the universe is open, closed, or flat.

Key takeaways

The Big Bang model: the universe began hot and dense and has been expanding and cooling ever since.
The CMBR — uniform microwave radiation at ~2.7 K from every direction — is relic radiation from the early universe, strong evidence for the Big Bang.
The CMBR’s existence favours the Big Bang model over the historical steady-state alternative.
Comparing the universe’s actual density to the critical density (ρc = 3H₀²/8πG) determines whether it will expand forever or eventually collapse.

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The Big Bang and the Expanding Universe

40 min Video by FuseSchool Module 5.5 Astrophysics and cosmology