Red-Shift and the Big Bang

This lesson covers red-shift, the expanding universe, and the Big Bang theory, as required by the AQA GCSE Physics specification (4.8.2). This is a Physics-only topic. You need to understand how observations of red-shift provide evidence for the expansion of the universe and how this supports the Big Bang theory.

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The Electromagnetic Spectrum and Light from Stars

Before understanding red-shift, you need to recall that light is part of the electromagnetic spectrum. White light from a star can be split into a spectrum of colours (red, orange, yellow, green, blue, indigo, violet) using a prism or a spectrometer.

Each chemical element in a star's atmosphere absorbs light at specific wavelengths, producing a pattern of dark lines called an absorption spectrum (or line spectrum). These dark lines act as a "fingerprint" for each element — by studying the pattern, scientists can identify which elements are present in a distant star.

Key points:

• Every element produces a unique pattern of spectral lines.
• If a star contains hydrogen, it will show hydrogen's characteristic absorption lines.
• The positions of these spectral lines can shift depending on the relative motion of the star and the observer.

Exam Tip: You do not need to memorise specific spectral line positions, but you must understand that each element produces a unique pattern and that shifts in the position of these lines provide evidence about the motion of stars and galaxies.

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What Is Red-Shift?

Red-shift is the phenomenon in which the light from distant galaxies is shifted towards the red end of the electromagnetic spectrum — that is, the wavelengths of the spectral lines are longer than expected.

This shift occurs because the galaxy is moving away from us. As the galaxy recedes, the light waves are stretched, increasing their wavelength. Since red light has the longest wavelength in the visible spectrum, this stretching shifts the light towards the red end — hence "red-shift."

Analogy: The Doppler Effect

Red-shift is similar to the Doppler effect for sound. When an ambulance moves towards you, the siren sounds higher pitched (shorter wavelengths). When it moves away, the siren sounds lower pitched (longer wavelengths). Red-shift is the equivalent effect for light from galaxies moving away from us.

Observation	What It Means
Spectral lines shifted towards red (longer wavelength)	The galaxy is moving away from us
Spectral lines shifted towards blue (shorter wavelength)	The galaxy is moving towards us
Greater red-shift	The galaxy is moving away faster

Exam Tip: Red-shift does not mean the light looks red. It means the entire spectrum of light from the galaxy is shifted towards longer wavelengths. A blue spectral line might be shifted to green, and a green line shifted to yellow. The key idea is that all spectral lines are shifted towards the red end.

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Evidence for the Expanding Universe

Edwin Hubble made a crucial observation in the 1920s: the light from nearly all distant galaxies is red-shifted. This means that nearly all galaxies are moving away from us.

Furthermore, Hubble discovered a relationship between a galaxy's distance from us and its speed of recession (how fast it is moving away):

The further away a galaxy is, the greater its red-shift, and therefore the faster it is moving away from us.

This relationship is known as Hubble's Law:

v = H x d

where:

• v = speed of recession (km/s)
• H = Hubble constant (a measure of the rate of expansion)
• d = distance to the galaxy

What Does This Mean?

If all galaxies are moving away from us, and more distant galaxies are moving away faster, this implies that the universe is expanding. The space between galaxies is stretching, carrying the galaxies further apart.

Important: this does not mean that Earth or our galaxy is at the "centre" of the expansion. Every point in the universe sees all other galaxies moving away from it. The expansion is happening everywhere — like dots on the surface of an inflating balloon all move apart from each other.

Observation	Conclusion
Light from distant galaxies is red-shifted	Galaxies are moving away from us
More distant galaxies have greater red-shift	More distant galaxies are moving away faster
All galaxies (beyond the local group) are receding	The universe is expanding

Exam Tip: A common exam question asks: "What does red-shift tell us about the universe?" A good answer states: (1) light from distant galaxies is shifted to longer wavelengths (red-shifted), (2) this means galaxies are moving away from us, (3) more distant galaxies show greater red-shift so are moving away faster, (4) this provides evidence that the universe is expanding.

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The Big Bang Theory

If the universe is currently expanding, then in the past the universe must have been smaller. If we trace the expansion backward in time, we reach a point where all the matter and energy in the universe was concentrated in an extremely hot, dense point.

The Big Bang theory states that:

• The universe began from an extremely hot, dense point approximately 13.8 billion years ago.
• Space itself began to expand rapidly from this point.
• As space expanded, it cooled, allowing matter to form — first subatomic particles, then atoms, then eventually stars and galaxies.
• The universe has been expanding ever since.

The Big Bang was not an explosion in space — it was the rapid expansion of space itself. There was no space "outside" the Big Bang for it to expand into. Space, time, matter, and energy all began at the Big Bang.

graph LR
    A["Big Bang (13.8 billion years ago)"] --> B["Rapid Expansion"]
    B --> C["Universe Cools"]
    C --> D["Subatomic Particles Form"]
    D --> E["Atoms Form (mainly H and He)"]
    E --> F["Stars and Galaxies Form"]
    F --> G["Universe Today (still expanding)"]

    style A fill:#e74c3c,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#f1c40f,color:#000
    style D fill:#2ecc71,color:#fff
    style E fill:#3498db,color:#fff
    style F fill:#9b59b6,color:#fff
    style G fill:#2c3e50,color:#fff

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Evidence for the Big Bang

There are two main pieces of observational evidence supporting the Big Bang theory:

1. Red-Shift of Galaxies

As described above, the observation that galaxies are moving away from us — with more distant galaxies receding faster — is consistent with the universe expanding from a single point. This was the original evidence that led to the Big Bang theory.

2. Cosmic Microwave Background Radiation (CMBR)

In 1965, Arno Penzias and Robert Wilson accidentally discovered a faint background radiation coming from every direction in space. This radiation is in the microwave region of the electromagnetic spectrum and has a temperature of about 2.7 K (-270.45 degrees C) — just above absolute zero.

This radiation — the Cosmic Microwave Background Radiation (CMBR) — is the remnant heat from the Big Bang. Shortly after the Big Bang, the universe was extremely hot and filled with high-energy radiation. As the universe expanded and cooled over 13.8 billion years, this radiation was stretched to longer wavelengths and cooled to its current microwave frequency.

Evidence	Description	How It Supports the Big Bang
Red-shift	Light from distant galaxies is shifted to longer wavelengths	Shows the universe is expanding, consistent with expansion from a single point
CMBR	Faint microwave radiation detected from all directions	Remnant thermal energy from the hot, dense early universe