Physics (9203) | Red shift and the expanding universe

Welcome to Space Physics: Red Shift and the Expanding Universe!

Hello future astrophysicists! This is one of the most exciting chapters in physics, as we tackle some of the biggest questions of all time: How big is the universe, and where did it come from?

Don't worry if these ideas seem massive! We are going to break down the concept of Red Shift – the key piece of evidence that tells us the universe is constantly growing – into simple, easy steps. By the end of these notes, you’ll understand the observational evidence for the Big Bang Theory.

Quick Prerequisite: Remembering Light Waves

To understand Red Shift, we must first remember that visible light is a wave, and different colours have different wavelengths:

• Red light has the longest wavelength and the lowest frequency.
• Blue/Violet light has the shortest wavelength and the highest frequency.

A shift in the wavelength of light means a shift in the observed colour!

Section 1: The Doppler Effect – The Sound of Movement

The concept of Red Shift is based on something you experience every day: the Doppler Effect.

What is the Doppler Effect?

The Doppler Effect describes how the observed frequency (or wavelength) of a wave changes if the source of the wave is moving relative to the observer.

Analogy: The Speeding Ambulance

Imagine an ambulance with its siren blaring:

1. Moving Towards You: As the ambulance approaches, the sound waves are compressed, making the pitch sound higher (a shorter wavelength, higher frequency).
2. Moving Away From You: As the ambulance drives past and away, the sound waves are stretched out, making the pitch sound lower (a longer wavelength, lower frequency).

Key Takeaway: Movement causes waves to be compressed or stretched. This applies to light waves too!

Section 2: Red Shift Explained

Applying the Doppler Effect to Light

When a source of light (like a galaxy) moves, the wavelength of the light we observe changes.

1. Red Shift (Moving Away)

If a light source is moving away from Earth, the light waves are stretched out (like the siren waves being stretched when the ambulance leaves).

• Stretching the wave means the wavelength gets longer.
• In the visible spectrum, a longer wavelength means the light is shifted towards the Red end.
• This is called Red Shift.

2. Blue Shift (Moving Towards)

If a light source is moving towards Earth, the light waves are compressed.

• Compressing the wave means the wavelength gets shorter.
• A shorter wavelength means the light is shifted towards the Blue end.
• This is called Blue Shift (and is rare for distant galaxies).

Caution: Common Misunderstanding! Red Shift does not mean the galaxy itself looks red. It means the specific features of its light spectrum are shifted towards the red end.

How We Measure Red Shift: Spectral Lines

We cannot just look at a galaxy and tell if it is Red Shifted. We need to look at its light spectrum.

1. When light from a star or galaxy passes through gases, certain wavelengths of light are absorbed.
2. These absorbed wavelengths appear as dark lines against the continuous spectrum (the rainbow). These are absorption spectral lines.
3. For elements like Hydrogen or Helium, these lines always appear in the exact same location if the source is stationary.
4. Observation: When astronomers look at distant galaxies, they find these spectral lines are shifted compared to the expected position.

The Crucial Finding

Almost all galaxies outside our own local group show a Red Shift. This means:

All distant galaxies are moving away from us.

Section 3: Red Shift and the Expanding Universe

The Relationship Between Distance and Speed

Astronomers (like Edwin Hubble, though you don't need to know his name) did more than just observe Red Shift; they measured how much the light was shifted.

The amount of Red Shift is directly related to the speed of the galaxy:

A bigger Red Shift = A faster speed of recession (moving away).

When scientists compared the speed of galaxies to their distance, they made an astonishing discovery:

• Galaxies close to us move away relatively slowly.
• Galaxies further away move away much faster.

The Conclusion: The Expanding Universe

If all galaxies are moving away from Earth, and the further they are, the faster they go, this means that the entire space itself must be expanding.

Analogy: The Expanding Balloon

Imagine you draw dots (representing galaxies) on the surface of a deflated balloon. Now, inflate the balloon:

• All the dots move away from each other.
• A dot that is far away from your starting point moves a greater distance (and appears to move faster) in the same amount of time than a dot that is close.

This analogy helps us understand that the universe doesn't necessarily have a central point that everything is expanding from; rather, the expansion is happening everywhere equally.

Did you know? We often say galaxies are "receding" from us. This means they are moving away because the space between us is stretching.

Section 4: The Big Bang Theory

The Origin of the Universe

If the universe is currently expanding, what happens if we run the clock backwards?

If everything is moving apart now, in the past, everything must have been much closer together. If we go back far enough (about 13.8 billion years), the evidence suggests that all the matter and energy in the universe was concentrated into a tiny, extremely hot, and dense point.

The theory that describes this initial moment of rapid expansion is the Big Bang Theory.

Key Evidence for the Big Bang

The primary evidence supporting the Big Bang Theory, required by your curriculum, comes directly from our observations of Red Shift:

The fact that nearly all galaxies exhibit Red Shift proves that the universe is expanding. An expanding universe implies a starting point of expansion—the Big Bang.