GCSE · Physics · Edexcel · Spec 1PH0

Big Bang vs Steady State accounting for red-shift

Two rival models of the universe both explain the same famous clue: galaxies are moving apart. So how did scientists ever choose between them?

Two models, one contest

Who can explain it?

Two models of the universe are in a contest: the Big Bang and the steady state. For each clue, tick every model that can account for it. Tick both if both can. Then check.

A Big Bang model
B Steady state model

Galaxies are moving apart from each other
Light from distant galaxies is redshifted
New matter is constantly being created as galaxies move apart
The average density and temperature of the universe never change
The universe started incredibly hot and dense, in a tiny region of space
The universe has cooled and become less dense as it expanded
Microwave radiation (the CMBR) is detected from every direction in space

What redshift looks like

The same lines, shifted

View: Source moving away from us. Showing 5 layers: Spectra, Reference lines, Lines, shifted, Where they were, The shift

View

Layers

Explore

Flip between the two views, then tap a part to read what it shows.

The bottom strip is light from a source moving away from us: its lines have shifted towards the red end.

Flip between the two views, then tap a part to read what it shows.

Predict, then check

Light from a star in our own galaxy and light from a very distant galaxy are each split into a spectrum.

Which spectrum has the greater redshift, and why?

From redshift to a beginning

Reason it through

Why does the way galaxies are moving point back to a beginning?

Link 1 of 4

First link · your turn

Start with what redshift tells us. What are nearly all galaxies doing?

Locked — reveal the link above first

Check your thinking

Which idea sounds right to you?

Which of these is closest to what you think right now?

Redshift proves the Big Bang and disproves the steady state model.Both models fit redshift. The CMBR, which only the Big Bang explains, helped scientists choose.The Big Bang was an explosion of matter that was already there.A theory is just a guess, so the Big Bang is only one guess among many.

How sure are you?

I'm sureI think soI'm guessing

Redshift proves the Big Bang and disproves the steady state model.

This is a common misconception

Easy trap. Redshift is the famous clue, so it feels like the winner. But redshift shows that galaxies are receding, and both models account for that, so it can't disprove the steady state model. What favoured the Big Bang was the CMBR, which the steady state model could not explain.

Both models fit redshift. The CMBR, which only the Big Bang explains, helped scientists choose.

This matches the evidence

Yes. Redshift fits both models. The CMBR could not be explained by the steady state model, which was eventually rejected. Quasars, ultra bright objects, were also more common in the distant past, and the steady state model could not easily explain that either. It suggests the universe has been changing over time.

The Big Bang was an explosion of matter that was already there.

This is a common misconception

The word 'bang' makes that tempting, but it isn't what the theory says. The Big Bang was an expansion of space combined with the creation of all the matter in the universe. It was not existing matter blowing up.

A theory is just a guess, so the Big Bang is only one guess among many.

This is a common misconception

In science a theory is not a guess. It is our best understanding based on all the evidence, and it is subject to rigorous testing. The Big Bang theory is accepted over the steady state model because it explains the observations, including the redshift of galaxies and the CMBR.

Redshift: both models can live with it. CMBR: only the Big Bang explains it.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Galaxies are moving apart. Two rival models can both explain that, so what actually decided the argument?

What you need to know

A spectrum of starlight has dark absorption lines, and because all stars contain hydrogen, their hydrogen lines can be compared.
Redshift is an increase in the wavelength of light, and a decrease in its frequency, caused by the source moving away from us.
In redshift the absorption lines shift towards the red end of the spectrum, and a faster source gives a bigger shift.
Have a goGalaxy X is moving away from us much faster than galaxy Y. Whose absorption lines have shifted further towards the red end?
Galaxy X's lines have shifted further.
The greater the speed of the source away from us, the greater the shift. It's about speed, not about the light 'turning red'.
Light from nearly all galaxies is redshifted, so nearly all galaxies are moving away from us, and the distant ones are receding faster.
Hubble's Law says distance is directly proportional to velocity, and nearly every galaxy moves away from every other one, so there's no centre.
Have a goJo says: 'Everything is racing away from us, so we must be at the centre of the universe.' Jo is confidently wrong. What do you tell them?
Nearly every galaxy is moving away from every other galaxy, so there is no centre of the universe.
Galaxies receding from us doesn't make us special. They are receding from each other too, which is why nowhere is the middle.
Galaxies spreading apart means they were closer together in the past: about 13.8 billion years ago, all in the same space.
The Big Bang theory says the whole universe started in a hot, dense state, then expanded and cooled.
The steady state model says new matter is constantly created as galaxies move apart, so density and temperature never change.
Have a goQuick sort: 'The universe was hotter in the past than it is now.' Is that a Big Bang claim or a steady state claim?
A Big Bang claim.
The Big Bang theory says the universe started hot and dense and then cooled, while the steady state model says the average temperature doesn't change.
Both models account for galaxies moving apart, but only the Big Bang explains the CMBR, which the steady state model could not.
A scientific theory is our best understanding from all the evidence, and the Big Bang theory explains redshift and the CMBR.

The big picture

Light from nearly all galaxies is redshifted, so they are moving away from us, and both the Big Bang and steady state models account for that. Redshift alone could not pick a winner. The CMBR did, because only the Big Bang theory explains it.

Key points

1Redshift is an increase in wavelength caused by a source moving away from us, so the absorption lines shift towards the red end; a faster source gives a bigger shift.

2Light from nearly all galaxies is redshifted. Galaxies are moving away from us, and distant galaxies recede faster (Hubble's Law: distance is directly proportional to velocity).

3Both the Big Bang and steady state models account for galaxies moving apart, so redshift cannot separate them.

4Only the Big Bang theory explains the CMBR. The steady state model could not, and was eventually rejected.

5Quasars being more common in the distant past was another observation the steady state model could not easily explain.

6The Big Bang was an expansion of space combined with the creation of all matter, not an explosion of existing matter.

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Worked example

Problem

Explain why the discovery of cosmic microwave background radiation (CMBR) led scientists to reject the steady state model.

⚠ Watch out

Writing that redshift proves the Big Bang and disproves the steady state model. Redshift shows galaxies are receding, and both models account for that. The CMBR, which only the Big Bang explains, is what favoured the Big Bang.

🧠

Memory hook

Redshift is the alibi both suspects share. The CMBR is the clue that only fits one of them.

✓

Check yourself

Can you explain, in your own words, why redshift couldn't settle the argument, and name the observation that could?

Flashcards

(15)

What is an absorption line?

A dark line at a specific wavelength in a spectrum, where gases in a star's outer layers have absorbed light. All stars contain hydrogen, so their hydrogen lines can be compared.

What is redshift?

An increase in the wavelength (and a decrease in the frequency) of light, caused by a star or galaxy moving away from us.

Which way do the absorption lines move in redshift, and what does a bigger shift mean?

Towards the red end of the spectrum. The greater the speed of the source away from us, the greater the shift.

What does the redshift of light from nearly all galaxies show?

That nearly all galaxies are moving away from us, with distant galaxies receding faster.

State Hubble's Law.

The further away a galaxy is, the greater its velocity away from us: distance is directly proportional to velocity.

Why is there no centre of the universe?

Nearly every galaxy is moving away from every other galaxy, so no point is the middle.

When do the galaxies' velocities suggest all galaxies occupied the same space?

About 13.8 billion years ago (about 14 billion years ago).

What does the Big Bang theory say about the early universe?

All the matter in the universe occupied the same tiny region of space, incredibly dense and incredibly hot. The universe then expanded and cooled.

Was the Big Bang an explosion?

No. It was an expansion of space combined with the creation of all matter in the universe, not existing matter blowing up.

What are the features of the steady state model?

Matter is constantly created to keep the density constant, galaxies move further apart, and the average temperature and density do not change.

What does the Big Bang theory predict about temperature and density?

Both have decreased over time as the universe expands. It does not predict a constant density or constant creation of matter.

What is cosmic microwave background radiation (CMBR)?

Electromagnetic radiation produced in the hot early universe, stretched by expansion to the microwave part of the spectrum, and detected from every direction in space.

Which model explains the CMBR?

Only the Big Bang theory. The steady state model could not, and it was eventually rejected by scientists.

What else could the steady state model not easily explain?

Quasars, ultra bright objects, being more common in the distant past, which suggests the universe has been changing over time.

What is a scientific theory?

Our best understanding based on all the evidence, and it is subject to rigorous testing.

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Who can explain it?

Which idea sounds right to you?

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