Predicting the microwave background
Definition
Cosmic microwave background radiation
Cosmic microwave background radiation is faint microwave radiation of almost the same intensity that reaches the Earth from every direction in space.
Definition
Electromagnetic spectrum
The continuous range of electromagnetic waves arranged according to wavelength or frequency.
- The Big Bang theory starts the Universe in an extremely hot state, and anything that hot emits intense electromagnetic radiation.
- That radiation was released in every direction at once, so it should still be crossing space in every direction today.
- Space has expanded enormously since then, and the expansion stretches the wavelength of any wave travelling through it.
- Radiation that left as visible and infrared light has therefore been stretched all the way into the microwave part of the electromagnetic spectrum.
- The theory therefore predicted a weak microwave signal filling the whole sky, corresponding to a temperature only a few degrees above absolute zero.
- The Steady State theory makes no such prediction, because it never has a hot dense early stage to leave any radiation behind.
Finding the background radiation
- In 196519651965 two radio astronomers, Penzias and Wilson, were testing a large horn shaped aerial and could not get rid of a faint hiss in their receiver.
- The signal did not fade when they pointed the aerial elsewhere, did not change with the time of day and did not change with the seasons.
- That behaviour ruled out any source on the Earth, in the atmosphere, in the Solar System or in our own galaxy, because each of those would appear in one direction or vary through the year.
- What was left was radiation coming uniformly from all of space, at microwave wavelengths and at a temperature close to 3 K3\ \text{K}3 K.
- A separate team of physicists recognised that this matched the signal the Big Bang theory had predicted years earlier.
- Later satellites mapped the radiation across the whole sky and confirmed both its temperature and its evenness far more precisely.
Why the Big Bang became accepted
Definition
Big Bang theory
The Big Bang theory states that the Universe began from a very small, extremely hot and dense region that has been expanding and cooling ever since.
Definition
Steady State theory
The Steady State theory states that the Universe has always existed and always looks much the same, because new matter is created continuously as it expands.
- Red-shift had never separated the two theories, since both of them describe an expanding Universe.
- The microwave background did separate them, because it was predicted by one theory and not by the other.
- Supporters of the Steady State theory could only account for the signal by adding extra assumptions that had no independent support of their own.
- The Big Bang theory now had two independent pieces of evidence behind it, the red-shift of galaxies and the microwave background, while the Steady State theory had one.
- Because there is more evidence supporting the Big Bang theory than the Steady State theory, the Big Bang is the currently accepted model for the origin of the Universe.
- The word currently matters, because a model is accepted only while it fits the evidence and would be replaced if a future observation contradicted it.
- Acceptance also took time rather than happening at once, since other groups first had to repeat the measurement with different equipment before the result was trusted.
Example
Tracking down the aerial noise
- The first suspicion was equipment fault, so the receiver was rebuilt and retested, and the hiss stayed.
- The second suspicion was interference from nearby New York, so the aerial was pointed away from the city, and the hiss stayed.
- The third suspicion was contamination inside the horn, so the pigeon droppings were cleaned out, and the hiss stayed.
- Eliminating each local explanation in turn is what turned an annoying noise into a measurement of something real.
- Only then could the signal be matched against a prediction that had been published before it was found.
Example
How far the radiation has stretched
- Take radiation that left the hot early Universe with a wavelength of 1.0×10−6 m1.0 \times 10^{-6}\ \text{m}1.0×10−6 m, which is in the infrared.
- It is detected today at a wavelength of about 1.1×10−3 m1.1 \times 10^{-3}\ \text{m}1.1×10−3 m, which is in the microwave region.
- The stretch factor is 1.1×10−31.0×10−6=1100\dfrac{1.1 \times 10^{-3}}{1.0 \times 10^{-6}} = 11001.0×10−61.1×10−3=1100.
- Space has expanded by roughly that same factor since the radiation was released, which is why the signal now sits so far from the visible part of the spectrum.
Exam technique
Explaining why the model changed
- State that the microwave background was predicted by the Big Bang theory but not by the Steady State theory, because that contrast is the answer to the question.
- Mention that the radiation arrives from all directions, since a question often awards a mark for that feature alone.
- Write currently accepted model rather than correct model, because the examiners expect you to leave room for the theory being replaced.
- Say that more evidence supports the Big Bang theory rather than saying the Steady State theory was disproved.
- In a longer answer, order it as prediction, discovery, then acceptance, so each stage earns its own credit.
Common Mistake
- Do not write that the microwave background proves the Big Bang theory, because evidence supports a theory rather than proving it.
- Do not say the radiation comes from the edge of the Universe, since it arrives evenly from every direction.
- Do not claim the radiation was always microwaves, because it was released as much shorter wavelength radiation and has been stretched since.
- Do not treat the Steady State theory as unscientific, since it made testable predictions and was rejected because those predictions failed.
Self review
- Explain why the Big Bang theory predicts radiation filling the whole sky.
- Describe how the wavelength of that radiation has changed since it was released.
- State two observations that showed the signal found by Penzias and Wilson was not from a local source.
- Explain why the discovery of the microwave background counted against the Steady State theory.
- State which model of the origin of the Universe is currently accepted and give the reason.
