7.2.1 Big Bang and Steady State theories

Two theories of the Universe

1. Astronomers agree that the Universe is expanding, which means that the distances between distant galaxies are increasing.
2. Two rival theories were built to explain that expansion, the Big Bang theory and the Steady State theory.
3. Both were serious scientific theories, and both were taken seriously through the middle of the twentieth century because both accounted for the expansion that had been measured.
4. They part company over one question, whether the Universe had a beginning or has simply always been here.

What the Big Bang theory says

1. All of the matter and energy in the Universe began packed into a region that was extremely small, hot and dense.
2. That region began to expand, and space itself has been growing ever since rather than matter flying outwards through empty space.
3. The expansion has spread the same fixed amount of matter through a steadily larger volume, so the Universe has become less dense as it has aged.
4. Expansion also spreads out the energy present at the start, so the Universe has cooled continuously from that first hot state.
5. The theory therefore gives the Universe a finite age, currently taken as about $13.8$ billion years.
6. It also means the Universe looked very different in the past, since everything was closer together, hotter and more crowded.

What the Steady State theory says

1. The Universe has always existed, so there was no starting event and there is no age to calculate.
2. The Universe is still expanding, so this theory agrees with the Big Bang theory that galaxies are moving apart.
3. To stop the expansion thinning everything out, new matter is created continuously in the gaps that open up between the galaxies.
4. That new matter forms new galaxies at the same rate as old ones drift apart, so the average density stays constant.
5. The overall appearance of the Universe is therefore unchanging, which is where the name of the theory comes from.
6. An observer looking at the Universe billions of years ago, or billions of years in the future, would see much the same picture as one looking today.

Comparing the two theories

1. Both theories accept that the Universe is expanding and that distant galaxies are moving away from us.
2. Both also accept that the Earth holds no special place, so an observer in any galaxy would see the other galaxies receding.
3. They differ on the beginning, because the Big Bang theory has a single starting event while the Steady State theory has none.
4. They differ on age, because the Big Bang theory gives a finite age while the Steady State theory makes the Universe infinitely old.
5. They differ on matter, because the Big Bang theory keeps the total amount fixed while the Steady State theory creates more of it as space grows.
6. They differ on density, because the Big Bang theory has it falling with time while the Steady State theory holds it constant.
7. They differ on how the Universe changes over time, because the Big Bang theory says the distant past looked hotter and more crowded while the Steady State theory says it looked the same.
8. Those last differences are what make the two theories testable, since a theory that predicts a changing Universe can be separated from one that predicts an unchanging Universe by observation.

7.2.2 Evidence for the Big Bang

What counts as evidence

1. A theory about the Universe cannot be tested in a laboratory, so it is judged by whether its predictions match what telescopes actually record.
2. Two observations are treated as the supporting evidence for the Big Bang theory, the red-shift of light from distant galaxies and the cosmic microwave background radiation.
3. Each of these is a measurement made with instruments rather than an argument, which is what makes them evidence rather than opinion.
4. Both have been repeated by many independent teams using different telescopes, so neither depends on a single result that might be a fault in one instrument.

Red-shift of distant galaxies

1. Light from a galaxy is split into a spectrum, and that spectrum carries a pattern of dark absorption lines produced by the elements in the outer layers of its stars.
2. The same elements produce the same pattern in a laboratory on Earth, so the two spectra can be laid side by side and compared directly.
3. In the light from a distant galaxy the whole pattern sits at longer wavelengths, which means it has moved towards the red end of the visible spectrum.
4. The spacing and order of the lines are unchanged, so the elements in the galaxy are the same as those on Earth and only the wavelengths have shifted.
5. Almost every galaxy shows this shift, and the more distant the galaxy the larger the shift measured.
6. A red-shift is produced when a source of light moves away from the observer, so the measurement shows that these galaxies are receding from us.

The cosmic microwave background

1. A faint signal in the microwave part of the electromagnetic spectrum arrives at the Earth from every direction in the sky.
2. It does not come from any star, galaxy or other object that can be pointed at, and it is present even where the sky looks completely empty.
3. Its intensity is almost exactly the same in every direction, with variations of only a tiny fraction of the total.
4. The radiation matches what a body at a temperature just under $3K$ would emit, which is only a few kelvin above absolute zero.
5. The Big Bang theory accounts for it as radiation released when the young Universe was extremely hot, stretched to microwave wavelengths by the expansion of space since then.
6. On that account the microwave background is leftover heat radiation that has been cooling for the whole life of the Universe.

Why this evidence carries weight

1. The microwave background was predicted before it was found, so the Big Bang theory was committed to it in advance rather than fitted to it afterwards.
2. A prediction that is confirmed later is stronger support than an explanation invented once the measurement is already known.
3. The two pieces of evidence are independent, since one uses visible light from individual galaxies and the other uses microwaves from empty sky.
4. Agreement between independent measurements is what turns a plausible idea into an accepted theory.
5. Neither observation counts as proof, because a scientific theory is always open to being replaced if a later measurement contradicts it.

7.2.3 The Doppler effect and red-shift

The Doppler effect

1. A stationary source sends out waves as a set of evenly spaced circles, so every observer around it measures the same wavelength and the same frequency.
2. Once the source starts to move it travels a short distance between sending out one wave and sending out the next, so each new wave leaves from a slightly different place.
3. Ahead of the moving source the waves are squashed together, so an observer in front measures a shorter wavelength and a higher frequency.
4. Behind the moving source the waves are stretched apart, so an observer behind measures a longer wavelength and a lower frequency.
5. Only the relative motion of source and observer matters, so the same change is measured whether the source approaches the observer or the observer approaches the source.
6. The source itself never changes what it emits, since the change is created entirely by the motion and appears only in what the observer receives.
7. Wavelength and frequency shift in opposite senses because the wave speed is set by the material the wave travels through, and $v=f\times \lambda$ must still hold.

Sound from a moving source

1. The everyday example is an ambulance siren heard from the pavement as the ambulance drives past.
2. While the ambulance approaches, the sound waves reaching you are compressed, the frequency you hear is higher than the siren produces, and the note sounds higher pitched.
3. The moment the ambulance passes, the waves reaching you are stretched instead, and the note drops to a lower pitch even though the siren has been set to one steady note throughout.
4. The driver hears no change at all, because the ambulance and the siren are not moving relative to each other.
5. The speed of sound in the air stays at about $340m/s$ the whole time, so a shorter wavelength has to mean a higher frequency.
6. The faster the source travels, the larger the change in the observed frequency, which is why a passing racing car produces a far more dramatic drop than a passing bicycle bell.

Light from a moving source

1. Light is a wave, so a light source moving relative to an observer produces the same change in observed wavelength and frequency as a moving sound source does.
2. A source moving away gives a longer observed wavelength and a lower observed frequency, and the light is described as red-shifted because red sits at the long wavelength end of the visible spectrum.
3. A source moving towards the observer gives a shorter observed wavelength and a higher observed frequency, and the light is described as blue-shifted.
4. The shift is detected by comparing the wavelengths of absorption lines from a star with the wavelengths the same elements produce in a laboratory.
5. Everyday speeds give shifts far too small to notice, because light travels at $3.0\times 10^{8}m/s$ and a car is slower than that by a factor of many millions.
6. The size of the shift depends on how fast the source is moving along the line of sight, so a larger shift means a larger speed of approach or recession.

7.2.4 Red-shift as evidence for an expanding Universe

Red-shift and distance

1. When the light from a distant galaxy is compared with a laboratory spectrum, every absorption line is found at a longer wavelength than it should be.
2. This red-shift is seen in almost every galaxy that has been measured, in whichever part of the sky the telescope is pointed.
3. The size of the shift is not the same for every galaxy, and the further away a galaxy is, the larger its red-shift.
4. The pattern is close to a proportional one, so a galaxy twice as far away shows roughly twice the shift.
5. Since a larger shift means a faster speed along the line of sight, the more distant galaxies are moving away from us more quickly.
6. A few of the nearest galaxies are blue-shifted instead, because their own motion within the local group of galaxies is larger than the general recession at such small distances.

Why this shows expansion

1. The red-shift measurements show that the distance between our galaxy and almost every other galaxy is increasing.
2. The same pattern would be measured from any galaxy, so there is no galaxy at the centre and our own position is nothing special.
3. Galaxies flying outwards through fixed space from one point would put us at the centre of the motion, and the observations rule that picture out.
4. What fits instead is that space itself is expanding, stretching the gaps between the galaxies and carrying them apart with it.
5. Expanding space also stretches the light waves travelling through it, which is why the wavelength arriving at the telescope is longer than the wavelength that left the galaxy.
6. Light from a more distant galaxy spends longer crossing the expanding space, so it is stretched more, which is exactly why distance and red-shift rise together.
7. The expansion is a property of the whole Universe rather than of one region, so red-shift is evidence about the Universe as a whole.

How both theories explain it

1. Red-shift on its own does not decide between the two theories, because both of them include an expanding Universe.
2. The Big Bang theory explains it as expansion that started at a single event, so the galaxies have been separating ever since and the light between them has been stretched throughout.
3. On that account the red-shift is a record of an expansion with a beginning, which is why it can be traced back to a hot, dense start.
4. The Steady State theory explains the same red-shift as expansion that has always been going on and always will, with no beginning to trace back to.
5. New matter forms new galaxies in the widening gaps, so the galaxies still separate and still show red-shift while the Universe as a whole looks unchanged.
6. Both theories therefore predict red-shift that increases with distance, and both match the measurements equally well.
7. Separating them needs an observation the two theories disagree about, which is why red-shift is quoted as evidence for expansion rather than as evidence for the Big Bang alone.

7.2.5 The CMB and the accepted model

Predicting the microwave background

1. The Big Bang theory starts the Universe in an extremely hot state, and anything that hot emits intense electromagnetic radiation.
2. That radiation was released in every direction at once, so it should still be crossing space in every direction today.
3. Space has expanded enormously since then, and the expansion stretches the wavelength of any wave travelling through it.
4. Radiation that left as visible and infrared light has therefore been stretched all the way into the microwave part of the electromagnetic spectrum.
5. The theory therefore predicted a weak microwave signal filling the whole sky, corresponding to a temperature only a few degrees above absolute zero.
6. 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

1. In $1965$ 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.
2. 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.
3. 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.
4. What was left was radiation coming uniformly from all of space, at microwave wavelengths and at a temperature close to $3K$.
5. A separate team of physicists recognised that this matched the signal the Big Bang theory had predicted years earlier.
6. 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

1. Red-shift had never separated the two theories, since both of them describe an expanding Universe.
2. The microwave background did separate them, because it was predicted by one theory and not by the other.
3. Supporters of the Steady State theory could only account for the signal by adding extra assumptions that had no independent support of their own.
4. 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.
5. 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.
6. 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.
7. 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.