The Big Bang

1. The Big Bang

2. Few theories are so widely known by the public as the ‘Big Bang’ origin of the Universe.For GCSE science in the UK, you need to understand the evidence for this theory.

3. The theory states that about 13 billion years ago, the Universe expanded at a stupendous rate from being very small to very large.

4. So, what is the evidence for the theory and what research is going on to develop it?Make a short note about each piece of evidence

5. Try making a table like this

6. At the beginning of the 20th Century, most scientists thought that the Universe had existed for ever. In 1905 Einstein created equations that explained the nature of time and space. However they predicted an expanding Universe. Convinced of un unchanging Universe, Einstein changed his equations. He later described this unscientific behaviour as “my biggest blunder”.

7. You should have noted something like this:

8. The Big Problem • Space is Big • I mean really big • It took a long time to realise exactly how really big it is • Measuring how far away things are is difficult • The first person to find a method to measure things on the galactic scale was an American woman

9. We have speeded up time so that one second is a day Can you see an odd star?

10. Henrietta Leavitt Henrietta Leavitt was very interested in this type of star, called a Cepheid variable. In 1912 she carefully measured the brightness and period of hundreds of them in the Small Megellanic Cloud; a satellite galaxy to the Milky Way.   Leavitt wanted to know if there was a relationship between the brightness of the stars and how fast they pulsed. Nobody knew how far away the cloud was but Leavitt reasoned that as the cloud was a long way off, all the stars in the cloud would be roughly the same distance away.

11. Leavitt measured the brightness and period of hundreds of Cepheid variables. Here is a sample of her data Just to be awkward, astronomers measure brightness in a scale called magnitude, where the smaller the magnitude, the brighter the object. What do you think the relationship is between brightness and period?

12. This is what her data actually looked like. Is there a relationship between the brightness of the star and the period of its pulsing? How sure are you of your conclusion? Why could the data be so spread out from the line of best fit?

13. The brighter the star, the shorter its period. There is a strong correlation The data is spread out indicating other variables are having an effect. Later it was found that there are several types of Cepheid variables.

14. Leavitt had discovered that the brightness of one type, Cepheid variables, was linked to their pulsation period.  

15. In 1666, Isaac Newton discovered a basic relationship about brightness; something twice as far away is a quarter as bright. This is called the inverse square rule. When Leavitt combined this fact with he discovery about Cepheids she had created a tool for estimating the relative distance of objects.   This tool was key to studying the Universe. Her achievement was recognised by naming a crater on the Moon in her honour.

16. A Cepheid with a period of 2 days in the Andromeda Galaxy is 100 times dimmer than a Cepheid with the same period in the Small Megellanic Cloud How much further away is the Andromeda Galaxy compared to the Small Megellanic Cloud? Image T.A.Rector and B.A.Wolpa/NOAO/AURA/NSF

17. A year later in 1913, Danish astronomer Ejnar Hertzsprung measured the distance to ‘nearby’ Cepheid variables. He needed to look at a Cepheid in relation to very distant background stars from two different vantage points. He could then measure the shift of the star against the background and calculate its distance.

18. The solar system is hurtling through interstellar space at a rate of 15km per second in the direction of the constellation Hercules. At that rate we cover 300 million km in about eight months. By looking at the positions of Cepheid's on photographic plates taken several years apart, Hertzsprung was able to detect and measure a shift for 13 stars. With this, the SMC was placed at approximate 200,000 light years distant – twice the diameter of the Milky Way.

19. Approximately 2 million light years For example, how far away is the Andromeda Galaxy? However in 1913 most astronomers thought that Andromeda was just a cloud of gas in our galaxy

20. In 1923, American astronomer Edwin Hubble used the 100 inch diameter telescope on Mount Wilson in California to see if there were any Cepheid's in Andromeda. He found them but they were far fainter than he expected. • He calculated that it was at least a million light years distant.

21. Where are we? • So we now have a method of finding out how far away things are. • The next chapter in the story of the Big Bang is about working out the relative velocity of things.

22. Fingerprints of light In 1802 the Englishman William Hyde Wollaston noticed dark lines in the Sun’s spectrum. In 1814 the German Joseph Von Fraunhofer recognised that these lines were missing wavelengths. In 1857 two friends in Germany, Robert Bunsen and Gustav Kirchhoff discovered that the missing wavelengths were the light fingerprints of different elements which they were able to identify. You still use one of the bits of equipment Bunsen invented to do this – his Burner!

23. If your school has some spectrometers, see if you can identify some elements in Sun light Here you can see HRH the Prince of Wales using a simple spectrometer to identify elements in the Sun. You don’t even need a sunny day.

24. In 1872  Annie Jump Cannon, at Harvard University, lead a team that analysed the spectra of 250,000 stars. She developed the basis for the star classification system that we still use today.  O B A F G K M .   Generations of astronomers have learnt this sequence with the pneumonic 'Oh, be a fine girl (guy), kiss me!'

25. Fe He H Surface Temperature O Class 35,000 K B 22,000 K A 11,000 K F 7,000 K G 6,000 K K 5,000 K M 4,000 K Ca Na Here is the spectrum of Cannon’s seven classes of star. A few of the most important fingerprint absorption lines are labelled. From the information in the chart, can you explain the differences between star types?

26. Fe He H Surface Temperature O Class 35,000 K B 22,000 K A 11,000 K F 7,000 K G 6,000 K K 5,000 K M 4,000 K Ca Na There are two main difference; the colour spread and the lines. As things get hotter, they go from red hot, through orange, yellow white and blue. M will appear as reddish stars and O as bluish. Our own star is in the G class, and viewed from far off, would appear as a yellow star.The lines differ only in whether they are there or not. They do not change position. Eg the Iron (Fe) line appears at the same point in every spectrum.

27. SOS website challenge Map the temperature of the Sun with Cambridge University SOHO researcher Helen Mason.

28. Where are we? • We now know that when a star gives off light. the spectrum is ‘bar-coded’ with a series of absorption lines. • Elements create these lines at specific wavelengths. • If something changes that wavelength, we’ll be able to detect it.

29. Have you ever noticed that the noise of a passing motor vehicle, train or plane changes in pitch? Have a listen (if you can’t hear anything, switch the speakers on!) They make a sort of “neeow” sound.

30. Christian Doppler The first person to explain this effect was the Austrian maths teacher Christian Doppler in 1842. We now call it the Doppler effect. He followed up his idea with experiments involving putting musicians on open railway carts with another musician on the station platform. The musicians could compare notes played to notes heard.  

31. Christian Doppler Doppler showed that the effect was the same regardless of whether the note was played on the moving train or the platform.   It was all to do with relative movement.  Goodness knows what the train travelling public thought of these experiments!

32. What Doppler said was that waves travel at a particular speed so they cannot move away from the object any faster or slower.  

33. So when sound waves are given off in the same direction as the object is moving, they get bunched up.   This makes them a shorter wavelength and a higher frequency.  Those given off behind get spread out.   This makes them have a longer wavelength and a lower frequency.

34. Doppler realised that this should also effect light waves. He said that if the object is coming towards you and making sound, the pitch will be higher.  If it's making light the colour will be bluer. If the object is going away from you and making sound the pitch will be lower.   If it's making light its colour will be redder. Doppler hoped to use this method to detect the orbiting of double stars around each other but his measuring techniques were not up to the task.

35. You don't notice a colour change because the change is so tiny on something going a million times faster than sound.   However the fingerprints of a star’s elements should also be shifted towards the red or blue end of the spectrum and this is measurable.

36. In 1913 Vesto Slipher found that the Andromeda galaxy is blue shifted. It and the Milky way are rushing towards each other at 300km per second. At the time Slipher did not know how far apart the galaxies are. You do. How long have we got until they collided? – take a class vote. Distance 20 billion billion km Closing velocity 10 billion km/year • A 20 million years • B 2 billion years • C 20 billion years • D 2 million years

37. B 2 billion years

38. Whilst Slipher found Andromeda was blue shifted, most of the other galaxies were red shifted. What was his conclusion about the movement of galaxies relative to the Milky way?

39. Most galaxies are moving away from the Milky way! This is peculiar – what’s wrong with us? It makes us appear to be at the centre of the Universe. It could be evidence for the creation of the Universe – was it created here in a big explosion?

40. Where are we? Using Cepheid variables, we can now roughly measure how far away a galaxy is. Using the redshift of absorption lines, we can quite accurately measure how fast a galaxy appears to be moving away from us.

41. We now go back to Edwin Hubble on Mount Wilson. It is now 1929 and Hubble is doing a survey to find out if there is a relationship between how far away a galaxy is and how fast it is hurtling away from us. If we really are at the centre of an explosion, all the galaxies should be going away from us at the same velocity.

42. Here is some of Hubble’s data Make a quick sketch graph of this data What is the relationship between distance and velocity How confident are you of the relationship? (a Mega parsec is roughly 3 million light years)

43. Here is the complete set of Hubble’s data. It is even worse than the five data points you used. However there is a significant relationship. The further away, the faster the galaxy is going away from us. Because Hubble mistook clouds of stars for individual stars in his distance measurements, he thought the galaxies were ten times closer than they really were. However that does not change the fact that there is a relationship.

44. What is the explanation for the relationship? • The Universe could not have started with an explosive event where we are in space. If that had happened, all the galaxies would be rushing away from us at the same velocity. • To explain increasing velocity, we need another model.

45. Imagine the Universe as a balloon Stick some bits of white paper on it. These represent galaxies. We’ll ask Edwin Hubble to blow it up for us What do you think will happen?

46. Imagine the Universe as a balloon The galaxies stay the same size but the distances between them open up as the rubber separating the expands Imagine that one of the galaxies is you in the Milky Way. What would the movement of the other galaxies look like to you?

47. The further away the galaxy, the faster it appears to be moving away. Why does this happen? Milky Way Closer galaxy Further galaxy

48. There is more expanding space/rubber between more distant galaxies. Milky Way Closer galaxy Further galaxy

49. Ca2+ K 786.8nm Redshift is worked out using this formula:    observed wavelength minus the original wavelength divided by the original wavelength.    So in this spectrum from a galaxy, the Calcium II K line, which is usually at 393.4 nm (in the far blue region of the spectrum)  is observed at double that at 786.8 (into the infrared part of the spectrum) the redshift (z) = 786.8-393.4/393.4 = 1   

50. The resulting redshift scale is logarithmic so that the difference between redshift 1 and 2 is billions of years whilst between 7 and 8, only a couple of hundreds of thousands of years .    Whilst redshift 7 puts a galaxy at 800 thousand years after the big bang,  redshift 12 puts it at only 400 thousand years.