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the age of the

WE’RE OBSESSED WITH measuring time. We celebrate new , anniversaries and birthdays. We dedicate entire sports to seeing how long someone takes to get from one point to another. We take joy in counting the minutes before the end of a particularly boring English lesson (or before the beginning of a fantastically engrossing session of Physics). So it was only a matter of time before we made the ultimate measurement: how old is time itself? What is the age of the Universe?

THE NIGHT SKY is dark. It might appear brighter than daylight. Of sound like an obvious statement, course, we know this isn’t true: but someone had to point it out if you take this sheet of paper before we realised that it actually to the middle of a field, in the has quite profound implications. middle of the night, and with no We used to think that the artificial light, you’re not going Universe was infinite - that is, it to be able to read it. Which can was always in existence, and went only mean one thing: the Universe on, forever, in every direction. But isn’t infinte at all. It must have a in 1823, the German astronomer finite age, and the light from the Heinrich Wilhelm Olbers realised most distant hasn’t had the that if this was true, then the time to reach us. night sky would be filled with an infinte number of stars and Heinrich Wilhelm Olbers

Right: the original NEARLY A HUNDRED years later, velocity-distance graph the American astronomer Edwin plotted by Hubble Hubble used a technique known (below left). Although as (see “painting the he had the right idea, his sky red”) to investigate the speed actual results were quite at which distant were inaccurate; we have since been able to produce more moving away from us. He plotted velocity-distance accurate results using the a graph (left) Hubble Space Telescope and found that the distance of a (below right). was more or less directly proportional to its speed; that is, a galaxy twice as far away from us as another galaxy, was moving twice as fast. This was an astonishing discovery, and not what anyone expected. What’s more, using some pretty basic physics and maths, it leads to quite a startling conclusion. If an object emitting a wave is painting the sky red: redshift moving away from you, then the You know that a wave, such as the light smaller frequency than blue light. So wave appears “stretched”, and the emitted from a , has a wavelength as the wavelength gets longer, and the wavelength appears longer. - the distance between two consecutive frequency gets smaller, the light appears peaks or troughs. However, if the object redder - which is why this phenomenon is moving away from you, then the wave is known as redshift. If you measure the seems stretched and so the wavelength value by which the frequency decreases, appears longer. In turn, this means that you can then use a simple formula to its frequency appears smaller. calculate the speed at which the object is moving away from you - and this The frequency of visible light determines is what Hubble used to determine the what colour it is - red light has a speed of distant galaxies.

IMAGINE THAT YOU’RE standing you are, that you’re observing further reading! on the side of a particularly long something similar to what Hubble If you want to find out more about stretch of road, when, at exactly saw in the stars: their speed away the origin of our universe, and its 12:00pm, two cars pass you at from you is proportional to their potential fates, check out A Brief exactly the same time. One is distance from you. But you have History of Time by , travelling at 30mph, and the an extra piece of information: or An Elegant Universe by Brian other is travelling at 60mph. At there was a time when both cars Greene. Although be warned: the first you take no notice, but then where at the same place at the former isn’t that brief, and the latter at 1:00pm, exactly one hour later, same time. Applying the same isn’t always that elegant! you decide to look through your logic to the stars, there must have telescope to see where they are. been a time when everything in references As expected, the first car is 30 the Universe was also at the same BW Carroll, DA Ostlie, An Introduction miles away, and the second car is place at the same time. We call it to Modern Astrophysics, 2nd Edition (2007, Addison Wesley, San Francisco). 60 miles away. You notice, being The . the particularly bright person that

If the distance to several objects is proportional to the speed at which they’re moving away from you, then there must have been a time when they were all in the same place at the same time.

THE BIG BANG was when believed to be about 2.33 x 10-18 everything came into existence s-1. This means we now have a – so if we can calculate how relation between v and r, known long ago that was, then we’ll as Hubble’s Law – which states, know how old the Universe is. quite simply, that v = H0r. If we You know that quantities that rearrange the formula, we can are proportional to each other see that the Hubble Constant is are related by a proportionality equal to speed over distance (1, constant, and in this case, the left). Take the reciprocal, and we speed, v, and the distance, r, are have something that is equal to related by the Hubble Constant, distance over speed… or time(2). H0. We can find it by calculating So if we take the reciprocal of the the gradient of the graph, which Hubble Constant, we’ve got what turns out to be quite a simple we’re looking for. The age of the task, since it’s a straight line Universe. 14 . - using data obtained from the Hubble Space Telescope, it is Who wants to buy the candles? LIST OF IMAGE SOURCES:

Portrait of Olbers: Images of the used http://upload.wikimedia.org/ for backgrounds: wikipedia/commons/a/a9/ http://www.astropix.com/HTML/ Heinrich_Wilhelm_Olbers_2.jpg D_SUM_S/MILKYWAY.HTM

Portrait of Hubble: http://www.depts.ttu.edu/ http://lh4.ggpht.com/_ communications/news/stories/ opzY8FwgzCI/RxcpXLtIEwI/ images/milkyway_brunier-large. AAAAAAAAACA/y-d9N7wpREA/ jpg edwin+hubble.bmp http://apod.nasa.gov/apod/ The Hubble Space Telescope: image/0501/milkyway_garlick.jpg http://miguellopes.files. Portrait of Olbers: wordpress.com/2007/01/hubble. jpg

Hubble’s Original Diagram: http://spiff.rit.edu/classes/ phys240/lectures/expand/ hubble_fig1_small.gif