# How old is the Universe? Presented by Miles Hudson All images courtesy of NASA and Hubblesite.org.

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How old is the Universe? Presented by Miles Hudson All images courtesy of NASA and Hubblesite.org

This picture shows many galaxies. It was taken by the Hubble Space Telescope But when did it all come into being?

Age of the Universe? Any guesses? Three hundred years?300 y A thousand years?1 000 y Five million years?5 000 000 y Two billion years?2 000 000 000 y Eight trillion years?8 000 000 000 000 y 3 x 10 2 1 x 10 3 5 x 10 6 2 x 10 9 8 x 10 12 Maths shorthand (‘standard form’) On a scientific calculator, the EXP button means ‘x10’

Did you know that the Universe is expanding? The light from distant galaxies shows that they are moving away from us. The Sun Every galaxy is moving away…

The Sun Many years later….

So if we run the clock backwards…. Many years ago The beginning of the Universe This is the basis of the Big Bang theory.

In 1929, the American astronomer Edwin Hubble did a survey of many galaxies. He plotted a graph v (km/s) How fast it’s moving How far away it is: d (x10 19 km) and found that the further away a galaxy is, the faster it moves away. This gave us Hubble’s Law: v = H 0 d Hubble’s Law

Hubble’s Law: v = H 0 x d So how fast a galaxy is moving away from us is always so many times how far away it is. The ‘how many times’ is H 0 and is called the Hubble Constant. And… how old the Universe is can be found from the Hubble constant. If we follow in Hubble’s footsteps and draw the graph he did, we can find H 0 and so we can find out how old the Universe is. Let’s do it! It’s 1 ÷ H 0

Step 1: Draw graph axes Put your paper upright (portrait). v (km/s) d (x10 19 km) 0 1100 (go up in 100’s so 11cm across) 0 21000 (go up in 1000’s means 21cm up) Step 2: Plot the points Galaxy cluster Distance, d (x10 19 km) Speed, v (km/s) Pegasus 1203800 Perseus 1905200 Coma 3506800 Hercules 32010400 Leo 95018800 Gemini 110020600 Step 3: Draw the best fit line through the points 1000 2000

Finding H 0 Finding the gradient, H 0 Pick two points far apart on the best fit line. Find the change in v:  y Find the change in d:  x Remember, d is x10 19 so put that on your value for  x. Gradient: H 0 =  y ÷  x yy xx v d Finding the age of the Universe Your answer for H 0 should be ‘something’ x 10 -18 We want to find 1 ÷ H 0 Your calculator may have a button marked ‘1/x’ or ‘x -1 ’. Pressing this will do the ‘1÷’ function we want. Your answer will be in seconds. There are 31536000 seconds in one year. So take the number of seconds and divide by 31536000 to find the answer in years. Remember ‘x10 9 ’ means billions. Alternatively, just type in the calculation with your value of H 0.

We’ve used data that Hubble used, and our answer for the age of the Universe is around 16 to 18 billion years old. Since Hubble plotted his graph, there have been huge improvements in the accuracy of the distance measurements, d. The current best answer for the age of the Universe is…... 13.8 billion years 13 800 000 000 years The errors in the data at his time meant that the answer could be anywhere from 10 to 20 billion years.

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