{"id":261,"date":"2016-10-30T16:54:31","date_gmt":"2016-10-30T16:54:31","guid":{"rendered":"http:\/\/www.mrsphysics.co.uk\/higher\/?p=261"},"modified":"2025-11-25T07:38:24","modified_gmt":"2025-11-25T07:38:24","slug":"the-expanding-universe","status":"publish","type":"post","link":"https:\/\/mrsphysics.co.uk\/index.php\/2016\/10\/30\/the-expanding-universe\/","title":{"rendered":"The Expanding Universe Practical"},"content":{"rendered":"\n

A great little practical with washers, that was used as an exam question!<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n
IOP-Elastic-Band-Universe-Student-Activity-Instructions<\/a>Download<\/a><\/div>\n\n\n\n
\"\"<\/figure>\n\n\n\n

Try the following practical<\/p>\n\n\n\n

http:\/\/www.schoolsobservatory.org.uk\/astro\/cosmos\/uniball<\/a><\/p>\n\n\n\n

expanding universe school observatory<\/a><\/p>\n\n\n\n

<\/p>\n\n\n\n

Expanding Universe Experiment<\/h2>\n\n\n\n

To understand how the redshift<\/a> of galaxies<\/a> is due to the expansion<\/a> of the Universe<\/a>, try the following experiment.<\/p>\n\n\n\n

You will need the following items:<\/strong><\/p>\n\n\n\n

    \n
  1. A round balloon (do not use a long, thin one).<\/li>\n\n\n\n
  2. Some coloured stick-on dots (at least 5 different colours).<\/li>\n\n\n\n
  3. A piece of string about 50cm long.<\/li>\n\n\n\n
  4. A ruler.<\/li>\n\n\n\n
  5. A stopwatch or other timer.<\/li>\n<\/ol>\n\n\n\n

    Step 1 : Setting Up<\/h3>\n\n\n\n

    You will need to work in teams of at least two, one to blow up and hold the balloon and the other to make the measurements.<\/p>\n\n\n\n

    Before you start, draw a table for your results like the one below with the colours of your five dots in the 1st column:<\/p>\n\n\n\n

    Colour of Dot<\/strong><\/td>First Distance D1<\/em> in cm<\/td>Second Distance D2<\/em> in cm<\/td>Change in Distance        D2 – D1<\/em> in cm<\/td>Speed v<\/em> in cm\/second<\/td><\/tr>
    Red<\/td> <\/td> <\/td> <\/td> <\/td><\/tr>
    Green<\/td> <\/td> <\/td> <\/td> <\/td><\/tr>
    Blue<\/td> <\/td> <\/td> <\/td> <\/td><\/tr>
    White<\/td> <\/td> <\/td> <\/td> <\/td><\/tr>
    Yellow<\/td> <\/td> <\/td> <\/td> <\/td><\/tr>
    Time to fully inflate the balloon:        seconds.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    Step 2 : Making the Measurements<\/h3>\n\n\n\n

    Putting dots on the small balloon<\/p>\n\n\n\n

    Blow up the balloon a little bit<\/em> and hold the “nozzle” closed, but do not tie it up.<\/p>\n\n\n\n

    Stick your five dots onto the balloon. Try to spread them out over the whole balloon.<\/p>\n\n\n\n

    \"stick<\/figure>\n\n\n\n

    Each of the dots represents a whole galaxy<\/a>, with the surface of the balloon being the Universe that they exist in.<\/p>\n\n\n\n

    Choose one of the dots to be your “home”. You can choose any of them.<\/p>\n\n\n\n

    Step 3<\/h3>\n\n\n\n

    Use string to measure the distance between two dots<\/p>\n\n\n\n

    \"measure<\/figure>\n\n\n\n

    While one of you holds the balloon, the other one can use the string to measure the distance from your “home” dot to one of the other dots.<\/p>\n\n\n\n

    Now measure the string distance with a ruler.<\/p>\n\n\n\n

    When you have measured the distance, write it down in your table in the D1<\/em> column.<\/p>\n\n\n\n

    Step 4<\/h3>\n\n\n\n

    Measure the distances from the “home” dot to all the other dots as well and fill in that column of the table.<\/p>\n\n\n\n

    Note:<\/em> The distance from your “home” dot to itself is zero.<\/p>\n\n\n\n

    Step 5<\/h3>\n\n\n\n

    Now carefully blow the balloon right up, using the stopwatch to time how long it takes. Write down the time in seconds.<\/p>\n\n\n\n

    Step 6<\/h3>\n\n\n\n

    Now re-measure all the distances from “home” to all the other dots and write then down in the D2<\/em> column of your table. Don’t forget that the distance from your “home” dot to itself is zero.<\/p>\n\n\n\n

    You now need to work out the speed of each galaxy. Remember that:<\/p>\n\n\n\n

    Here, the Distance travelled<\/strong> is the difference between D1<\/em> and D2<\/em>, so calculate D2 – D1<\/em> for each of our dots and write them in the 4th column on the table.<\/p>\n\n\n\n

    Step 7<\/h3>\n\n\n\n

    The Time taken<\/strong> is the time to blow the balloon up. Work out the speed V<\/em> for each dot and put it into the 5th column. Because your “home” dot has not moved, its speed will be zero.<\/p>\n\n\n\n

    Step 8<\/h3>\n\n\n\n

    We are studying how the speed<\/strong> that galaxies seem to have gets larger for galaxies that are further away.<\/p>\n\n\n\n

    The best way to see this is to plot a graph showing the distance<\/strong> along the bottom axis and with the speed<\/strong> up the side.<\/p>\n\n\n\n

    This means that you need to plot a graph with axes like the one below:<\/p>\n\n\n\n

    \"speed<\/figure>\n\n\n\n

    Put the points for all your dots on the graph using D2<\/em> as the Distance.<\/p>\n\n\n\n

    Step 9 : What does it all mean ?<\/h3>\n\n\n\n

    Use the ruler to draw a straight line that goes as close to as many of the points as possible (don’t forget the “home” dot!)<\/p>\n\n\n\n

    Think about the following questions and discuss them:<\/p>\n\n\n\n