How do we use light to work out that the universe is expanding?

06 September 2016

Question

i had a question concerning a subject you were talking about-size of the universe I did not have education but ..don't think that's the reason that I don't get it--'dark' (mysterious) energy (do we know by any measure, a ratio how much this speed has increased?.. meanwhile taking with this speed objects further and further away from us? )pushes the universe with increasing speed wider and wider-expanding it......in practice,meaning that when an object has emitted light , let's say 10 billion light yrs ago and we receive the light now (and know thus by the composition of that light it was 10 billion yrs ago and the distant is as well)....all those light years when its light was on its way to us...that object has moved away from us with increasing speed. that means in my view that we cannot state and determine the size and age of the Universe---we have only by the light that has received--do we know the speed of expansion at the edge of the Universe we can perceive? Can somebody brainwash me, so that I can think clean and clearly? Greetz from fremantle

Answer

We put this question to PhD student Adam Townsend...

Adam - So, most of us know what's called the Doppler Effect. So this is when a fire engine or a police car or an ambulance, and I'm familiar with all of these because I live in London. It comes straight past your window and I and you hear it go. And like, "Neno, neno, nenooooo, .." As it comes pass you, that's a great sound effect though - as it comes past you, the pitch drops, the frequency drops. The same thing happens with light. Therefore, as the stars move away from us - I think Hubble actually spotted this. Hubble was looking at sort of nearby stars and spotting what they were made of by looking at their colours. And then he looked at stars that were slightly further away and found that they were sort of similar but everything was just a little bit more red. And then he looked at ones that were really, really far away. And saw that they were even more red. Therefore, he was able to sort of see that okay, everything is being shifted towards the red spectrum. This is called red shift.

Kat - Red light has longer wavelength than blue light.

Adam - Right. So then basically, by looking at the change in the colour of light where it's like the galaxies are going past us going..

Adam - Yeah and in the same way that say, as the fire engine, goes past the frequency has decreased, they found that it went more red so the frequency has decreased.

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