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Waves - The Expanding Universe

This GCSE Physics quiz on waves looks at the expanding universe. The universe in which we live is a puzzling and unimaginably large place. It has been in existence for an incredibly long time and we humans are only able to observe it from a single point in space and time which makes it extremely difficult to understand and explain. It would be a bit like an intelligent microbe trying to work out the detailed structure of the human body from a single point inside of a bone! Cosmology is the name given to the study of the origin and nature of the universe and there have been many theories developed, ranging from creation by an all-powerful being to the currently accepted scientific explanation we call the Big Bang.

Possibly the most important moments in the history of cosmology were when Einstein came up with his two theories of relativity.

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Within about 20 years or so, scientists had realised that the universe was expanding at an enormous rate. Using a device called a spectrometer (a device that splits light into its component wavelengths), an astronomer called Vesto Slipher had observed the phenomenon of redshift. He did this because every star and galaxy has certain dark lines on its spectrum. He noticed that when he took the spectrum of distant galaxies, these lines were not in their normal positions, they were always displaced towards the red part of the spectrum. The significance of this was identified by another astronomer, Edwin Hubble. He looked carefully at Slipher's findings and realised that the further away the galaxy was, the greater the redshift. Whilst Hubble was doing this, a Belgian, Georges Lemaitre, had worked out that the universe should be expanding using Einstein's theories. His ideas were the start of what was to be called the Big Bang theory.

Redshift arises because of the Doppler effect. As an object recedes from an observer (or vice versa), any waves leaving the object will be changed by the motion. The crests of successive waves are emitted slightly further away than if the object was stationary and so they have a longer wavelength (and therefore lower frequency). You can think of the movement as stretching the waves. For light, longer wavelengths mean redder light, hence the name redshift.

Explaining the expanding universe gave rise to several theories, the two most favoured ones were the Steady State and the Big Bang. In science, when a theory is put forward, it is then tested. This is done by making predictions and then testing the prediction. One of the predictions of the Big Bang theory was that there would be an 'echo' of the massive explosion that created our universe, the cosmic microwave background radiation. This was discovered in 1964.

Another prediction of Einstein's theories is the existence of black holes. These are the remains of massive stars that have run out of nuclear fuel and collapsed in on themselves under the influence of gravity. They are so compact that their gravitational attraction is enormous, not even light can travel fast enough to escape it so they can't actually be seen directly. There is still no definite proof that these strange objects exist but scientists are looking for evidence. This is done in several ways. Matter falling into a black hole will be heated to such high temperatures that it will emit high energy radiation like X-rays just before it enters the black hole. Objects near to a black hole would be affected by its gravity so astronomers are looking for objects which appear to be orbiting something that they can't see. The gravity of a black hole is so great that it can bend light, exactly the same as a lens does. This is called gravitational lensing and astronomers are looking for this too. One of the latest ideas is that at the centre of many galaxies, including our own, there is a super-massive black hole pulling the stars in towards itself.

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  1. How long does it take the light from the sun to reach us?
    Due to the distance between us and the Sun and the fact that light travels at 3 x 108 m/s, it takes around 8 minutes for the light emitted by the Sun to reach us
  2. What happens to the wavelength of light from stars that are moving away from us?
    Each successive light wave crest or trough starts from a little further away than it would have done if the star was stationary so the wavelength is a little longer
  3. The faster a star or galaxy is moving (relative to us), we say that the redshift is …
    The light waves are 'stretched' more by faster moving objects
  4. The shift seen when an object is moving towards us is not red, it is …
    The faster the object is approaching, the greater the blueshift
  5. What types of radiation are emitted by the Sun?
    The Sun emits many kinds of radiation - not just the light that we can see. We can look at the Sun with special telescopes designed to see different wavelengths of light
  6. "In space no one can hear you scream" was a famous movie quote, but is that actually true and why?
    Sound is a longitudinal wave and requires a material medium so you could not hear a scream directly in space. Electromagnetic radiation can travel through a vacuum so if the person screaming had their communications radio switched on, you would be able to hear their scream. Assuming the spacesuit of the person screaming was still pressurised, if you touched your helmet to theirs, you could hear them scream. But how would that work? And would you really want to be that close ... it could be your turn next!
  7. How do we know that the Universe is currently expanding?
    When an object moves away from an observer, the waves that the object emits have a longer wavelength due to the speed of the object. Using this fact we can see that distant objects are moving away
  8. How do we know black holes exist if we cannot see them?
    The idea of black holes was first suggested in 1916 by a scientist called Scwarzchild
  9. What is blueshift?
    Each new wave is emitted from a point a little closer than if the object was stationary so the wavelength appears to be shorter than it would have been
  10. If scientists find new evidence that doesn’t support the Big Bang Theory, what should other scientists do?
    This is the scientific method

Author: Martin Moore

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