The Universe
Section: Space Physics | Syllabus: Cambridge IGCSE Physics (0625)
Structure of the Universe
The Universe is everything that exists - all matter, energy, space and time - organised on a hierarchy of scales from planets, to solar systems, to galaxies, to the Universe itself.
- The Milky Way is one of many billions of galaxies that make up the Universe
- The Milky Way's diameter is approximately 100 000 light-years
- Galaxies are each made up of many billions of stars
- Astronomical distances are measured in light-years - the distance light travels in one year through a vacuum
Redshift
When scientists analyse the light from distant galaxies, they find that the spectral lines are shifted to longer wavelengths - towards the red end of the spectrum - compared to the same lines measured in a laboratory on Earth. This is called redshift.
- Redshift is an increase in the observed wavelength of electromagnetic radiation emitted from a star or galaxy that is receding (moving away from the observer)
- Light emitted from distant galaxies appears redshifted in comparison with light emitted on the Earth
- The greater the distance to a galaxy, the greater the redshift, and the faster the galaxy is moving away from us
- Galaxies in all directions show redshift, meaning they are all moving away from us
Absorption lines in light from a receding galaxy are shifted towards longer, redder wavelengths
The Big Bang Theory
The observation that all distant galaxies are moving away from us - and that more distant galaxies move faster - is strong evidence that the Universe began from a single point in an explosive event known as the Big Bang.
- Redshift in the light from distant galaxies is evidence that the Universe is expanding
- An expanding Universe means that, working backwards in time, all matter was once concentrated in a single, extremely hot, dense point
- After the Big Bang, the Universe rapidly expanded and cooled, eventually forming hydrogen and helium, then stars, galaxies, and planets
- Redshift therefore supports the Big Bang Theory
As the Universe expands, every galaxy sees all other galaxies moving away from it
- Chain of reasoning to remember: redshift observed in distant galaxies → galaxies are moving away from us → the Universe is expanding → this supports the Big Bang Theory
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