The Universe
Section: Space Physics | Syllabus: Cambridge AS Level Physics 9702
Structure of the Universe The Universe is everything that exists - all matter, energy, space and time. It is organised on a hierarchy of scales from planets, to solar systems, to galaxies, to the Universe itself.
The Milky Way Our galaxy - a spiral galaxy containing approximately 200 billion stars including our Sun. Its diameter is approximately 100 000 light-years . The Milky Way is one of many billions of galaxies that make up the Universe The Universe contains an estimated 200 billion (2 × 10 11 ) galaxies Galaxies are each made up of many billions of stars Other stars that make up the Milky Way are much further from Earth than the Sun is 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 notice 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 An increase in the observed wavelength of electromagnetic radiation emitted from a star or galaxy that is moving away from the observer. This is similar to how a siren sounds lower in pitch as it moves away from you (the Doppler effect).
Light from distant galaxies is redshifted compared to light emitted from the same elements on Earth The greater the distance to a galaxy, the greater the redshift Greater redshift means the galaxy is moving faster away from us Galaxies in all directions show redshift - they are all moving away from us This means the Universe is expanding Figure: Redshift of Spectral Lines Two horizontal spectra shown side by side.
Left spectrum: light from a stationary laboratory source showing dark absorption lines at their expected positions (e.g. hydrogen lines). Right spectrum: light from a distant, receding galaxy showing the same pattern of absorption lines, but shifted noticeably towards the right (longer wavelength / red end).
A double-headed arrow labelled "redshift" shows the distance the lines have moved. The visible spectrum from violet to red is shown along the bottom. 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 light from distant galaxies is evidence that the Universe is expanding If the Universe is currently expanding, then working backwards in time, all matter was once concentrated in a single, extremely hot, dense point The Big Bang occurred approximately 13.8 billion years ago After the Big Bang, the Universe rapidly expanded and cooled, forming hydrogen and helium, then stars, galaxies, and eventually planets Redshift supports the Big Bang Theory Figure: The Expanding Universe Two side-by-side diagrams illustrating expansion.
Left diagram - "The Universe at an Earlier Time": a smaller rectangle containing several galaxy symbols (spiral shapes), spaced relatively close together. Our galaxy is marked with a star symbol in the centre.
Right diagram - "The Universe Today": a larger rectangle of the same proportions, with the same galaxies but now spaced further apart. Arrows on each galaxy point outward from our central galaxy, showing that all galaxies appear to recede from us.
Galaxies further away have longer arrows (faster recession). A key note states: "Every galaxy sees all other galaxies moving away - there is no special centre to the expansion." A small inset shows the analogy of dots on an inflating balloon - as the balloon expands, all dots move apart from every other dot.
Exam Tip Learn the chain of reasoning: Redshift observed in distant galaxies → galaxies are moving away from us → the Universe is expanding → this supports the Big Bang Theory . Be able to state each step and explain how it follows from the previous one.
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