The universe from the big bang to today and the evidence that it is expanding, then Earth's energy balance: sunlight absorbed and reflected, energy flowing between the spheres, and the greenhouse effect. Concept tutorials with a timeline of the universe, expanding space, and a live energy-balance model.
The big bang model and redshift as evidence for expansion (AC9S10U03).
The big bang theory: space, time, matter and energy began about 13.8 billion years ago in an extremely hot, dense state, and the universe has expanded and cooled ever since: from particles to atoms, stars, galaxies and planets.
Light from distant galaxies is redshifted: shifted to longer wavelengths, so they are moving away. Farther galaxies move away faster (Hubble's law), exactly what an expanding universe predicts, from any galaxy. Run backwards, everything was once together: evidence for the big bang.
A few quick questions on the scale of the universe, the cosmic microwave background and matter proportions, and how the evidence fits together. Taught in class; use this to check yourself.
Sunlight in, energy balance between the spheres, and the greenhouse effect (AC9S10U04).
The sun is Earth's main energy source. Some sunlight is reflected straight back to space and the rest is absorbed and warms the planet. How much is reflected (the albedo) depends on the surface: ice reflects most, oceans and forests absorb most.
Earth is in energy balance when the energy it absorbs from the sun equals the energy it gives off to space as infrared. Energy moves between the spheres by radiation, conduction and convection, and the oceans store heat. Upset the balance and Earth warms or cools until it is restored.
Earth gives off infrared. Greenhouse gases (water vapour, CO₂, methane, nitrous oxide) absorb some of it and send part back down, slowing the energy going out. Without them Earth would average about −18 °C. Adding more upsets the balance, and Earth warms until it is restored.
A few quick questions on how human activity creates an energy imbalance, the indicators that show warming, and mitigation versus adaptation. Taught in class; use this to check yourself.
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