Understandings: The Big Bang model; Hubble’s law; redshift (z)
Applications and skills: Describing both space and time as originating with the Big Bang; Solving problems involving Hubble’s law; Estimating the age of the universe by assuming a constant expansion rate
Utilization: Doppler effect
In 1823 German amateur astronomer Heinrich Wilhelm Olbers asked the question "Why is the sky dark at night?" His argument was that if we live in an infinite static universe, that has existed for an infinite length of time then the sky - at night time or in the day - should be as bright as the surface of a star. Indeed, in such a scenario the radiation flux at all points in space would be infinite!
The solution to the dark sky paradox is to reject one or more of the premises. Either the universe is not infinite or it is not static or it has a finite age - or more than one of these statements is true. We now believe the last two statements, and maybe the first as well, to be true. But the first evidence for this (apart from that the universe exists at all) had to wait until the early twentieth century.
NOTE: The Doppler Effect is part of the AHL Topic, Wave Phenomena. I suggest reviewing that content, but focus on the Electromagnetic Doppler effect and equation, not the equations for sound.
In the 1920s Edwin Hubble discovered, building on the work of Henrietta Leavitt and Cepheid variables, that galaxies exhibited redshift caused by the electromagnetic redshift. Initially this indicated that they are moving away from us, but careful consideration has led us to the conclusion that this apparent motion is the result of the universe expanding, the space between every galaxy or cluster of galaxies is expanding.
The implication of an expanding universe, in which everything is moving away from everything else, is that in the past everything in the universe was closer together. If this trend is replayed to its logical beginning, then the implication was that the early universe was a hot, dense place. The idea that the entire universe was born in an explosion of some kind is known as the Big Bang model. Additionally, if we know the expansion rate of the universe (as measured by Hubble's constant) then some relatively simply mathematics allows us to calculate (or at least estimate) the age of the universe to be around 13 - 14 billion years ago.
There is evidence beyond the expansion of the universe for the Big Bang model, which we look at next class.
Oxford Physics: pages 660 - 662 including worked examples
Hamper HL (2014): pages 557 - 560
Pages 223 - 224 conceptual questions about Big Bang, pages 237 - 239 for Hubble's law and constant