In today's lecture we are going to provide an overview of the history of the expanding universe, with a look to the supporting evidence for the Big Bang theory and a short discussion of the cosmological principle.
Once you're settled down we can begin.
calculations of the age of the universe itself come from the value of the Hubble constant, being 67.4 Kms/MPc to an error of only plus or minus 0.5 kms/MPc. Calculating this value required a vast amount of work done in distance calculation of specific notable astronomical objects, using the cosmic distance ladder. These include, but are not limited to type 1a supernovae, cepheid variables and spiral arm galaxies. Each has an intrinsic brightness which can be calculated according to other properties. The cepheid variables essentially acts like a metronome based on its luminosity, brightening and dimming according to the mass of the star, rising up and down in a periodic fashion allowing calculation of its distance. Spiral arm galaxies work in a similar way. Galaxy rotation curves, as well as providing evidence for the existence of dark matter have a period relating to their intrinsic luminosity according to the Tully-Fischer relation. This will all be covered in later lectures.
Once a distance had been established, the redshift of notable absorption lines in the stars or novae, most often the Balmer series, were used to give a value for the Hubble constant. The reciprocal of this giving an estimate for the age of the universe, being 13.8 billion years.
To begin with, as I'm sure most of you are aware the universe was a singularity, not dissimilar to a black hole. More accurately one might describe it as a white hole, a structure, as postulated by Einstein and Rosen in response to Swartzchild's original solution to the field equations, which mathematically was shown to be a time-reversed black hole. That structure is entirely theoretical and realistically relativity can't take us back before the Planck epoch. What we can theorise however, is that energy and matter at the beginning must have been evenly distributed.
Activity in that point of infinite density must have effectively been a constant, which would quickly change as expansion began.
I'm discussing these time periods as if they lasted for a notable length, however one must appreciate that these epochs lasted less than femtoseconds. As expansion started we began to observe minor quantum fluctuations, as dictated by the uncertainty principle. At first, of course the energy was a constant, though through these minor fluctuations the structure of the universe was born. This lead to an increase in density in some areas and a notable drop in others. A hint of emptiness in an otherwise infinitely dense area of space. A picture of this can be seen in the cosmic microwave background, a radiation relic of the early universe, showing the surface of last Thompson scattering before coupling of fermions.
This coupling could only begin as the universe began to cool. What was originally this hot state of perfectly symmetrical density began to cool down. As it cooled, and the emptiness from the original fluctuation grew and the particles that had been whizzing around started to calm down a little and form pairs. The expansion after this point, now in the epoch of cooling, has often been likened to baking. Everything needed for the bread was there at the beginning, all in one spot, though as it expands the gas from the yeast expands. This expansion will start to not only increase the size of the bread, but the increase in the size of the holes. That already established structure, those tiny fluctuations growing into the emptiness.
As time passes we move to the structure forming epoch. The first stars, the first planets, the first galaxies. We see gravity pulling together hydrogen clouds, swirling in, gathering together. The closer they would get, the faster they would spin, drawing in more and more matter. Letting everything around sink into the growing ball of gas.
