 We have taken the history of the universe back to the first few microseconds of its expansion. At each stage, with barogenesis through nucleosynthesis to recombination and decoupling, we had the universe in thermal equilibrium. At this point, there are two notable issues with the flat lambda-cold-dark matter theory. One is that nothing we have covered so far gives us the anisotropy we see in the CMB radiation. And two, the universe appears to have been too large to be in thermal equilibrium during recombination. This one is called the horizon problem, and it's a showstopper. You'll recall that the horizon distance is the furthest distance that light could have traveled in the time available. Assuming a matter and radiation dominated universe appropriate for the time, we get a horizon distance that is 2% of the distance across the region. In other words, there is no way for the vast majority of the particles in recombination to be in causal contact as required for thermal equilibrium. To solve this problem, a theory of cosmic inflation was suggested by cosmologist Alan Guth in 1982. We know from quantum field theory that the universe is permeated with any number of matter and force fields. Like Higgs proposed a new field to explain W and Z boson behavior, Guth proposed that the universe contains a scalar energy field with a very large vacuum energy, much larger than today's cosmological constant. The field is called the inflation field, and its force particle is called an inflaton. If we assume that this large potential energy isn't changing appreciably, the large Hubble parameter would be constant, and as we have seen before, the universe would expand exponentially. Here's a graph that relates the potential energy density of space against the changes in the energy of the field. A large enough vacuum energy density would cause the universe to double in size once every 10 billion trillion trillions of a second. At the end of the plateau, there is a sharp fall in the potential energy. We don't know what triggered the start of inflation, and we don't know what triggered this end. But during this split second fall, the inflation theory has it that all the potential energy and all the inflatrons were converted into all the heat, matter, and energy in the universe. This is the fiery Big Bang we had entering the Baryogenesis epoch. The horizon distance at the start of inflation would have been submicroscopic. The horizon distance at the end of inflation, a tiny fraction of a second later, would have been the size of a whale. And the horizon distance at the time of last gathering would have been 652 million light years, 800 times larger than without inflation. This puts every particle in the decoupling process that created the CMB into causal contact with every other particle, easily enabling thermal equilibrium. The zero point energy quantum fluctuations are short, small, and create localized deviations in energy density. Under normal circumstances, the restorative force returns the deviation to normal almost instantly. But an exponentially expanding space weakens the restorative force. Each wave stretches with the expansion and freezes once it reaches the size of the horizon. So we have a large, ambient population of waves of every wavelength undergoing this expansion. This way large numbers of small, short localized energy deviations become small, long-lived extended deviations. In fact, these quantum fluctuations persist long after the inflation ends. We can tweak the variables to produce an energy density deviation on the order of 10 to the minus 5, the amount of the temperature deviation we found in the CMB. These tiny quantum fluctuations are the origin of the anisotropy in the CMB that eventually led to the large galaxy superclusters and great voids we see around the universe today. The ability to solve big bang problems like the horizon, CMB, anisotropy, and others have given the theory of inflation a great deal of credibility among cosmologists.