Cosmological Inflation

• Inflation is a particle physics theory on the exponential expansion of the universe
• In 1929, Edwin Hubble observed that distant galaxies are moving away from Earth and concluded that the universe is expanding
• The theory of inflation is introduced by Alan H. Guth introduced in 1980 as a resolution to the unresolved problems aroused from the Big Bang Theory
• The Grand Unified Theories of particle physics suggests that the universe underwent a dramatic burst of expansion at a very early age
• Occurred approximately 10^-36 seconds after the Big Bang
• Increased size of the universe by 10^50

Alan H. Guth on Cosmic Inflation Part I

Alan H. Guth on Cosmic Inflation Part II

The Flatness Problem

• In order to be a flat universe, the total density of the universe has to be extremely close to the critical density
• According to the Big Bang theory, a flat universe would be unstable. As time has gone on, the geometry of our universe should have deviated away from being flat. The only way for the universe today to be flat would be if the density of our young universe was extremely close to the critical density at that time
• Our universe's total density is within a percent of the critical density
• This problem is solved by proposing that during the inflationary period, the universe is formed into a flat geometry
• The inflationary period must last long enough for the universe become almost perfectly flat
• Once inflation ends, the time from when it ends and the present is to short of a period for the universe to move significantly away from its shape in the inflationary period. So the universe today is very close to flat

The Horizon Problem

• Why does the universe look the same in opposite directions?
• This problem was pointed out in the 70’s in reference to the Standard Model
• How can two different regions of the universe that have not contacted each other because of the great distances between them have the same temperature and similar physical characteristics?
• We expect that different parts of the universe would have different characteristics because the universe is not old enough for everything to have interacted with everything else yet. “Information” (which means physical interaction) can travel no faster than the speed of light and the universe is only 13.7 billion years old
• Contrary to our expectation, however, the universe is extremely homogenous
• The cosmic microwave background radiation (a form of electromagnetic radiation), which fills the space in between stars and galaxies, is almost the same temperature everywhere in the sky, about 2.725 K
• If the universe had started with even slightly different temperatures in different areas, there is no way it could have evened itself out so quickly
• Physics demand that there should actually be a difference in characteristic
• Inflation solves this problem because of the rapid expansion that increased the size of the universe by 10^50 in a period of 10^-32 seconds
• It was during this period that the physical properties probably evened out

• In 1981 a physicist named Alan Guth figured out that theories called “grand unified theories” had the potential to answer the three questions that were previously unanswered by the Big Bang models
• The grand unified theories predict that the separation of the GUT force and the strong force should have released extremely large amounts of energy- which would cause the universe to dramatically expand in a really short period of time- in less than 10-36 second
• Inflation would have dramatically increased the wavelengths of the quantum fluctuations. The quantum ripples would have stretched from tiny to large sizes due to inflation- which would allow them to become places where galaxies could later form
• Inflation explains uniformity because although two separate regions can’t have had contact since the time of inflation- they had contact before the time of inflation. This means that before inflation occurred, radiation that traveled at the speed of light bounced between the two regions and the exchange of energy that occurred equalized the temperatures and densities
• Inflation explains the balance in the universe by its effect on universal geometry. When the universe inflated, the overall flattening of space would have been so large that any curvature there was previously would only be noticeable on size scales much larger than the actual observable universe. It predicts that overall the universe should appear flat- so the overall density of matter and energy should be equal to the critical density

• We are unable to test inflation at its original occurrence in the universe, but we can test it through predictions made based on the universe and how it is today
• The best tests of inflation currently come from the WMAP satellite, which gives a detailed map of the cosmic microwave background. The map illustrates the universe when it was roughly 400,000 years old and the variances of temperature related to variances of density that were present
• Studies have shown that the variances in density were created much earlier in universe’s life due to tiny quantum ripples, which formed the universe’s structure. These ripples were the result of inflation. With this information, the variances in temperature in the microwave background give us a glimpse at the structure of the universe at the very beginning