Open questions about our universe

• Why is the universe so large and old?
• Why is the universe so homogeneous and synchronized?
• What is the origin of primordial fluctuations?

Slow-roll inflation

• Right after the Big Bang, the universe doubled in size every fraction of a second. Unless the amount of matter is matched precisely with the rate of expansion, the universe would either become completely empty or recollapse again. 
• The fact that our universe continued to expand for 14 billion years and is certainly not empty means that “something clever” had to have happened before the Big Bang. 
• According to Professor Zaldarriaga, an attractive proposal is that the universe went through a phase of exponential expansion called slow-roll inflation, creating the conditions for the hot Big Bang. 
• The inflationary model says the universe expanded at least 60-fold in a short period of time.
• Quantum mechanical principles offer an appealing account for the origin of primordial fluctuations.
  • We need to fill the universe with a material that makes it expand at the appropriate speed during the inflationary period that also acts like a clock. 
  • Inflation needs a clock because it has to end at the Big Bang.
  • Because the universe is so small at the time, quantum mechanics implies that the clock cannot be completely accurate. The clock must fluctuate. 
  • Therefore, the universe cannot be perfectly homogenous (although it can nearly be so). 
  • We can only calculate a probability distribution for the primordial seeds. 
  • The statistical properties of these quantum fluctuations are consistent with our observations of the density fluctuations of the cosmic microwave background radiation.
• Are there other potential fossils besides the perturbations of the cosmic microwave background? Gravitational waves might be left over from even before the hot Big Bang. 
• We could detect gravitational waves indirectly by measuring the pattern of polarization of the cosmic microwave background. 

Before slow-roll inflation

• The history of the universe would be incomplete by just postulating slow-roll inflation before the Big Bang. Slow-roll inflation itself requires precise initial conditions, forcing us to speculate about what happened before (currently without any observational support). 
• We never know if the pursuit of understanding what happened before is never-ending or if there’s truly a first something that explains everything.
• One popular proposal of what happened before slow-roll inflation is eternal inflation within a multiverse. 
• This proposal solves the same types of equations and physics that we’re already using (e.g., general relativity and quantum mechanics), and offers an interpretation to solutions that we otherwise can’t make sense of in slow-roll inflation, the hot Big Bang, or the history of the universe since then.