19-01 The Inflation Hypothesis

• How can the CMB be so isotropic over the sky? -- the horizon problem.
  [The cosmic light horizon; the observable universe]
  
   
• How can the cosmic energy density be so close to unity? -- the flatness problem.
   
• To solve the above two problems, the idea of inflation was proposed independently by Alexei Starobinsky in Russia and by Alan Guth in the US in early 1980s.
  [During a very brief period of time (~ 10^-32 s) shortly after the big bang, the universe expands by a factor of 10⁵⁰.]
  
   
• There are alternative models without inflation.
  Future observation of CMB polarization may help to discriminate these models.