Inflation and the Early-Universe Origin of Structure
A Brief Epoch of Rapid Expansion
Motivation for inflation
Inflation is a period of accelerated expansion in the early Universe. It helps explain the horizon problem, flatness problem, and the absence of relics such as unwanted monopoles. More importantly, quantum fluctuations during inflation can be stretched to cosmic scales and later grow into the observed density perturbations.
Big picture
Inflation turns microscopic quantum fluctuations into macroscopic seeds for galaxies and clusters.
Perturbation generation
In many models, the inflaton field slowly rolls down a potential, producing nearly scale-invariant scalar perturbations and a small tensor component. A useful slow-roll parameter is , and inflation requires for accelerated expansion.
Which problem is inflation designed to address?
Inflation explains why widely separated regions of the CMB have nearly the same temperature.
Correct answer: The horizon problem
What condition on is required for inflation?
Inflation is accelerated expansion, which occurs when the first slow-roll parameter satisfies .
Correct answer: epsilon < 1
Linking inflation to observations
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1
Assume nearly scale-invariant primordial perturbations.
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2
Propagate them through radiation and matter eras.
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3
Compute the CMB power spectrum and matter power spectrum.
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4
Compare with data to constrain the inflationary model.