The universe is expanding – this we know from looking at red shifts of distant galaxies – but the acceleration of the universe's expansion is harder to measure. It requires measuring the change of recession velocity over time, and it's done by comparing Type Ia supernovas as standard candles at different distances, which is how we know the universe is accelerating!
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Type Ia supernovae
Hubble diagram for type Ia supernovas (relatively close to earth): http://www.pnas.org/content/101/1/8/F3.expansion
Ryan Foley talking about WFIRST & Dark Energy
Dark Energy FAQ:
Why does dark energy make the universe accelerate?
David Spergel on dark matter & WFIRST
Permlutter article (old article but quite well explained & goes into future directions near the end)
Mukhanov’s Physical Cosmology
Nobel Prize Summary
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Minute Physics provides an energetic and entertaining view of old and new problems in physics -- all in a minute!
Created by Henry Reich