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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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REFERENCES
Type Ia supernovae
http://hyperphysics.phy-astr.gsu.edu/...
https://en.wikipedia.org/wiki/Type_Ia...
Luminosity Distance
https://en.wikipedia.org/wiki/Luminos...
Hubble diagram for type Ia supernovas (relatively close to earth): http://www.pnas.org/content/101/1/8/F...
http://www.astronomy.ohio-state.edu/~...
Straightforward references:
Ryan Foley talking about WFIRST & Dark Energy
https://youtu.be/BhOAwXYKdmw?t=23m
Dark Energy FAQ:
http://www.preposterousuniverse.com/b...
Why does dark energy make the universe accelerate?
http://www.preposterousuniverse.com/b...
David Spergel on dark matter & WFIRST
https://tvr2c.com/2016/06/01/53116-sh...
Slightly-technical references:
Permlutter article (old article but quite well explained & goes into future directions near the end)
http://supernova.lbl.gov/PDFs/Physics...
Technical references:
Mukhanov’s Physical Cosmology
http://www.astro.caltech.edu/~george/...
Nobel Prize Summary
http://www.nobelprize.org/nobel_prize...
Cosmological redshift
https://en.wikipedia.org/wiki/Redshif...
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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
How Do We Know The Universe Is ACCELERATING? ─ minutephysics
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