Dark Energy

Conclusions from Hubble’s Law The universe is expanding Space itself is expanding Galaxies are held together by gravity on “small” distance scales R. Knob - Creative Commons

Expansion Causes Redshifting Cosmological redshift (vs doppler shift or gravitational redshifting) Observed evidence of expanding universe and big bang R. Knob - Creative Commons

More evidence – CMB radiation Cosmic Microwave Background radiation T space = 2.7 K Emitted when universe was very hot, then was redshifted over billions of years Evidence of continued cosmological redshift NASA

What the CMB Tells Us Critical density (ρ 0 ) – density of the universe that would result in a critically expanding universe Average density (ρ) – actual density of the universe Ω 0 = ρ/ρ 0 If ρ < ρ 0, then Ω 0 < 1  negative curvature If ρ = ρ 0, then Ω 0 = 1  flat (no curvature) If ρ > ρ 0, then Ω 0 > 1  positive curvature Our universe NASA/WMAP Science Team

Curvature of the Universe On VERY large scales Could be: – Positive (>180 ° ) [closed universe] – Negative (<180 ° ) [open universe] – Flat (=180 ° ) [critical universe]

Possible fates of the universe Closed – expansion then collapse, (rinse,) repeat? Critical – expansion will stop at exactly time = ∞ Open – continually decelerating expansion Constant – expansion at a constant rate Accelerating – continually accelerating expansion NASA (modified)

Our Universe There is no center… There is no edge… Creative Commons

Methods of Measuring Distance Redshift Uses redshift due to Hubble’s Law expansion Based on Hubble’s constant at the time the light was emitted Supernovae Uses known brightness of certain types of supernovae Based on physical distance to the supernovae now NASA Creative Commons

Possibilities…

Our universe is… accelerating! Why? We don’t know… But there must be some source of energy causing the acceleration. We call it “dark energy.”

Dark Energy Activity