Dark Energy Conclusions from Hubbles Law The universe

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 • Tspace = 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 (ρ0) – density of the universe that would result in a critically expanding universe • Average density (ρ) – actual density of the universe Ω 0 = ρ/ρ0 NASA/WMAP Science Team • If ρ < ρ0, then Ω 0 < 1 negative curvature Our universe • If ρ = ρ0, then Ω 0 = 1 flat (no curvature) • If ρ > ρ0, then Ω 0 > 1 positive curvature

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

Possibilities… • If decelerating, Hubble’s constant (redshift) would have been higher in the past. • If constant, Hubble’s constant (redshift) would have been the same in the past. • If accelerating, Hubble’s constant (redshift) would have been lower in the past.

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