Dark Energy

Expanding Universe Galaxies in the universe are spreading out over time. –Hubble’s law From Einstein’s general relativity this happens as space itself expands. –Dimensions expand

Cosmological Principle If the universe were 2D (flat): –Possibly like a balloon –Moves all points through a third dimension –No edge and no center Our universe is 3D: –Possibly a hypersphere –Moves all points through a fourth dimension –Universe with no edge and no center The universe is similar for all observers.

Shape of Space Space can be flat or curved. –3D not just 2D The Ω parameter determines the direction and extent of curvature. –Positive curvature closed –Negative curvature open

Collapsing Universe Matter contributes to gravitation. –Pulls the universe together The orange line represents a universe dominated by high density matter. –Few billion years old –Eventual collapse

Critical Density At critical density the matter is sufficient for a flat universe. –Green curve At lower density gravity cannot stop expansion. –Expansion slows but continues –Blue curve

Dark Energy Dark energy represents energy tied in space itself. –Red curve with large fraction This energy drives expansion. –Acts to oppose gravity –Fraction sets the rate

Fate of the Universe The Hubble constant expresses the outward velocity. –Gravity slows the expansion rate by pulling inward –If gravity is too weak the universe expands forever –If gravity is strong the universe will eventually collapse Recent data indicates that the universe is accelerating! It will expand forever!