Dark Energy Expanding Universe Galaxies in the universe

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 2 D (flat): – Possibly like a balloon – Moves all points through a third dimension – No edge and no center • Our universe is 3 D: – 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. – 3 D not just 2 D • 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!