4.2 SPACE AND TIME; MATTER AND ENERGY

1. 4.2SPACE AND TIME; MATTER AND ENERGY

2. The Cosmological Principle 0 Considering the largest scales in the universe, we make the following fundamental assumptions: 1) Homogeneity: On the largest scales, the local universe has the same physical properties throughout the universe. Every region has the same physical properties (mass density, expansion rate, visible vs. dark matter, etc.) 2) Isotropy: On the largest scales, the local universe looks the same in any direction that one observes. You should see the same large-scale structure in any direction. 3) Universality: The laws of physics are the same everywhere in the universe.

3. Critical Density • The expansion of the universe should be slowed down by the mutual gravitational attraction of the galaxies. • The eventual fate of the universe depends upon the matter density within it. • The critical density (ρc) is the average density of matter required for the universe to just halt its expansion, but only after an infinite time. Critical Density ρc= 8πH0 / 3G H0 = Hubble constant ~ 70 km/s/Mpc G = Gravitational constant ~ 6.67 x 10-11 m3/kgs2

4. Critical Density • Generally, astronomers will compare any density they are measuring to the critical density. • We call this ratio, Omega (Ω), after the last letter in the Greek alphabet. Critical Density Ratio Ω = ρ/ ρc

5. Model Universes 0 r<rcuniverse will expand forever Maximum age of the universe: ~ 1/H0 Size scale of the Universe r>rcuniverse will collapse back If the density of matter equaled the critical density, then the expansion of the universe would come to a halt at an infinite time in the future, but it would never re-collapse.

6. Model Universes

7. Dark Matter • Combined mass of all “visible” matter (i.e. emitting any kind of radiation) in the universe adds up to much less than the critical density. • Gravitational lensing shows some clusters contain ten times as much mass as directly visible. Can dark matter be composed of normal matter? • If so, then its mass would mostly come from protons and neutrons (baryons). • The density of baryons right after the Big-Bang leaves a unique imprint in the abundances of deuterium and lithium. • Density of baryonic matter is only 4% of the critical density. • Most dark matter must be non-baryonic!

8. Dark Matter • Some theorists thought neutrinos, particles that are predicted to be very abundant in the universe yet are not baryons, might have enough mass to make up the dark matter, but modern measurements show neutrinos are not massive enough. • Some particle physics theories predict the existence of new type of particles labeled WIMPS(weakly interacting massive particles). • These have not been detected with certainty in the laboratory.