PHYS 205 Big Picture Why hasnt the universe

PHYS 205 Big Picture Why hasn’t the universe collapsed? Either the universe is infinite, or it is expanding faster than gravity can stop it. If the universe is infinite, eternal and static than the sky must be full of stars, but it is not, so why? (Olber’s Paradox) Is the universe static? Hubble and Humason showed that the universe is expanding and not static. But, the universe is homogeneous (no preferred direction of observation) And the universe is isotropic (no preferred place in the universe) Cosmological Principle With the addition: The universe is static (not) Perfect Cosmological Principle

PHYS 205 Curvature of Spacetime

PHYS 205 Positive vs. Negative Curvature

PHYS 205 Expanding Universe

PHYS 205 Cosmic Background Radiation The universe is expanding The universe was more crowded at the beginning The universe was hotter at the beginning Hot stuff should radiate at higher frequency. But now, the universe is cooler We should observe the radiation at lower frequencies Penzias and Wilson discovered a background radiation at radio frequencies. This radiation corresponds to a thermal radiation of 3 K. This radiation is observed uniformly through the universe. This supports the idea that there was a beginning which is called a BIG BANG.

PHYS 205 Big Bang vs. Steady State • Cosmic background radiation suggest a leftover heat from an earlier time. Steady state theory does not have a beginning. • Active galaxies and quasars were happening more in the past (no near quasars, only far away ones) evolution • The number of quasars drop off for very large redshifts. The Hubble Law says that these are large look-back times at the beginning there were no quasars there was a beginning! • When we measure the amount of hydrogen, helium and lithium in the universe, we cannot explain their amounts by fusion, Big Bang theory allows for the production of hydrogen, helium and lithium at the beginning.

PHYS 205 Fate of the Universe Depends on the mass of the Universe

PHYS 205 Curvature of the Universe

PHYS 205 Mass of the Universe If the mass of the universe is less than a critical mass, the gravity is not strong enough to stop the expansion, hence the universe will expand forever open universe If the mass of the universe is greater than a critical mass, the gravity is strong enough to stop the expansion, hence the universe will collapse onto itself closed universe So what is the mass of the universe? ? If we add up all the mass we can see + all the mass we can understand calculate (like the Dark Matter) we do not have enough mass to stop the expansion. However all the measurements we make besides the mass suggests that the universe is close to being “flat” (critical mass), but the jury is still out…