1. The Fate of the Universe What property determines the ultimate fate of the universe?

2. Affect of Density on the Universe Must include ALL mass and energy in the Universe, because of mass-energy equivalence. Define Cosmic Density Parameter: Ωo Possible Universes: Open – Unbound, infinite, Ωo < 1 Closed – Bound, finite, Ωo > 1 Flat – Marginally Bound, infinite, Ωo = 1 - The expansion will continue forever, but approach an expansion speed of zero! Turns out the expansion of the Universe is Accelerating, WHY?

3. We aren't sure, but we call it dark energy. Repulsive nature suggests that universe may expand forever.  Largest contributor to cosmic density, more than normal matter or even dark matter. Recent Progress: Cosmological constant (additional “vacuum pressure” ) - One possible source of dark energy - Effect of nonzero vacuum energy (pair creation)‏ Knew it was repulsive, but couldn't calculate strength Recent work yields value consistent w/ observations

4. “Note that, while dark energy opposes the force of gravity in its effect on cosmic expansion, it still adds to the total density (and Ω 0 ) in its effect on the curvature of space. Based on all available data, the best estimate is that matter accounts for 27 percent of the total, dark energy for the remaining 73 percent.”

5. But, consensus & best evidence we now have points to a FLAT universe, Ωo ~ 1, when dark matter and dark energy are included in density calculations. Strange, no reason to expect Ωo to EXACTLY = 1 ? !

6. Two “Problems” with Original Big Bang Model: 1) Horizon Problem 2) Flatness Problem

7. The Horizon Problem Cosmic Microwave Background Radiation (CMBR) - Leftover from Big Bang Blackbody spectrum Cooled by expansion of universe More energy than all stars and galaxies that ever existed! CMBR is uniform across sky No reason why CMBR should look the same in regions A and B. Each lies outside the other’s communication horizon

8. The Flatness Problem The cosmic density appears to be fairly close to the critical value In terms of space-time curvature, the universe is remarkably close to being flat Why is this a problem? No obvious reason why our universe should have exactly the critical density! And if starts out near, but not exactly on critical density, quickly deviates.

9. The Theory of Inflation A short period of unchecked expansion The universe swelled in size by a factor of about 10 50 in only 10 -32 seconds! Solves both horizon and flatness problems! How?

10. Inflation and the Horizon Problem Initially, regions A and B could communicate Inflation => expand at speeds much greater than that of light. (GR only resticts matter & energy, not the universe itself)‏ Possible Mechanisms Cosmological constant? Inflaton – repulsive particle that decayed almost immediately?

11. Inflation and the Flatness Problem Solved by taking a curved surface and expanding it enormously in size. Example: As balloon expands, curvature becomes less pronounced on small scales, Earth looks flat to us. Any curvature the universe may have had before inflation has been expanded so much that space is now perfectly flat on all scales we can ever hope to observe.

12. Question The point in time when the temperature of the universe first dropped low enough to allow atoms to form is called the epoch of ______. A) inflation B) expansion C) contraction D) decoupling

13. Question The properties of the cosmic background radiation imply that in the past the universe was ___ and ___. A) transparent, cooler B) transparent, hotter C) opaque, cooler D) opaque, hotter