Galaxy Evolution 1) Density fluctuations in the primordial matter 2) galaxies grew by repeated merging of smaller objects - evidence: galaxies at large red shifts (-> large distances -> far back in time) are smaller and more irregular 3) Quasars, most luminous objects known, supermassive balck hole with lots of fuel -> less fuel becomes AGN -> runs out of fuel becomes “normal galaxy”. Black hole remains at center, but no longer giving off vast amounts of energy. Basically, the difference between a normal galaxy and an active one (or a Quasar) seems to be mainly a matter of fuel supply.

Many, many galaxies are not alone -> mergers change galaxies, this is how they may evolve! Computer simulations: Two large Spirals can merge to become an Elliptical Small galaxy passing through a galaxy can cause it to become a Spiral! AND, merging galaxies are active galaxies: - food for BHs -star formation

Dark Matter “Upward of 90 percent of the matter in the universe is dark” Two types: 1) normal matter that we can't “see” 2) exotic subatomic particles that are still a mystery How do we even know it's there? Gravitational Effects! (Animation)‏

Recall the Cosmological Principle: the universe is homogeneous (the same everywhere -> no edge) and isotropic (the same in all directions -> no center). But NOT unchanging, the Universe is EXPANDING.

Hubble's Law imply that at some time in the past all galaxies in the universe lay on top of each other! Big Bang: Beginning of the Universe Expansion of Universe is Expansion of Space Itself Therefore, can have expanding Universe AND Cosmological Principle. time = velocity/distance = distance/(Ho * distance) = 1/Ho So, if Ho ~ 70 km/sec/Mpc -> Universe is 14 Billion yrs old

The Big Bang Primordial explosion creating our universe – all matter and space, “beginning” of time. Hot and dense conditions => like a “fireball” Evidence: Specific prediction for initial amounts of hydrogen and helium Cosmic Microwave Background Radiation & 2006 Nobel Prize in Physics

Primordial Nucleosynthesis  Creation of base level of helium (~25%) in the early universe. Supports big bang model!  Stopped when temperature and density became too low for fusion to continue

The Formation Of Atoms Early universe hotter and opaque to radiation Universe expanded and cooled Electrons & nuclei combined => neutral atoms Epoch of Decoupling Universe becomes transparent to EM radiation. Period during which nuclei and electrons combined to form atoms

The Epoch of Decoupling After atoms of hydrogen and helium formed, only certain wavelengths of radiation— those corresponding to the spectral lines of those atoms, could interact with matter. The universe became nearly transparent to radiation. Universe continued to expand and radiation cooled becoming CMB

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

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?

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

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 !

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

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

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.

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

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?

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.

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

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