Cosmology What is Cosmology Study of the universe

Cosmology What is Cosmology? • Study of the universe as a whole – Its past, present, and future • Is the Universe finite or infinite? • Did it have a beginning? • Will it ever end? • Where did everything come from? • • Stellar systems Stellar Clusters Galaxies Galaxy Groups and Clusters • Galaxy Superclusters • The Universe

The Shape of the Universe In Einstein’s General Theory of Relativity, space and time can be curved • The amount of matter and its motion affects the curvature of space-time • If there’s a lot, then the universe is like a sphere and finite: “Closed” • If there’s less than that, it’s infinite: “Open” • If there’s just the right amount, it’s infinite: “Flat”

The Ultimate Number: • The density, denoted , is the amount of stuff in the Universe • The critical density c, is the amount of stuff you need to make the universe flat • The ratio is called : (aka 0) • If > 1, universe is closed and finite • If < 1, universe is open and infinite • If = 1, universe is flat and infinite Q. 104: The Flat Universe

What is ? First Attempt • Add up contribution from everything we know of: Stuff Cont. to Stars 0. 012 Gas, Dust 0. 036 Dark Matter 0. 259 Neutrinos 0. 001 • Total < 1* • Universe is Open* • Universe is Infinite* *It’s lies, all lies Total 0. 308

Hubble Expansion Revisited • Universe is expanding • In the past, everything was together • When was everything together? • Will it expand forever?

Age of the Universe: Round 1 • To find age of universe, find how long ago any galaxy left us • Let its distance be d • Hubble’s Law gives us the velocity: v = H 0 d • If you move at constant speed, then: d = vt 0 • Solve for t 0: • Compare: oldest known stars – About 13 Gyr Q. 105: Age of Universe Flawed Argument

Age of the Universe: Round 2 • We assumed the Universe always expanded at a constant rate • But there is gravity – Gravity slows things down • So the universe was expanding faster in the past • Which means it should be younger than we previously calculated • The formula gets a little more complicated: • • If = 0. 30, t 0 = 11. 7 Gyr If = 1, t 0 = 9. 6 Gyr • Younger than the oldest stars: The age problem

Age of the Universe: Round 2 Now • If you ignore gravity, you get one age distance • When you include it, universe is younger No gravity Matter, = 1 t = 9. 6 Gyr t = 14. 4 Gyr

Measuring Deceleration • White dwarf supernovae are so bright you can see them more than half way across the universe • You are seeing the universe at an earlier era • Universe may have been expanding at a faster rate • Should be able to see this • How you do this: – Measure distance to very distant white dwarf supernovae – Compare to Hubble’s Law – Fit to curve for various • Drum roll please. . .

And the Results Are In • No model (with only ordinary matter) worked • It looked like universe has recently been accelerating • Best fit: Universe decelerated early on, but now accelerating m = 0. 3, = 0. 7 m = 0. 3, = 0. 0 m = 1. 0, = 0. 0

Dark Energy • Universe is currently accelerating – This is impossible with ordinary matter or dark matter • There must be another unknown contribution to the universe’s gravity • This mysterious stuff is called “Dark Energy” • We know almost nothing about Dark Energy • But dominant view is that it is – Cosmological Constant – Also known as Vacuum Energy Density

Vacuum Energy Density Best guess for nature of dark energy is vacuum energy density • Particle physics: A combination of relativity and quantum mechanics • Predicts you can create particles and anti-particles out of nothingness – For a brief time • For this reason, “empty” space (the vacuum) has energy – We don’t know how to calculate how much • Vacuum energy density predicted to have repulsive gravity – Empty space repels itself – causes expansion

What is ? Second Attempt • Add up contribution from everything we know of: Stuff Cont. to • Total 1* Stars 0. 012 • Universe is Flat, or close to Flat* Gas, Dust 0. 036 • Universe is Infinite or very large* Dark Matter 0. 259 *No longer lies Neutrinos 0. 001 Dark Energy 0. 693 Total 1. 001 Q. 106: The Universe Is Made of

Age of the Universe: Round 3 • At present, universal expansion is accelerating – Vacuum energy density is the most important contribution – Therefore, in the past, the expansion was slower • Early on, universal expansion was decelerating – Matter had a high concentration and was most important – Therefore, early on, the expansion was faster • By a numerical coincidence, the average speed is almost exactly the current speed • Our first estimate of age was pretty close: t 0 = 13. 8 Gyr • Oldest stars: 13 Gyr – No age problem

Age of the Universe: Round 3 distance Now Matter & Energy m = 0. 3 e = 0. 7 No gravity Matter, = 1 t = 13. 8 Gyr

The Standard Cosmological Model The standard cosmological model: CDM • Big bang started it all • The universe is formed from: – Dark energy ( ) – Cold (slowly moving) dark matter (CDM) – Ordinary matter • The total density parameter is = 1 (or close to 1) • All structure grew from tiny density perturbations – With a simply described distribution

Composition of the Universe H 0 = 21 km/s/Mly 0, 1 1, 2 3. 6 t 0 = 13. 8 Gyr =1 Neutrinos [VALUE]. 9 Stars Gas, Dust Dark Matter Dark Energy 69. 2