Cosmology What is Cosmology? Stellar Systems Stellar Clusters Galaxies Galaxy Groups and Clusters Galaxy Superclusters The Universe 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?

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

What is ? First Attempt Add up contribution from everything we know of: Stuff Cont. to  Stars 0.012 Gas, Dust 0.038 Dark Matter 0.250 Neutrinos 0.001 Total 0.301 Total  < 1* Universe is Open* Universe is Infinite* *It’s lies, all lies

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 = H0d If you move at constant speed, then: d = vt0 Solve for t0: Compare: oldest known stars About 13 Gyr What’s wrong with this computation? A) You assumed constant speed B) You ignored peculiar velocities C) Both A and B D) There’s nothing wrong, this is correct

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, t0 = 11.7 Gyr If  = 1, t0 = 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 When you include it, universe is younger No gravity distance 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 WD 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 = 0.0,  = 0.0 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  Stars 0.012 Gas, Dust 0.038 Dark Matter 0.250 Neutrinos 0.001 Matter Subtotal 0.301 Dark Energy 0.701 Total 1.002 Total  = 1, or close* Universe is Flat, or close* Universe is Very Large or Infinite* *No longer lies

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: t0 = 13.8 Gyr Oldest stars: 13 Gyr No age problem

Age of the Universe: Round 3 Now Matter & Energy m = 0.3 e = 0.7 No gravity distance 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 H0 = 21 km/s/Mly t0 = 13.8 Gyr  = 1

The Future of Expansion “Vacuum” is the biggest part of the mass density now As the universe expands, you have more vacuum, but no more matter – vacuum will be forever the dominant part Vacuum causes universe to accelerate its expansion Universe should expand exponentially – forever Are we sure? No – recall, we don’t know what the dark energy is If it behaves differently, there are other possibilities Modest eternal growth Big Crunch Big Rip

Cosmic Eschatology – End Times Giant Asteroids Global Warming Death of Sun Isolated Universe Super Galaxies No New Stars Stars Die Galaxies Evaporate Matter Decays Black Holes Decay What events will destroy life on Earth? Can we survive the destruction of the Earth? How long can we live around other stars? How long will energy continue to be produced somewhere in the universe? Current age – 13.8 Gyr = 1.381010 yr Recall: Universe in era of eternal expansion

Act 1: Destruction of Life on Earth 100 Myr = 108 yr – Giant Asteroid impact Probably can deflect it with minimal improvement in technology 1 Gyr = 109yr – Runaway Global Warming Sun is gradually getting brighter If it gets too bright, Earth gets like Venus Move Earth (?) or leave for space 5 Gyr = 5109 yr – Sun becomes Red Giant Earth melts, all life eradicated Leave Earth for other stars Giant Asteroids Global Warming Death of Sun Isolated Universe Super Galaxies No New Stars Stars Die Galaxies Evaporate Matter Decays Black Holes Decay

Act 2: Galaxies Merge or Separate Giant Asteroids Global Warming Death of Sun Isolated Universe Super Galaxies No New Stars Stars Die Galaxies Evaporate Matter Decays Black Holes Decay 1011 yr – Superclusters or clusters separate from each other Other Supercluster Other Supercluster Virgo Supercluster Super Galaxy Other Supercluster Other Supercluster 1012 yr – All galaxies in a cluster or supercluster merge into super-galaxies

Act 3: The Stars Die Giant Asteroids Global Warming Death of Sun Isolated Universe Super Galaxies No New Stars Stars Die Galaxies Evaporate Matter Decays Black Holes Decay 1014 yr – No gas left for making stars Will have to switch to long lived stars 1014 yr later – Low mass stars die Low mass stars much dimmer They burn fuel more efficiently What remains – White Dwarfs, Neutron Stars, Black Holes, and Brown Dwarfs

Act 4: The Galaxies Die Giant Asteroids Global Warming Death of Sun Isolated Universe Super Galaxies No New Stars Stars Die Galaxies Evaporate Matter Decays Black Holes Decay 1019 yr – Near collisions of dead stars cause some to get thrown out of galaxy, others thrown into center Black hole at center grows, gets super big Galaxies disappear

Act 5: Matter Dies Giant Asteroids Global Warming Death of Sun Isolated Universe Super Galaxies No New Stars Stars Die Galaxies Evaporate Matter Decays Black Holes Decay 1034 - 1046 yr – Protons and neutrons decay Speculative, but most theorists agree What remains – Small and large black holes, isolated electrons and anti-electrons

Act 6: Black Holes Die Giant Asteroids Global Warming Death of Sun Isolated Universe Super Galaxies No New Stars Stars Die Galaxies Evaporate Matter Decays Black Holes Decay According to Quantum mechanics, black holes don’t last forever, they spontaneously emit radiation – very slowly 1060 years – Stellar black holes evaporate 1099 years – Galactic black holes evaporate What remains – Isolated electrons and anti-electrons in a super-cold universe