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Cosmology

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?

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Cosmology

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  1. 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?

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

  3. 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 andfinite • If  < 1, universe is open andinfinite • If  = 1, universe is flat andinfinite

  4. What is ? First Attempt • Add up contribution from everything we know of: StuffCont. 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

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

  6. 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: oldestknown 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

  7. 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 alittle more complicated: • If = 0.30, t0 = 11.7 Gyr • If  = 1, t0 = 9.6 Gyr • Younger than the oldest stars: The age problem

  8. 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

  9. 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 . . .

  10. 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,  = 0.0 m = 1.0,  = 0.0 m = 0.3,  = 0.7

  11. 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

  12. 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

  13. What is ? Second Attempt • Add up contribution from everything we know of: StuffCont. 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

  14. 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

  15. Age of the Universe: Round 3 Now Matter & Energym = 0.3e = 0.7 No gravity distance Matter,  = 1 t = 13.8 Gyr

  16. 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

  17. Composition of the Universe H0 = 21 km/s/Mly t0 = 13.8 Gyr  = 1

  18. 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

  19. 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 theEarth? • How long can we live around other stars? • How long will energy continue to beproduced somewhere in the universe? • Current age – 13.8 Gyr = 1.381010 yr • Recall: Universe in era of eternalexpansion

  20. 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

  21. 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 SuperGalaxy Other Supercluster Other Supercluster • 1012 yr – All galaxies in a cluster or supercluster merge into super-galaxies

  22. 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

  23. 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

  24. 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

  25. 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

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