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Lecture 23: Cosmology Astronomy 111 The first three minutes The First Three Minutes: A Modern View of the Origin of the Universe by Steven Weinberg The Big Bang Bang’s s hot past • Today: – Universe is low-density low density and very cold (2.7 K) – Steadilyy expanding p g • ~15 Gyr Ago: – Universe was smaller smaller, denser, denser & hotter – Expanding at a somewhat faster rate • How far back can we go? ASTR111 Lecture 23 Early (expanding) Universe Average temperature decreases with expansion expansion. ASTR111 Lecture 23 Loosing & binding • Binding Energy: – Energy gy needed to unbind ((break up) p) matter. • Binding Temperature: – Temperature p equivalent q to the binding g energy. gy – Matter at this temperature “melts” (unbinds) • Example: – In massive stars, nuclei melt at T~10 Billion K. ASTR111 Lecture 23 Typical sizes & binding energies At Atoms Binding Size Energy 3 10 m 10 K 10 10 K m Nuclei 10 p&n 11 10 m 10 K Quarks 10 m 1013 K ASTR111 Lecture 23 Fundamental forces of nature • Gravitation: – Long-Range • Electromagnetic Force: – Long-Range, g g 10 stronger g than g gravity y • Weak Force: – Range g <10 meters,, 10 g gravity y • Strong Force: – Range a ge <10 0 meters, ete s, 10 0 g gravity a ty ASTR111 Lecture 23 Unification of the forces • Electroweak Force: – EM & Weak forces unifyy at high g energies g (1015K) – Verified in particle accelerator experiments. • Grand Unified Theory (GUTs): – Strong & Electroweak Forces unified. – Predicted, but no experimental basis (yet?) ASTR111 Lecture 23 “Dreams Dreams of a final theory theory” • What about Gravity? – Gravity should unify with the GUTs force at very high energies. – Much higher than in any possible accelerator. accelerator – However, these energies could occur in the early Universe Universe. • Problem: – We have no quantum theory of Gravity! ASTR111 Lecture 23 The cosmic timeline • Physics gives us a framework within which to describe the Big Bang from the earliest phases to the present. – Particle accelerators probe matter at states similar to some of these early phases. • Large Hadron Collider will soon begin experiments –A Astronomers t look l k for f evidence id iin th the presentt Universe (e.g., Cosmic Background, primordial deuterium & helium,, dark energy) gy) ASTR111 Lecture 23 LHC ASTR111 Lecture 23 LHC instrument ASTR111 Lecture 23 Planck epoch • Before t=10 sec: – All 4 forces unified into a single Superforce – 1 force rules all of physics • Can’t say much else, since we don’t yet have a quantum theory of gravity to guide us. ASTR111 Lecture 23 Planck epoch t<10-43 sec T>1032 K No theory of quantum gravity All forces may have been unified ASTR111 Lecture 23 Grand unification epoch • At t=10 sec, T=1032 K (???): – Gravity separates from the Superforce – Strong & Electroweak Forces still unified. • 2 forces rule physics: – Gravity & GUTs • Universe is a soup of quarks, antiquarks & photons photons. ASTR111 Lecture 23 Grand unification epoch 10-43<t<10-38 sec 1032>T>1029 K Gravity becomes distinct from other forces Era ends when strong & electroweak forces decouple; inflation? ASTR111 Lecture 23 Inflationary epoch • t=10 sec (?), T=1027 K (?): – Strong force separates from GUTs force – EM & Weak forces still unified • 3 forces rule physics: – Gravity, Strong, and Electroweak forces • R Rapid id separation ti ttriggers i a rapid id “inflation” of the Universe ASTR111 Lecture 23 Inflationary epoch 10-48<t<10-20 sec 1029>T>1015 K Gravity, strong, & electroweak forces are distinct. distinct Era ends when electrostatic & weak forces decouple decouple. ASTR111 Lecture 23 The inflationary Universe • Universe grows by a factor of 1050 between 10 and 10 seconds! • Expansion p rate g greatly y slows after this brief burst of inflation. • Helps to explain why the universe is so very smooth on large scales. ASTR111 Lecture 23 Four forces at last • t=10 sec, T=1015 K: – Electroweak separates p into EM & Weak forces – All forces are now separate • 4 forces rule physics: p y – Gravity, Strong, Weak & Electromagnetic • Conditions becoming right for free matter to begin to exist separate from photons. ASTR111 Lecture 23 Four forces at last ASTR111 Lecture 23 Quark freeze-out • At t=10 t 1066 sec, T=10 T 1013 K: K – Free quarks combine into hadrons (primarily protons & neutrons) – particle-antiparticle pairs & photons in equilibrium: q pp nn ASTR111 Lecture 23 Nucleon freeze-out • At t=0.01 sec, T=1011 K – protons & neutrons decouple from photons and exist as free particles. – electrons & p positrons in equilibrium q with photons – neutrinos & nucleons in equilibrium • Free neutrons are stable during this epoch. h ASTR111 Lecture 23 Freeze-out As universe cools, particles and anti-particles anti particles annihilate, but fortunately for us there are slightly more particles: ASTR111 Lecture 23 Freeze-out 10-10<t<0.001 sec 1015>T>1012 K Amount of matter & antimatter nearly equal About 1 extra proton for every 109 protonantiproton pairs. Inequality very important! ASTR111 Lecture 23 Neutrino decoupling • At t=1 sec, T=1010 K – neutrinos decouple from matter – stream out into space freely – cosmic neutrino background (not yet observed) • Free neutrons are no longer g stable: – Decay into protons, electrons & neutrinos – Left with about 1 neutron for every 7 protons ASTR111 Lecture 23 Epoch of nucleosynthesis • t~3 min, T=109 K: • Fusion of protons & remaining free neutrons: – Formation of 2H ((Deuterium)) & 4He – End up with ~75% H, 25% He – Traces of D, Li, Be, B • We cannot observe this directly, but we products of these events. can look for the p ASTR111 Lecture 23 Epoch of nucleosynthesis 0.001 s<t<5 min 1012>T>109 K Begins when matter annihilates remaining antimatter. antimatter Nuclei begin to fuse ASTR111 Lecture 23 Helium nuclei formed at ~3 minutes Epoch of nuclei • 3 min<t<300,000 yrs, 109>T>3000 K • Universe cools to leave Hydrogen and Helium nuclei ASTR111 Lecture 23 Epoch of nuclei 5 min<t<380 min<t<380,000 000 yrs 109>T>3000 K Universe becomes too cool for photons to break helium apart (f (free-streaming) t i ) ASTR111 Lecture 23 Epoch of recombination • t=300,000 yr T=3000 K: • Electrons & nuclei combine into neutral atoms – Universe becomes transparent – Photons stream out into space – Origin of the Cosmic Background Radiation • Earliest we can see back directly using light. ASTR111 Lecture 23 Recombination ASTR111 Lecture 23 Epoch of recombination 380,000 , yrs y <t<109 yyrs 3000>T>30 K Atoms formed. formed Cosmic background radiation released released, Universe becomes transparent to electrons (Surface of last scattering) ASTR111 Lecture 23 ASTR111 Lecture 23 Epoch of galaxies • Galaxy formation: t=109 yrs, T~30 K – Quasars – First generation of stars. – First metals from first supernovae. supernovae • Present: tt=10 1010 yrs, T T=2.726 2.726 K – Galaxies, stars, planets, us... – Metals from supernovae of massive stars stars. ASTR111 Lecture 23 Cosmic timeline ASTR111 Lecture 23 What about the beginning? • Our physics can not yet probe earlier than the end of the Planck epoch (t=10 sec). • Some would say we have problems back before the Electroweak epoch p ((t=10 sec). ) • This will be the astrophysics of the 21st Century (or maybe the 22nd…) ASTR111 Lecture 23 Summary: • Physics of the early Universe – Informed by experimental & theoretical physics • The cosmic timeline: – Observations go back to t~3 minutes – Reasonably firm physics back to tt~10 10 sec – Speculative back before t~10 sec sec – Present theories stop at t~10 ASTR111 Lecture 23 ASTR111 Lecture 23