Lecture 24: Cosmology: The First Three Minutes. Astronomy 111 Monday November 27, 2017

Transcription

1 Lecture 24: Cosmology: The First Three Minutes Astronomy 111 Monday November 27, 2017

2 Reminders Last star party of the semester tomorrow night! Online homework #11 due Monday at 3pm

3 The first three minutes The First Three Minutes: A Modern View of the Origin of the Universe by Steven Weinberg

4 The Big Bang s hot past Today: Universe is low-density and very cold (2.7 K) Steadily expanding ~15 Gyr Ago: Universe was smaller, denser, & hotter Expanding at a somewhat faster rate How far back can we go?

5 Early (expanding) Universe Average temperature decreases with expansion.

6 Loosing & binding Binding Energy: Energy needed to unbind (break up) matter. Binding Temperature: Temperature equivalent to the binding energy. Matter at this temperature melts (unbinds) Example: In massive stars, nuclei melt at T~10 Billion K.

7 Typical sizes & binding energies Binding Size Energy Atoms 10 m 10 3 K Nuclei 10 m K p & n 10 m K Quarks 10 m K

8 Fundamental forces of nature Gravitation: Long-Range Electromagnetic Force: Long-Range, stronger than gravity Weak Force: Range <10 7 meters, 10 2 gravity Strong Force: Range <10 meters, 10 gravity

9 Unification of the forces Electroweak Force: EM & Weak forces unify at high energies (10 15 K) Verified in particle accelerator experiments. Grand Unified Theory (GUTs): Strong & Electroweak forces unified. Predicted, but no experimental basis (yet)

10 Dreams of a final theory What about Gravity? Gravity should unify with the GUTs force at very high energies. Much higher than in any possible accelerator. However, these energies could occur in the early Universe. Problem: We have no quantum theory of Gravity!

11 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. Astronomers look for evidence in the present Universe (e.g., Cosmic Background, primordial deuterium & helium, dark energy)

12 LHC ASTR111 Lecture 24

13 LHC instrument, ATLAS ASTR111 Lecture 24

14 Planck epoch Before t=10 3 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.

15 Planck epoch t<10-43 sec T>10 32 K No theory of quantum gravity All forces may have been unified

16 Grand unification epoch At t=10 3 sec, T=10 32 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.

17 Grand unification epoch <t<10-38 sec >T>10 29 K Gravity becomes distinct from other forces Era ends when strong & electroweak forces decouple; inflation?

18 Inflationary epoch t=10 3 sec (?), T=10 27 K (?): Strong force separates from GUTs force EM & Weak forces still unified 3 forces rule physics: Gravity, Strong, and Electroweak forces Rapid separation triggers a rapid inflation of the Universe

19 Inflationary epoch <t<10-20 sec >T>10 15 K Gravity, strong, & electroweak forces are distinct. Era ends when electrostatic & weak forces decouple.

20 The inflationary Universe Universe grows by a factor of between 10 3 and seconds! Expansion rate greatly slows after this brief burst of inflation. Helps to explain why the universe is so very smooth on large scales.

21 Four forces at last t=10 2 sec, T=10 15 K: Electroweak separates into EM & Weak forces All forces are now separate 4 forces rule physics: Gravity, Strong, Weak & Electromagnetic Conditions becoming right for free matter to begin to exist separate from photons.

22 Four forces at last ASTR111 Lecture 24

23 Quark freeze-out At t=10 6 sec, T=10 13 K: Free quarks combine into hadrons (primarily protons & neutrons) Particle-antiparticle pairs & photons in equilibrium: p n p n

24 Nucleon freeze-out At t=0.01 sec, T=10 11 K protons & neutrons decouple from photons and exist as free particles. electrons & positrons in equilibrium with photons neutrinos & nucleons in equilibrium Free neutrons are stable during this epoch.

25 Freeze-out As universe cools, particles and anti-particles annihilate, but fortunately for us there are slightly more particles:

26 Freeze-out <t<0.001 sec >T>10 12 K Amount of matter & antimatter nearly equal About 1 extra proton for every 10 9 protonantiproton pairs. ASTR111 Lecture 24 Inequality very important!

27 Neutrino decoupling At t=1 sec, T=10 10 K neutrinos decouple from matter stream out into space freely cosmic neutrino background (not yet observed) Free neutrons are no longer stable: Decay into protons, electrons & neutrinos Left with about 1 neutron for every 7 protons

28 Epoch of nucleosynthesis t~3 min, T=10 9 K: Fusion of protons & remaining free neutrons: Formation of 2 H (Deuterium) & 4 He End up with ~75% H, 25% He Traces of D, Li, Be, B We cannot observe this directly, but we can look for the products of these events.

29 Epoch of nucleosynthesis s<t<3 min >T>10 9 K Begins when matter annihilates remaining antimatter. Nuclei begin to fuse. Helium nuclei formed at ~3 minutes.

30 Epoch of nuclei 3 min<t<380,000 yrs, 10 9 >T>3000 K Universe cools to leave Hydrogen and Helium nuclei

31 Epoch of nuclei ~3 min<t<380,000 yrs 10 9 >T>3000 K Universe becomes too cool for photons to break helium apart (free-streaming)

32 Epoch of recombination t=380,000 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.

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34 Recombination ASTR111 Lecture 24

35 Epoch of recombination 380,000 yrs <t<10 9 yrs 3000>T>30 K Atoms formed. Cosmic background radiation released, Universe becomes transparent to electrons (Surface of last scattering)

36 Epoch of galaxies Galaxy formation: t=10 9 yrs, T~30 K Quasars. First generation of stars. First metals (heavy elements) from first supernovae. Present: t~10 10 yrs, T=2.726 K Galaxies, stars, planets, us... Lots of metals from many generations of supernovae of massive stars.

37 Cosmic timeline ASTR111 Lecture 24

38 What about the beginning? Our physics can not yet probe earlier than the end of the Planck epoch (t=10 3 sec). Some would say we have problems back before the Electroweak epoch (t=10 2 sec). This will be the astrophysics of the 21 st Century (or maybe the 22 nd )

39 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 t~10 6 sec Speculative back before t~10 2 sec Present theories stop at t~10 3 sec

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