PHYSICS 20B Lecture Notes - Lecture 20: Baryon, Radioactive Decay, Dark Energy
29 May 2018
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PHYSICS 20B - Lecture 20 - Dark Matter (Part 1)
Early Universe Timeline
●Light elements form (i.e. H, He nuclei)
●H and He atoms form; photons able to stream freely (CMB!)
Main Points from Big Bang Nucleosynthesis
●The big bang model makes a strong prediction for the relative abundance of hydrogen
and helium in the early Universe. Observations confirm this relative abundance
●The universe started out as primarily H with some Helium (and trace amounts of Li, D)
●Heavier elements (C,N,O, etc.) form as part of stellar evolution
●Quasars - extremely bright emissions from a supermassive black hole at the center of a
galaxy
○Can act as cosmic flashlights, allowing us to study gas along the line-of-sight
○With distant quasars, we can probe cold gas at high redshift, observing
absorption features in the observed spectrum of the quasar
○These absorption features allow us to measure the abundance of H, He, D, Li in
pockets of pristine gas, unsoiled by star formation. The abundance agree at high
precision with the prediction with the predictions of BBN
4 Fundamental Forces of Nature
●Gravitational
○Range: infinite, strength: 01 −39
○Planetary attraction & orbits, large-scale structure of universe
○On the largest scale in astrophysics gravity always wins
●Electromagnetic
○Range: infinite, strength: 01 −2
○Electricity, chemistry, why objects seem solid
○Acts on charges. Opposite charges attract (e.g. electrons & protons)
Document Summary
Physics 20b - lecture 20 - dark matter (part 1) Light elements form (i. e. h, he nuclei) H and he atoms form; photons able to stream freely (cmb!) The big bang model makes a strong prediction for the relative abundance of hydrogen and helium in the early universe. The universe started out as primarily h with some helium (and trace amounts of li, d) Heavier elements (c,n,o, etc. ) form as part of stellar evolution. Quasars - extremely bright emissions from a supermassive black hole at the center of a galaxy. Can act as cosmic flashlights, allowing us to study gas along the line-of-sight. With distant quasars, we can probe cold gas at high redshift, observing absorption features in the observed spectrum of the quasar. These absorption features allow us to measure the abundance of h, he, d, li in pockets of pristine gas, unsoiled by star formation.
