1	Astr 1050 Mon., Dec 8, 2003 Today: Finish Ch. 15, Cosmology Start Solar System
2	Refining the Big Bang Flatness Problem – why so close to a critical universe? Horizon Problem – why is background all same T? SOLVED BY AN “INFLATIONARY UNIVERSE” “Grand Unified Theories” of combined Gravity/Weak/Electric/Nuclear forces predict very rapid expansion at very early time: “inflation” When inflation ends, all matter moving away with v=v_escape (flat universe – curvature forced to zero) Also solves horizon problem – everything was in causal contact
3	Implications of Slowing Expansion Rate Our calculation of age T=1/H_o = 13.6 billion years assumed constant rate Gravity should slow the expansion rate over time If density is high enough, expansion should turn around If expansion was faster in past, it took less time to get to present size For “Flat” universe T = 2/3 * (1/H_o) = 9.3 billion years contradiction with other ages if T is too small
4	Is the expansion rate slowing? Look “into the past” to see if expansion rate was faster in early history. To “look into the past” look very far away: Find “H_o” for very distant objects, compare that to “H_o” for closer objects Remember – we found H_o by plotting velocity (v_r) vs. distance We found velocity v_r from the red shift (z) We found distance by measuring apparent magnitude (m_v) of known brightness objects We can test for changing H_o by measuring m_v vs. z
5	Measuring deceleration using supernovae Plot of m_v vs. z is really a plot of distance vs. velocity If faint (Þdistant Þearlier) objects show slightly higher z than expected from extrapolation based on nearby (present day) objects, then expansion rate was faster in the past and has been decelerating Surprise results from 1998 indeed do suggest accelerating expansion May be due to “cosmological constant” proposed by Einstein AKA “Dark energy” or “Quintessence”
6	“Cosmological constant” General Relativity allows a repulsive term Einstein proposed it to allow “steady state” universe He decided it wasn’t needed after Hubble Law discovered Is the acceleration right? Could it be observational effect – dust dims distant supernova? Could it be evolution effect – supernova were fainter in the past? So far the results seem to stand up Still being determined: 1) density, 2) cosmological constant With cosmological constant included, can have a “flat universe” even with acceleration. Given “repulsion” need to use relativistic “geometrical” definition of flatness, not the escape argument one given earlier. Energy (and equivalent mass) from cosmological constant may provide density needed to produce flat universe.
7	Chapter 16: Origin of the Solar System Solar Nebula Hypothesis Context for Understanding Solar System Extrasolar Planets Dust Disks, Doppler Shifts, Transits and Eclipses Survey of the Solar System Terrestrial Planets Jovian Planets Other “Stuff” including apparent patterns with application to the nebular hypothesis
8	Patterns in Motion All planets orbit in almost the same plane (ecliptic, AKA Zodiac) Almost all motion is counterclockwise as seen from the north: All planets orbit in this direction Almost all planets spin in same direction with axes more-or-less perpendicular to ecliptic Regular moons (like Galilean satellites and our own moon) orbit in this direction too Planets are regularly spaced steps increasing as we go outward
9	Solar Nebula Model Planets form from disk of gas surrounding the young sun Disk formation expected given angular momentum in collapsing cloud Naturally explains the regular (counterclockwise) motion Makes additional explicit predictions Should expect planets as a regular part of the star formation process Should see trends in composition with distance from sun Should see “fossil” evidence of early steps of planet formation
10	Extra-Solar Planets Hard to see faint planet right next to very bright star Two indirect techniques available (Like a binary star system but where 2nd “star” has extremely low mass) Watch for Doppler “wobble” in position/spectrum of star Watch for “transit” of planet which slightly dims light from star About 100 planets discovered since 1996 See http://exoplanets.org/ Tend to be big (³Jupiter) and very close to star (easier to see)
11	Characteristics of “Planets” Two types of planets Terrestrial Planets: small, rocky material: inner solar system Jovian Planets: large, H, He gas outer solar system Small left-over material provides “fossil” record of early conditions Asteroids – mostly between orbits of Mars and Jupiter Comets – mostly in outermost part of solar system Meteorites – material which falls to earth
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13	Patterns in Composition Terrestrial Planets Relatively small Made primarily of rocky material: Si, O, Fe, Mg perhaps with Fe cores (Note – for earth H₂O is only a very small fraction of the total) Jovian Planets Relatively large Atmospheres made of H₂, He, with traces of CH₄, NH₃, H₂O, ... Surrounded by satellites covered with frozen H₂O Within terrestrial planets inner ones tend to have higher densities (when corrected for compression due to gravity) Planet Density Uncompressed Density (gm/cm³) (gm/cm³) Mercury 5.44 5.30 Venus 5.24 3.96 Earth 5.50 4.07 Mars 3.94 3.73 (Moon) 3.36 3.40
14	Equilibrium Condensation Model Start with material of solar composition material (H, He, C, N, O, Ne, Mg, Si, S, Fe ...) Material starts out hot enough that everything is a gas May not be exactly true but is simplest starting point As gas cools, different chemicals condense First high temperature chemicals, then intermediate ones, then ices Solids begin to stick together or accrete snowflakes Þ snowballs (“Velcro Effect”) Once large enough gravity pulls solids together into planetesimals planetesimals grow with size At some point wind from sun expels all the gas from the system Only the solid planetesimals remain to build planets Composition depends on temperature at that point (in time and space) Gas can only remain if trapped in the gravity of a large enough planet
15	Growth of the Planetisimals Once a planetisimal reaches critical size gravity takes over
16	Evidence of Assembly Process? Craters
17	Craters evident on almost all small “planets”
18	Clearing of the Nebula Radiation pressure (pressure of light) See present day effects in comets Solar Wind Strong solar winds from young T Tauri stars See present day effects in comets Sweeping up of debris into planets Late Heavy Bombardment Ejection of material by near misses with planets Like “gravity assist maneuvers” with spacecraft Origin of the comets
19	Chapter 17: Terrestrial Planets Earth History, Interior, Crust, Atmosphere The Moon In particular origin Mercury Venus Mars Including water (and life ?)
20	“Comparative Planetology” Basis for comparisons is Earth Properties of Earth Similarities and differences with Mars and Venus help us understand Earth better (e.g., life, greenhouse effect, etc.) Won’t spend much class time on basic properties (size, gravity, orbital period, length of day, etc.) but you should have some relative ideas about these (see “Data Files” in text). There will be a few exam questions!!!
21	Four Stages of Planetary Development
22	Earth’s Atmosphere: Greenhouse Effect
23	The Moon and Mercury No atmosphere Cratering is evidence of final planet assembly – lots to be learned from craters
24	Patterns in Geologic Activity Judge age of surface by amount of craters: more craters Þ more ancient surface (for some objects, have radioactive age dates) Moon “dead” after about 1 billion years Mercury “dead” early in its lifetime Mars active through ~1/2 of its lifetime Venus active till “recent” times Earth still active Big objects cool off slower Amount of heat (stored or generated) proportional to Volume ( so R³) Rate of heat loss proportional (roughly) to Surface Area (so R²) Heat/(Unit Area) µ R³/R² = R so activity roughly proportional to R Same reason that big things taken out of oven cool slower than small things (cake cools slower than cookies)
25	Formation of an impact crater Crater caused by the explosion Impactor is melted, perhaps vaporized by the kinetic energy released Temporary “transient” crater is round Gravity causes walls to slump inward forming “terraces” Movement of material inward from all sides (trying to fill in the hole) may push up central peak in the middle. Final crater is typically ~10 times the size of the impactor
26	Examples of craters on the moon Images on line at The Lunar and Planetary Institute: http://www.lpi.usra.edu/expmoon/lunar_missions.html Detailed record of Apollo work at: http://www.hq.nasa.gov/office/pao/History/alsj/frame.html
27	Moon: Giant Impact Hypothesis Explains lack of large iron core Explains lack of “volatile” elements Explains why moon looks a lot like earth’s mantle, minus the volatiles Explains large angular momentum in the earth-moon system
28	Venus