AST4320 - Cosmology and extragalactic astronomy Lecture 1 Introduction/Background 1
Relevant information: Pre-Midterm Schedule Week 1 (22/8-26/8) Week 2 (29/8-02/9) Lecture 1,2 Week 3 (05/9-09/9) Lecture 3,4,5 Week 4 (12/9-16/9) assignment week I Week 5 (19/9-23/9) Lecture 6,7,8 Week 6 (26/9-30/9) Lecture 9,10, 11(?) Week 7 (05/9-09/9) assignment week II Week 8: Midterm Monday: 14:15-16:00; Tuesday 10:15-12:00; Wednesday 12:15-14:00 2
Relevant information: Literature Books: we ll follow the syllabus: https://dl.dropboxusercontent.com/u/16515147/ast4320/syllabusv0.1.aug30.pdf Additional reading material that may be helpful will be mentioned in syllabus. Today: Chapter 1 of Loeb & Furlanetto How did the first stars & galaxies form? Chapter is available on website of Prof. Loeb: https://www.cfa.harvard.edu/~loeb/lf/ 3
Relevant information: Examination/Requirements Exams: midterm: October 13 (30% of grade) final: December 12 (times to be announced, 50% of the grade ) class presentations: week to be determined (20% of the grade) Group sessions/assignment sessions led by Max Gronke. Work in pairs. Handing in of work is mandatory, at the end of assignment weeks. Contact information: mark.dijkstra@astro.uio.no (room 205), max.gronke@astro.uio.no 4
Class Presentations Presentations of 20-25 minutes by each student + 5 minutes of questions Topic can be picked by students after discussion with me. List of suggested subjects: Breakthroughs obtained with Hubble Space Telescope The Hubble Deep Fields Direct Collapse Black holes/origin of super massive black holes. Supermassive Stars Observational constraints on variation fundamental constants.... 5
Relevant information: Grade Scale A 92-100% B 77-91% C 58-76% D 46-57% E 40-45% F 0-39% 6
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How the Universe started & developed Origin of Universe has been pondered upon for millennia For example, book of Genesis 1. Universe was created 2. Light separated from darkness 3. water separated from sky 4. continents separated from water 5. vegetation appeared 6. stars formed 7. life emerged 8. humans appeared 8
How the Universe started & developed Origin of Universe has been pondered upon for millenia Current cosmological model 1. Universe was created in Big Bang 2. Light dominates Universal energy content 3. Matter dominates Universal energy content 4. stars (& planets) formed 5. life emerged 6. humans appeared Cosmology is a now a mature field, in which we can explore this picture critically with observations. 9
How the Universe started & developed Importantly, finite light speed allows us to look back on younger Universe. Cosmic Microwave Background (CMB) from Planck Satellite. Universe ~ 0.4 Myr old. 10
How the Universe started & developed Importantly, finite light speed allows us to look back on younger Universe. Galaxy surveys. Universe ~ 1-14 Gyr old. 11
How the Universe started & developed Importantly, finite light speed allows us to look back on younger Universe. Galaxy surveys. Universe ~ 1-14 Gyr old. 12
How the Universe started & developed We currently have one snapshots of the Universe at age tuni ~0.4Myr,.. and many at tuni ~1-14 Gyr Homogenous plasma at ~ few thousand K galaxies... What happened in between? How did stars, galaxies and black holes form? 13
How the Universe started & developed We currently have an incomplete photo-album of the evolution of our Universe. not just scaled up version The situation that astronomers face is similar to having a photo album containing the first ultrasound image of an unborn baby and some additional photos of that same person as a teenager and an adult (Loeb, 2006) - 14
How the Universe started & developed Observational efforts are being developed to fill in our `missing pictures, and to address how stars etc actually formed. `JWST James Webb Space Telescope successor to Hubble & Spitzer space telescopes To be launched in 2018. Science goals include observing the first stars & galaxies. `SKA Square Kilometer Array Low frequency radio `interferometer. Construction in 2018. First observations in 2020. Science goals include observing intergalactic atomic HI directly in the young Universe 15
Standard Cosmological Model 16
Standard Cosmological Model Einstein applied his `field equations from general relativity to Universe as a whole. To simplify the analysis, Einstein assumed/ asserted that the Universe was isotropic (same in all directions) homogeneous (same in every location) 17 Exercise: think of distributions which are isotropic, but not homogeneous and vice versa
Standard Cosmological Model At the time, it was thought that the entire Universe consisted of the Milky Way, which was/is not expanding. Einstein could not reproduce a static solution, and needed to introduce his cosmological constant. He later realized that this solution was not stable however. 18
Standard Cosmological Model At the time, it was thought that the entire Universe consisted of the Milky Way (MW), which was/is not expanding. Einstein could not reproduce a static solution, and needed to introduce his cosmological constant. He later realized that this solution was not stable however. Less than a decade later, Hubble discovered that `spiral nebulae - initially thought of as constituents on the MW - were moving away from us with velocity v = H0r Hubble constant. Hubble also resolved individual stars in these `spiral nebulae which unambiguously determined their distances. Hubble s observations indicated that the Universe was expanding. 19
Einstein s Cosmological Principles Einstein s cosmological principles of isotropy and homogeneity have been remarkably successful. Isotropy CMB looks the same in all directions to within 10-5 (also see galaxy distribution, X-ray background,...) homogeneity Isotropy without homogeneity possible, if inhomogeneity in spherical shells. This has been ruled out by galaxy surveys. 20
Einstein s Cosmological Principles Observations thus indicate that the Universe is indeed the simplest way we could have imagined it to be. Why? It has been demonstrated the `cosmic inflation - a brief period of accelerated expansion - naturally gives rise to the conditions postulated by the cosmological principle. 21
The Expanding Universe: The Past Expansion of the Universe implies that it was denser in the past. Evidence that the Universe was indeed denser & hotter in the past include. CMB.. CMB has a blackbody spectrum, which implies matter and radiation are tightly coupled. This requires extremely dense & ionized (and therefore hot) gas. 22
The Expanding Universe: The Past Expansion of the Universe implies that it was denser in the past. Evidence that the Universe was indeed denser & hotter in the past include. Existence of certain nuclei, such as D, Li could only have been synthesized during conditions of Big Bang. There are no known post-big Bang processes which can produce significant amounts of deuterium. 23
The Expanding Universe: The Future Simple insight into future evolution of Universe (and structure formation in general!) can be obtained from Kirchhoff s theorem. In spherically symmetric Universe, when considering sphere of matter inside of it, when we can ignore gravitational influence of everything inside the sphere. Universe If sphere behaves like the rest of the Universe (homogeneity), then we can deduce expansion history of Universe as a whole by examining its behavior. 24
The Expanding Universe: The Future Total energy of gas element per unit mass blackboard Universe 25
The Expanding Universe: The Future Total energy of gas element per unit mass Universe 26
The Expanding Universe: The Future Total energy of shell of material < 0. Shell is gravitationally bound, and collapses back to R=0. Total energy of shell of material = 0. Shell keeps expanding until it becomes stationary (v=0), at infinitely large R. Total energy of shell of material > 0. Shell is not gravitationally bound, and continues to expand forever. Future evolution of the Universe as a whole depends critically on its matter content. 27
The Expanding Universe: The Future Einstein s field equations connects dynamics of matter to geometry of space. Universe is `closed (positive curvature) Universe is `flat (no curvature) Universe is `open (negative curvature) How can we tell geometry? 28
The Expanding Universe: The Future How can we tell geometry? From acoustic peaks in the CMB. Physics of `primordial plasma is straightforward and well understood. Any perturbation inside the plasma generates `soundwaves that travel at 0.57c. 29
The Expanding Universe: The Future How can we tell geometry? From acoustic peaks in the CMB. Physics of `primordial plasma is straightforward and well understood. Any perturbation inside the plasma generates `soundwaves that travel at 0.57c. Once the Universe cools enough to recombine (~0.4 Myr post Big-Bang), the sound waves practically `freeze, and leave a pattern on the matter distribution that we can observe as acoustic peaks in the CMB. 30
The Expanding Universe: The Future How can we tell geometry? From acoustic peaks in the CMB. Powerspectrum is a more compact, complete representation of the CMB Location of peaks provides direct measure on curvature of the Universe. 31
The Expanding Universe: The Future Universe is isotropic & homogeneous, and its geometry is (very close to) flat. 32
The Flat Universe Description The space-time (4D) line element is For flat space 33
The Flat Universe Description The space-time (4D) line element is For flat space The scale factor, which quantifies the expansion of Universe Comoving coordinates, coordinate system that moves together with expanding Universe. Natural to express observables like number density of galaxies in comoving coordinates. 34
Expansion & Comoving Coordinates Galaxies maintain their `comoving positions on the sphere, but the expansion of the sphere as a whole increases their separation (ignore that the geometry is not flat..) 35
Expansion & Comoving Coordinates Scale factor plays a key role in cosmology. Some useful relations part I Hubble constant (parameter) Photon frequency Photon wavelength Energy NR-particle Scale factor - redshift relation 36
Expansion & Comoving Coordinates Scale factor - redshift relation Most distance known galaxy (spectroscopically confirmed) at z~8.7. 37
Expansion & Comoving Coordinates Scale factor plays a key role in cosmology. Some useful relations part II Hubble constant (parameter) Time evolution Hubble parameter Matter domination Radiation domination Vacuum domination 38
Observables in Expanding Universe `Most of our information on astronomical objects are derived from the radiation we receive from it, or by the absorption it causes in the light of background sources. Mo, VdB, White This information is encoded within the objects Spectral Energy Distribution (SED) Measure flux (energy/time/area) in the frequency range Often expressed as (AB)-magnitude: (CGS units)
Absolute Magnitude Absolute magnitude is magnitude of source if it were located 10 pc away. Apparent magnitude of sun is mv~-26. Absolute magnitude... of sun is MV~4.8.
Absolute Magnitude Absolute magnitude is magnitude of source if it were located 10 pc away. Apparent magnitude of sun is mv~-26. Absolute magnitude... of sun is MV~4.8. of a massive O-star is MV~-5
Absolute Magnitude Absolute magnitude is magnitude of source if it were located 10 pc away. Apparent magnitude of sun is mv~-26. Absolute magnitude... of sun is MV~4.8. of a massive O-star is MV~-5 of a (Type 1a) supernova explosion MV~-19.3
Absolute Magnitude Absolute magnitude is magnitude of source if it were located 10 pc away. Apparent magnitude of sun is mv~-26. Absolute magnitude... of sun is MV~4.8. of a massive O-star is MV~-5 of a (Type 1a) supernova explosion MV~-19.3 of our Milky way as a whole MV~-20 brightest elliptical galaxies MV~-23
Absolute Magnitude Absolute magnitude is magnitude of source if it were located 10 pc away. Apparent magnitude of sun is mv~-26. Absolute magnitude... of sun is MV~4.8. of a massive O-star is MV~-5 of a (Type 1a) supernova explosion MV~-19.3 of our Milky way as a whole MV~-20 brightest elliptical galaxies MV~-23 brightest quasars MV~-30
Absolute Magnitude Absolute magnitude is magnitude of source if it were located 10 pc away. Apparent magnitude of sun is mv~-26. Absolute magnitude... of sun is MV~4.8. of a massive O-star is MV~-5 of a (Type 1a) supernova explosion MV~-19.3 of our Milky way as a whole MV~-20 brightest elliptical galaxies MV~-23 brightest quasars MV~-30 gamma-ray bursts MV~-36
Material Content of Universe We know the Universe is geometrically flat, and that Omegatot=1 But...ordinary matter (baryons) only accounts for ~ 5% of the Universal energy density! 46
Overwhelming Evidence for `Dark Components Galaxy rotation curves: evidence for dark matter 47
Overwhelming Evidence for `Dark Components Kinematics of galaxies inside galaxy clusters 48
Overwhelming Evidence for `Dark Components Growth of structure, see next lectures... 49
Overwhelming Evidence for `Dark Components Supernovae type 1a are thought to be `standard candles (luminosity is known and same everywhere) S The total flux we observe from a SN1a at a given z then depends on cosmological parameters only. 50
Open Questions How did (the first) stars, galaxies and black holes form? Why do galaxies & stars the properties (e.g. mass), they have? Origin of black holes. Is there a difference between stellar mass & super massive black holes? How can we test our theoretical models? 51
Some Topics to be Covered Structure formation Linear, Newtonian perturbation theory: quantitative describes growth of structure (Jeans stability) Non-linear collapse in the spherical top-hat model Lyman-a Forest, gravitational lensing `Virialization Gas cooling & condensation. Galaxy colors, spectra, IR, optical, UV, X-ray,... Observational probes Supersonically driven star formation Black holes & their formation 52