Chs. 28 and 29: Cosmology The expansion of the Universe: ----------------------------- Not an explosion into space. Space itself is expanding (like balloon demo). This explains why all but the nearest galaxies are receding away from us. (We can't be the center of the Universe, right?) No matter which galaxy you're in, they appear to be expanding away from you. This (and the balloon demo) also show why Hubble's law works: v = H_0 D where v = a galaxy's recession velocity D = its distance H_0 (pronounced "H naught") = a constant of proportionality, "the Hubble parameter" From any given point on an expanding balloon (or Universe), any other point will expand away from it, at a speed proportional to its distance from the first point. Run the movie backwards => The Big Bang ------------ Age of the Universe t_0 = 1 / H_0 (neglecting deceleration, by gravity). Hubble Space Telescope observations indicate: H_0 = 70 +/- 7 km/s-Mpc so that: t_0 = 13 - 14 billion yr. The First Three Minutes: ----------------------- The Universe was as hot as a stellar core. H -> He fusion everywhere left the Universe 25% He by mass (observed). Affected abundances of deuterium (D) and lithium (Li) (also observed). The Cosmic Background Radiation (CBR): ------------------------------------- Background of microwaves, coming from whole sky *Very* smooth: (Delta T)/T < 10^-4 Fits a 3 K blackbody extremely well: it's the echo (or waste heat) of Creation, greatly redshifted (z = 1500). CBR was made during the recombination epoch, at t_0 = 300,000 yr, when the Universe cooled enough to become transparent to light for the first time. Electrons combined with ions, making the first atoms. COBE satellite (launched 1989) - found ripples in the CBR, (Delta T)/T < 10^-(4-5) => Seeds of galaxy formation? Galaxy formation: ---------------- Recall the concept of look-back time. Hubble Deep Field (HDF), taken in 1995, can see galaxies as far back as t_0 = 1 billion years. Elliptical galaxies are more common now than in the past, because of galaxy clustering and collisions. Possible route of galaxy evolution: globular clusters -> dwarf spheroidals -> dwarf ellipticals (? needs confirmation) -> spirals -> giant ellipticals -> cD galaxies (observed) In HDF, we also see bright blue knots in galaxies, in distant past (z = 5, near 1st billion years). Is this Pop III, the first stars? Since we find no zero-metallicity stars anywhere, this implies there was am early population of supernovae, and massive stars to make them. Pop III burned out quickly, leaving only the metals - we only expect Pop III to be observable in z > 5 galaxies. The star formation rate throughout the Universe peaked near z = 2 (about half the Universe's present age). The Sun formed 4.65 billion yr ago, *after* this peak. Next Generation Space Telescope (NGST): To explore the "Dark Age", from z > 6 to z = 1500, and observed the origin of galaxies. Chapter 27: Active Galactic Nuclei and Quasars ---------------------------------------------- 1943 - Carl Seyfert found ~1% of spiral galaxies have bright, star-like nuclei (centers). Type I have broad (1000s of km/s) emission lines Type II have narrow (100s of km/s) emission lines -> Probably an inclination effect. Broad line region (BLR) is toroidal (doughnut-shaped). LINERS - low-ionization, weak AGNs: more common. Radio galaxies - mostly ellipticals, like NGC 5128 = Cen A Have jets and radio lobes, from their centers. May be kpc long! Quasars (discovered 1963) - "Quasi-stellar radio sources" ------- Looked like stars (unresolved points) Most (90%) now known are radio quiet, so technically are QSOs (quasi-stellar objects), but everyone still calls them "quasars." They are *highly* redshifted => at cosmic distances e.g., z = 0.37 for 3C 273, the nearest z = 2, the "quasar epoch", when they were most common - when the Universe was about half its present age. Aside from the redshifts, how do we know quasars are at cosmic distances? - The "Lyman alpha forest" (absorption from intervening H clouds) - Found in galaxy clusters with the same redshift - Their spectra are very similar to those of Seyfert galaxies - no dichotomy - The smoking gun: We can now see the host galaxies, with Hubble Space Telescope! => Quasars are the most luminous objects in the Universe. What powers them? Clue from rapid variability: can vary over just a few hours => only light-hours (few AU) across This is *very* small. It's like finding a flashlight as bright as a city. (No kidding, this is an accurate analogy.) This is consistent with accretion into *supermassive (10^9 solar mass) black holes* in their centers, like the 10^6 one in the MWG (Sgr A*) and other normal galaxies (inferred from their rotation curves) Often, jets form, as in other systems with accretion disks. Blazars, or BL Lac objects: highly polarized, often optically violent variables (OVV) - probably jets coming right at us. These sometimes show *apparent superluminal expansion*: blobs in jets appear to be moving faster than light! -> Thought to be just a perspective effect, from jets coming nearly at us. Gravitational lensing: seen in quasars and galaxy clusters. Galaxies can act as a lens: GR effect. This serves as a probe of dark matter; also, I hope, extrasolar planets!