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Gravitational Lensing and Dark Matter

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Gravitational Lensing and Dark Matter 4/16/20 ASTR 1040: Stars & Galaxies Our Schedule Lensing: Abell 2218 • Third Mid-Term Exam next Tues April 21 on our Canvas/Quizzes site (11:00am-12:15pm) • Review Session on Mon 4pm-6pm by Max Weiner on Zoom – will send info on Mon. Review Set #3 posted • Homework #12 due today, new HW #13 posted • Today examine dark matter in galaxies • Read: 23.2 Evidence for Dark Matter • Then turn to Cosmology: models of the universe • Overview read: Chap 22 Birth of Universe • Discovery of cosmic microwave background CMB Prof. Juri Toomre TAs: Daniel Sega, Max Weiner implies a big-bang beginning Lecture 26 Thur 16 Apr 2020 zeus.colorado.edu/astr1040-toomre Now to Case for Dark Matter Individual galaxies show it • ~ 80+% of mass of universe is dark matter • Rotation curves: (invisible, missing matter) motions of stars in the galaxy • Detectable ONLY via its gravitational • Reveal that dark forces on light matter (gas and stars) matter extends beyond visible part • Note -- this dark matter is NOT the same of the galaxy, mass as black holes, brown/black dwarfs, or is 10x stars and gas dust Spiral galaxy ROTATION • Flat rotation CURVES curve of galaxy • Discovered by Vera Rubin in the 1970s • High speeds far • Highly controversial until from luminous many rotation curves center: means confirmed there is dark matter in the outer regions 1 4/16/20 # 2: Hot x-ray emitting gas in cluster Galaxy Clusters: reveal dark matter in three ways • # 1: Galaxy velocities too large to be explained by gravity of visible galaxies • Expected ~100 km/sec for a typical cluster, found 1000 km/sec! • Gas between galaxies is also moving because of • Discovered in 1930s by Fritz Zwicky (they didnt gravity of dark matter: gets very hot believe him, either) • 1000 km/sec à 100 million K: emits x-rays! # 3: Gravitational Lenses REMINDER Effects of strong gravity on light • Dark (& visible) (Einstein GRT) matter warps space à acts like a lens and distorts and can act like lens magnifies the view of more distant galaxies • Can form circular can redshift light arc segments Questions or Comments Gravitational lensing: how it works 2 4/16/20 Lensing by massive galaxy cluster Abell 1689 Bending of light by cluster Abell 2218 Quasars acting as gravitational lenses Einstein Cross: gravitational lensing of (SDS + VLT) (one) very distant quasar Medley of most distant gravitational lensing with HST The Beast 4 or 5 different galaxies! Red arc at the bottom: Z = 4.8 Erica Ellingson -- HST 3 4/16/20 Gravitational lens drifts across your harbor view (Boston) Effects of gravitational lensing on background galaxies How much dark matter overall? Questions or Comments • All cluster methods generally agree • About 5 times as much dark matter as normal matter overall in the universe • Is DM measurable in our solar system? Big Puzzle: What is Dark Matter? MACHO Searches STAR • Two possible flavors for Dark Matter: • Use gravitational lensing • Possibility 1. MACHOs • When a MACHO • Massive Compact Halo Objects floats in front of a MACHO star, the star suddenly • Very faint, actual things; baryonic matter brightens OBSERVER • Brown dwarfs, black holes, black dwarfs … etc. • Focusing effect • May be floating through the galaxy halo unnoticed of compact massive object 4 4/16/20 Possibility 2. MACHO hunt WIMPs results • Weakly Interacting Massive Particles • MACHOs are • Non-baryonic à subatomic particle (possibly detected by made in Big Bang?) brightenings • Neutrinos? probably not…. they move too fast and cannot be collected into stable galaxy halos • But not enough to explain all • Slower (unknown) particles: Cold Dark Matter ……. BIG SEARCHES underway dark matter REVIEW Cosmological (Big) Redshifts Redshift is expansion factor (from expansion of universe) Alternative definition of redshift : 1 + Z also measures how much universe has Z = redshift expanded = change in wavelength/ normal wavelength and wavelength of light is effectively stretched 1 + Z = observed wavelength / normal wavelength 1 + Z = distance between galaxies now redshifts always have Z > 0 distance between galaxies then (redder light has larger wavelengths) REVIEW Knowing distances reveals large-scale Relativistic galaxy clustering redshifts Find clusters + super-clusters : If Z is big (~1+), use relativistic formula sheets and voids to get velocity v like `bubble bath 5 4/16/20 Telescopes Lookback time (in expanding universe) are lookback • Say it takes 400 TIME million years for light time machines to get from galaxy A to us in Milky Way • Yet during travel in spacetime, both A and MW have A changed positions MW by expansion • Thus distance is a Today, we see Andromeda fuzzy concept – as she was 2.54 M years ago ! LOOKBACK TIME is better DISTANCE Poll 1: gravitational lens Lensing • If you measure the redshifts of the yellowish and blue • B. The blue objects, youll find: images are a single BACKGROUND A. The yellow galaxies have galaxy being lensed similar redshifts, all higher by the foreground than the blue galaxies cluster (yellow galaxies) B. The blue galaxies have the same redshift, which is higher than the yellow • The blue galaxy galaxies (spiral) is farther from C. Yellow and blue galaxies us and thus will have have similar redshifts a higher redshift EARLY DEVELOPMENTS Predictions of General Relativity Theory (GRT) Models of our • Einstein in 1917 realized GRT (1915) predicted universe universes in motion, but preferred `steady state – added `cosmological constant (CC) as repulsive Dark matter has force in space-time to counteract attractive force of gravity big influence on open vs • Willem de Sitter (A, Dutch, 1917) solves GRT closed equations with no CC and low density of matter : showed universe must expand • Alexander Friedmann (M, Russian, 1920) solves GRT with no CC but any density of matter : universes can expand forever, or collapse again, If enough mass, gravity eventually wins! depending on mean matter density 6 4/16/20 Very important diagram More on … Predictions of GRT Average distance • Georges Lemaitre (P, Belgian, 1927) rediscovers between galaxies Friedmann solutions, told Hubble (observing = 1 / expansion factor redshifts since 1924) that cosmic expansion = 1 / ( 1 + Z ) suggests more distant galaxies should have greater redshifts (Hubble publishes V = Ho d in 1929) NOW is fixed in time ( Z=0 ) Hubble constant NOW sets • Einstein visited Hubble in 1932, said CC biggest slope of line = how fast blunder universe is expanding Big Bang = when distance zero NOW Z = infinity Dark Matter and Fate of the Universe Four models for fates of universe Expansion begins with Big Bang – but what evidence? NOW CLOSED OPEN Several different models for Past and Future 7.
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