SOME NEGATIVE EFFECTS OF THE EXPANSION OF THE UNIVERSE. Lecture 22 Hubble s Law and the Large Scale Structure of the Universe PRS: According to modern ideas and observations, what can be said about the location of the center of the Universe? 1. The Sun is at the center. 2. The Milky Way is at the center. 3. An unknown, distant galaxy is at the center. 4. A black hole is at the center. 5. The Universe does not have a center. 1 The Cosmological Redshift Hubble discovered most galaxies seemed to be moving away from the Milky Way The speed of recession is from the Doppler formula: v = c λ λ In the early 1900 s it was discovered that most galaxies appeared to have large Doppler shifts. 2 Hubble s Diagram 3 Cepheid Variables 4 Recession Velocity 1000 km/s 500 km/s 0 0 1 Mpc 2 Mpc Distance Note: nearly all galaxies are shifted to longer wavelengths, although a few nearby ones are blueshifted. Used to calibrate Hubble s Law. Luminous yellow supergiants with unstable atmospheres that pulsate: a stable instability Longer Periods of oscillation happen in more Luminous stars. Measure P, get L, and then get the distance. PRS: Distance Uncertainties Suppose we learned tomorrow that Cepheids are more luminous than we had previously thought. How would that affect our estimates of the distances and sizes of galaxies? The distances would be and the sizes. (1) farther; smaller (3) nearer; smaller (2) farther; bigger (4) nearer; bigger Answer: 2 5 Standard Candles for distance A light source whose luminosity we know. Apply distance-brightness law: L d = 4πB High L stars can be used to distances >10 Mpc. Also used to calibrate Hubble s Law. Higher L supernovae can be used out to billions of parsecs (remember Type Ia are all the same L). 6 1
Hubble s Law (see Lec 22, 23) Since galaxies follow a straight line in Hubble s diagram, we can write an equation for the slope: v = H o d Hubble s Constant H o is the slope of the line. We can determine a galaxy s distance from its redshift velocity: d = v/ H o Note that individual galaxies may deviate from the line if they have local gravitational influences. 7 PRS: Redshift Distance Relationship Assuming a Hubble constant of 70 km/sec per Mpc, how far away is a galaxy that is traveling at 2100 km/sec? (1) (2100 x 70) km (2) (2100 / 70) km (3) (2100 x 70) Mpc (4) (2100 / 70) Mpc Hubble' s law : v = H 0 d Answer: 4 8 The Cosmic Distance Ladder At larger distances we have to find new approaches because the previous techniques don t work anymore. At smaller distances it is not always possible to apply the techniques we use far away 9 Hubble s observed redshift is not really a Doppler shift. v = H o x d In GR, space can move. In fact, GR predicts that the space of the universe can expand. z = λ λ 10 Cause of the Cosmological Redshift 11 Light waves are stretched out as they travel through expanding space. If we see a galaxy with light redshifted to 5 its original wavelength, it left the galaxy when the universe was 5 smaller! z = λ λ Are we at the center of the universe? It seems like the Milky Way occupies a special position in the universe, but not really. Consider raisins in the expanding dough of raisin bread. A raisin may see a nearby neighbor traveling away at 2 cm/hr. A raisin 5 times farther away travels at 10 cm/hr, etc. If the edge cannot be seen, it looks to each raisin as if it is at the center! 12 2
Lookback Time 13 14 From the redshift of a galaxy, and using a cosmological model, the time since the light left the distant galaxy can be determined. Since the universe is expanding, the true distance today is farther than just the lookback time as light years (space expands behind the photon). 14 billion ly 46 billion ly. The Hubble Deep Field is a Time Machine! HDF Galaxies by Redshift shows Galaxy evolution 15 What Are Quasars? 16 The HST shows that quasars are located in the centers of galaxies. The galaxies often appear to be interacting. Overall morphology is like radio galaxies. Quasars are produced by very active supermassive black holes. Computer Modeling of Galaxy Interactions 17 Formation of an Elliptical from a Group of Galaxies 18 Model individual particles interacting gravitationally. Two galaxies near to each other will disturb the orbits within each galaxy. N-body simulations are more and more realistic as computers become faster. Note all the other spiral, bar and irregular shapes during the process of interaction. 3
Mergers in the Milky Way Analysis of stars around the Milky Way shows several streams These appear to be small galaxies that have been cannibalized by the Milky Way 19 The Milky Way and M31 20 The Milky Way is falling toward M31, so in about 10 billion years What is our cosmic address? 21 Our Place in the Milky Way 22 UMass Amherst Massachusetts USA Earth Solar system? Spiral arm?? We live on the inner edge of the Orion spiral arm about 2/3rds of the way out in the disk. (photo is of a similar type of galaxy as the Milky Way) X Outward from the Milky Way 23 Observing M31 Directly 24 ~50 kpc away The 2-Micron All Sky Survey mapped the whole sky at infrared wavelengths. Note the Magellanic Clouds M31 is the most distant object visible with the naked eye. Galaxies look very dim because the stars are widely spaced. The details we see in images are based on long exposures. 4
25 M31-Great Nebula in Andromeda 26 M31-Great Nebula in Andromeda dwarf galaxies orbiting M31 M31 is about 2 million light years away and bigger than the Milky Way. The Cepheids measured by Hubble indicated that M31 is ~2 million light years away and bigger than the Milky Way. It is the most distant object visible with the naked eye. D~700 kpc R~20 kpc The Local Group 27 The Virgo Cluster 28 ~3 dozen D~700 kpc R~5 kpc D~50 kpc R~2kpc The Local Group is about 15 Mpc from a dense cluster of galaxies in the constellation Virgo Clusters of Galaxies... and of gas! 29 Clusters of Galaxies 30 Galaxies are found clustered, from rich groups with 100s to 1000s of galaxies, to poor groups with only a few. Clusters are dynamic! Although we see only a snapshot in time, different clusters show galaxy mergers, acquisition of galaxies, interactions. Because the galaxies interact, clusters tend to contain few spirals and mostly ellipticals. Composite Chandra X-ray / Kitt Peak optical image of Abell 2029, showing intra-cluster gas. Gravity holds the galaxies together, and they can fall through the center in the process, gas is stripped out of them into the intra-cluster medium. 5
31 32 Clusters can even cluster! By measuring redshifts of many galaxies, we can trace structure as in an MRI scan. In the slice at right each dot is a galaxy with a distance from its redshift. Note the voids and superclusters Mapping Out Large Scale Structure Redshift Surveys 33 Huge new surveys are revealing structure out to billions of light years distance. 6