Published on August 29, 2007
“Observing the Expansion of the Universe”: 'Observing the Expansion of the Universe' Joe Mohr Chandra Fellow Department of Astronomy and Astrophysics University of Chicago Special thanks to Sara Bittinger and Dennis Gordon Lecture 1, April 3 Spring ‘99 Compton Lecture Series 'Exploring the Mysteries of Our Evolving Universe: Observational Tests of Big Bang Cosmology' Outline: Outline 1. Introduction Our place in the universe Earth Solar system Galaxies Edwin Hubble and his discovery 2. Observing the expansion Measuring recession velocities Nature of light and emission Doppler effect Galaxy spectroscopy Estimating large distances Standard candles and rods Supernovae as standard candles Results from Type Ia supernovae 4. Review 3. Interpreting the expansion The bad neighbor hypothesis Homogeneous expansion Slide3: NASA Image of Planet Earth: NASA Image of Planet Earth A view of Earth from space. Antarctica and South America are easily visible. Our Place in the Solar System: Our Place in the Solar System The Earth is one of nine planets orbiting our Sun. It is the third closest to the Sun, and so far it is the only place in the universe where we know there is life. M31, the Andromeda Galaxy: M31, the Andromeda Galaxy If this picture were of the Milky Way, our solar system would lie here. Our solar system lies within the Milky Way, a gal- axy much like M31. These galaxies con- tain billions of stars similar to our sun. Light emitted in the center of the Milky Way takes 25,000 years to reach us. Therefore, when we look at the gal- actic center we are looking 25,000 yrs back in time. Deep Hubble Space Telescope Image: Deep Hubble Space Telescope Image A deep image of an 'empty' portion of the sky with the Hubble Space Telescope reveals that the universe is filled with galaxies- many just like our own. The light we see from the most distant galaxies has traveled approx- imately 10 billion years to reach us. Edwin Hubble at Mt. Wilson: Edwin Hubble at Mt. Wilson Hubble guiding the Hooker 100 inch telescope in 1923. The Hooker 100 inch telescope atop Mt. Wilson near Pasadena, CA. It was the largest telescope in the world from 1917-1947. Photos courtesy Mt. Wilson: http://www.mtwilson.edu/History Hubble’s observations at the 100 inch during the 1920’s led him to the conclusion that the universe is expanding, and that an object’s recession velocity is proportional to its distance from the observer. Slide9: Photo courtesy Colleen Gino of Mt. Wilson The View from Mt. Wilson The Hubble Space Telescope: The Hubble Space Telescope Nature of Light and Light Emission: Nature of Light and Light Emission Light is a wave phenomenon it has a wavelenth l, a frequency n and it travels at speed c Can use prism or grating to disperse light into its colors Consider two kinds of light sources incandescent- heated filament emits light of all colors fluorescent- excited gas emits light at specific colors or wavelength pattern of emission lines provides unique fingerprint for each element Stars in distant galaxies composed of same elements as Sun elements emit same unique pattern of lines everywhere in universe l Using the Doppler Effect to Measure Velocity: Using the Doppler Effect to Measure Velocity Doppler Shift- applies to sound and light If source is moving with respect to observer, the observer experiences a shift in wavelength l Velocities away from observer shift light to longer l (redshift) Velocities toward observer shift light to shorter l (blueshift) The higher the velocity the larger the shift- provides velocity measure Blueshift Redshift Galaxy Spectroscopy: Galaxy Spectroscopy Spectra of a nearby star and a distant galaxy Star is nearby, approximately at rest Galaxy is distant, traveling away from us at 12,000 km/s Wavelength l Intensity Spectrum courtesy Bob Kirshner Calcium Magnesium Sodium Galaxy Spectrum Stellar Spectrum Spectra of nearby and distant galaxies Nearby galaxy travels at 261 km/s Distant galaxy travels at 6,400 km/s Distance Cues in Astronomy: Distance Cues in Astronomy Apparent size of a 'standard ruler' Standard ruler is an object whose intrinsic size is known Apparent (angular) size q provides distance d given intrinsic size r Apparent brightness of a 'standard candle' Standard candle is source whose intrinsic brightness is known Apparent brightness b provides distance d given intrinsic brightness B r d q Standard Rulers in Everyday Life: Standard Rulers in Everyday Life The STOP sign is an everyday 'standard ruler'. Because we know STOP signs are all the same size, the apparent size of a STOP sign provides us with distance information. Some Known Standard Candles and Rulers: Some Known Standard Candles and Rulers Standard rulers Elliptical galaxies Galaxy clusters Standard candles Certain types of stars (Cepheid variables) Spiral galaxies Certain types of supernovae (Type Ia SNe) Exploding white dwarfs Emit as much light as an entire galaxy, so can be detected at great distances Type Ia Supernova 1998bu in M96: Type Ia Supernova 1998bu in M96 Observations from the CfA Supernova Group: Kirshner, Garnavich, Challis and Jha SNe look like bright stars superimposed on galaxies. They brighten toward maximum and then fade away over time as the hot material expands and cools. Type Ia Supernova: Type Ia Supernova Observations from the CfA Supernova Group: Kirshner, Garnavich, Challis and Jha Repeated observations yield a light curve- measured brightness versus time- which astronomers use to determine the peak apparent brightness and distance of the SN and parent galaxy. This is SN 1998aq. A spectrum of the light emitted by SN 1998bu. The features in this spec- trum identify SN 1998bu as Type Ia. Testing Expansion with Type Ia Supernovae: Testing Expansion with Type Ia Supernovae Find SNe in distant galaxies (rare objects) Take spectrum to confirm they are Type Ia SNe For each SN Ia Measure the recession velocity of the parent galaxy vr Measure the maximum apparent brightness and compare that to the intrinsic brightness to calculate the distance d Place that point on Hubble diagram Hubble Diagram Distance Velocity A single supernova vr d Type Ia Supernovae Measurements: Type Ia Supernovae Measurements Distance measurements to 19 SNe Riess, Press andamp; Kirshner ApJ 1996 Blue points: 19 SNe Red line: Hubble Law with Ho=19.6 km/s/MLy Type Ia supernovae and every other distance indicator used provides results consistent with the Hubble Law: other galaxies are receding from us, and their recession velocities are pro- portional to their distances, in other words, the farther away the galaxy, the faster it travels away from us. Interpreting the Expansion: Interpreting the Expansion Galaxies are receding from us, and their recession velocities are proportional to their distances from us Two interpretations Bad neighbor hypothesis We are at the center of the universe, and the rest of the universe is trying its best to get away from us. Homogeneous expansion hypothesis The whole universe is expanding, and observers on any other planet in any other galaxy would note the same proportionality between recession velocity and distance- the Hubble Law. Interestingly, the second hypothesis has garnered more attention in the science community! Review: Review Observations: galaxies tend to travel away from us, and their recession velocities vr are proportional to their distances d- the Hubble Law: Our universe is expanding homogeneously and is not static. The universe was denser in the past The universe had a beginning? Expansion is generic to the Big Bang model.