Visible universe is an infinitesimal fraction of the material from the big bang

2. Visible universe is an infinitesimal fraction of the material from the big bang

4. What we know about the universetoday

5. Edwin Hubble at Mt. Wilson 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. 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

6. The View from Mt. Wilson Photo courtesy Colleen Gino of Mt. Wilson

7. The Hubble Space Telescope

8. Thermal and Non-thermal Emission • Light • wavelength l, frequency n , speed c • Dispersion: Use (prism or grating) to determine spectrum • Two kinds of sources • thermal emission: incandescent heated filament: blackbody radiation…the Planck Law • non-thermal emission: eg: excited gas emits light at specific colors or wavelength. eg laser, radio, line spectra • pattern of emission lines provides unique fingerprint for each element • Identify spectral lines in distant galaxies • universality of spectral lines: laws of physics invariant in space and time. l

9. 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

10. z • No recession: z=0 • v=0.5c, z=0.86 • Most distant galaxy:z=6 • Cosmic Background: z=1000 • z=Dl/l=Da/a where a is characteristic scale size of universe

11. 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 Stellar Spectrum Intensity Spectrum courtesy Bob Kirshner Sodium Magnesium Galaxy Spectrum Calcium Wavelength l • Spectra of nearby and distant galaxies • Nearby galaxy travels at 261 km/s • Distant galaxy travels at 6,400 km/s

12. Astronomical Distance Measurement • Apparent size of a “standard ruler” • Standard ruler is an object whose intrinsic size is known • Apparent (angular) size qprovides 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 q d

13. Standard Rulers in Everyday Life The STOP sign is an everyday “standard ruler”. If we know STOP signs are all the same size, the apparent size of a STOP sign provides us with distance information.

14. 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

15. 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.

16. Type Ia Supernova Observations from the CfA Supernova Group: Kirshner, Garnavich, Challis and Jha A spectrum of the light emitted by SN 1998bu. The features in this spec- trum identify SN 1998bu as Type Ia. 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.

17. 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 vr Velocity d Distance A single supernova

18. Type Ia Supernovae Measurements Distance measurements to 19 SNe 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. Riess, Press & Kirshner ApJ 1996 Blue points: 19 SNe Red line: Hubble Law with Ho=19.6 km/s/Mly =64km/s/Mpc

19. Interpreting the Expansion • Galaxies are receding from us, and their recession velocities are proportional to their distances from us • Two interpretations • Bad neighbour 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. • BNH violates the Cosmological Principle. • Only linear homogeneous expansion is universally internally consistent

20. Summary • 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.

24. What we know and what we don’t • Matter density is dominated by cold dark matter that we know nothing about. • Perturbations which give rise to structure formation arise in the inflation era due to ultra-high energy processes about which we know nothing! • The universe is dominated by a property of space called dark energy, cosmological constant or quintessence (also called the cosmo-illogical constant!). We know nothing about it! • Baryon asymmetry arises in the GUT or Electroweak era due to CP violation but the details are unknown