Five pieces of evidence for a Big Bang 1. Expanding Universe

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1 Five pieces of evidence for a Big Bang 1. Expanding Universe More distant galaxies have larger doppler shifts to the red, so moving faster away from us redshift = z = (λ λ 0 )/λ 0 λ 0 = wavelength at rest λ = observed wavelength Measure of expansion rate is Hubble Constant H 0 = V/R = velocity of motion away from us/distance from us Vt = R velocity x time = distance -> H 0 proportional to 1/time If the Universe has always been expanding at the same rate then all observable galaxies can be traced back to the same point in time given by 1/H 0 Determination of H0: measurements of velocities from spectra is easy and accurate more difficult to measure distances 1/H 0 ~ 13 billion years

2 Five pieces of evidence for a big Bang 2. Universe has an age oldest globular clusters and stars in galaxies are given ages based on the main sequence turn-off point faintest white dwarfs indicate time available for these objects to cool ages ~ 14 billion years It stands to reason that the Universe is older than the stars in it!

3 Five pieces of evidence for a Big Bang 3. Universe is evolving. age of universe 3 Gyr age of universe 6 Gyr many more quasars at z ~ 2 many more blue, fragmented galaxies at z ~ 1 implies most star formation occurred at z ~ 1-2 when Universe was 1/2-1/3 it s present age record highest redshift observed today z ~ 6 11 time age of universe 1 Gyr 500 Myr

4 Five pieces of evidence for a Big Bang 4. Microwave Background Radiation. T = 2.7 o K extremely uniform around sky In detail, 2 kinds of anisotropies are seen 1. dipole: thought to be due to a mo7on of 600 km/s w.r.t. cosmic rest frame 2. small scale irregularities : ΔT/T ~ 10-5 K due to irregulari7es at the `surface of last scaeering # Cosmic microwave background comes to us from the time when the Universe made a transition from a hot, ionized state to a cool, neutral state. # Going back in time in the Universe is like going spatially into the Sun. The deeper into the Sun, the hotter and denser it gets until, at the core, there is thermonuclear fusion. Retracing back out, there is a transition at the photosphere, the apparent surface of the Sun. Inside, photons scatter off free electrons analogous to a fog. Outside, photons come directly to our eye because scattering is no longer important. Same with the Universe: light (photons) were scattered during the hot phase but come unimpeded to us once conditions were sufficiently cool. T ~ 3000K at z ~ 1000 Blackbody at recombination peaked ~1 micron, today redshifted to ~1 millimeter.

5 Five pieces of evidence for a Big Bang 5. Abundance of Elements. Everywhere we look, we find 23-24% Helium by mass and almost all the rest Hydrogen. Heavier elements account for about 2% by mass in places like the Sun where there has been processing of materials Big Bang model nicely predicts that this H & He is almost all primordial and all heavier elements were synthesized in stars, particularly supernova explosions. (trace elements of Lithium Li and Deuterium D provide refining constraints on the Big Bang model)

6 Geometry of the Universe - effects with look-back time

7 static, flat space metrics expanding hyperbolic: k<0 flat: k=0 spherical: k>0

8 effects at large distance

9 Evidence for a Flat Universe Irregularities in surface of last scattering (as Universe cooled below 3000 K and electrons attach to atoms) were due to acoustic oscillations with dimensions 0.6x age of Universe Universe was 400,000 years old at recombination and gave acoustic oscillation features we see today on scales of about 1 degree We are looking at a ruler of known length (0.6x400,000 lt yrs) at a known lookback in the size of the Universe (factor 1000) so the observed scale (1 degree) just depends on the geometry of the Universe (larger angle --> closed; smaller angle --> open)

10 Models of the Expanding Universe Horizons Only a limited part of the universe is within our horizon. As the universe ages, we gain access to a larger volume of the universe as light from distant places enters our horizon.

11 Looking Back: Overview