The Early Universe Thursday, January 24 (planetarium show tonight: 7 pm, 5 th floor Smith Lab)

Hot, dense, opaque objects emit light. Color (or wavelength) of light depends on object’s temperature.

Temperature is inversely proportional to wavelength. red-hot iron = 1000 K lightbulb filament = 2900 K Sun’s surface = 5800 K

Early universe was hot, dense, and opaque: it emitted light. In 1965, two astronomers (Penzias & Wilson) discovered faint “static” in their microwave antenna.

This static was the “leftover light” from hot, dense, opaque early universe. microwave Its spectrum peaks at λ ≈ 1 mm; this is microwave radiation. Microwave Scientists call the “leftover light” the Cosmic Microwave Background (CMB). Intensity

Temperature implied by CMB spectrum is T ≈ 3 K. (That’s COLD!)

The universe became transparent at a temperature T ≈ 3000 K. visible & infrared microwave But…objects at T ≈ 3000 K produce visible & infrared light (think “lightbulb filament”), not microwave light.

visible & infrared microwave How did the cosmic background change from visible & infrared light (λ ≈ mm) to microwave light (λ ≈ 1 mm)? 3000 K3 K How did its temperature drop from 3000 K to 3 K?

The universe is expanding. Distance between galaxies increases. Wavelength of light Wavelength of light (distance between wave crests) increases.

Wavelength of cosmic background light has increased by a factor of Why? Why? Because the universe has expanded by a factor of 1000 since the time it became transparent mm 1 mm

We now have two ways to think about a galaxy’s redshift. Doppler shift 1) The redshift is the result of a Doppler shift. expansion 2) The redshift is the result of expansion stretching the wavelength.

Example: z Example: a galaxy has a redshift z ≡ (λ-λ 0 )/λ 0 = ) Radial velocity of the galaxy is 1% the speed of light: v = 0.01 c = 3000 km/sec d = v/H 0 = 42.9 Mpc.

now 2) The distance to the galaxy now is 1% greater than it was when the light we observe was emitted: d now = 42.9 Mpc d then = 42.9 Mpc / 1.01 = 42.5 Mpc

The Right Way ?? So, which way of thinking about redshift (Doppler or expansion) is The Right Way ?? identical In the limit of small redshift (v << c) they are identical. Let’s see why!!

Light from galaxy has traveled d average = 42.7 Mpc = 139 million light-years in a time t = 139 million years. During that time, distance to the galaxy has expanded by 0.01 d average = 1.39 million light-years. Average radial velocity = 1.39 million light-years / 139 million years = 0.01 c.

very Some galaxies have very high redshift. The arrowed galaxy at left has z = (λ-λ 0 )/λ 0 = 5.7 expansion For very distant galaxies, it’s best to think of redshift as being due to expansion (no guarantee of constant radial velocity!)

Astronomers are fascinated by galaxies at high redshift. “A telescope is a time machine.” Larger redshift → larger distance → longer light travel time.

Astronomers the Cosmic Microwave Background

anything The CMB has highest redshift of anything we can see (z = 1000). When we look at the CMB, we look at the surface of the glowing “fog” that filled the early universe!

When we look at the CMB, we see a message direct from the early universe. What is this message telling us? Messages are often (1) hard to read & (2) hard to interpret.

1) “Reading” the CMB above Water vapor in Earth’s atmosphere absorbs microwaves: go above the atmosphere! COBEWMAP

WMAP (Wilkinson Microwave Anisotropy Probe) is at the L 2 point, beyond the Moon’s orbit.

2) “Interpreting” the CMB isotropic Observation: Temperature of CMB is nearly isotropic (the same in all directions). homogeneous Interpretation: early universe was nearly homogeneous (the same in all locations).

hottercooler Observation: Temperature of CMB is slightly hotter toward Leo, cooler toward Aquarius (on opposite side of sky). cooler → ← hotter Temperature fluctuation = 1 part per 1000.

Doppler shift Interpretation: difference in temperature results from a Doppler shift. Earth orbits Sun (v = 30 km/sec) Sun orbits center of our galaxy (v = 220 km/sec) Galaxy falls toward center of Local Group (v ≈ 50 km/sec) Local Group falls toward Virgo Cluster (v ≈ 200 km/sec)

Net motion: toward Leo, with a speed v ≈ 300 km/sec ≈ c. Cosmic light from direction of Leo is slightly blueshifted (shorter wavelength, higher temperature).

Observation: After subtracting the effect of our motion through space, CMB still shows hot & cold spots, about 1° across. Temperature fluctuation = 1 part per 10 5.

density Interpretation: observed temperature fluctuations result from density fluctuations in the early universe. density temperature Regions that were compressed had higher density, but also higher temperature (gases heat up as they are compressed).

Hot cold Hot spots in the CMB are higher in temperature than cold spots by only 1 part per 100,000. densityfluctuations Implication: the density fluctuations in the early universe were also small (about 1 part per 100,000). Why do we care Why do we care about such tiny density fluctuations?

The Rich Get Richer, the Poor Get Poorer. slightly much A region that was only slightly denser than average will eventually become much denser than average; it’s compressed by its own gravity.

Great Oaks from Tiny Acorns Grow. smallmore massive A dense region that initially has a small mass will become more massive with time; its gravity attracts surrounding matter.

Computer simulations of the growth of density fluctuations: 1234

Gravity sucks. Since gravity sucks (and doesn’t spew), tiny density enhancements become dense, massive clusters of galaxies.

Tuesday’s Lecture: Reading: Chapter 4 Problem Set #3 due!! The VERY Early Universe