Lecture 35: The Whispers of Creation: Testing the Big Bang

Readings: Section 28-4, 28-5 and 29-5

Key Ideas

Fundamental Tests of the Big Bang

Primordial Nucleosynthesis

Primordial Deuterium & Helium

Primordial Light Elements (Li, B, Be)

Cosmic Background Radiation

Relic blackbody radiation from Big Bang

Temperature today: T=2.725 +/- 0.001K

The Three Pillars of the Big Bang

Expansion of the Universe

Explains the observed Hubble Law

Age is consistent with ages of the oldest stars

Primordial Nucleosynthesis

Creation of the original light elements

Deuterium (²H), Helium, traces of Li, Be & B

Cosmic Background Radiation:

Relic blackbody radiation from hot early phases

The Hot Big Bang

What we see Now:

The Universe is cold & low density

Galaxies (matter) are getting further apart as space expands between them.

As the Universe expands, it cools further

In the past:

Universe was smaller, hotter, & denser

Is there any evidence of this early hot, dense phase in the past?

Where Did Helium come from?

Metal-rich stars (such as the Sun)

70% H, 28% He & ~ 2% metals

Metals come from earlier supernovae (& AGB stars)

Metal-poor stars (=old stars):

75% H, 25% He, & < 0.01% metals

Where did all the He in metal-poor stars come from?

If from the first stars, where are all the metals that would have formed along with it?

Haven’t found a pure H star, cloud, anything. Always He as well.

Primordial Nucleosynthesis

When the Universe was only 1 second old:

Temperature: 10 Billion K

Too hot for atomic nuclei to exist

Only protons, neutrons, electrons & photons

1 neutron for every 5 protons

General hot, dense soup of subatomic particles & photons

As it expanded, it cooled off

Primordial Deuterium Formation

When the Universe was 2 minutes old:

Temperature dropped to 1 Billion K

Neutrons & protons fuse into Deuterium (²H)

All of the free neutrons go into making deuterium nuclei

Leftover protons stay free as ¹H nuclei

Proportions: about 1 ²H for every 4 protons (¹H)

Soup of mostly ¹H and ²H along with a mix of photons, electrons & other particles.

Primordial Helium Formation

Most of the ²H fuses to form ⁴He nuclei

Other reactions make ³He, Li, Be and B in very tiny quantities.

When the Universe was 4 minutes old:

Much of the deuterium turned in ⁴He

Left with tiny traces of deuterium and other light elements

The Universe cooled so much that fusion stopped.

Aftermath

When Primordial Nucleosynthesis stopped:

Predictions

⁴He/H-20-26%

D/H=0.0001-0.1%

Observations:

⁴He/H=22-25%

D/H=0.001-0.02%

Agrees!

Information about Dark Matter

The amount of ²H is extremely sensitive to the density of “baryonic”=ordinary matter

The higher the density of baryonic matter, the lower the predicted amount of ²H out of the Big Bang

The relatively high amount of ²H shows that all of the dark matter cannot be baryonic.

Current Status

Predictions of Primordial Nucleosynthesis agree well with current observations:

Observations:

Need refinement of the primordial abundances

Very difficult observations to make.

Need to look at high redshift or areas with little star pollution

Theory

Need to know average density of p&n

Light-element reaction rates need refinement

The Hot Early Universe

After Nucleosynthesis, the Universe stays hotter than 3000 K for a long time

Electrons & nuclei cannot combine to form neutral atoms

Universe remains fully ionized

Free electrons easily scatter photons

Universe is opaque to light during this time.

Filled with a hot ionized “fog” of ions & free electrons

Blackbody Radiation

The Early Universe is filled with a hot opaque ionized gas:

Has a perfect blackbody spectrum

With a characteristic temperature, T

As the Universe expands, it cools:

Photons redshift

The peak of the spectrum shifts redward

The blackbody temperature drops

Epoch of Recombination

When the Universe is ~300,000 years old

Temperature drops below 3000 K:

Electrons & nuclei combine to form atoms

Not enough free electrons to scatter photons

Universe suddenly becomes transparent:

Photons stream out through space

Photon Spectrum: 3000 K blackbody

Cosmic Background Radiation

After Recombination, the Universe is filled with diffuse, “relic” blackbody radiation.

As the Universe expands further:

Blackbody photons redshift

Spectrum peak shifts to redder wavelengths, and hence cooler temperatures

By today, the spectrum is redshifted by a factor ~1000 down to T~3K

Discovery

1965: Penzias & Wilson (Bell Labs)

Mapping sky at microwave wavelengths

Found a faint microwave background noise

First thought it was equipment problems (noisy amplifiers, pigeons in the antenna)

Finally determined it was cosmic in origin

Won the Nobel Prize in 1978 for discovering the Cosmic Background Radiation

But, is it Blackbody Radiation?

The Big Bang model makes very specific predictions:

The spectrum is a perfect blackbody

Characterized by a single temperature

Observations:

Need to work with very cold instruments at the South Pole, high altitude or in orbit

Experiments with balloons, rockets, radio antennas, and satellites

See Figure 28-7

Spectacular Confirmation

Current data all spectacularly confirm the predictions of the Big Bang

Perfect blackbody spectrum

A single temperature T=2.725=0.001K

Uniformly fills the Universe

Details

Fine structure at a part in 10⁵ level is related to the large-scale structure we see in the galaxy distribution

Currently an object of intense study

See Figure 28-14

Evidence for the Big Bang

Expansion of the Universe: CONFIRMED

Hubble’s Law

Age is consistent with the oldest stars

Primordial Nucleosynthesis: CONFIRMED

Deuterium & Helum in the right amounts

Cosmic Background Radiation: CONFIRMED

Perfect blackbody with a single temperature