skip navigation
Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 37: The Whispers of Creation

Readings: Ch 28, sections 28-4 & 28-5; Ch 29, section 29-4

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
Perfect blackbody spectrum with a Temperature of 2.725±0.001 K

The Three Pillars of the Big Bang

The "Big Bang" is our name for our physical model of the expanding Universe. It makes specific predictions that can be tested by making observations of the Universe.

Three fundamental pieces of observational evidence in favor of the Big Bang Model are:

The Expansion of the Universe:

Primordial Nucleosynthesis:

The Cosmic Background Radiation:

The Hot Big Bang

What we see Now:

In the past:

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

Where Did all the Helium come from?

Pop I Stars (and the Sun):

Metal-poor Pop II Stars:

Where did the He in Pop II stars come from?

Primordial Nucleosynthesis

When the Universe was only 1 second old:

General hot, dense soup of subatomic particles and photons.

As it expanded, it cooled off.

Primordial Deuterium Formation

When the Universe was 2 minutes old:

Neutrons & Protons fused into Deuterium (2H) Nuclei:

A dense, hot soup of mostly protons (H) and deuterium (2H) in a 4:1 proportion, and a mix of photons, electrons and other subatomic particles.

Primordial Helium Formation

Most of the newly-formed Deuterium fuses to form 4He nuclei

By the time the Universe was 4 minutes old:

At this point, the Universe had cooled down so much that fusion stopped and no further heavy elements were formed.


After Primordial Nucleosynthesis stopped when the Universe was about 4 minutes old:



Current Status

The predictions of Primordial Nucleosynthesis calculations agree very well with current observations of the primordial abundances of the light elements relative to Hydrogen.

There are still a few remaining issues to be settled:

Observational Issues:

Theoretical Issues:

Hot Early Universe

After the Epoch of Primordial Nucleosynthesis, the Universe stays hotter than 3000 K for a long time:


Blackbody Radiation

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

As the Universe expands it cools:

Epoch of Recombination

When the Universe is about 300,000 years old:

The temperature drops below 3000 K:

The Universe suddenly becomes transparent:

Cosmic Background Radiation

After Recombination, the Universe is filled with a diffuse, "relic" blackbody radiation from the initial hot, dense, opaque phases.

As the Universe expands further:

By today, the spectrum will have been redshifted by a factor of 1000 down to a temperature of only about 3 Kelvin.

Wein's Law tells us that a blackbody of this temperature would emit primarily at microwave wavelengths.


1965: Arno Penzias & Robert Wilson (Bell Laboratories) What they had discovered, almost by accident, is the Cosmic Microwave Background Radiation.

This work won Penzias and Wilson the 1988 Nobel Prize in Physics.

But, is it Blackbody Radiation?

The Big Bang model makes very specific predictions for the nature of the Cosmic Background Radiation:


You can browse through all of the data from the main space missions devoted to studies of the Cosmic Microwave Backgroun on the Legacy Archive for Microwave Background Data Analysis webpage at the NASA Goddard Space Flight Center.

Spectacular Confirmation

The current state of observational work has spectacularly confirmed and greatly strengthened the observational case for the Big Bang Model:


Subsequent experiments, like the Wilkinson Microwave Anisotropy Probe (WMAP) and specialized experiments conducted from the South Pole and Northern Chile are studying the details of these temperature fluctuations, which can tell us a great deal about the Universe. We'll discuss these results separately in a later lecture.

Evidence for the Big Bang

Expansion of the Universe:

Primordial Nucleosynthesis:

Cosmic Background Radiation:

Together, these constitute strong empirical confirmation of many of the specific predictions of the Big Bang Model for the Expanding Universe.

This is why all astronomers take the Big Bang so seriously, and in fact it has so far provided us with a remarkably robust framework in which to explain a large number of otherwise inexplicable observations about our Universe.

Return to [ Unit 5 Index | Astronomy 162 Main Page ]
Updated: 2006 February 28
Copyright Richard W. Pogge, All Rights Reserved.