Did you know that, every second, you are experiencing microwave radiation from
space? Yes, that's right: microwaves. The same radiation heating your leftovers is
what's heating the universe. The first time humans saw this cosmic radiation
was while channel surfing on our first television sets. We didn't know it then,
but we were watching the oldest radiation. Within this radiation is our
history, it describes the conditions of the early
universe and today is the strongest evidence for a Hot Big Bang model. Let's
slow down--to understand how all this works, we need to travel back in time one
month after the beginning of the cosmos, 13.8 billion years ago. Less than a
second after the hot big bang, the universe underwent a period of massive
inflation. During this period of inflation, everything grew by 60-e
foldings. After this short-lived inflation epoch, the universe has
continued to expand, and has become a dense soup of particles about a billion
degrees Celsius. That's roughly a million times hotter than the sun today- so hot
that matter and energy are essentially the same thing. Our universe soup is
called the photon-baryon fluid because it's made of mostly photons and baryons
with a dash of electrons. Photons are the light particles which carry the energy
of radiation. They travel in the form of waves: short wavelengths carry high
energy, long wavelengths carry less energy.
When a photon is radiated away, it retains information about the source
such as its temperature, direction, and even its chemical composition. They carry
this information until they interact with matter, which means that the photons
we detect today provide us with the history of their origins. Baryons include
protons and neutrons which make up the nucleus of the ATOM. The atom is the
fundamental building block for all matter. The simplest atom, Hydrogen,
requires one electron orbiting a single proton. But because the cosmos is so
small and hot, the photons are energetic enough to blast electrons out of orbit--
thus no atoms are formed and nothing "matters". The universe pretty much stays
as this hot cosmic soup for a while, so let's fast forward. It is now 380,000
years later. As the universe expands, it stretches the wavelength of photons
resulting in a distribution of energy across space. The dispersion of this
energy has caused the cosmic soup to cool to about 3000 degrees Celsius, roughly
half the temperature of the sun today. Since the photons are traveling with
longer wavelengths they are not energetic enough to scatter off
electrons in orbit. The photon-baryon fluid has evaporated. Let there be light...
and matter. This instant is recognized as the surface of last scattering, the first
light of the cosmos. A snapshot of the young universe. Photons can now move much
longer distances and carry the information about this time further.
Electrons are free to orbit baryonic nuclei forming the basic blocks of all
life. As we move forward in time, the photon wavelength continues to stretch.
At 100 million years, matter condenses to form stars. At 600 million years, our home
galaxy. The Milky Way is born. At 8.6 billion years, the sun forms, giving birth to our solar system. Today the universe is
13.8 billion years old. It has continued to expand and the photons of the early
cosmos have continued to stretch, becoming invisible low-energy microwaves,
which are nearly radio waves. In the year 1964 (modern time), two scientists at Bell Labs detect
this radiation as noise in their Radio Telescope. After cleaning up the "white
dielectric material"-- bird poop-- they think is interfering with their data, they
discover this cosmic radiation is constant from every direction in the sky.
After a paper publication, this apparent noise from the sky is recognized as
remnant radiation from the earliest observable moment in the evolution of
the universe, a baby picture of the cosmos, called
the Cosmic Microwave Background (CMB). This radiation from the CMB is what heats the
empty voids of space to 3 degrees above absolute zero. 2.725 Kelvin.
Today we know the CMB is the heat radiation from the evaporation of
the early cosmic soup, it is evidence that the universe was once a hot dense
fluid that has been expanding. We interpret this as evidence of the
big bang, but we don't know what happened before this picture. Cosmologists
consider a number of origins for the cosmos
based on this CMB radiation. But the most widely accepted model is the Hot Big Bang.
If the universe weren't expanding, we wouldn't be here and it would still
be a hot soup of energy and matter.