18 From Quarks to Protons and Neutrons...
139
forth between two photons to electron–positron production therefore went
on until the universe’s temperature fell below the mass of electron and the
age of the universe was a few hundred thousand seconds. After that particular
period of the universe, electrons and positrons became scarcer and the photons
floated freely as the universe cooled. When the universe was about 40 years
old, electron–photon collision became too weak to change the energy of the
photons. The light wavelength after that got stretched out with the universe’s
expansion. Light got less and less energetic until the present time when the
temperature is about 2.7 K. This process led to about 400 photons per cubic
centimeter of the present universe. Such low energy (or microwave energy)
photons are present everywhere in the universe today and were detected by
the famous Penzias–Wilson Bell lab experiment in 1964. They used a horn
antenna in Holmsdale, New Jersey, to detect radiation from between galaxies.
They detected a persistent noise, which turned out to be a remnant of the
red shifted radiation from the early stage of the universe. In recent years, this
experiment has been improved further, leading to finer details in the radiation
patterns. More precise studies by the recent WMAP and Planck experiments of
the cosmic radiation show that it is now highly isotropic. There is however an
anisotropy (or direction dependence) of radiation, about one part in 100,000.
The later anisotropy is linked to the formation of structure, which induces
non-uniformities to the radiation.
Does one expect a cosmic neutrino background similar to the cosmic
microwave background (CMB)? The answer is yes. In many ways, the evolution of neutrinos and photons is similar, except for the timing of when they
became “decoupled” from the cosmic soup and started their free expansion.
The neutrinos “fell behind” the cosmic expansion when the age of the universe
was one second. Ever since, they have been freely and slowly expanding, being
less and less energetic as their wavelength stretches with the expansion of the
universe. They also became less abundant as the universe got older. At the
time when the photons decoupled from the cosmic soup, the temperature of
the neutrinos went down a bit, and the current temperature of the neutrino
background is about 2 K compared to the photon temperature, which is about
3 K now. Just like the microwave background, there is a neutrino background
now pervading the entire universe. In principle, they could be detectable, but
due to their low energy and very weak interaction, they rarely scatter against
matter, making their detection much harder.
139
forth between two photons to electron–positron production therefore went
on until the universe’s temperature fell below the mass of electron and the
age of the universe was a few hundred thousand seconds. After that particular
period of the universe, electrons and positrons became scarcer and the photons
floated freely as the universe cooled. When the universe was about 40 years
old, electron–photon collision became too weak to change the energy of the
photons. The light wavelength after that got stretched out with the universe’s
expansion. Light got less and less energetic until the present time when the
temperature is about 2.7 K. This process led to about 400 photons per cubic
centimeter of the present universe. Such low energy (or microwave energy)
photons are present everywhere in the universe today and were detected by
the famous Penzias–Wilson Bell lab experiment in 1964. They used a horn
antenna in Holmsdale, New Jersey, to detect radiation from between galaxies.
They detected a persistent noise, which turned out to be a remnant of the
red shifted radiation from the early stage of the universe. In recent years, this
experiment has been improved further, leading to finer details in the radiation
patterns. More precise studies by the recent WMAP and Planck experiments of
the cosmic radiation show that it is now highly isotropic. There is however an
anisotropy (or direction dependence) of radiation, about one part in 100,000.
The later anisotropy is linked to the formation of structure, which induces
non-uniformities to the radiation.
Does one expect a cosmic neutrino background similar to the cosmic
microwave background (CMB)? The answer is yes. In many ways, the evolution of neutrinos and photons is similar, except for the timing of when they
became “decoupled” from the cosmic soup and started their free expansion.
The neutrinos “fell behind” the cosmic expansion when the age of the universe
was one second. Ever since, they have been freely and slowly expanding, being
less and less energetic as their wavelength stretches with the expansion of the
universe. They also became less abundant as the universe got older. At the
time when the photons decoupled from the cosmic soup, the temperature of
the neutrinos went down a bit, and the current temperature of the neutrino
background is about 2 K compared to the photon temperature, which is about
3 K now. Just like the microwave background, there is a neutrino background
now pervading the entire universe. In principle, they could be detectable, but
due to their low energy and very weak interaction, they rarely scatter against
matter, making their detection much harder.