A period of primordial nucleosynthesis followed
from
to
during which light element abundances were synthesized to form
24% helium with trace amounts of deuterium, tritium, helium-3,
and lithium. Observations of these relative abundances represent
the earliest confirmation of the standard model. It is also
during this stage that neutrinos (produced from proton-proton and
proton-photon interactions, and from the collapse or quantum
evaporation/annihilation of topological defects) stopped
interacting with other matter, such as neutrons, protons, and
photons. Neutrinos that existed at this time separated from these
other forms of matter and traveled freely through the Universe at
very high velocities, near the speed of light.
By
, the matter density became equal to the radiation density as the
Universe continued to expand, identifying the start of the
current matter-dominated era and the beginning of structure
formation. Later, at
(
), the free ions and electrons combined to form atoms,
effectively decoupling the matter from the radiation field as the
Universe cooled. This decoupling or post-recombination epoch
marks the surface of last scattering and the boundary of the
observable (via photons) Universe, and plays an important role in
the history of the Cosmic Microwave Background Radiation (CMBR).
Assuming a hierarchical Cold Dark Matter (CDM) structure
formation scenario, the subsequent development of our Universe is
characterized by the growth of structures with increasing size.
For example, the first stars are likely to have formed at
from molecular gas clouds when the Jeans mass of the background
baryonic fluid was approximately
, as indicated in Figure
2
. This epoch of pop III star generation is followed by the
formation of galaxies at
and subsequently galaxy clusters. Though somewhat controversial,
estimates of the current age of our Universe range from
to
, with a present-day linear structure scale radius of about
, where
is the Hubble parameter (compared to
typical for the virial radius of rich galaxy clusters).
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