140 Thermal History of the Universe
The^3 He isotope can be seen in galactic star-forming regions containing ionized
hydrogen (HII), in the local interstellar medium and in planetary nebulae. Because
HII regions are objects of zero age when compared with the age of the Galaxy, their
elemental abundances can be considered typical of primordial conditions.
The^7 Li isotope is observed at the surface of the oldest stars. Since the age of stars
can be judged by the presence of metals, the constancy of this isotope has been inter-
preted as being representative of the primordial abundance.
The strongest constraint on the baryonic density comes from the primordial deu-
terium abundance. Ultraviolet light with a continuous flat spectrum emitted from
objects at distances of푧≈ 2 − 3 .5 will be seen redshifted into the red range of the
visible spectrum. Photoelectric absorption in intervening hydrogen along the line of
sight then causes a sharp cut-off at휆= 91 .2nm,theLyman limit. This can be used to
select objects of a given type, which indeed are star-forming galaxies. Deuterium is
observed as a Lyman-훼feature in the absorption spectra of high-redshift quasars. A
recent analysis [5] gives
훺b(^2 H)ℎ^2 = 0. 020 ± 0. 001 , (6.99)which is more precise than any other determination. The information from the other
light nucleids are in good agreement. The values of휂and훺bin Table A.6 come from
a combined fit to^2 H data, CMB and large-scale structure. We defer that discussion to
Section 8.4.
In Figure 6.5 the history of the Universe is summarized in nomograms relating the
scales of temperature, energy, size, density and time. Note that so far we have only
covered the events which occurred between 10^11 Kand10^3 K.
Nuclear synthesis also goes on inside stars where the gravitational contraction
increases the pressure and temperature so that the fusion process does not stop with
helium. Our Sun is burning hydrogen to helium, which lasts about 10^10 yr, a time span
which is very dependent on the mass of the star. After that, helium burns to carbon in
typically 10^6 yr, carbon to oxygen and neon in 10^4 yr, those to silicon in 10yr, and sili-
con to iron in 10h, whereafter the fusion chain stops. The heavier elements have to be
synthesized much later in supernova explosions, and all elements heavier than lithium
have to be distributed into the intergalactic medium within the first billion years.
To sum up, Big Bang cosmology makes some very important predictions. The
Universe today should still be filled with freely streaming primordial photon (and
neutrino) radiation with a blackbody spectrum [Equation (6.10)] of temperature
related to the age of the Universe and a polarization correlated to the temperature.
This relic CMB radiation (as well as the relic neutrino radiation) should be essentially
isotropic since it originated in the now spherical shell of the LSS. In particular, it
should be uncorrelated to the radiation from foreground sources of later date, such
as our Galaxy. We shall later see that these predictions have been verified for the pho-
tons (but not yet for the neutrinos).
A very important conclusion from BBN is that the Universe contains surprisingly
little baryonic matter! Either the Universe is then indeed open, or there must exist
other types of nonbaryonic, gravitating matter.