2.3. Variations On A Theme: Non-Standard BBN
Before moving on, let's take a diversion to which we'll return
again in Section 5. Suppose the standard
model is modified through the addition of extra relativistic particles
(
N
> 0; SSG).
Equivalently (ignoring some small differences), it could be
that the gravitational constant in the early universe differs
from its present value (G
G'
G). Depending
on whether G' > G or G' < G, the early
universe expansion
rate can be speeded up or slowed down compared to the standard
rate. For concreteness, let's assume that S > 1. Now, there
will be less time to destroy D and 3He, so their relic
abundances will increase relative to the SBBN prediction. There is less
time for neutrons to transform to protons. With more neutrons
available, more 4He will be synthesized. The changes in
7Li are more complex. At low
there is less
time to destroy
7Li, so the relic 7Li abundance increases. At high
there is less time to produce 7Be, so the relic
7Li (mass-7) abundance decreases.
Since the 4He mass fraction is relatively insensitive to the
baryon density, it provides an excellent probe of any changes
in the expansion rate. The faster the universe expands, the
less time for neutrons to convert to protons, the more 4He
will be synthesized. The increase in Y for "modest" changes
in S is roughly
Y
0.16(S - 1)
0.013
N
.
In Figure 2 are shown
the BBN-predicted Y versus the BBN-predicted Deuterium abundance
(relative to Hydrogen) for three choices of
N
(N
3 +
N
).