Physical Foundations of Cosmology

3.5 Nucleosynthesis 111

10 −^2

10 −^11

10 −^10

10 −^9

10 −^8

10 −^7

10 −^6

10 −^5

10 −^4

10 −^3

10 −^2

10 −^1

10 0

10 −^1

10

1 101 102 103

final abundance

(^3) He
(^7) Li
D
(^4) He
η
Fig. 3.8.
sources which give comparable contributions. First, the greater the number of ultra-
relativistic species, the faster the universe expands for a given temperature. This
means neutrons freeze out earlier, leading to largerX∗n. Second, if there are more
light species, the nucleosynthesis temperature is reached sooner and more neutrons
avoid decay. Thus, givenη 10 , we can put rather strong bounds on the number of
unknown light species using the observational data on helium-4 abundance. We will