| Briefly explain the significance of any TWO of the following in the establishment of the Hot Big Bang theory as the best cosmological model: | |
| Note that if you answer all three parts, you will only get the marks for your best two answers. | |
| the abundances of 2H and 4He; | [2.5] |
|
Deuterium is a very fragile nucleus, destroyed, not made, by stellar fusion processes.
Helium-4 is made by stars, but its observed abundance is extremely high relative
to heavier elements such as carbon and iron. Therefore it is difficult to account for
the abundances of these isotopes using stellar nucleosynthesis. In the Hot Big Bang, there is a brief period ~100 s after the Big Bang in which the temperatures are high enough for nuclear reactions to take place but not so high that the nuclei are destroyed by high-energy photons. This allows the protons and free neutrons to combine to make light elements. Heavy elements are not made because the lack of stable isotopes with mass 5 or mass 8 prevents further build-up (the universe at this time is not dense enough for quasi-three-body reactions like the triple-alpha process). The relative abundances of deuterium and helium-4 can be predicted using this model and agree with observations, for a physically sensible value of Ωbaryon. The support for the Big Bang model lies in the detailed numerical agreement of the observed abundances and the theoretical prediction. |
|
| radio source counts; | [2.5] |
|
The received flux from a radio source of given luminosity L is proportional to
1/r2 where r is its distance. The number of sources within radius r
is proportional to r3 if they are evenly distributed. Therefore, the
number of sources with received flux greater than some value f should be proportional
to f -3/2. Observationally we find that there are more faint sources than expected. This is a potential problem for the Steady State because it expects that such sources will be evenly distributed (the universe looks the same at all times, therefore at all redshifts). It is not a problem for the Big Bang because cosmic populations evolve, and it is perfectly possible that radio sources are more common in the young universe. This is fair evidence against the Steady State, and therefore favours the Big Bang. It is not conclusive because the faint sources, if not positively identified, could in principle belong to a local, Galactic, population of intrinsically faint objects. Later, when more radio sources were identified with optical counterparts whose redshifts could be measured, it became apparent that the excess of faint sources really did correspond to an excess of sources at high redshift; at this point the evidence against the Steady State becomes solid. |
|
| the blackbody spectrum of the cosmic microwave background. | [2.5] |
|
In the Big Bang model, the early universe is hot, dense, fully ionised, and opaque to
radiation. In these conditions we expect that radiation will be in thermal equilibrium
with matter (because photons interact readily with charged particles), producing a
blackbody distribution. When the universe cools enough for neutral atoms to form, it
quite suddenly becomes transparent, allowing these blackbody-distributed photons to
travel long distances without interaction. The subsequent expansion has no effect on
the form of the distribution except for reducing the average temperature by a factor
of (1+z) – therefore the radiation now is at 3 K whereas it was emitted at around
3000 K. Therefore a blackbody background radiation is expected and natural in the Big Bang. The Steady State, by definition, has no such hot, dense early epoch. Although it is possible to produce a background radiation field in a Steady State model, e.g. from scattered starlight, or by assuming that the unknown physics that creates the necessary matter also creates radiation, it is extremely difficult to get a blackbody distribution, since a superposition of blackbody-distributed photons streaming in from different distances/redshifts produces an overall distribution that is not blackbody. One must resort to unconvincing special pleading to rescue the blackbody spectrum (e.g. assuming that intergalactic space is full of special microwave-absorbing dust). The increasing evidence for the blackbody nature of the microwave background in the late 1960s was what finally killed off the Steady State. |
(2005 Q2.)