The Big Bang

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	Time: 20 hours Level: Advanced

	Introduction Introduction Resource In this unit, we present the three main lines of experimental evidence pointing to the big bang origin of the Universe: (i) the recession of the galaxies; (ii) the microwave remnant of the early fireball;...

	1 Introducing cosmology 1 Introducing cosmology Resource General relativity has a very different conceptual basis from that of Newtonian mechanics. Its success in accounting for the precession of Mercury's orbit, and the bending of light by massive objects like...

	2 Radiation from the galaxies 2 Radiation from the galaxies Resource Stars occur in great collections called galaxies. The distribution and motion of galaxies provide us with the first important experimental information on which we shall build our understanding of the type...

	3 Distances of galaxies 3.1 First steps towards a distance scale Resource As you will see from Table 2, when it comes to astronomy and cosmology, one is called on to deal with a wide range of distances. (Note that a light-year (ly) is the distance light travels in one year,... 3.2 Some general properties of galaxies Resource Firstly, we note that galaxies tend to occur in clusters rather than singly. The mutual gravitational attraction of galaxies naturally tends to hold them on paths that remain close to each other. Typically... 3.3 Extending the distance scale Resource Having reviewed some of the properties of galaxies, we are now in a position to return to the question of how we are to develop further our methods of measuring distance.

	4 The variation of redshift with distance 4.1 Hubble's discoveries Resource In this section, we bring together two important features of galaxies – their redshifts and their distances. 4.2 Evidence for a big bang Resource Having interpreted the redshift as indicating a recessional speed proportional to distance, one may extrapolate into the future to predict how the positions of the galaxies will evolve with time. One can...

	5 The microwave background radiation 5.1 A second major discovery Resource In the introduction to this unit, we said that there were three pillars of evidence for the big bang. We now turn to the second. It rests on a discovery that ranks in importance with that of Hubble's law.... 5.2 The origin of the 3 K radiation Resource In speaking of the radiation as having a cosmic origin, what do we have in mind? Essentially this: 5.3 The redshift of the 3 K radiation Resource The temperature, T, of the radiation is proportional to the most probable photon energy, E, which as we have said is proportional to f, and hence inversely proportional to the wavelength λ. Thus,

	6 The angular distribution of the 3 K radiation 6.1 Basic isotropy Resource As we have said, the photons in the 3 K background have been practically free from interaction with anything since about 4 × 105 years after the instant of the big bang. The present angular distribution... 6.2 The effect of the motion of the Earth Resource The speed of the Earth in its orbit round the Sun is 29.8 km s−1, in a heliocentric frame. But to specify the velocity vector, it is not sufficient to specify the Sun as the origin of the coordinate system;... 6.3 Anisotropies in the Universe itself Resource Having subtracted the dipolar anisotropy due to the motion of the Earth relative to the 3 K radiation, we are left with radiation that is exceedingly isotropic. So, we have to ask whether there are any...

	7 The primordial nuclear abundances 7.1 Introduction Resource So far we have presented two pieces of evidence pointing to the occurrence of a big bang: the redshift of the galaxies (indicating the continuing expansion of the Universe), and the 3 K radiation (the... 7.2 The temperature of matter and radiation Resource The different reactions by which neutrons and protons came together soon after the instant of the big bang to produce heavier nuclei will have proceeded at different rates according to the energies of... 7.3 The formation of light nuclei Resource It is the very high temperatures that make the early stages of the big bang relatively simple to calculate. When it comes to the formation of the first nuclei, we are looking at a temperature that has... 7.4 Nuclear abundances as evidence for the big bang Resource What we have seen is that a theoretical model based on the assumption that there was a big bang, and incorporating an assumption about the present-day value of the baryonic density, ρb,now, leads to definite...

	8 Conclusion 8 Conclusion Resource We have seen that there are three independent pieces of evidence, all of which point to the occurrence of a big bang: (i) the recession of the galaxies; (ii) the cosmic microwave background; and (iii)...

	Summary Summary Resource 1 The visible matter in the Universe, stars and interstellar gas, is concentrated into galaxies, which are collections of ∼1011 stars. The galaxies themselves are usually to be found in clusters with typically...

	References and Acknowledgements Acknowledgements Resource