Smeenk starts out with posing two major problems in cosmology. The first one is the flatness problem, which states that the Hubble constant needs to be fine-tuned with extraordinary accuracy. The second problem is the horizon problem: the universe is very homogeneous, implying that different areas have been in thermal contact with each other, although it seems like these areas have been causally disconnected. The theory of inflation, that can explain these issues, was proposed by Guth in 1981 to highlight these problems. However, it turned out that inflation was a powerful theory in predicting the structure of the universe.
This is the first topic Smeenk discusses: how quantum fluctuations were “stretched out” to classical density perturbations by inflation. However, structures could also have arisen from topological defects in the early universe. Smeenk continues with a discussion of these, before concluding with empirical evidence for the topological defects theory.