Note to the third reprint

In this printing, a number of misprints have been corrected. These will inevitably not be the last, and I thank those readers who are kind enough to keep reporting them. I have also taken the opportunity of bringing the contents up to date in a few places, specifically new results on the supernova Hubble diagram (Fig. 5.4) and CMB anisotropies (Fig. 18.2).

Given the pace of cosmological research, I am surprised, but pleased, to see that the basic framework described in the original text survives without the need for revolutionary change. Nevertheless, some very significant developments have occurred since the first printing. Here is a personal list of recent highlights:

(1) Results on atmospheric neutrinos show that the $\mu$ neutrino oscillates, probably to a $\tau$ neutrino. If so, the $\tau$ neutrino mass is $\sim 0.06$~eV, and hot dark matter is unimportant (hep-ex/9912007).

(2) The supernova Hubble diagram now argues very strongly for vacuum energy, and an accelerating expansion (see the new Fig. 5.4 and astro-ph/0005229).

(3) Small-scale CMB data show a clear first peak in the power spectrum, requiring the universe to be very close to spatially flat. Combined with the supernova results, a matter density parameter of $\Omega \simeq 0.35$ is indicated (see the new Fig. 18.2 and astro-ph/0007333).

(4) If vacuum energy is a reality, a cosmological constant presents a fine-tuning problem. `Quintessence' models attempt to solve this by using scalar-field dynamics to `switch on' negative pressure automatically at matter-radiation equality (astro-ph/9901388).

(5) Given that the $\Lambda$CDM model works so well on large scales, it is essential to understand the discrepancy with the form of galaxy correlations. Galaxy-formation models are now starting to pinpoint the origin of the required scale-dependent bias (astro-ph/9910488).

(6) Nevertheless, worries about the basic CDM paradigm persist, and are most severe on small scales, where the structure of galaxy-scale dark matter haloes appears not to match observations (astro-ph/9901240 and astro-ph/9907411). The nature of dark matter continues to be perhaps the greatest uncertainty in cosmology.