OBSERVATIONAL evidence strongly supports the hypothesis of an expanding and cooling universe. General Relativity tells us that an expanding universe had a definite beginning a finite time into the past --- the famous Big Bang. At that instant, Thermal Physics says that the universe had an infinite average temperature that has been decreasing since. Given average kinetic energy increases with temperature, it follows that particle collisions were highly energetic in the universe shortly post Big Bang. Thus do Particle Physics and Cosmology meet in the arena of the Early Universe!
The Cosmic Microwave Background (CMB) consists of relic photons that decoupled from matter about a million years after the Big Bang. Information on the state of the universe at this instant should therefore be encoded in these photons. The COBE satellite has begun mapping the CMB pattern across the sky. Two other satellites, MAP and PLANCK, should provide more information about the CMB in several years' time, which will help answer some fundamental questions about the role of Particle Physics in the Early Universe: What is the nature of the "dark matter" and its role in the development of large-scale structures such as galaxy clusters? Is the Cosmological Constant zero? How much baryonic matter is there in the universe?
Going back further in time to about one second post Big Bang, we arrive at the epoch of Big Bang Nucleosynthesis (BBN), when isotopes of hydrogen, helium and lithium are synthesised. Neutrinos play a crucial role in this process. Firstly, relativistic neutrinos contribute to the pressure driving the expansion of the universe. The precise expansion rate is pivotal in determining the light element abundances. Secondly, the relative electron neutrino/antineutrino concentrations affect the nuclear reaction rates involved in elemental syntheses. Neutrino masses and oscillations (see Neutrino Physics article) may thus strongly impact on BBN.
Stepping back again, we reach the hypothetical epoch of the "Electroweak Phase Transition" (EWPT). Envisage the pre-EWPT universe as a blob of water. As water is heated through boiling point, bubbles of steam form and expand. Before the EWPT, like electromagnetism, the weak interactions had an infinite range. During the transition, bubbles of a new phase began to form and expand where the weak interactions suddenly became the familiar short-ranged forces. It turns out that, for reasons too numerous to explain here, the extraordinary conditions that presumably existed during EWPT may have nurtured the gross matter-antimatter asymmetry we now observe in the universe. CP Violation is a crucial factor here (see B Physics and CP Violation article).
Closer still to the moment of Creation... well, the story is uncertain. It may contain yet more phase transitions where exotic objects like Cosmic Strings and Magnetic Monopoles formed. Perhaps an "inflationary period" occurred, during which the universe expanded exponentially, possibly explaining the size and homogeneity of the universe today. The Very Early Universe may have left an imprint in gravitational waves, whose detection is a major goal of science. Such is the synergy between the Physics of the Very Small and of the Very Large!
Tip: Check out the Encyclopedia of Cosmology.