The ``hot'' in the hot big-bang cosmology makes fundamental physics an inseparable part of the standard cosmology. The time - temperature relation, kBT ~ 1 MeV (t/sec )-1/2, implies that the physics of higher energies and shorter times is required to understand the Universe at earlier times: atomic physics at t 1013 sec, nuclear physics at t ~ 1 sec, and elementary-particle physics at t < 10-5 sec. The standard cosmology model itself is based upon Einstein's general relativity, which embodies our deepest and most accurate understanding of gravity.
The standard model of particle physics, which is a mathematical description of the strong, weak and electromagnetic interactions based upon the SU(3) SU(2) U(1) gauge theory, accounts for all known physics up to energies of about 300 GeV (Gaillard et al. 1999). It provides the input microphysics for the standard cosmology necessary to discuss events as early as 10-11 sec. It also provides a firm foundation for speculations about the Universe at even earlier times.
A key feature of the standard model of particle physics is asymptotic freedom: at high energies and short distances, the interactions between the fundamental constituents of matter - quarks and leptons - are perturbatively weak. This justifies approximating the early Universe as hot gas of noninteracting particles (dilute gas approximation) and opens the door to sensibly speculating about times as early as 10-43 sec, when the framework of general relativity becomes suspect, since quantum corrections to this classical description are expected to become important.
The importance of asymptotic freedom for early-Universe cosmology cannot be overstated. A little more than twenty-five years ago, before the advent of quarks and leptons and asymptotic freedom, cosmology hit a brick wall at 10-5 sec because extrapolation to early times was nonsensical. The problem was twofold: the finite size of nucleons and related particles and the exponential rise in the number of ``elementary particles'' with mass. At around 10-5 sec, nucleons would be overlapping, and with no understanding of the strong forces between them, together with the the exponentially rising spectrum of particles, thermodynamics became ill-defined at higher temperatures.
The standard model of particle physics has provided particle physicists with a reasonable foundation for speculating about physics at even shorter distances and higher energies. Their speculations have significant cosmological implications, and - conversely - cosmology holds the promise to test some of their speculations. The most promising particle physics ideas (see e.g., Schwarz & Seiberg 1999) and their cosmological implications are:
Spontaneous Symmetry Breaking (SSB). A key idea, which is not fully tested, is that most of the underlying symmetry in a theory can be hidden because the vacuum state does not respect the full symmetry; this is known as spontaneous symmetry breaking and accounts for the carriers of the weak force, the W± and Z0 bosons, being very massive. (Spontaneous symmetry breaking is seen in many systems, e.g., a ferromagnet at low temperatures: it is energetically favorable for the spins to align thereby breaking rotational symmetry.) In analogy to symmetry breaking in a ferromagnet, spontaneously broken symmetries are restored at high temperatures. Thus, it is likely that the Universe underwent a phase transition at around 10-11 sec when the symmetry of the electroweak theory was broken, SU(2) U(1) -> U(1).
Grand unification. It is possible to unify the strong, weak, and electromagnetic interactions by a larger gauge group, e.g., SU(5), SO(10), or E8. The advantages are twofold: the three forces are described as different aspects of a more fundamental force with a single coupling constant, and the quarks and leptons are unified as they are placed in the same particle multiplets. If true, this would imply another stage of spontaneous symmetry breaking, G -> SU(3) SU(2) U(1). In addition, grand unified theories (or GUTs) predict that baryon and lepton number are violated - so that the proton is unstable and neutrinos have mass - and that stable topological defects associated with SSB may exist, e.g., point-like defects called magnetic monopoles, one-dimensional defects referred to as ``cosmic'' strings, and and two-dimensional defects called domain walls. The cosmological implications of GUTs are manifold: neutrinos as a dark matter component, baryon and lepton number violation explaining the matter - antimatter asymmetry of the Universe, and SSB phase transitions producing topological defects that seed structure formation or a burst of tremendous expansion called inflation.
Supersymmetry. In an attempt to put bosons and fermions on the same footing, as well as to better understand the `hierarchy problem,' namely, the large gap between the weak scale (300 GeV) and the Planck scale (1019 GeV), particle theorists have postulated supersymmetry, the symmetry between fermions and bosons. (Supersymmetry also appears to have a role to play in understanding gravity.) Since the fundamental particles of the standard model of particle physics cannot be classified as fermion - boson pairs, if correct, supersymmetry implies the existence of a superpartner for every known particle, with a typical mass of order 300 GeV. The lightest of these superpartners, is usually stable and called ``the neutralino.'' The neutralino is an ideal dark matter candidate.
Superstrings, supergravity, and M-theory. The unification of gravity with the other forces of nature has long been the holy grail of theorists. Over the past two decades there have been some significant advances: supergravity, an 11-dimensional version of general relativity with supersymmetry, which unifies gravity with the other forces; superstrings, a ten-dimensional theory of relativistic strings, which unifies gravity with the other forces in a self-consistent, finite theory; and M-theory, an ill-understood, ``larger'' theory that encompasses both superstring theory and supergravity theory. An obvious cosmological implication is the existence of additional spatial dimensions, which today must be ``curled up'' to escape notice, as well as the possibility of sensibly describing cosmology at times earlier than the Planck time.
Advances in fundamental physics have been crucial to advancing cosmology: e.g., general relativity led to the first self-consistent cosmological models; from nuclear physics came big-bang nucleosynthesis; and so on. The connection between fundamental physics and cosmology seems even stronger today and makes realistic the hope that much more of the evolution of the Universe will be explained by fundamental theory, rather than ad hoc theory that dominated cosmology before the 1980s. Indeed, the most promising paradigm for extending the standard cosmology, inflation + cold dark matter, is deeply rooted in elementary particle physics.