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We examine the possibility that a significant component of the energy density of the universe has an equation-of-state different from that of matter, radiation or cosmological constant (Λ). An example is a cosmic scalar field evolving in a potential, but our treatment is more general. Including this component alters cosmic evolution in a way that fits current observations well. Unlike Λ, it evolves dynamically and develops fluctuations, leaving a distinctive imprint on the microwave background anisotropy and mass power spectrum. PACS number(s): 95.35.+d,98.70.Vc,98.65.Dx,98.80.Cq Inflationary cosmology predicts that the universe is spatially flat and that the total energy density of the universe is equal to the critical density. This prediction is consistent with current measurements of the cosmic microwave background (CMB) anisotropy and may be verified with high precision in the next generation of CMB satellite experiments. At the same time, there is growing observational evidence that the total matter density of the universe is significantly less than the critical density. 1 If this latter result holds and the CMB anisotropy establishes that the universe is flat, then there must be another contribution to the energy density of the universe. One candidate that is often considered is a cosmological constant, Λ, or vacuum energy density. The vacuum density is a spatially uniform, time-independent component. Cold dark matter models with a substantial cosmological constant (ΛCDM) are among the models which best fit existing observational data. 1 However, it should be emphasized that the fit depends primarily on the fact that the models have low matter density and are spatially flat; the fit is not a sensitive test of whether the additional energy contribution is vacuum energy.In this paper, we consider replacing Λ with a dynamical, time-dependent and spatially inhomogeneous component whose equation-of-state is different from baryons, neutrinos, dark matter, or radiation. The equation-ofstate of the new component, denoted as w, is the ratio of its pressure to its energy density. This fifth contribution to the cosmic energy density, referred to here as "quintessence" or Q-component, is broadly defined, allowing a spectrum of possibilities including an equationof-state which is constant, uniformly evolving or oscillatory. Examples of a Q-component are fundamental fields (scalar, vector, or tensor) or macroscopic objects, such as a network of light, tangled cosmic strings. 2 The analysis in the present paper applies to any component whose hydrodynamic properties can be mimicked by a scalar field evolving in a potential which couples to matter only through gravitation. In particular, we focus on equations-of-state with −1 < w < 0 because this range fits current cosmological observations best. [3][4][5][6][7] This has motivated several investigations 3,4,8,9 of components with w < 0 in which a spatially uniform distribution has been assumed, e.g., a decaying Λ or smooth component.In this Letter, we begin by arguing t...

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Recent observations suggest that a large fraction of the energy density of the universe has negative pressure. One explanation is vacuum energy density; another is quintessence in the form of a scalar field slowly evolving down a potential. In either case, a key problem is to explain why the energy density nearly coincides with the matter density today. The densities decrease at different rates as the universe expands, so coincidence today appears to require that their ratio be set to a specific, infinitessimal value in the early universe. In this paper, we introduce the notion of a "tracker field," a form of quintessence, and show how it may explain the coincidence, adding new motivation for the quintessence scenario.A number of recent observations suggest that Ω m , the ratio of the (baryonic plus dark) matter density to the critical density, is significantly less than unity.1 Either the universe is open, or there is some additional energy density ρ sufficient to reach Ω total = 1, as predicted by inflation. Measurements of the cosmic microwave background, the mass power spectrum, 1-3 and, most explicitly, the luminosity-red shift relation observed for Type Ia supernovae, 4 all suggest that the missing energy should possess negative pressure (p) and equationof-state (w ≡ p/ρ). One candidate for the missing energy is vacuum energy density or cosmological constant, Λ for which w = −1. The resulting cosmological model, ΛCDM, consists of a mixture of vacuum energy and cold dark matter. Another possibility is QCDM cosmologies based on a mixture of cold dark matter and quintessence (−1 < w ≤ 0), a slowly-varying, spatially inhomogeneous component.7 An example of quintessence is the energy associated with a scalar field (Q) slowly evolving down its potential V (Q).5-8 Slow evolution is needed to obtain negative pressure, p = 1 2Q 2 − V (Q), so that the kinetic energy density is less than the potential energy density.Two difficulties arise from all of these scenarios. The first is the fine-tuning problem: Why is the missing energy density today so small compared to typical particle physics scales? If Ω m ∼ 0.3 today the missing energy density is of order 10 −47 GeV 4 , which appears to require the introduction of new mass scale 14 or so orders of magnitude smaller than the electroweak scale. A second difficulty is the "cosmic coincidence" problem:9 Since the missing energy density and the matter density decrease at different rates as the universe expands, it appears that their ratio must be set to a specific, infinitessimal value in the very early universe in order for the two densities to nearly coincide today, some 15 billion years later.What seems most ideal is a model in which the energy density in the Q-component is comparable to the radiation density (to within a few order of magnitude) at the end of inflation, say. If there were some rough equipartition of energy following reheating among several thousands of degrees of freedom, one might expect the energy density of the Q-component to be two or so orders of ma...