Density Parameter, Ω

The galaxies we see in all directions are moving away from the Earth, as evidenced by their red shifts. Hubble's law describes this expansion. Remarkably, study of the expansion rate has shown that the universe is very close to the critical density that would cause it to expand forever. It is customary to express the density as a fraction of the density required for the critical condition with the parameter Ω = ρ/ρ_critical so that Ω = 1 represents the condition of critical density.

The expansion of the universe has been studied in a number of different ways, but the WMAP mission completed in 2003 represents a major step in precision and the results quoted here will be mainly those from WMAP. 宇宙的膨胀已通过多种不同的方法进行了研究，但2003年完成的WMAP任务标志着精度的一个重大进步，此处引用的结果主要来自WMAP。

	The mass density at the current time was assessed by WMAP to be Ωm,0 = 0.27 +/- 0.04. But the assessment of ordinary or baryonic matter was only 0.044 +/- 0.004, so baryonic matter constitutes ony 17% of the matter of the universe, the remainder being classified as "dark matter". 目前时间的物质密度据WMAP评估为Ωm,0 = 0.27 ± 0.04。但普通物质或重子物质的评估仅为0.044 ± 0.004，因此重子物质仅占宇宙物质的17%，其余部分被归类为“暗物质”。
	The equivalent mass density of the relativistic particles, made up of electromagnetic energy and neutrinos, is assessed by WMAP to be Ωrel,0 = 8.24 x 10-5. This implies that in the comparison with mass, the mass is strongly dominant over radiation in the current era. At early times in the universe, radiation was dominant over both mass and dark energy. 相对粒子的等效质量密度，由电磁能和中微子组成，据WMAP测定为Ω rel,0 = 8.24 x 10 -5 。这表明在当前时代，质量在与辐射的比较中占据主导地位。在宇宙早期，辐射在质量和暗能量之上都占据主导地位。
	The fraction of the effective mass of the universe attributed to "dark energy" or the cosmological constant is ΩΛ,0 = 0.73 +/- 0.04. With 73% of the influence on the expansion of the universe in this era, the dark energy is viewed as the dominant influence on that expansion. The previous history of the big bang is viewed as being at first radiation dominated, then matter dominated, and now having passed into the era where dark energy is the dominant influence. 宇宙中归因于‘暗能量’或宇宙常数的有效质量分数是 Ω Λ,0 = 0.73 ± 0.04。在这一时期，暗能量对宇宙膨胀的贡献占73%，因此暗能量被视为对膨胀的主要影响因素。大爆炸之前的史观则首先是以辐射为主导，随后以物质为主导，现在已进入暗能量成为主导影响的时期。

The sum of the contributions to the total density paramater Ω₀ at the current time is