The Age of the Universe

How old is the universe? This provocative question will be addressed only by describing the models we have of the expanding universe and the Big Bang process. If we could take the presently observed expansion and project backward to the beginning of that process, then it would seem reasonable to call that the age of the universe.

宇宙有多大？这个问题只有通过描述我们对膨胀宇宙和大爆炸过程的模型才能回答。如果我们能将目前观测到的膨胀过程倒推回其开始，那么似乎合理地将那个时间点称为宇宙的年龄。

If we were dealing with a simple expansion as suggested at left in the illustration, and could measure the expansion as a function of time, then it might be straightforward to calculate the time of origin of the expansion. But our time span of observation is so brief compared to the process as a whole that we must draw our conclusions from an almost instantaneous view. We can see clear evidence of expansion from the Doppler red shift, and the fact that more distant stars are redshifted more. The Hubble law and the value of the Hubble constant give us a parameter that we can translate into an expansion time but there are other factors involved. Because of the uncertainties in distance measurement to other galaxies, there have been considerable uncertainties about the value of the Hubble constant.

如果我们正在处理一种简单的膨胀，如图左侧所建议的那样，并且能够将膨胀作为时间函数进行测量，那么计算膨胀的起源时间可能是直接的。但是，我们的观测时间跨度与整个过程相比实在太短了，因此我们必须从几乎瞬间的视角中得出结论。我们可以从多普勒红移现象中清楚地看到膨胀的证据，并且更远的恒星表现出更大的红移。哈勃定律和哈勃常数的值给我们提供了一个参数，可以转换为膨胀时间，但还有其他因素需要考虑。由于对其他星系距离的测量存在不确定性，哈勃常数的值也存在相当大的不确定性。

Recent measurements of the cosmic background radiation by the COBE and WMAP satellites have provided values of the Hubble constant which are much more precise.

COBE和WMAP卫星对宇宙背景辐射的最新测量结果提供了更为精确的哈勃常数值。

Even with the a known value for the Hubble constant, the value projected for the age of the universe is model dependent. Much evidence suggests that the universe is very nearly "flat", i.e., it is expanding at just about the right rate to expand indefinitely. Perfectly flat would mean that it would asymptotically approach a zero expansion rate, and not collapse back. A flat universe, termed an "Einstein-de Sitter universe" requires a critical mass density. We presume that we have very close to that critical mass density, but it is much more than the mass density we see, hence the presumption of "dark matter" to make up the bulk of the critical mass.

即使已知哈勃常数的值，宇宙年龄的预测值仍取决于模型。许多证据表明，宇宙几乎“平坦”，即它以几乎正确的速率膨胀，从而无限膨胀。完全平坦意味着它会渐近接近零膨胀率，而不会坍缩回。被称为“爱因斯坦-德西特宇宙”的平坦宇宙需要临界质量密度。我们假设我们非常接近这种临界质量密度，但实际观测到的质量密度远低于此，因此假设有“暗物质”来填补临界质量的大部分。

With the presumption of a flat universe, we can project an expansion time by making use of the current cosmic background temperature and the critical mass density of the universe. For a Hubble constant of 71 km/s/Mpc the critical density is 5.33 GeV/m³. When this is substituted into the equation for expansion time you get 13.77 x 10⁹ years.

假定宇宙是平坦的，我们可以利用当前宇宙背景温度和宇宙的临界质量密度来计算膨胀时间。对于哈勃常数为71 km/s/Mpc的情况，临界密度为5.33 GeV/m³。当这个值代入膨胀时间的方程中时，得到13.77 x 10⁹年。