Cosmologists attempt to understand the origin and structure of the universe as a whole. They begin their search with an assumption about the nature of the universe—namely, that in looking out from our vantage point in the cosmos, we see essentially the same kind of universe that an observer stationed in any other part of it, no matter how remote, would see. As far as our telescopes can reach, we see galaxies and clusters of galaxies distributed in more or less the same way in every direction. This assumption that the universe is uniform on a large scale is called “the cosmological principle.”


One thing that is certain is that the universe is expanding. In every direction we look, distant galaxies are moving away from each other. Until the 1960s, the expansion of the universe was the primary fact of cosmological significance that cosmological theories had to accommodate. There were two general classes of cosmological theories that fit with the expanding universe: the evolutionary (Big Bang) theory and the steady-state theory.


The essential idea of the evolutionary cosmology is that there was a beginning—a moment of creation at which the universe came into existence in a hot, violent explosion—the Big Bang. In the beginning, the universe was very hot, very dense, and very tiny. As the explosion evolved, the temperature dropped, the distribution of matter and energy thinned, and the universe expanded. From the current observed rate of expansion, we conclude that the creation event occurred between ten and twenty billion years ago.


The steady-state theory is based on an idea called the “perfect cosmological principle.” It is “perfect” in that it maintains that the universe is uniform not only in space but in time. Thus it is the hypothesis that the large-scale universe has always been the way it is now and will be this way forever in the future. This view is consistent with philosophical approaches that reject the notion of an absolute beginning of the universe as unacceptable. The steady-state universe would have no beginning and no end.


In an expanding universe, the galaxies move away from each other, spreading matter more thinly over space. On the other hand, the perfect cosmological principle requires that the density of matter in the universe remain constant over time. To make the steady-state theory compatible with the expanding universe, its proponents introduced the notion of continuous creation. As the universe expands and the galaxies move farther apart, new matter—in the form of hydrogen—is introduced into the universe. The rate at which the hypothesized new matter is created is far too small for this creation to be detected with available instruments, but continuous creation provides just enough matter to form new stars and galaxies that fill in the space left by the old ones. Thus in the steady-state universe there is evolution of stars and galaxies, but the general character and the overall density of the universe remains unchanged over time. In this special sense, the steady-state universe itself does not evolve.


Both of these views—steady-state and Bing Bang—allow for cosmic expansion. However, the discovery in the 1960s of a comparatively small star-like objects called quasars tipped the scales in favor of the Big Bang cosmology. Astronomers determined that almost all quasars are very distant.


Given how bright quasars appear even at such great distances, astronomers concluded that quasars typically have an output of light that is 1,000 times greater than that of a whole spiral galaxy composed of billions of stars.


Quasars are such distant objects that the light now reaching us from quasars left them billions of years ago. This means that when we observe quasars today we are seeing that state of the universe billions of years ago. Thus the fact that almost all quasars are very far away implies that earlier in the history of the universe quasars were developing more frequently than they are now. This evolution is consistent with the Big Bang theory. But it violates the perfect cosmological principle, and so it is inconsistent with the steady-state view.

宇宙学家试图了解整个宇宙的起源和结构。他们开始寻找宇宙的本质，即我们从宇宙的有利位置向外看，我们看到的基本上就是，无论站在哪里观看，无论多远，我们都能看到同样的宇宙。就我们的望远镜可以看到的范围而言，我们看到星系和星系团各个方向上或多或少地分布着。这种宇宙在很大程度上是统一的假设被称为“宇宙学原理”。 有一点可以肯定的是宇宙正在扩张。在我们看到的每个方向上，遥远的星系正在远离着彼此。直到20世纪60年代，宇宙的膨胀才是宇宙学理论必须适应的主要事实。宇宙学理论有两大类与不断膨胀的宇宙相适应：进化论（大爆炸）理论和稳态理论。 演化宇宙论的基本观点是，有一个开端——宇宙诞生的那一刻，在一场激烈的、猛烈的爆炸中，宇宙诞生了—大爆炸。一开始，宇宙非常热，密度非常大，而且很小。随着爆炸的演变，温度下降，物质和能量分布变薄，宇宙开始膨胀。根据目前观察到的扩张速度，我们得出结论，创造事件发生在100亿年至200亿年前。 稳态理论是建立在一个称为“完美宇宙论原则”的理论基础上的。它是“完美的”，因为坚持认为宇宙不仅在空间上，而且在时间上是一致的。因此，这是一个假设，即大宇宙一直都是它现在的样子，将来也会永远如此。这种观点与拒绝将宇宙绝对开端的观念视为不可接受的哲学方法是一致的。稳定状态的宇宙没有起点也没有终点。 在膨胀的宇宙中，星系彼此远离，在空间上更稀薄地传播物质。另一方面，完美的宇宙学原理要求宇宙中物质的密度随时间而保持不变。为了使稳态理论与膨胀的宇宙相适应，其支持者引入了连续创造的概念。随着宇宙的膨胀和星系之间的距离不断扩大，新的物质——以氢的形式——被引入到宇宙中。假设新事物产生的速率太小，无法用现有仪器检测，但连续创造物提供了足够的物质来形成新的恒星和星系，以填充旧星留下的剩余空间。因此，在稳定的宇宙中，恒星和星系的演化是存在的，但是宇宙的整体特征和整体密度随时间而变化。在这个特殊的意义上，稳态宇宙本身并没有进化。 这两种观点 - 稳态和爆炸- 允许宇宙膨胀。然而，20世纪60年代一种被称为类星体的相对较小的恒星状物体的发现，使天平倾向于大爆炸宇宙学。天文学家认为几乎所有类星体都是非常遥远的。 考虑到在如此遥远的距离内，如何出现类星体，天文学家得出结论认为，类星体的光输出量通常是由数十亿颗恒星组成的整个螺旋星系的1000倍。 类星体是如此遥远的物体，而类星体的光早在数十亿年前就已到达我们地球。这意味着，当我们今天观察类星体时，我们其实已经看到数十亿年前的宇宙状态。因此，几乎所有类星体都离我们很遥远，这就意味着，在宇宙的早期，类星体的发展比现在要频繁得多。这种演变与大爆炸理论是一致的。但它违背了完美的宇宙学原理，因此它与稳态观点不一致。