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At night, I like to gaze at the stars in the sky. Far from the city and at the right time of the year, when your eyes get used to the darkness you see the core of the Milky Way appear majestically. The heavens are a phenomenon that has always intrigued mankind.
Allegedly, primeval people seem to have thought that all those distant lights were campfires from distant tribes. Early civilizations explained stars as holes in a vast spherical firmament, which supposedly shielded the Earth at its center from an outside eternal light. This idea is understandable, since stars do not appear to move relative to each other, and to the naked eye the starry sky appears to have no depth.
It is a perception that persisted until in the late eighteenth century William Herschel, after endlessly staring through a telescope, first proposed the spatial positions of many stars. What did he discover? Stars were not lying on a sphere, but rather floating within one! The realization dawned that all stars were part of a single structure, the Milky Way. Even a young Albert Einstein was taught that the Milky Way was the entire universe.
Distances between stars are unimaginably vast. For example, our Sun is an average star. Its nearest neighbours are Alpha Centauri A and B, two stars roughly the same size as the Sun, and of course red dwarf Proxima Centauri. If the Sun were a soccer ball, then the Earth would be a grain of sand about 25 meters away. Now if this miniature Sun were in Amsterdam, Alpha Centauri A and B would be two soccer balls somewhere in … Detroit. The Milky Way is not just unfathomably large, it is also incredibly empty.
Edwin Hubble discovered about a hundred years ago that several nebulae supposedly in the Milky Way, actually are other galaxies far beyond our own star system. In the decades that followed, the universe was found to be teeming with galaxies. Latest estimates are that there can be as many as two trillion of them. Our Milky Way, a typical galaxy, is estimated to consist of upto four hundred billion stars. In other words, for every grain of sand on Earth, there are perhaps tens of thousands of stars out there. Let that sink in for a moment.
Now you are probably wondering why, if there are that many stars, it is still dark at night? This is an enigma that is called Olbers' paradox, but actually is much older. Scientists and philosophers, assuming that the universe were static and eternal, could not explain why the night sky is mostly dark. The modern answer follows from the fact that these assumptions are incorrect.
You see, the cosmos is not eternal, but had a beginning in time, the Big Bang. Hubble's work shows that the universe is not static, but continuously stretching since it came into existence. Space stretches everywhere at the same rate, and one can then demonstrate that the rate of stretching is a function of distance - the further away an object is from us, the faster it appears to move away.
People tend to not understand this. How could everything fly away from us? This is not the case however, but an effect of expanding space. Imagine observing a still object at astronomical distance, then it would appear to move away from you. If you could move by half a circle around that object, to the position opposite but keeping a same distance, it would not come towards you as you would expect, but move away from you as well in the direction of where you were first. Another concept you may want to contemplate for a bit.
This elongation of space also lengthens light waves, so what we observe is that the more remote an object is, the more its colour is red-shifted when it reaches us. The furthest objects can only be observed in infrared, which our eyes cannot see, and that is one explanation for the dark night.
In fact, the sky actually is glowing uniformly with the earliest light in the universe. The source is now so far away however, that this light is stretched to invisible cosmic background radiation. If you happen to have a vintage analog television set, take a look at the snow on the screen when the receiver is not tuned - a tiny part of that noise comes from this cosmic background. This way you can get a glimpse of the oldest light in the universe, after all.
Next to red-shifting, there is another explanation for invisible stars. The velocity of light is constant everywhere in the expanding cosmos. This causes a real horizon, since very distant light has to swim against a too strong current so to speak, and could never reach us. Red-shifting makes objects invisible to our eyes though could be detected by instruments, but this horizon leaves objects beyond it truly out of sight.
With that, we know of two reasons why the distant parts of the cosmos are forever invisible to us, so we understand when looking up, that visible stars twinkle against a background as black as midnight.
Top picture: The Sun, captured by NASA's Solar Dynamics Observatory (source: NASA).
