As you look out into a dark night sky, you can see incredible distances. You can also see incredible spans of time back into the past. That’s because in the vast emptiness of the cosmos, light itself takes measurable time to travel from place to place. For instance, sunlight takes about eight minutes to reach Earth. The image of the sun that you see in the sky shows you where the sun actually was eight minutes earlier. When you look at the western sky just before sunset, you can therefore still see the sun even after it has “really” set (see figure).
Looking out to greater distances involves ever-greater time lags, offering us glimpses into the more and more remote past. The basic unit of astronomical distance, in fact, is called the light year, which is the distance that light travels in one year. A light year is approximately 10 trillion kilometers. That is an incomprehensible distance to us humans, but it is very modest on the grand scale of things. Outer space is almost completely empty within a few light years of our solar system.With telescopes and other modern instruments, astronomers can peer much farther than the naked eye. If you pay attention to astronomical discoveries, you will hear stories about supernovae hundreds of millions of light years away, gamma ray bursts a billion years old, or archaic galaxies and quasars over 10 billion years old! Some of these are extinct astronomical structures that have not even existed for billions of years. Their ghosts are strangely imprinted on our sky.
Yet that’s where the time-travel ends. You can stargaze all you want, but you will never find a galaxy a trillion years old, or 100 billion, or even 15 billion. Nothing is that old, not even light itself. The oldest light ever observed is a little less than 14 billion years old. (About 13.8 billion years, to be as precise as measurements allow). 1
The universe is expanding, with all regions of space receding away from each other simultaneously. If we ran time backward and watched space contract in reverse, we would see all the galaxies rushing toward each other, closer and closer, until the whole universe was compacted into the same place at the same time. This was the moment that the universe, as we know it, began. All that is and ever was can be traced back to this beginning point. The 14 billion year age is a 20th-century discovery; a complete history of the universe was not even possible until recent decades.
- European Space Agency press release, 3/20/13, http://www.esa.int/For_Media/Press_Releases/Planck_reveals_an_almost_perfect_Universe, accessed 9/05/13. ↩
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