A hundred years ago, physicists and astronomers thought that the Universe was infinitely old. For the Universe to have come into being at some point in the past seemed just too human, more like mythology than science. Instead, the general belief was that the Universe on large scales was static, staying more or less the same over time.
The discovery eighty years ago that the Universe is expanding, destroyed that static image. And about forty years ago, the discovery of the "echo" or "afterglow" of the Big Bang in the form of microwave background radiation clinched any remaining controversy: it was clear that our Universe had a beginning. For a long time, the best estimates for the age of the Universe were in the ten to twenty billion years ball park. Cosmology, the study of the Universe at the largest scales in space and time, had never been a particularly exact science.
All that has changed in the last ten years, when new amazingly accurate observations inaugurated what is now called the era of `precision cosmology' -- almost a contradiction in terms, for those who studied astronomy in the twentieth century. For example, we now know that the Universe is 13.7 billion years old, with an uncertainty of only about one percent. In fact, astronomers know the age of the Universe too a better degree of accuracy than they know the ages of most of their friends! For more background, on precision cosmology, see the NASA web site "http://map.gsfc.nasa.gov/m_uni/uni_101ouruni.html".
All nice and fine, you might say, to have such an accurate number, 13,700,000,000 years, give or take 100,000,000 or so years, but it is hard to imagine how large or small that is. How can you wrap your mind around such a huge number? The best way to do so, is to make a connection between everyday life and the age of the Universe through a number of steps. A natural step is an increase by a factor of sixty, an invention of the Babylonians: we are all familiar with an hour containing 60 minutes, and a minute containing 60 seconds.
If we start with a second, the first such Babylonian step brings us to a minute, the second step to an hour, and the third step to 60 hours, or two and half days, about the time that you can reasonably stay awake. Compared to steps of a factor 60, let us not worry about making errors of small addition factors of 3/2 or 2/3, so that we can choose some durations that are easy to remember. So when the fourth step brings us to half a year, we can consider that to be roughly comparable with the duration of a human pregnancy. The fifth step brings us to 30 years, the length of a human generation.
Step 6 leads
us to about 2,000 years,
roughly the length of time that humans have used writing. Step 7 goes
years back, the time that Neanderthals were roaming through
So that's all, ten steps! Not as much as you might think, and indeed, a lot less than all the guesses made by the avatars in Ed's Cafe (at Videoranch), who understandably made guesses ranging from 15 to 60 steps. Interestingly, there are as many seconds in a human generation as there would be human generations in the life of the Universe if humans had been around that whole time.
To look at it from another angle, the first eight steps beyond a second all involve human references points, and even the ninth step involves life and death on Earth.
In a linear sense, a human lifetime of 100 years or less is a very small fraction of the age of the Universe, less than one millionth of one percent. But on a logarithmic scale, as we have used here, a single human life goes already more than halfway, from a second toward the age of the Universe, with the midpoint, five steps, being at the duration of a generation. Your parents were born earlier than you were, by an amount of time that is in between that of a heart beat and the age of the universe, in terms of steps from seconds to minutes to hours to (3) to (4) to (5) to (6) to (7) to (8) to (9) to back to the Big Bang.