To make a long story short
The history of the universe in one calendar year
Author’s note: this week I thought I’d share an adapted and revised short piece from a now-defunct pandemic project. The project, which I called the Big Numbers Blog, was an effort to help improve the ways that we think about big numbers. The premise was that the best way to understand a big number is to turn it to a small number — something manageable. It’s hard for me to have a sense of $1000, but I can think about something equivalent, say one decent laptop. I can’t picture $10,000 but I can picture one used car in reasonable shape. Astronomy — studying the vast expanses of our universe — demands we get comfortable with big numbers. So today, a tool for thinking about one particular very large astronomical number: 13.8 billion years.
One very useful, if slightly tired, way to think about the history of our universe is to use the analogy of a calendar. The idea is to take the 13.8 billion years astronomers think the universe has existed, and shrink it down to one year. Imaging the start of the universe, that blindingly hot instant of the Big Bang, occurring with the resounding explosion of confetti that heralds January 1: Happy New Year! or, in this case, New Universe. Now imagine the events of the universe unfolding throughout the year, in exactly the same sequence, squeezed down so that right now this instant the clock is ticking midnight, December 31.
It’s a dramatic scaling: on this scale, each month represents just over a billion years, each day 38 million years, and each second 438 years. So, on a “cosmic calendar,” one second ago no telescope had ever been pointed at the sky, people in western Europe were arguing about whether or not the earth goes around the sun or vice versa, and Pope Gregory was just in the process of standardizing the calendar we use today. Jesus was walking earth just under 5 seconds ago, and all recorded human history happens in its last 20 seconds.
Indeed, to think of the universe unfolding in a calendar year, to turn an impossibly long time to a timescale I’ve lived, is a powerful way of grasping the dramatic story our telescopes have begun to untangle from the past. Carl Sagan famously did this in his production Cosmos and book The Dragons of Eden. Neil DeGrasse Tyson and Ann Druyan kept the device for several important scenes in updated version of Cosmos, like this clip here. Every time I hear it I get a rush: an exhilarating thrill, as if flying. We soar past the formation of the first stars mere days into the year, the birth of our galaxy, not long after. We careen past generations of stars, black holes, and nebulae until early September, when our earth precipitates into existence, and rocket through the emergence of plants, microbes, multi-cellular life, not slowing for the reign of the dinosaurs in early December. It’s December 30 — the second to last day of the year — when primates finally appear, and after 11:50pm on December 31 that homo sapiens enter the picture. We ask to slow, but time it will not, and before we know it it’s midnight again, with me sitting typing characters I hope stand for something that matters.
But more than just taking us on grand, sweeping journeys, the cosmic calendar also helps us better understand how long different astronomical processes take. For example, consider some of the crazy differences in things like the lives of stars. The shortest lived stars last only a couple million years, just a couple of hours on the cosmic calendar. In contrast, we think the entire lifetime of our sun will be about 9 cosmic calendar months (and the longest lived stars we expect to last hundreds of cosmic calendar years into the future!).
Lots of other astronomical timescales are better understood this way. For example:
The time it takes to build a planet like earth (~15 million years) is just under half a day on the cosmic calendar (and occurred, for earth, about 8 months after the big bang, or 4 months before the present).
One orbit of the sun around the center of the Milky Way takes about 6 days, just shy of a week.
Mergers of galaxies can take about a month on the cosmic calendar (and the Milky Way will collide with Andromeda galaxy in another 4 months on this scale (less than a month before we expect the sun to turn into a red giant!).
This scale can show us that even things that are really really fast, like light, (which goes so fast it could travel around the earth 7x in one second!) are somewhat slow on cosmic scales. It would take light most of a work day (~7 hours) on the cosmic calendar to traverse the local group of galaxies, and nearly half a week to travel across our local supercluster of galaxies. (Then again, light’s 4 year journey to the nearest star would take less than 1/100th of a second on the cosmic calendar!).
And finally, we should pause to consider the calendar’s philosophical weight. My entire life, my parent’s, grandparents, and great grand parents lives all have occurred in the literal blink of an eye (about 1/3 of a second) on the cosmic calendar. As the psalmist writes, “what is man that you are mindful of him?”
And yet, I still type. I still hope. I still wonder. I note that the cosmic calendar is just one way to think about time, any philosophical and scientific extensions derived from it things to be handled with caution. I note that just because something happens quickly doesn’t mean it isn’t important, or meaningful. Consider the oscillations of visible light, occupying a mere one 500 trillionth of a second, and yet upon these depend all the colors I see. Consider the comparative eternity of the flap of a hummingbird’s wing: upon these moments, even a 2 inch creature might fly.
Given these caveats, given that it might feel a little too constricting, given that it tends to the dramatic, and given its whiffs of nuance lost and ideas incomplete: I’m still glad for this analogy — the cosmic calendar. Glad for this metaphorical device that, like a blink, gives us a new, clearer way to see.