Universe Today
This is Part 1 in a series on the age of the universe.
When I say that the universe is 13.77 billion years old, it sounds rather authoritative. And it’s not just because of the values after the decimal point. That just makes it should precise (and we’re very proud of achieving that level of precision, thank you very much). No, it’s the extreme amount of confidence that I can just sit here, look you in the eyes, and say in no uncertain terms that we estimate the age of the universe is 13.77 billion years.
But…how? How do we know, like REALLY know, how old the universe is? And what does it even mean for the whole UNIVERSE to have an age?
So let’s dig in. In today’s episode I’m going to tell you how we calculate it and how we get so dang confident, and then I’m going to present three potential challenges to our method of calculating the universe’s age.
And I’m going to knock down those challenges with the sheer force of my confidence.
And science. Mostly science.
Okay, first objection: can the universe really have a “clock”? Like, didn’t special relativity, which is kind of important and kind of well tested for over a century, say that there’s no such thing as a universal clock? When we go way back in time to the pre-Einstein era, our conceptions of the structure of the universe were rooted in Newton. How do objects know when and where to interact with each other? For Newton, what allowed this was a universal clock and a master rule, some set of standards against which everything else could be measured.
No, there wasn’t literally a giant clock sitting behind some nebula ticking away cosmic time, and there’s Bureau of Universal Standards and Measurements that has, I don’t know, a bar of gold that says “this is exactly one meter, against which all other measurements shall now and forever be made.”
It’s a conceptual idea, a way to frame the mathematics so that everything lines up and makes sense. For Newton, objects and events have no intrinsic sense of time or space; they have to rely on some external entity (we’ll go ahead and call that entity spacetime for convenance’s sake, even though he didn’t think of it that way) to know where and when they are at any given place and time.
So Einstein destroyed all that, as was his usual style. He said, and so far he’s been right, that there are NO universal clocks. There are NO master rulers. Every single object in the universe has its own individual special unicorn frame of reference, with its own special way of measuring the passage of time and the distances between objects.
That sounds…like chaos. And it is. Under special relativity, synchronization is a joke, something that can happen for brief moments when objects are close together. As soon they move, it all goes away. But ever the salesman, old Uncle Albert provided the solution to the problem that he created: the mathematics of special relativity, which tell us HOW to translate one frame of reference to another. No, we’re never going to agree on how long a second is or how wide a meter is, but at least we have the machinery to check our notes against each other’s.
So it’s there no master ruler or master clock in the universe, if there’s no absolute frame of reference, if everybody and everything has their own version of the passage of time, then how can we say with Any confidence at all that the universe is 13.77 years old? Doesn’t that depend on who’s doing the measuring?
And the answer is: nah. Special relativity says that the universe doesn’t HAVE to have a preferred reference frame, a special absolute scale. But that doesn’t mean there CAN’T be one.
Enter general relativity, which is like special relativity, but with extra sauce. And that extra sauce is the ability to capture the behavior of spacetime when things get really complex. Complex as in, for example, the evolution of the entire universe.
In cosmology we’re faced with a straightforward observational fact: distant galaxies appear to be receding away from us. This seems like a complex problem involving gravity and the nature of spacetime, so we turn to general relativity to help us build a model of what’s going on. And the simplest and simultaneously most comprehensive model (in physics we like to use the word parsimonious: what’s the most we can explain with the simplest possible model, aka how cheap can we be and get away with it) is called the FLRW metric.
Those letters stand for Friedmann, Lemaitre, Roberston, and Walker, four folks who in the early 20th century put all of cosmology into a single coherent picture. And in this picture, this model of how the universe works, makes one key distinction that allows for a break from special relativity: stuff.
The universe is full of stuff. Stars. Planets. Your lost socks. There’s all sorts of matter just laying around, but it’s not just laying around, it’s moving around. And it’s moving around in a very particular way: it’s expanding.
As time goes on, the stuff in the universe expands, gets thinner, and cools off. Aha! That gives us a SHARED HISTORY. Epochs in the universe are DIFFERENT. Yesterday, the universe was a little bit smaller, a little big thicker, and a little bit warmer. Tomorrow, the cosmos will be bigger, thinner, and cooler.
Because the universe is expanding, the symmetry is broken: the past is different than the future. And any observer in the universe, no matter what galaxy they live, if they become sophisticated enough to develop cosmology and their version of the FLRM metric, will come to the same conclusion, and they’ll all agree that the universe was different in the past, and they’ll be able to calculate HOW MUCH different the universe was in the past, and from there calculate how long ago “the past” was.
Which means we can build a universal clock.
To be continued…
