This is Part 4 in a series on the age of the universe. Read Parts 1, 2, and 3.
But wait, wait wait wait wait wait wait wait. Isn’t the FLRW metric way generic? It lays out the basic assumptions and tells us how the universe should behave, but it doesn’t say WHAT the universe is made of. And the stuff IN the universe dictates EXACTLY how the universes evolves.
It’s like…the FLRW metric is the map. It tells you where all the roads are, all the possible paths of the universe can drive through. It doesn’t tell you exactly which road the universe will take in its evolution – but it mays down the rules for how the roads are made, where they are allowed to curve and wind, and how long they can stretch.
Our universe is following one particular path, one road. And that road is dictated by the ingredients of the universe. How much curvature, how much matter, both regular matter and extra spicy dark matter, how much radiation, and how much dark energy. And who knows, maybe some other components or ingredient that we haven’t even dreamed of yet (although, for various and sundry technical reasons, matter, radiation, dark energy, and raw curvature pretty much cover all the possibilities that can “fit” within the FLRW metric).
The FLRW metric ALONE doesn’t give us the age of the universe. It gives the formula we need to convert “a detailed list of observations and measurements about the ingredients and expansion history of the cosmos” INTO a finite age – a proper time, if you want to be precise – since the big bang.
And that, my dear friends, is up for debate. The current favored model of what the universe is made of, the particular path that the universe is followed, is called lambda-CDM, for dark energy (we use the Greek letter lambda because Einstein did it once way back when and who the heck is going to one up that?) and CDM for cold dark matter (dark matter particles that are generally moving slower than the speed of light).
According to the LCDM model – this path on the FLRW metric map – the universe is 13.77 billion years old. But of COURSE we don’t understand dark matter. Or dark energy. So how in the world can we be confident in the output of a model if we’re not confident at all about the inputs?
That’s because while we don’t know what dark matter and dark energy ARE to any degree of certainty whatsoever, we DO know two very critical things: we know what they DO and we know HOW MUCH of them we got.
Listen, dark matter is just matter, but dark. A pound of hydrogen and a pound of, I don’t know, axions, do the same exact thing to the expansion of the universe: they want to slow it down. They plug in to the same part of the formula in the FRLW metric. So all we have to do is try to measure the TOTAL amount of mass in the universe (which we do, often and repeatedly) and we get to fill in the “matter” column and let later generations sort out what’s going on this particular bucket.
The same is true for dark energy. Who the heck knows what it is, but we can all agree on what it does: it accelerates cosmic expansion. As long as the universe appears to be accelerating – and trust me, it does – then we just have to measure how much of the dark stuff there is, plug it into FLRW, and away we go.
Now let’s assume that someday we come across some theory or idea that does throw a monkey wrench into the LCDM model (which is almost guaranteed to happen at some point, because we KNOW we’re missing a lot of nuance with that model) or even the FLRW metric (less likely because so far it’s survived every observational test), then wouldn’t that change the estimated age of the universe?
Yes, but probably not by very much.
That’s because our models are SO successful at explaining all sorts of different cosmological observations that new ones have to be somewhat “tuned” to get to line up. If you introduce a new idea, you have to face the same gauntlet of observations that LCDM has already gone through and survived.
AND, there are other, independent checks on the age of the universe: Go out and find the oldest star. The universe can’t be younger than that. Go out and find the coldest white dwarf. The universe has to be older than that. Go look at the amount of decaying radioactive elements. The universe has to stick around long enough to make all those. Very generously, those estimates pin the age of the universe to be somewhere between 12 and 15 billion years old. So any model you concoct has to fit within those ranges that are derived from other, independent experiments.
The upshot of all this is that once you survive the battery of tests and constraints and observations, you generally find that you wind up making the same broad-brush predictions as LCDM, except with some minor tweak here or there (like ooh look voids should be a little thicker around the middle).
You might end up changing the number a little. For example, if we take the recent DESI cosmology results at face value, which hint at evolving dark energy, the age of the universe goes up from 13.77 billion years to 14.1 billion years – that’s about a two and a half percent difference.
Two and a half percent! I mean, come on! Think about it: with all of our models, and all of our assumptions, and all the observations, to think that we’re likely to be within a few percent of the current answer is pretty mind-blowing.
Nice job, science.