One of the most surprising things about the Universe? As vast as it is, it hasn’t been around forever. In fact, if you take our plain little rocky home (Earth), it looks like the Universe is only about 3 times as old as we are. It’s surprising, considering how huge, expansive, and full of interesting things the Universe is.
And yet, our Sun, Earth, and entire Solar System, at 4.5 billion years, represents a significant fraction of the age of the Universe, about 13.7 billion years.
But how well do we know that number? If you look at the best data, you find that the Universe is 13.73 billion years old, with an uncertainty of about 0.13 billion years in either direction.
But that’s not entirely accurate. There are some assumptions made in figuring this out, including:
- The Hubble parameter is the same everywhere in the Universe,
- The Universe consists only of photons, normal, dark matter, cosmological constant, and low-mass neutrinos, and
- There’s no funny stuff going on.
This may or may not be true, but there are some caveats here. The uncertainty quoted about was determined with “likelihood calculations,” which muffle some of the real uncertainties. For example, the Hubble constant is quoted as 71 km / s / Mpc. But nobody would really be surprised if it was 65, or if it was 75. And yet, just changing that one parameter gives you an age range of 12.5 to 14.4 billion years!
We know this from the observed relation (above) that (the age of the universe) * (the Hubble parameter) = 1.0. This can make a huge difference, to be sure, and yet it’s perfectly within reason at this point. Similarly, the parameters of normal matter, dark matter, and cosmological constant density can be tweaked as well; so long as the data for the CMB, supernovae, and large-scale structure are fit to the model, there’s quite a substantial workable range.
So when I see people writing about discussing the uncertainty in the age of the Universe, I am hopeful that this is what they’re talking about. You see, the uncertainties are much larger than are reported, and actually leave more than a billion years of wiggle-room. So go ahead, ask your second-favorite cosmologist (because I know I’m your favorite) if they’d be shocked if the Universe were either a billion years younger or older than the best estimate; if they’re any good, the answer will be “no.”
But the flip-side of this is that it would be shocking if the Universe were 11 billion or 16 billion years old; those are pretty firmly excluded. So we know this age business pretty well, but not necessarily as well as most people think!