How We Know How Old Everything Is

Reading time ~7 min · Bring your three answers to Block 4

Somebody will tell you the universe is 13.8 billion years old, and the honest response is: how could anyone possibly know that? Nobody was there. Nothing has been running that long that we can read like a clock.

The answer is that three completely separate lines of evidence, using different instruments and different physics, all point at the same number. That agreement is what makes the claim strong. Any one of them alone would be interesting. Three of them landing together is hard to explain any other way.

Line one: everything is moving apart

In the 1920s astronomers measured the light from distant galaxies and found something odd. The light was stretched toward the red end of the spectrum, and the further away the galaxy, the more stretched it was. That stretch is called redshift.

The obvious reading is that the galaxies are flying away from us and we sit at the center of the fleeing. That reading is wrong, and the correction is the most important idea in this reading.

The galaxies are not moving through space. Space itself is expanding, and carrying them along. Imagine dots drawn on a balloon that is being inflated. Every dot moves away from every other dot. No dot is the center. Nothing is traveling across the surface. The surface is getting bigger.

That also explains why more distant galaxies recede faster: more space between us means more space to expand. The relationship between distance and speed is steady enough to be written as a number, and that number is what lets you do something remarkable.

You can run the film backwards. If everything is separating at a measurable rate, ask how long ago it was all in the same place. That calculation gives an age of about 13.8 billion years. There is real uncertainty in it, because the expansion rate has not been perfectly constant, but it is uncertainty of a few per cent, not of a factor of two.

Line two: the leftover heat

If the universe was once compressed, it was once extremely hot. And a universe that expands also cools. So there ought to be radiation left over from that hot early state, stretched by all the expansion since, arriving now as faint microwaves from every direction.

That prediction was made before anyone found it. Then it was found, and it is called the cosmic microwave background. It fills the sky. It is astonishingly uniform, and it matches the temperature the theory said it should have.

Notice what makes this evidence rather than a story. Somebody said in advance what should be there, and it was there. A theory that explains what we already knew is cheap. A theory that tells you where to point a telescope is not.

Line three: the recipe of the early universe

The third line is the one people skip, and it is the one I find most convincing.

Run the early universe forward from first principles and you can calculate what should have been made in the first few minutes: a lot of hydrogen, a specific proportion of helium, a trace of deuterium and lithium, and nothing heavier. The numbers come out of the physics, not out of observation.

Then go and look. The oldest stars and the most distant galaxies show almost exactly those proportions. And the helium in particular is the giveaway: there is far more of it in the universe than stars could have produced in the time available. Something else made it, early, everywhere, all at once.

Deuterium is even sharper. It is fragile and stars destroy it rather than make it, so any deuterium we see is left over from the beginning, and how much survived depends on how dense the universe was at the time. Measure deuterium today and you get a reading on conditions in the first minutes.

Why this belongs in a course about habitable planets

Two reasons, and both come back in your Goldilocks Report.

The first is that looking out is looking back. Light takes time. A star 1,800 light-years away is being seen as it was 1,800 years ago; there is no way to see it as it is now. Every observation in astronomy is a delayed report. When you evaluate an exoplanet in Block 6, you are working from a photograph of a place as it was when the light left it.

The second is that ages constrain what is possible. On Earth, life shows up in the record within roughly the first billion years, and it takes billions more before anything complex appears. If a planetary system is very young, some outcomes have not had time to happen yet. Age is not a habitability criterion on its own, but it sets the ceiling on what you can reasonably claim.

And underneath all of it is the move this whole unit is training you in. Nobody measured the age of the universe directly. They took three independent things they could measure, checked whether the answers agreed, and trusted the convergence more than any single measurement. That is the same reason your report evaluates three planets instead of one.

Sources

• Expansion, redshift and the age calculation: OpenStax, Astronomy 2e, §29.1 "The Age of the Universe" [open-licensed university textbook]
• Models of an expanding universe: OpenStax, Astronomy 2e, §29.2 [open-licensed university textbook]
• Early nucleosynthesis, helium and deuterium: OpenStax, Astronomy 2e, §29.3 [open-licensed university textbook]

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