One of the great achievements of science is that we’ve figured out not only what the universe is made of, but where it came from and how old it is.
Counting from the start of the hot big bang, we now know that 13.8 billion years have elapsed since that moment until the present day, with an uncertainty of less than 1 percent.
And yet, when we talk about the distance from our location to the edge of the observable universe, the answer isn’t the 13.8 billion light-years you might expect, but much greater: 46.1 billion light-years.
How did scientists arrive at this counterintuitive conclusion? It was only possible thanks to Einstein’s general theory of relativity.
General relativity is sometimes explained as matter and energy telling spacetime how to curve, and curved spacetime telling matter and energy how to move. But there's another piece to the story: spacetime is free not only to curve, but also to expand or contract.
In 1922, Alexander Friedmann proved that as long as you have a universe that is uniformly filled with one or more types of energy, your universe can’t simply be curved — it must either expand or contract. Friedmann’s equations — precisely as he wrote them down more than 100 years ago — dictate how a universe filled with matter and energy expands and evolves.
Today, we’ve not only measured how quickly the universe is expanding, but we’ve also measured how that expansion rate has changed throughout cosmic time. This profound piece of information means that measuring the speed of the universe's expansion, as well as how that expansion rate has changed over cosmic history, enables us to solve Friedmann’s equations.
Because those equations relate the matter-and-energy contents of the universe to its expansion and evolution, we can then measure the expansion rate and evolution to determine what the universe is made of.
To the best of our knowledge, our universe is made of roughly two-thirds dark energy, about one-third matter (both normal matter and dark matter combined), along with a tiny amount of radiation: about 0.01 percent worth.
If the universe is expanding today, that implies it’s getting larger and less dense as time progresses. That also means that if we extrapolate what happened backwards in time, we’d find that it was smaller and denser in the past. We can take that extrapolation as far back as we like: all the way back to the start of the hot big bang. The universe begins by expanding rapidly, and all of the different species of energy respond to that expansion in a unique way:
Matter: Both normal and dark matter simply dilutes in an expanding universe. As the volume increases, the total mass stays the same, so the energy density drops as ~1/r³, where r is the radius of the observable universe.
Radiation: Radiation behaves similarly to matter, as it’s also made of a fixed, unchanging number of particles and dilutes as ~1/r³. However, the energy of each quantum of radiation is determined by its wavelength, which stretches (and lengthens) proportionally to ~r as the universe expands. Therefore, its energy density drops as ~1/r⁴, or faster than the matter density drops.
Dark energy: This is a form of energy inherent to space itself. Even as the universe expands, its energy density doesn’t dilute but rather remains constant (mathematically, it evolves as ~r⁰).
Putting this all together, it tells us that very early on, some 13.8 billion years ago, the hot big bang began. For the first 10,000 years or so of cosmic history, the universe was dominated by radiation. It expanded rapidly at first, but the expansion rate dropped very quickly early on.
Once the radiation density fell below the matter density, the universe became matter-dominated. During this phase, which lasted for about 9 billion years, the universe continued to expand, but the expansion rate dropped more slowly than it did during the radiation-dominated stage (for the simple reason that matter doesn’t drop in energy density as rapidly as radiation does).
Finally, the matter density dropped sufficiently so that dark energy began dominating the expansion of the universe, which it’s now done for longer than the age of planet Earth: 4.5 billion years. During this phase, the expansion rate approaches a constant, positive non-zero value and will continue expanding at that rate (as far as we know) for all of eternity.
In a universe where 13.8 billion years have elapsed since the start of the hot big bang, Friedmann’s equations allow us to solve for how far away the most distant visible objects are within the observable universe.
In any universe we can imagine, every possible combination of radiation, matter, and dark energy will lead to a different amount of total expansion that has occurred:
If the universe were made 100 percent of radiation, with no matter or dark energy, we’d be able to see for 27.6 billion light-years in all directions.
If the universe were made 100 percent of matter, with no radiation or dark energy, we’d be able to see for 41.4 billion light-years in all directions.
And in our universe, made of two-thirds dark energy, one-third matter, and about 0.01 percent radiation, we can presently see for 46.1 billion light-years in all directions.
That’s why we can see so far away — 46.1 billion light-years — in a universe that’s only 13.8 billion years old: because the expanding universe has carried those objects to that distance while the light they emitted long ago has been journeying to our eyes for all this time.
The observable universe isn’t even larger than it already is because the universe would be larger if it were older. As it ages, it continues to expand. In fact, the distance to our cosmic horizon grows by 3.3 light-years with each new year that passes: a value that will gradually increase with time as the expansion of the universe pushes the most distant objects ever farther.
The universe would also be larger if it contained more dark energy (and, for balance, less matter) than ours does. If our universe had just half of the matter density (and a correspondingly larger amount of dark energy), we’d be able to see more than 50 billion light-years away at present.
It’s only because we live in the here-and-now, 13.8 billion years after the start of the hot big bang, in an expanding universe that’s two-thirds dark energy, one-third matter, and not much of anything else, that we can presently see an impressive 46.1 billion light-years away.
As for what's out there beyond the observable universe? It may be impossible to know, but various cosmological models suggest it's at least seven times bigger than what we can see and could extend trillions of light-years. It boggles the mind, but what's most amazing is that human minds can learn these things about our universe through ingenuity and science.
A theoretical astrophysicist by training, Ethan Siegel took the unconventional path of ditching research to become full-time science communicator. He is the creator and writer of the science site Starts With a Bang.
