Originally posted to 13.7 Cosmos & Culture on 6/7/2017
Today we address the composition of the universe, in the final essay of our trilogy on cosmic questions.
As the great German astronomer Johannes Kepler once wrote in the early 17th century: "When the storm rages and the shipwreck of the state threatens, we can do nothing more worthy than to sink the anchor of our peaceful studies into the ground of eternity."
So, let's sink our anchor, once again, into the study of the heavens. Previously, we discussed both what an expanding universe means, and whether we can make sense of the hardest question of them all — the origin of all things. Our question today? How much do we know of the stuff that fills up the universe?
In the interest of clarity, I'll break the cosmic recipe into its three main ingredients: the stuff we are made of; dark matter; and dark energy.
The stuff we are made of
In school, we learn that everything in the world is made of 92 naturally occurring atoms — from hydrogen to uranium — and that each atom is made of only three particles — protons, neutrons and electrons — in different numbers. A hydrogen atom has one proton and one electron, while one of uranium has 92 of each.
To have been able to reduce the material composition of the stuff around us, including the stuff we are made of, to only three particles is an amazing achievement of 20th-century physics. The reduction doesn't stop there, since protons and neutrons are, themselves, made of more elementary constituents, the up (u) and down (d) quarks. A proton is a u-u-d composite, while a neutron is a u-d-d composite. So, we can proudly state that the stuff that makes up the world is made of two quarks and electrons in different arrangements and numbers. Three particles to describe everything, from water to bones to leaves to rocks to air to planets to stars.
Of course, the details are a bit more complex, since the particles need to interact — attract and repel — to make stuff up. These are the forces between the particles — gravity and electromagnetism being the two more familiar, and the strong and weak nuclear forces acting only at very short distances, within an atomic nucleus. All four forces are described in terms of the exchanges of particles. A helpful image is of two ice-skaters gliding along a lake together, each carrying a bag full of tennis balls. As they throw balls at one another they recoil, mimicking a repulsive force between them. (Like two equal electric charges.) So, modern particle physics (aka high-energy physics) describes matter and forces as made of elementary constituents. If we count all of them, we get 12 particles of matter plus the Higgs boson, the particle that gives mass to all others (with the exception of the photon, the particle of light, responsible for transmitting the electromagnetic force), and the particles responsible for the forces.
It's amazing that this simple picture holds so well, and that it holds across the known universe and its history. A star 5 billion light-years away is still made of mostly hydrogen, as is our sun. All the atoms that make up the 92 naturally occurring chemical elements originate from stars. (Light nuclei, such as helium and lithium, and some of their isotopes, together with the hydrogen isotopes deuterium and tritium, were also made when the universe was very young.) In a sense, stars are true alchemical furnaces, transmuting the lightest element into all the others, as they struggle against the pressure from their own gravity. As we like to say in astronomy, people is what happens to hydrogen when you wait long enough! The atoms in your body, the iron in your blood cells, the calcium in your bones, the oxygen and hydrogen that make the water that is about 60 percent of you, come from stars that died billions of years ago, before the sun and Earth formed. We are animated agglomerates of very old matter.
There are also other particles out there, such as the ghostly neutrinos (three kinds of them) that go through you by the trillions every second. And photons, the particles of light — some remnants from when the first hydrogen atoms formed only about 400,000 years after the Big Bang, and others from burning stars and hot gases in our galaxy.
The amazing thing is that all this stuff that is made of the 92 atoms, neutrinos, and photons — that is, everything that we see around us and in the skies — amounts to a pitiful five percent of the total. What total? The rest of the stuff the fills the universe. Here's where the mystery begins.
In the 1930s, American-Swiss astronomer Fritz Zwicky noticed that when galaxies clustered in large groups, they moved about with speeds much larger than what he could explain with the gravitational pull they ought to be exerting onto one another. Zwicky conjectured that galaxies carried with them much more mass than what we could see.
He called this invisible matter "dark matter": It pulled matter that shined around but it didn't shine. Now, people and planets don't visibly shine, but they are not dark matter. They can still reflect visible light, and do shine, albeit in invisible frequencies such as infrared. (Our eyes see only a small window of the whole spectrum of electromagnetic waves.) Dark matter is truly dark, not emitting or reflecting light. The trouble is that whatever dark matter is, it's not made of quarks and electrons, or photons — and presumably not neutrinos either. (Although neutrinos have been proposed as dark matter candidates for many years, chances that they are remain slim.)
To make things more exciting, current measurements indicate that about 26 percent of the cosmic stuff is made of dark matter. That's five times more than the amount in ordinary matter. And we don't know what it is or could be. There is no shortage of candidates, of course, some by yours truly, as the mystery of dark matter inspires the imagination of thousands of physicists around the world. But so far, no luck, despite a ferocious hunt for such particles, in ground and space-based detectors. Some even propose that there is no dark matter, that it is our understanding of gravity that is limited. Either way, solving the mystery of dark matter would, no doubt, open a whole new window into the universe and its constituents, one that seems tantalizingly close, but that has proven immensely deceitful thus far.
Well, we still have 69 percent of stuff filling the cosmos to deal with. Here we find the greatest of all puzzles in current cosmology — "dark energy." Discovered in 1998 by two teams of astronomers, dark energy is generally described as being responsible for accelerating the cosmic expansion, apparently acting as a kind of stretchy force on the very fabric of space. (See our first essay on how the cosmic expansion is due to the stretchiness of space.) It makes distant galaxies move away from one another as if they were on steroids.
Current measurements attribute their amount to the 69 percent of missing stuff. We have no clue what dark energy is, although we model it as a very smooth fluid that fills the whole of space, like water in a bathtub. But a strange and nasty kind of water, almost imponderable, yet capable of pushing everything apart. People are puzzled by this cosmic repulsion, asking how come we don't feel it. The answer is that the effect of dark energy is truly cosmological, that is, it is only felt over gigantic distances of many millions of light-years apart. Our solar system, our galaxy (only about 100,000 light years in diameter) are glued together by local gravitational forces much stronger that the dark energy repulsion.
Another curious fact about dark energy is that it seems to have fired up around five billion years ago, not too far from when the sun and our solar system formed. This is often called the "coincidence problem." Why then?
Although we've made enormous strides in understanding the nature and composition of the physical universe, it is clear that much remains to be learned. And this is how it should be. After all, it is the very nature of scientific inquiry to generate new questions to each of our answers. We move forward, humbled by what we don't know, as we confront some of the hardest questions we can ask. Not a bad way to spend a lifetime.