The chart of everything

Some 13.8bn years ago, the universe came into existence in the Big Bang. Its first moments were a period of pure energy with intense, possibly infinite, heat and density. As the universe expanded, it cooled and objects began to emerge, like snowflakes forming in cold air. In less than a nanosecond the elementary particles—photons, electrons, quarks and gluons—condensed from the maelstrom. As the universe cooled further, the quarks coalesced into neutrons and protons. Then these composite particles formed atomic nuclei and, pulling electrons into orbit around them, became atoms of hydrogen and helium. Over hundreds of millions of years, gravity pulled these simple atoms together to form stars; in the cores of those (and later) stars, smaller nuclei fused to create the nuclei of heavier elements. Those elements ended up in dust, planets, people, whales, galaxies, and black holes.

“The whole history of the universe can be understood as a sequence of condensations,” says Charles Lineweaver, a physicist at the Australian National University. Each new class of fundamental particles was formed at a time when the entire cosmos had that specific density. At the moment when protons emerged, the entire universe was the density of a proton. Protons are still with us as distinct objects because—at roughly the same time—the strong nuclear force was able to pull quarks together. This froze protons (and neutrons) in time at a higher density than their surroundings. That is all any object is, cosmologically: something made from particles left at a higher density as the universe has become less dense around them.

Noting that, Dr Lineweaver and a fellow physicist, Vihan Patel, devised a single chart onto which every known object in the history of the universe could be plotted. They wondered what they might learn if they categorised everything in terms of its size and mass.

The solid white diagonals represent the edges of the known universe. The top is bounded by gravity, which imposes a limit on how dense an object can get. As more mass is concentrated into the same volume, its gravity will increase until, eventually, the force overcomes everything else and the mass will collapse into a black hole. The size at which any given mass will collapse is its “Schwarzschild radius”.

Black holes of all sizes sit on the “gravitational limit” line. Gravity forbids anything from existing above this line; an object of a specific size can never have more mass than a black hole of the same size.

Small, old black holes, created just after the Big Bang, sit towards the bottom section of the gravitational-limit line. Move up and right, and you find black holes that are the remnants of massive stars; farther along are the supermassive black holes that sit at the centres of galaxies, such as Sagittarius A* in the Milky Way.

At the top, where the line meets the modern day, is the largest possible object in the universe—the universe itself. This is represented by the mass inside the Hubble radius—the distance from Earth at which galaxies are receding at the speed of light. The fact that this object fits on the gravitational-limit line raises the question: is our universe itself a black hole?

At the bottom of the chart are the fundamental particles—quarks, bosons (eg Higgs, w+, w-, z), protons, neutrons, electrons and neutrinos. A particle’s “Compton wavelength” is used here as a proxy for its size—the more massive a particle, the smaller its wavelength.

Top quarks and protons have small wavelengths; electrons have big wavelengths, around the size of atoms; visible-light particles (photons) have bigger wavelengths still. All these particles sit along a line, the Compton limit, which is a lower boundary, determined by quantum physics, for what you can call an object.

Quantum mechanics says that the more precisely a particle’s location is known, the bigger the uncertainty in how much energy it has. Albert Einstein showed that energy and matter are equivalent (E=mc2), so pinpointing the tiniest particles would require so much energy that new particles would be conjured into existence from the vacuum. Below the Compton limit, therefore, is a realm of uncertainty in which it makes no sense to talk of individual objects.

In the triangle bounded by these two solid lines—on one side forbidden by gravity and the other enforced by quantum uncertainty—lie all objects in the known universe. Start at the white dot on the left and move horizontally to the right and follow a timeline of the universe from the Big Bang to the cosmos as it is today.

That white dot where the gravitational- and Compton-limit lines meet would be an instanton: dense enough to be governed by gravity but small enough to be wholly quantum. Given its position in the map, “the instanton may be the same thing as the origin of a universe,” says Dr Lineweaver.

Picoseconds (trillionths of a second) after the Big Bang, the first elementary particles began to form: quarks, bosons, and gluons.

Mere nanoseconds later, the new universe’s density continued to drop and the quarks coalesced into neutrons and protons.

Around 15 minutes later, these composite particles began to stick together too and, pulling electrons into their orbit, became atoms of hydrogen and helium.

Over several more billions of years, the atoms became stars. They fused into more complex elements and ended up in everything else that surrounds us.

Everything that exists (or has existed) in the universe that has the same density appears on the same positively-sloping, dotted line. Viruses, fleas, humans, whales, Earth and the Sun appear on the same line as atoms, since that is what they are all made from. This line is the most populated thing on the chart because scientists know more about atoms (and the objects they create) than anything else.

Other constant-density lines correspond to other condensation events—bosons and protons, for example. They are more sparsely populated and the only known examples of objects at these densities exist almost exclusively at the ends of the lines.

The chart also provides pointers for the unknown. Dark matter, which makes up around 27% of the universe (normal matter makes up only 5%), is not on the chart because scientists don’t know what it is. But the galaxies and superclusters at the top right might provide clues. These will be overwhelmingly composed of dark matter so—just as dotted lines of equal density connect atoms and stars—could the densities of galaxies tell us something about the place of fundamental dark-matter particles along the Compton limit?

And what of the vast triangle of emptiness to the left of the Higgs boson? No forces or laws of physics have been found that would dominate here in order to create objects. But that doesn’t mean nothing could exist there. “It’s a wonderful wild west of particle existence,” says Dr Lineweaver.