A little less than one-third of the universe—around 31 percent—consists of matter. A new calculation confirms that number; astrophysicists have long believed that something other than tangible stuff makes up the majority of our reality. So then, what is matter exactly?
One of the hallmarks of Albert Einstein’s theory of special relativity is that mass and energy are inseparable. All mass has intrinsic energy; this is the significance of Einstein’s famous E=mc2 equation. When cosmologists weigh the universe, they’re measuring both mass and energy at once. And 31 percent of that amount is matter, whether it’s visible or invisible.
That difference is key: Not all matter is alike. Very little of it, in fact, forms the objects we can see or touch. The universe is replete with examples of matter that are far stranger.
What is matter?
When we think of “matter,” we might picture the objects we see or their basic building block: the atom.
Our conception of the atom has evolved over years. Thinkers throughout history had vague ideas that existence could be divided into basic components. But something that resembles the modern idea of the atom is generally credited to British chemist John Dalton. In 1808, he proposed that indivisible particles made up matter. Different base substances—the elements—arose from atoms with different sizes, masses, and properties.
But if only it were that simple. Since the early 20th century, physicists have known that tinier building blocks lurk within atoms: swirling negatively charged electrons and shrouded nuclei, made from positively charged protons and neutral neutrons. We know now, too, that each element corresponds to atoms with a certain number of protons.
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And it’s still not that simple. By the middle of the century, physicists realized that protons and neutrons are actually combinations of even tinier particles, called quarks. To be precise, protons and neutrons both contain three quarks each: a configuration type that physicists call baryons. For that reason, protons, neutrons, and the matter they form—the stuff of our daily lives—are often called “baryonic matter.”
Strange matter in the sky
In our everyday world, baryonic matter typically exists in one of four states: solid, liquid, gas, and plasma.
Again, matter is not that simple. Under extreme conditions, it can take on a menagerie of more exotic forms. At high enough pressures, materials can become supercritical fluids, simultaneously liquid and gas. At low enough temperatures, multiple atoms coalesce together, creating the Bose-Einstein condensate. These atoms behave as one, acting in all sorts of odd quantum ways
Such exotic states are not limited to the laboratory. Just look at neutron stars: Their undead cores aren’t quite massive enough to collapse into black holes when they go supernova. Instead, as their cores crumple, intense forces rip apart their atomic nuclei and crush the rubble together. The result is essentially a giant ball of neutrons—and protons that absorb electrons, becoming neutrons in the process—and it’s very, very dense. A single spoonful of a neutron star would weigh a billion tons.
There are, potentially, hundreds of millions of neutron stars in the Milky Way alone. Deep in their centers, some scientists think, pressures and temperatures are high enough to rip neutrons apart too. Those neutrons may break the quarks that form them.
Physicists study neutron stars to learn about these objects—and what happened at the beginning of the universe. The matter we see around us did not always exist; it formed in the aftermath of the big bang. Before atoms formed, protons and neutrons swam alone through the universe. Even earlier, before there were protons and neutrons, everything was a superheated quark slurry.
Scientists can recreate that state, in some fashion, in particle accelerators. But that disappears in a flash that lasts a fraction of a second. It’s no comparison to the extremely long-lasting neutron stars “You have a lab that basically exists forever,” says Fridolin Weber, a physicist at San Diego State University.
Matter in the grand scheme of the universe
Over the past several decades, astronomers have developed several ways to understand the universe’s basic parameters. They can examine its large-scale structure and identify subtle fluctuations in the density of the matter they can see. They can watch how objects’ gravity bends passing light.
A specific way to measure matter density—the proportion of the universe made up of visible and invisible matter—is to pick apart the cosmic microwave background of the big bang. From 2009 to 2013, the European Space Agency’s Planck observatory prodded the afterglow to give scientists the best calculation of the matter density yet, 31 percent.
The most recent research used a different technique called the mass-richness relation, essentially examining clusters of galaxies, counting how many galaxies exist in each cluster, using that to calculate each group’s mass, and reverse-engineering the matter density. The technique isn’t new, but until now it was raw and unrefined.
“When we did our work, as far as I know, this is the first time that the mass-richness relation has been used to get a result that’s in very good agreement with Planck,” says Gillian Wilson, an astrophysicist at the University of California Riverside, and one of the authors of a paper published in The Astrophysical Journal on September 13.
Yet remember, it’s not that simple. Only a small fraction—thought to be around 15 percent of matter, or 3 percent of the universe—is visible. The rest, most scientists think, is dark matter. We can detect the ripples that dark matter leaves in gravity. But we can’t observe it directly.
Consequently, we aren’t certain what dark matter is. Some scientists believe it is baryonic matter, just in a form that we can’t easily see: Perhaps it is black holes that formed in the early universe, for instance. Others believe it consists of particles that must barely interact at all with our familiar matter. Some scientists believe it is a mix of these. And at least some scientists believe that dark matter does not exist at all.
If it does exist, we might see it with a new generation of telescopes, such as eROSITA, the Rubin Observatory, the Nancy Grace Roman Space Telescope, and Euclid, that can scan ever greater swathes of the universe and see a wider variety of galaxies at different times in cosmic history. “These new surveys might change our understanding of the whole universe [and its matter],” says Mohamed El Hashash, an astrophysicist at the University of California Riverside, and another of the authors. “This is what I personally expect.”