Where did all the matter in the cosmos come from? This is an intriguing question as we know we live in a universe that has only existed for about fourteen billion years. Was matter simply born with the universe, or was it forged at some point during its fiery beginnings?
To explore this, we're going to have to journey back to the birth of the universe, into the Hot Big Bang, and to times where the cosmos was very different to the one surrounding us today. With each step backwards, we will find that the universe becomes weirder and that our cosmic ideas will become more speculative. And on the journey, we will find that the origin of matter is wrapped up with some of the biggest mysteries facing our understanding of the universe.
Let's begin with today. Over the last century, astronomical detective work has revealed that we live in an evolving universe, expanding and cooling from an extremely hot and dense state. A lot of stuff has happened over fourteen billion years, with gravity drawing gas into multiple generations of stars that lit up the universe.
We're interested in the stuff of the universe, and through our telescopes we spy atoms, either as stars, or gas, or planets. Of course, chemistry has revealed that all of the stuff out there, and within you, is built from around 92 natural elements, from hydrogen to uranium. We know the picture is a little messier with the existence of isotopes, and new elements created in laboratories, but all of the basic building blocks can be laid out in the Periodic Table.
There are many ways of representing the periodic table, but for astronomers their favourite is by coding the elements based on their origin. This only became clear in the 1940s, with advances in nuclear physics (building on the interest of nuclear weapons and power). In a classic paper, named Alpher–Bethe–Gamow after the authors, it was realised that the conditions in the early stage of the universe would only cook appreciable amounts of the two lightest elements, hydrogen and helium. The heavier elements were forged either in the hearts of stars, or in the violent events associated with stellar death.
So, in the first few minutes, the primordial elements were made, with everything else built up through the lifecycles of stars over billions of years. It seems we have our answer, and this will be a very short story. But, of course, this cannot be the end of the matter (pun intended!) as we still have to answer just how these first elements were formed.
To answer this question, we have to wonder about the conditions of the universe before atomic nuclei could exist. In the first minute, the temperature and density were so high that the existence of any atomic nucleus was fleeting, being rapidly torn apart in collisions with other particles. It was only once the universe had cooled sufficiently that the first helium nuclei to exist.
Combining the rules of nuclear physics with details of the expanding universe, cosmologists have been able to successfully trace the initial build up of elements. This idea, known as Primordial Nucleosynthesis, has proven extremely successful in explaining the presence of the primordial elements that still dominate the universe.
Of course, this cooking of the primordial elements depends upon the available raw material, the stuff that existed in the hotter and denser universe. Again, we can use the laws of nuclear physics to understand the make up of the cosmos at this stage, finding it to be a soup of the particles that comprise atomic nuclei, namely protons and neutrons, mixed with electrons and photons, zipping about.
In terms of time, we've reached back to about one second after the Big Bang. The universe, containing the protons and neutrons, is about ten billion kelvin with a density similar to water. But all we have done is push the original question, namely the origin of the matter in the universe, backwards. Now we need to find out where the protons and neutrons came from.
Again, lets push backwards in time, back to higher densities and higher temperatures. What happens to the state of matter when the universe was too hot and violent for protons and neutrons to exist? Getting back to about a millionth of a second we find temperatures at more than a thousand billion kelvin and the density of the universe was similar to the interior of an atomic nucleus. We now move into a regime where our physics becomes a little more speculative.
Before protons and neutrons could exist, the universe must have been composed of the building blocks of these, the quarks. But the physics of quarks is messy and complex, and is governed by the rules of quantum chromodynamics which describes the nuclear strong force, the force that binds quarks into protons and neutrons, and protons and neutrons into nuclei. The mathematics are complex, often requiring immense computations to describe the the exchange of the force particle, the gluon, between the sea of quarks.
We've ended up pushing the question back of the origin of matter to asking where the quark-gluon plasma, and the mix of electrons and protons came from. But we think that quarks and gluons and electrons are fundamental, so we cannot chop them into smaller pieces. So, at about a millionth of a second after the cosmic birth, you might think that our journey has come to an end. But really, we've only just begun.
What was the universe like before a millionth of a second? It must have been hotter and denser, but there was a lot of other physics underway which impacted on the origin of matter. And to understand, we are going to have to take a few more steps.
In their classic book, The Early Universe, Kolb and Turner laid out the time line of the universe in beautiful detail, and whilst a millionth of a second, the quark-hadron transition, might seem very close to the universal stars, they push a lot closer, to about 10^-42 seconds! Let's work our ways backward.
The next stop is about a millionth of a millionth of a second, 10^-12 seconds. This is the end of Electroweak Unification. Just what does this mean?
The first thing to remember is that there are four fundamental forces in the universe - gravity, electromagnetism, and the strong and weak nuclear forces. On the face of things, these all seem quite different, but mathematically, electromagnetism and the nuclear forces have something in common, namely that they can be written in the language of quantum mechanics. This is not a hand-wavy "they sort of look similar" kind of statement, but highlights the fact that these forces are written in the same language, namely that the forces are propagated via intermediary particles, known as the force bosons. For electromagnetism, the particle is the photon, for the strong nuclear force it is the gluons, and for the weak force, the most enigmatic force in the universe, it has three particles, the W+, W- and Z0.
These quantum forces have a peculiar property in that their strength depends upon the energy of the interaction. If you smash particles together at low energy, they will experience the forces at one strength, but if you increase the energy of the collision, the strengths of the forces changes. At the energy scale at the end of electroweak unification, around 10^15 K, the strengths of electromagnetism and the weak force are the same. In fact, before this point, these two forces were completely indistinguishable! There were not four fundamental forces before this point, but there.
The characteristics of the universe changed at this point when the two forces separated. A symmetry in the universe, the electroweak force, was broken in spontaneous breaking. But we have reached a point which we can test in our laboratories, at energies accessible to the Large Hadron Collider.
Before this point, we are in a bit of a physics desert. It's not a desert because we are certain there is nothing there. it is a desert because we lack a map, the mathematical laws of physics, to guide us through this unknown terrain. But we can start to put the pieces together. From here on in, temperatures and times are indicative only!
Let's continue above look at the forces. If our mathematical theories are correct (or, if at least we are on the right path with our mathematical theories), as we continue to push back, we eventually reach a temperature where the strength of the electroweak and strong nuclear forces becomes similar, and these become indistinguishable. This was about 10^-34 seconds, when it was 10^28 K, and before this point there were two fundamental forces, gravity and the Grand Unified Theory combination of the other forces.
What about gravity? In modern physics, gravity is distinct from the other forces, written in the language of Einstein's General Theory of Relativity, not quantum mechanics. The question of the existence of a Theory of Everything, where all of the forces can be bundled within a single mathematical framework, is still unsolved, but there is hope that it will be found one day. It is thought (or hoped!) that this will reveal that at some point in the extremely distant path the universe was hot enough that there was only a single super force that ruled over everything.
Let's return to the question of mass. Somewhere in the desert something happened that completely changed the nature of the universe, a something allows us to be here today. The question is why is there any matter in the universe?
This might seem like a strange question, but in heading back into the early hotter universe, in terms of the fundamental forces the cosmos has become simpler, with one force replacing four. In this simpler universe, there should have been a perfect balance between matter and antimatter. What this means that if an interaction as a chance to spit out an electron and some quarks, it should be equally likely in spitting out an anti-electron and some anti-quarks.
With a perfect balance, as the universe cooled every electron would annihilate with every anti-electron, and every quark with every anti-quark. After the first minute or so, the universe should have been nothing but a sea of radiation. Something must have upset the balance between matter and anti-matter!
And here we encounter a mystery, because we don't know how this happened. Our laws of physics must have been imperfect at these early times, with mathematical cracks that favoured matter over anti-matter. When we dig into the Standard Model of Particle Physics, the theoretical rulebook that governs how particles interact, we find such cracks, curiosities to do with the weak nuclear force. These cracks, known as violations of charge and parity, are interesting in their own right (and will form the basis of a future article), but they just seem so inconsequential compared to the cracks needed to explain the quantity of matter in the universe today.
There is more than one mystery at play here. There is the question of quarks over anti-quarks, a process known as baryogenesis, but it is not clear if this is related to the question of electrons over anti-electrons, known as leptogenesis, or if this was distinct. In the physics desert of the early universe, all of these processes remain unsolved problems.
There remains an elephant in the room, the dominant mass in the universe, dark matter. Astronomers and physicists are desperately searching for dark matter, trying to map its location through the cosmos and detect it within experiments, but its fundamental nature remains unknown. Most think that it is a particle, like electrons and quarks, but outside of the boundaries of physics as we understand it. And with so few clues to what dark matter really is, understanding how it fits in to the story of normal matter remains, well, mysterious.
Was dark matter just hanging about? Did it interact with other particles? Was there anti-dark matter that had to have similar cracked physics that allowed it to remain in the universe today? All problems that have yet to be solved.
At some point before the asymmetries kicked in and separated matter from anti-matter, the universe was a hot mix of the two. Temperatures were immense, and physics was seemingly simple. So perhaps we have we reached the end of our journey?
Like a child asking recursive "why" questions, we can still ask where did this "perfect" mix of matter come from? Perhaps the universe was just born with this mix, but because we lack a theory of everything we cannot describe the physics of the first instances of the cosmos and understand where this apparent perfection comes from.
There is one more hypothetical aspect of the early universe that comes into play, namely the notion of inflation. The full story of inflation is too long to go into here, but in summary it is thought that the universe underwent a rapid burst of expansion when it was about 10^-33 seconds old. It was originally proposed to rid the universe of another break in symmetry, particles known as magnetic monopoles, magnetic versions of electrical charges that might have existed in the earliest stages of the cosmos. This rapid expansion dilutes the density of monopoles so that there is about one per observable universe, and so they become "out of sight and so out of mind".
Whilst inflation is very popular amongst theories, seemingly solving several cosmological problems, not everyone is happy. Part of the problem is the motivation behind inflation, namely its source, a proposed field called the inflaton, and the process whereby inflation began, proceeded and eventually ended. But if inflation did occur, it would have supercooled the universe!
This might seem strange, but just like a gas cool when it expands, the universe cooled as it expanded. And as it cooled, it emptied, emptied to such an extent that there was basically nothing, no electrons, no quarks, no dark matter, nothing. So we are back to the question of why there is any matter in the universe today?
But there is a way! Inflation is in driven by a field, the inflaton, whose presence kicks off the rapid expansion. But as inflation comes to an end, the inflaton decays away, dumping its energy back into the universe. This energy is converted into particles, all of the dark matter, quarks, electrons, and everything else, in an event known as reheating.
Reheating is complex and messy, with the mathematics that describes the mass condensing from energy involving preheating, parametric resonance, tachyonic instability and other very cool sounding processes. If you are interested, there are some extensive reviews, but like everything else in the desert, mysteries remains, but in terms of what we see around us today, we have reached the end of our journey.
Before inflation, we are truely in "Here be dragons" territory, and we really don't know what was really happening in the universe. But let's try and summarise.
Look at your hand. It is built from molecules which are composed of atoms. Their story began in reheating at the end of inflation, where the initial quarks and electrons were formed, and these survived the intensive annihilation during the genesis of matter in the universe. In the expanding and cooling universe, eventually quarks were bound into protons and neutrons, and protons and neutrons into nuclei, before being into forged into elements, first in the Big Bang and then in the hearts of stars. Many atoms would have been through the hearts of a number of stars, before eventually finding themselves on this planet, and into the hand in front of you.
The bits and pieces of the atoms around you today have been witness to the entire history of the universe, from about 10^-33 seconds to now. The story of matter is, quite frankly, amazing.