These science challenges help to guide what STFC invests in across our research portfolio.
Challenge A: how did the universe begin and how is it evolving?
Understanding how the universe began and how it is evolving are two of the most fundamental questions we seek to answer. Adding knowledge of the structure and constituents of the universe reveals deeper questions and mysteries about how nature fits together.
The universe ‘began’ in a very hot, dense, state, at temperatures and energies well beyond anything we can currently test in a laboratory. In order to explore this, we need to understand how the theories of matter and particle physics interface with gravity.
We also need to understand how any universe could begin so we can predict how our universe that we see today can evolve. For example, how do galaxies form? One of the key questions we have is, why is so much of our universe dark?
The present-day composition of the universe is only 5% ordinary (baryonic matter), 25% in the form of dark matter and remaining 70% is the illusive dark energy. Dark energy is thought to cause the accelerating expansion of the universe, a phenomenon whose observation was awarded a Nobel Prize in Physics in 2011.
Determining the source and explanation of these phenomena is of crucial importance to completing the picture of the cosmos.
Understanding the universe takes us from exotic conceptual theories of physics, to experiments underground trying to detect dark matter. This includes state-of-the-art mega-simulations of galaxies forming and evolving to space missions attempting to understand the nature of dark energy.
The goal of understanding our origins encompasses all areas of theory, experiment, and observation, and can truly be considered a fundamental science challenge.
A1: what are the laws of physics operating in the early universe?
To fully understand the universe we live in today, we must understand the laws of physics that govern how the universe began and how it has evolved.
There are a number of ways in which we can investigate this. Like the cosmic microwave background, the cosmic neutrino background has the potential to provide us with a snapshot of the very early universe.
The detection of gravitational waves and the Higgs boson both open up new possibilities for understanding our universe. The detection of primordial gravitational waves would allow us to understand what happened at the time immediately after the universe was born.
In addition, the Higgs boson may play a central role in the evolution of the universe and help us to understand how the universe expands and why there is matter-antimatter asymmetry, some of the key cosmological questions of our time.
Exploring these new kinds of physics through experiments and coupling them with the latest theories will give us a more precise understanding of how our universe began.
A2: how did the initial structure in the universe form?
We would not have any galaxies or stars if the Big Bang had produced a uniform initial state. However, small density differences (fluctuations) in the early universe have created the rich mix of clusters and galaxies that we observe today.
It is still a big challenge to understand what created these initial fluctuations and how they developed to become the mix of galaxies and stars, and indeed for life itself to be created.
The cosmic microwave background provides a crucial constraint on our understanding. But the challenge is merging our theory of how the universe evolved, including massive neutrinos and gravity (possibly with modifications to our current theory), with our observations of galaxy clustering, gravitational lensing, the clumping of the inter-galactic medium and the abundance of galaxy clusters. All of which play an important role in understanding the origin of first structures.
A3: how is the universe evolving and what roles do dark matter and dark energy play?
Our current understanding of the universe tells us that only 5% of it is made of ordinary (atomic) matter. The rest is thought to be made of dark matter (25%) and dark energy (75%), but what are they?
We know dark matter exists on galactic scales. We know this from studying the curves of spiral galaxies and, on much larger scales, from studying the gravitational lensing effect where light reaching us from distant galaxies is ‘bent’ around a clumpy dark matter.
Clusters of galaxies contain a large amount of dark matter so investigating these would allow us to explore what dark matter might be. We can do this by studying the geometry of the universe using ‘standard candles’ (supernovae type Ia) and ‘standard rulers’ (baryonic acoustic oscillations in the galaxy distribution). Coupling this information with data about the growth of structure from weak gravitational lensing and from the abundance of clusters gives us insight into the nature of dark energy and tell us if Einstein’s theory was right.
A4: when and how were the first stars, black holes and galaxies born?
It is thought that in the early universe, stars with short lifetimes were formed and then were rapidly destroyed in supernovae or gamma-ray bursts. This creates a region, rich with many different atoms created in the nuclear reactions.
This was combined with newly forming galaxies and black holes to create the universe that we know today. Recent theories speculate that dark matter is made of primordial black holes, so studying these may give us a greater understanding of dark matter.
Whilst we can’t directly study this time of star formation, the crucial question of how stars are born can be tackled through two complementary approaches. The first involves directly observing how stars are born in clouds of interstellar gas in our own galaxy and nearby galaxies. The second involves using super-computers to simulate how stars are born. Comparing direct observation with simulation may allow us to test our understanding of the birth of stars.
Observing very distant galaxies and black holes allows us to look back in time. It is thought that the first galaxies merged to form large clusters of galaxies that are big enough to observe today with the most powerful telescopes. Studying these galaxy clusters may allow us a greater understanding of how they formed.
A5: how do stars and galaxies evolve?
Understanding the evolution of stars and galaxies from the time of their formation, through cosmic time, up to the present day is one of the major challenges in modern astrophysics. Whilst we do have a reasonable understanding of how stars evolve important gaps in our knowledge remain. For example, how does the speed at which stars rotate change, and how is material re-distributed inside stars, as they age?
We still only have a limited understanding of how stars are born and how they die. These are the very phases that impact most on galaxy evolution. We want to be able to reliably estimate properties of individual stars so we can understand the populations of different stars in our galaxy, and how the Milky Way has evolved over time. On a different scale we are also starting to understand the role of merging stars in galaxy construction and evolution.
The most massive stars end their lives as a supernova explosion that gives birth to a black hole or neutron star. In these extreme environments we can study exotic physics which pushes our theories of nature to the limit. Stars such as the Sun are not massive enough to end in this way but instead form white dwarfs, studies of these objects allow us to test our theories of gravity.
A new generation of telescopes and satellites is providing data that challenges our traditional approach to modelling stars and helping us to address some of these questions.
A6: how do nuclear reactions power astrophysical processes and create the chemical elements?
Shortly after the creation of the universe in the Big Bang, the cosmos contained mainly hydrogen and some helium but little else. All other chemical elements, including those such as carbon and oxygen essential for life, must have been produced by nuclear reactions between those light elements.
Energy is required for two nuclei to approach each other closely, against their mutual electrostatic repulsion, and undergo nuclear reactions. In the laboratory this energy is provided by particle accelerators.
In the universe, complex nuclear reaction networks occur through ambient thermal energy in sites such as stars, supernovae and during other explosive astrophysical events.
Production of light elements in the universe is relatively well understood. However, for heavy elements, while we know that they exist so processes must occur in the universe that manufacture them, many of the rates of critical nuclear reactions within those processes are unknown and, to make the challenge even harder, they depend on the characteristics of very short-lived radioactive isotopes, far from the stable species found on Earth.
Even where in the universe these reactions occur is not well determined. Recent electromagnetic observations, initiated by the gravitational waves detected from a neutron star merger, have indicated that such events are a key astrophysical site.
Studies of critical reaction rates between exotic isotopes in Earth-based laboratories, combined with observational data from astronomy, will be needed to understand the origin of the chemical elements in nature.
A7: what is the true nature of gravity?
Our current understanding of gravity comes largely from Einstein’s work on general relativity. The recent detection of gravitational waves confirms a long-standing prediction of this theory in which a bending or warping of space-time is thought to be responsible for gravitational forces.
This description is in stark contrast to the way that the other three fundamental forces are understood. Electromagnetic interactions and the strong and weak nuclear forces are seen as fundamentally arising from exchange of quantum particles in so-called quantum field theory.
Unfortunately, attempts to establish quantum theories of gravity run into serious difficulties. When the gravitational quanta are included in quantum field theories, the known properties of gravity lead to unphysical infinities in the calculations suggesting that a more radical, but as yet unknown, approach is necessary.
Finding a theory that describes gravity in this way would enable all the known forces to be understood on a common theoretical basis. Such a theory is also required if we are to properly understand small compact objects with strong gravitational effects. Shortly after the Big Bang, the universe was such an object so it will be impossible to fully understand it’s origin and evolution without understanding the true nature of gravity.
A8: what can gravitational waves and high-energy particles from space tell us about the universe?
The direct detection of gravitational waves has opened a new era of science, allowing us to probe exciting new phenomena. There is now much to be learnt from multi-messenger observations, combining gravitational waves and electromagnetic radiation from the same object, as illustrated by the binary neutron star event GW170817. Black hole binaries are not expected by most current theories to have electromagnetic counterparts, but nature may surprise us.
Gravitational waves originate in processes that involve extreme conditions. This allows them to be used to study both violent events occurring in the universe and cosmological properties. Gravitational waves may help us probe dark energy (for example as the speed of gravitational waves is observed now to be the same as the speed of light), as well as to estimate the Hubble constant (the rate of expansion of the universe) by an entirely new method.
Technology is also allowing the detection of high-energy particles from the most extreme astrophysical environments. Supernovae, for example, produce vast numbers of neutrinos which can, in certain circumstances, be detected.
It is expected that other extreme events will produce the highest energy photons which, while we cannot detect them directly, their impact with the Earth’s atmosphere eventually produces characteristic optical photons we can detect. These detections may also help us to identify the source of dark matter as well as allowing us to peer into supermassive black hole systems.
Challenge B: how do stars and planetary systems develop and how do they support the existence of life?
Is there life elsewhere in the universe?
We know that microbial life can survive in hostile and extreme environments such as deep sea geothermal vents. The consensus now is that where there is water, nutrients and an energy source such as our Sun there is the possibility for discovering life. The obvious first place to search for extra-terrestrial life is within our own solar system. Possible examples include Mars, which may have regions in its permafrost that could harbour microbial communities, and also in the subsurface water ocean of Jupiter’s moon, Europa.
The Sun is central to life on Earth. Its proximity to us means it can be studied to levels of detail not possible for other stars. As such the Sun has a special place in astronomy, underpinning studies of how stars and their planets form and evolve.
Satellites and telescopes are now also discovering thousands of planets orbiting other stars in our galaxy. These new observations are revealing many exoplanet systems, which will help us to place our solar system in a wider context. Studying them will help us understand how common solar systems like our own are and how many exoplanets might be capable of harbouring life. We may even be able to recognize the existence of life on these distant worlds.
B1: how does the Sun and other stars work and what drives their variability?
The Sun is highly active and this gives rise to a variety of phenomena that we can observe. These range from dark sunspots on its visible surface, to hot plasma and intense flares that are visible in its outer chromospheric and coronal layers.
This activity is associated with the Sun’s magnetic field, and regeneration of the field leads to variations of the Sun’s outputs and emissions over time. Some happen over a short timescale, such as those associated with flares and huge eruptions of plasma called coronal mass ejections. Others happen over decades and are associated with the cycle of the Sun’s magnetic field. There are even variations that happen over hundreds of years to millennia, which we can see on Earth by observing tree ring growth and radioactive isotopes in polar ice sheets.
Even though we cannot produce detailed images like we can for the Sun, observations of other stars show similar signs of activity, such as spots, flares and cyclic variability.
We need to understand what drives these changes, and how are they influenced by internal stellar structure and dynamics. This would give us clues to understand how stellar interiors link to stellar atmospheres where the active phenomena are observed. It may also help us explain how coronae are heated to million-degree temperatures. Furthermore, it may then explain why activity levels vary from one solar cycle to another, while some stars show no cycles at all.
B2: what effects do the Sun and other stars have on their local environment?
The Sun affects every aspect of our daily lives, from providing the energy to create and sustain life, to the impact it has on our ability to communicate around the globe.
The Earth and the other planets in our solar system are bathed not only in the Sun’s light but also streams of charged particles from the solar wind and eruptions called coronal mass ejections, which produce ‘space weather’. We are trying to understand the occurrence and size of coronal mass ejections, how the solar wind is accelerated, and how it interacts with the planets (and gives rise to geomagnetic storms).
Thousands of planets have now been discovered orbiting other stars, and we are able to study the interactions of these suns with the many different types of planets they host. The variety of systems now amenable to study is opening a window on the past and future of our own solar system.
B3: what processes govern how planetary systems form and evolve?
Stars are formed during the collapse of molecular clouds and, as a by-product, these gas disks also form planets. We still do not understand the actual processes that lead to the birth of stars and planets but we think are probably quite different. Stars may form during a gravitationally controlled collapse of the centre of a gas disk. On the other hand, planets may be formed in a number of ways.
We suspect that most planets are ‘grown’ by smaller particles gathering together but some may form through the gravitational collapse. By studying these processes we hope to understand how the composition of a planet reflects that of the gas disk from which it was formed.
One of the early discoveries from the study of exoplanets was the existence of short-period massive gas planets. As these don’t exist in our solar system this led to the realisation that solar systems were probably dynamic places with planets moving through the disk either by interactions with the disk itself or interactions with other planets (or stars). It may even lead to the ejection of planets from their planetary system or, indeed, the ingestion of planets by their host star. There is still much to understand in these processes.
We suspect that our own solar system had a violent beginning and that the orbits of solar system planets have significantly changed over time. How unique our solar system is remains to be determined. It is likely that planetary evolution is complex and much remains to be understood there.
B4: what are the conditions for life and how widespread are they?
We have begun the first searches for extra-terrestrial life. We can do this because we now know that habitable environments may indeed exist in places originally thought hostile to life. For example, at Jupiter’s distance the Sun’s warmth is greatly diminished, but more favourable conditions may exist in the oceans of its moon, Europa.
To know if life can exist on a planet (or if it has ever existed), we need to understand how the environment of a planet changes over both geological and astronomical timescales. The source of these changes could be external, for example the host star or the planetary system itself, or internal to the planet, such as plate tectonics.
We also need to understand how life interacts and evolves with its environment. In our solar system we can study some habitable environments in-situ. Studies of the conditions prevailing on the early Earth have shown that surface conditions have changed enormously over time. This may make it possible to detect life on Earth from space. For exoplanets, less direct atmospheric studies are possible.
In each case one of the most challenging problems we face is actually identifying the signs of life. At the moment, there is no consensus on what biomarkers to trust. It is possible that the next decade will bring the first claims of the detection of extra-terrestrial life, but this is likely to be controversial.
B5: how diverse are exoplanets and is our earth typical?
It is becoming more routine to discover exoplanets now. With this growing number of them we can ask increasingly detailed questions about the distribution of different types.
The main challenge is characterising them. For example, the easiest planets to detect are called short-period massive ‘hot-Jupiters’. As their composition is very simple, we expected them to have predictable physical parameters, such as density. In fact, we see Jupiter-mass planets with very different diameters. This indicates that we do not yet fully understand the internal processes occurring in these planets.
One of the most common types of planets seem to be the so-called super Earths. These are massive rocky planets with maybe five to 10 times the mass of Earth. Our attempts to understand this planetary class is hampered by the lack of an example in our solar system to study in detail.
The situation with smaller planets is just as complicated. Theory predicts a range of possible configurations and compositions with an Earth-like rocky structure being just one type. However, we still don’t know how common small rocky planets actually are because they are hard to detect.
Studying the atmospheres of exoplanets is a new and growing field but it is extremely technically challenging. The closest, largest planets have been studied the most as they have extremely thick atmospheres. This means we can study them indirectly using some of our most sophisticated telescopes. However, these thick atmospheres make it hard to probe their lower depths where we expect to see many interesting molecules.
By picking planets to study that are similar in size to the Earth we hope to be able to study solar systems that are similar to ours and discover whether our solar system is truly unique.
B6: what are the processes that drive space weather?
The Sun’s changing outputs and emissions give rise to space weather.
Swathes of charged particles travel through the heliosphere and interact with the Earth’s magnetic field. Understanding space weather is important because these interactions adversely affect radio propagation and global positioning system (GPS) signals, leading to significant challenges for the daily operation of airlines and other transport networks.
They can also induce electrical currents that are strong enough to disrupt electricity supplies and other ground-based infrastructure. The dangers posed by space weather now feature on the UK National Risk Register.
Of fundamental importance to mitigating these dangers is the need to better understand the physical processes that drive space weather, particularly those taking place from the solar interior to the near-Earth environment. This includes understanding what drives the Sun’s emissions, including coronal mass ejections. We need to know how those emissions propagate through the heliosphere to the Earth. An improved understanding of the processes may help us to reliably forecast these events.
Challenge C: what are the basic constituents of matter and how do they interact?
In order to understand the universe, we must understand what it is made of and what rules govern its behaviour. We study matter in experiments to understand what its most basic constituents are and how they behave. We also develop theories that explain this behaviour in terms of underlying laws of nature.
Our research has allowed us to identify the most basic building blocks of matter, quarks and leptons, and the forces and energy fields that affect them. We’ve made tremendous advances in understanding the fundamental role these particles have and in describing how these interact with each other within our theories. The recent discovery of the Higgs boson at the Large Hadron Collider was an example of how huge progress in experimental technology can be harnessed to address questions in this field.
However, many mysteries remain. We don’t yet understand why there is so little antimatter in the universe. Yet knowing this will help us understand how the universe evolved from the Big Bang to now. We don’t understand what dark matter is made of, or how it behaves, and if it influences normal matter at all.
We want to better understand how quarks combine to form larger particles and atomic nuclei, and how nuclear reactions power stars. Above all, we want a deeper and more complete understanding of our universe at these most fundamental levels.
C1: what are the fundamental particles and fields?
We describe the constituents of matter using quantum fields within the framework of quantum field theory. We do this as it is necessary at the subatomic level to allow for the equivalence between waves and particles, and between matter and energy. These particles include electrons and quarks (quarks make up protons and neutrons), photons (which carry energy from the Sun to the Earth), neutrinos (billions of which pass through each of us every second) as well as rarer particles like the Higgs boson.
Although we have identified six types of quarks and six types of leptons that form matter, there is much to find out about them. We do not know what the masses of neutrinos are, or if there are more types of fundamental particles in the universe. We are also trying to see if any particles exhibit unexpected behaviour that could indicate new laws of physics.
The universe contains dark matter but we still don’t know what this mysterious material is composed of. In trying to explain the accelerating expansion of the universe we are considering if there are new Higgs-like fields. We are also looking for signals of new forces of nature in addition to the electromagnetic, weak and strong force by searching for heavier new particles.
It is even possible that the particles we think of as fundamental today might turn out to be made of smaller constituents. Discovering and studying these would allow us to reveal an even deeper understanding of matter.
C2: what are the fundamental laws and symmetries of physics?
Identifying symmetries, behaviour that is unchanged when one aspect of a physical system changes, can reveal underlying laws of nature. For example, physical processes are observed to happen in the same way in systems moving relative to each other. This symmetry in behaviour is linked to the laws of energy and momentum conservation.
There are other abstract symmetries in nature called ‘gauge’ symmetries which govern our entire framework for understanding electromagnetic, weak and strong interactions and tell us that interaction-mediating particles must exist.
There is a direct relation between symmetry, laws of physics and the sets of particles that we observe. If we look for new symmetries it could lead us to a deeper understanding of the universe’s structure.
We study the difference in behaviour of matter and antimatter particles to deduce if there is an underlying symmetry that might explain the phenomenon. We search for evidence of supersymmetry, a symmetry between particles with difference quantum ‘spins’, and a description of nature that predicts new particles and may explain dark matter.
At a deeper level, symmetries could allow us to understand how forces are related to each other. We are trying to understand if the symmetries that separately describe the electromagnetic, weak and strong nuclear forces will unify into one new symmetry (and new force) at very high energy. It may even be possible to understand gravity on a quantum scale, and one day relate this to the other forces too.
C3: what is the nature of space-time?
Space-time is both the fabric of the universe and intimately connected with how the universe works. Einstein’s theory of general relativity says that gravity is a consequence of the curvature of space-time and that it depends on the distribution of matter and energy.
At the small distance scales of the quantum world, and the early universe, we need a unified description of gravity and the other fundamental interactions. String theory may provide such a description, but it operates in a space-time with many more dimensions than the four (three space and one time) that we normally experience.
These extra dimensions may be ‘curled-up’ over an infinitesimal distance and our universe may exist only on a membrane in a much larger and higher-dimensional space. The geometry of the extra dimensions is related to how the symmetries of the fundamental interactions emerge from this unified picture.
A key challenge is to understand how this works, and whether it provides constraints on our universe, or if there are many consistent possibilities (a ‘multiverse’) and we simply live in one of them.
Observational tests of the nature of space-time include tests of our understanding of gravity on both large and small scales, and studies of gravitational lensing and gravitational waves. We can also search for signs of large-scale topological structures that point to symmetry-breaking in the early universe and for the existence of extra dimensions. One way to do this is by looking for their impact on the results of high-energy particle collisions in our accelerators.
C4: what is the nature of dark matter and dark energy?
The matter that we’ve studied in our experiments makes up just a fifth of all the matter in the universe, and just 5% of the universe’s total energy. Most matter comes in the form of dark matter, an invisible, extremely weakly interacting type of matter that has never been seen directly. We know that dark matter exists from many types of astrophysical observations.
The nature of dark matter is still a mystery. Many hypotheses of what it is made of have been put forward. These range from extremely light axions to very heavy supersymmetric particles, to give just two examples. We search for these new particles in our experiments, and study the particles we know well for any signs of dark matter’s influence on their behaviour.
Dark matter is a tiny component of the universe when compared to dark energy. This makes up 75% of the universe’s energy and is thought to cause the accelerating expansion of the universe.
Dark energy is even more mysterious than dark matter. At the moment we have no real understanding of its nature. However, we hope that studying the universe’s evolution, large scale structure and how the density of matter varies may give us a better understanding of this antigravitational force which dominates the universe today.
The matter that we’ve studied in our experiments makes up just a fifth of all the matter in the universe, and just 5% of the universe’s total energy. Most matter comes in the form of dark matter an invisible, extremely weakly interacting type of matter that has never been seen directly. We know this dark matter exists from many types of astronomical observations.
The nature of dark matter is still a mystery. Many hypotheses of what it is made of have been put forward. These range from extremely light axions to very heavy supersymmetric particles, to give just two examples. We search for these new particles in our experiments, and study the particles we know well for any signs of dark matter’s influence on their behaviour.
In addition, dark energy that makes, at present, up to 75% of the universe’s composition and is thought to cause the accelerating expansion of the universe.
C5: how do quarks and gluons form hadrons?
The strong interaction binds quarks into hadrons, and protons and neutrons into nuclei. Its physics is notoriously complex. Quantum chromodynamics (QCD) is well established as the theory of this interaction, but calculating the masses and other properties of hadrons from QCD stretches our most powerful supercomputers.
Understanding QCD is critical to interpret our measurements of the observable hadron world in terms of the properties of the invisible quarks. Quark interactions probe fundamental physics and learning more about them is a key route to uncovering new physics.
The variety of types of quark and their possible combinations multiplies to give hundreds of different hadrons. Most exist only fleetingly and evidence for them must be reconstructed from the tracks of their decay products in our detectors.
A key challenge is to find clear evidence of particles that differ in makeup from our current ‘zoo’ of two-quark and three-quark hadrons. These could be four- or five-quark combinations or even zero-quark ‘glueballs’.
To understand how hadrons work, we need to build a detailed picture of their internal structure, the aim is a complete tomography. For example, we want to understand how the momentum of a moving proton is shared among its constituents and how its spin is made up from quarks’ spins and orbital motion.
The rich physics of hadrons has ramifications for nuclear matter as well as internal structure and dynamics. Understanding all its facets is a key part of the jigsaw of the subatomic world.
C6: what is the nature of nuclear matter?
Atomic nuclei carry much of the conventional mass in the universe (as opposed to dark matter and dark energy). They comprise protons and neutrons (collectively known as nucleons) bound together.
It is a great challenge to understand the nature of such material. This is partly because the force between nucleons is difficult to describe and is not fully understood. It is also because most nuclei are complex, many-body objects. They are composed of too many nucleons for simple calculations to work and too few for the simplifications due to statistical averaging to be sufficiently accurate.
Nuclear forces have some peculiar characteristics. For example, the interaction between two nucleons can be influenced by the presence of a third. Also, the forces have relatively long-range effects that can also lead to coherent motion of many nucleons together. We would expect our theoretical calculations to help us understand these forces and yet they don’t describe what is happening accurately.
Understanding such characteristics is very challenging and requires the creation and study of very short-lived exotic isotopes. These are very different from the stable systems found on Earth. In fact, it is these exotic isotopes that participate in astrophysical nuclear reactions responsible for creating the chemical elements. Their currently unknown characteristics influence the rates of these reactions and determine the abundances of the chemical elements seen in nature.
C7: are there new phases of strongly interacting matter?
Quantum chromodynamics describes the force that binds quarks and gluons into protons and neutrons. It is also responsible for the interaction that holds protons and neutrons together in atomic nuclei.
While quarks are ultimately confined by their strong mutual attraction, deep inside protons and neutrons they behave as though they are free particles. The strength of the interaction depends on the distance between particles. This leads to different phases of strongly interacting matter under different conditions.
At high temperatures and densities nuclei are known to melt, forming a strongly-coupled liquid of quarks and gluons called a quark-gluon plasma. It is thought that this phase replicates the state of matter at about a millionth of a second after the Big Bang.
Studying the quark-gluon plasma gives us insight into what happened in the early history of the universe when structure first appeared.
In the realm of low temperatures and high densities, a different phase of quarks and gluons may form, called a colour superconductor. Studying the behaviour of nuclear matter over a wide range of temperatures and densities may reveal the structure of matter at the very core of neutron stars and be testable from gravitational wave observations of binary neutron star coalescence.
C8: why is there more matter than antimatter?
We think the universe was formed of half matter and half antimatter at the time of the Big Bang. This very early universe was a seething mass of activity. Particles annihilated their antiparticle counterparts and produced photons that were energetic enough to make new pairs of particles and antiparticles.
However, after less than a second, this cycle of annihilation and creation stopped. Due to the universe’s expansion and cooling, everything inside had lost so much energy that new particles and antiparticles could no longer be created.
Our matter dominated universe is made of the matter particles left over from those last annihilations. We do not yet understand how there was so much less antimatter than matter at that point.
Up to now, we have studied the behaviour of quarks and antiquarks in our experiments. Although we have uncovered differences between these particles, the difference is not large enough to explain how a matter-dominated universe evolved. We think the answer might lie in the as yet unmeasured difference in behaviour between neutrinos and anti-neutrinos, or in the behaviour of new particles and antiparticles we have yet to discover.
C9: what will precision measurements of the Higgs boson reveal about the universe?
The discovery of the Higgs boson is a fundamental breakthrough in our understanding of matter. It reveals that basic properties like particle masses are not intrinsic, but arise from their interactions with the Higgs field. This overturns the model of matter we have held for two millennia.
Precision measurements of the properties of the Higgs boson will reveal whether it is a single particle, as predicted by the Standard Model, or part of a larger system of fields, as predicted in extensions of the Standard Model, for example using supersymmetry. These measurements are also sensitive to new particles in ways which are difficult to access by other means. This opens a new window to dark matter and other hypothesised ‘dark sector’ processes.
The Higgs, or a similar field, offers an explanation for why the universe appears to have undergone a period of extremely rapid expansion in its very earliest stage. It can also explain why the current expansion rate is increasing, rather than slowing under the influence of gravity.
It is important the understand the nature of the transition which occurred when the Higgs field ‘switched on.’ This could explain the process which created the baryonic matter which forms all atoms. The current measurements of Higgs properties imply that the entire universe exists very close to an unstable state. Future measurements should reveal whether in future a new transition to completely different physical state is possible, or whether as yet unknown effects mean that our universe is stable.