STFC Science Challenges highlight our key scientific challenges. The STFC Strategy is built around these questions and is intended to set future scientific opportunities and options out in a structured way.
Understanding how the Universe began and how it is evolving are two of the fundamental questions that astronomers and space scientists seek to answer. There is considerable knowledge of the structure and constituents of the Universe and ever increasing understanding 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 the remit of current tested theories of particle physics and gravity. To explore this regime, understanding is needed of how theories of matter and particle physics interface with gravity. Research is also needed to understand the possible ways in which any universe or cosmology could begin, so that scientists can effectively predict the Universe we see today.
The next phase in explaining the cosmos is to understand how it evolved from this ”beginning”, for example how do galaxies form, light up, and become the rich structures we see today? It is already understood that a crucial part of this picture is the existence of something about which little physics is known: Why is so much of the Universe dark? Observations tell us that most of the matter in the Universe is not the usual stuff of which we are made, but something else as yet unknown and detected by its gravitational effect. Current research shows that matter is not the most important part of the Universe. Instead, three quarters of the Universe is made up of “dark energy” a form of energy completely beyond normal experience and this is causing the Universe to move apart ever faster. 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; from 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.
Fully understanding the Universe we live in today requires that we understand how the laws of physics governed its origins and evolution.
Our current picture suggests that the cosmos began from an initial state that was extremely hot and dense and, via expansion and cooling, evolved to the structural form of the Universe that we observe today.
We strive to gain information from the residual echoes of the physical processes that forged the early Universe which are imprinted on the cosmic microwave background and on the large scale distribution of galaxies and intergalactic matter. Any theory encompassing the physics of the early Universe will have to be able to encompass and support the results of these measurements.
Via observations of the interactions of particles at the highest energy accelerators, at high energy densities and through precision measurements at low energy, we aim to progress towards directly studying the particles and forces that are normally not visible in everyday life, but were effective at the birth of the Universe.
We would not have any galaxies or stars if the Big Bang had produced a uniform initial state. The entire Universe would be a uniform mix of matter and energy. However, small differences (fluctuations) in the initial conditions have created the rich spectrum of clusters and galaxies that we observe today - although our understanding of what created the initial fluctuations and how they developed to become the anisotropic mix that allowed galaxies, stars and indeed life itself to be created is far from complete.
The most direct constraints on these fluctuations come from the cosmic microwave background, but confronting the theory for production with observations from galaxy clustering, weak gravitational lensing, clumping of the inter-galactic medium and the abundance of galaxy clusters will play an important role in understanding the origin of first structures.
Over the last decade a "standard model" of cosmology has emerged consisting of a nearly flat universe in which just 4% of the mass-energy is in the form of ordinary matter, the so-called baryonic matter, with remainder residing in two utterly mysterious forms, namely dark matter (21%) and dark energy (75%).
Dark matter is evident, in the nearby Universe, through its impact on the dynamics and gravitational binding of individual galaxies and groups of galaxies, although the particles which comprise the dark matter component as yet remain elusive although the particles which comprise the dark matter component remain elusive and yet have to be detected, either through their annihilation products or directly in the laboratory.
The impact of dark energy is more apparent on very large scales, for example it gives rise to the accelerating rate of expansion of the Universe. Both theory and observation offer routes to a deeper understanding of dark matter and dark energy but, at least for the moment, the dominance of the "dark Universe" remains a conundrum in modern cosmology.
Following the fireball of the hot Big Bang, the Universe expanded and cooled to the point where the normal matter, comprised almost entirely of hydrogen and helium, could take a neutral atomic form. This is the epoch of recombination, when the radiation which we currently detect as the cosmic microwave background was generated. There then followed a cosmic "dark age" until eventually the first stars lit up the Universe anew.
It is likely that the first stars, were massive objects with short lifetimes, which through their explosive destruction in supernovae and/or gamma-ray bursts, chemically enriched their surroundings. Rapid star-formation in the newly forming galaxies and the production of both stellar-mass and galaxy-size black-holes are further ingredients within this particular melting pot. One consequence of the birth of luminous sources and their associated hard radiation fields is that the neutral hydrogen of the Universe will be converted back to an ionized form during the "Epoch of Re-ionization". A better understanding of the cosmic Dark Age, first light and the re-ionization of the Universe are key goals of modern astronomy.
Both observations and theoretical simulations show that the Universe which emerged from the Big Bang was clumpy on large-scales. These large-scale structures are assumed to have grown gravitationally, under the influence of both dark matter and dark energy, from primordial seed perturbations, possibly generated during a period of inflation in the early Universe. It is within the highest density regions of this fabric that galaxies, the fundamental building blocks of the universe, and clusters of galaxies first formed. Understanding the subsequent evolution of galaxies from the time of their formation, through cosmic time, up to the present day represents a formidable challenge in modern astrophysics. However, using a wide range of complementary techniques and wavebands we are beginning to probe the underlying mechanisms of galaxy formation such as the role of mergers and interactions and the importance of feedback from outflows driven by supernova and active galactic nuclei.
How do stars form in collapsing clouds of interstellar gas?
This crucial question can be tackled through two complementary approaches. The first involves direct observation of stellar birth in dense molecular clouds within our own Galaxy and in neighbouring galaxies, whereas the second entails state-of-the art numerical simulations using supercomputers. Once born, stars spend time on the main-sequence, a reference to the famous Hertszsprung-Russell diagram, before following evolutionary tracks which lead for fairly massive stars to death in a supernova explosion or, for less massive stars, to a quieter end as a cooling white dwarf. One astrophysical consequence of the cycle of stellar birth, life and death is the chemical enrichment of the surroundings by heavy elements forged in nuclear processes in the stellar interiors and atmospheres. Galaxies are comprised of different populations of stars which reflect episodes of star formation through the period since their formation up until the present day.
By tracing these various stellar populations, astronomers have a tool for investigating how the chemical enrichment of the Universe has progressed over cosmic time.
The solar system is a diverse environment from the small rocky worlds at its centre (including Earth), to the gas giants with their own systems of moons and the numerous minor bodies such as asteroids and comets. Each of these bodies offers different clues to the formation of the solar system. In recent years we have been able to observe planets orbiting distant stars. The development of more advanced techniques and sophisticated telescopes and satellites will lead to an increased understanding of distant solar systems.
The Sun is central to life on Earth. As the nearest star, the Sun has a special place in astronomy as detailed measurements are not possible for its distant cousins. Current projects have the capabilities to explore the physics of the Sun, from its exterior regions that impact directly on conditions on the Earth, down to the processes that take place at its centre. It is a keystone on which much of the knowledge of stellar structure and evolution is based. Furthermore the Sun’s variability is of crucial consideration for climate models of the Earth.
A big question for humanity, both from the standpoint of science and philosophy, is whether or not there is life elsewhere in the Universe. Over recent years a paradigm shift has occurred in the debate regarding the probability of life, and in particular whether benign earth-like conditions are required. Microbial life has been discovered on Earth which survives well in the most hostile and extreme of environments. The consensus now is that, where there is water, nutrients and an energy source such as our Sun, or from within a planet’s sub-surface, then the possibility for discovering life (past or present) is considerable. The obvious first place to search for extra-terrestrial life is within our own solar system. Possible examples include Mars that may have regions in its permafrost that could harbour microbial communities, and also in the subsurface water ocean of Jupiter’s moon, Europa. Beyond our own solar system, exoplanets may also harbour life.
Until very recently the only examples of planetary systems were those within our own Solar System, and whilst many speculated as to their existence beyond our Solar System, none had been seen. With the advent of new detection techniques over 500 extrasolar planets have now been discovered. Whilst these numbers will continue to rise, we have yet to understand if planetary systems are truly common throughout the galaxy, and furthermore if any of these extrasolar planets are like ours.
The Sun affects every aspect of our daily lives from providing the energy to create and sustain life, the driving of our oceans and atmosphere, through to the impact it has on our ability to communication around the globe. It has the ability to both facilitate and destroy every element of modern life and thus its influence is immense. Nevertheless there are many mysteries to the Sun and the ways by which it affects our environment and daily lives. The Sun impacts upon all the worlds within its ‘reach’ and by studying these interactions we also gain a greater insight into the totality of the Sun’s influence and its effect upon ourselves.
For many centuries it has been thought that Earth was the only world capable of supporting life, and that the other planets within our Solar System and beyond were just far too hostile. We now know that where you find water, nutrients and an energy source such as heat from the Sun, then invariably you find life, even when the environmental conditions are extreme. Recent exploration missions such as those to Mars and the discovery of the many planets beyond our Solar System has shown that these life forming conditions do exist elsewhere in the Universe, and thus the search in on!
The ancient Greeks proposed that the Universe was made of tiny indivisible "atoms" and scientists have long searched for an answer to the question: what is the Universe made of? The search for an answer led to the discovery that atoms are made up of smaller particles, a nucleus of neutrons and protons surrounded by electrons, and that the neutrons and protons are in turn composed of quarks and gluons. Along the way has come the recognition of the periodic table of elements, atomic theory, nuclear physics and quantum mechanics. Such developments have been fundamental in understanding how the Universe evolved.
Experiments and theory research continues is ongoing as many puzzles remain unsolved. For example why do we see more matter than antimatter; what is the nature of the force that binds quarks, and nucleons, together; what are the limits of this force; are forces unified at high energies; where does the mass of particles come from; what is the missing dark matter which seems to make up about a quarter of the Universe; and what is the "Dark Energy" which may drive the expansion of the Universe? Particle accelerators along with arrays of sensitive underground detectors, and astrophysical observations, let scientists peer into the fundamental structure of nature and back towards the big bang.
The electron was discovered more than 100 years ago, and as far as experiment can tell us, it is a fundamental constituent of nature - it is not made up of anything else. The atomic nucleus, on the other hand, has revealed layer after layer of substructure down to the level of quarks. Quarks are as fundamental as electrons and along with neutrinos, plus two heavier copies of the electron (muons and taus), they make up the full cast list of fundamental players in the Standard Model of particle physics. Is this the whole story? There are hints, for example in the patterns of the masses and charges of the particles, that it may not be. Particle colliders are studying electrons and quarks at shorter and shorter distances, and may throw up new layers of substructure or new forces at any time. As well being important for its own sake, knowing the basic cast list of physics is a requirement for addressing many of the other "Science Challenges".
This question covers fundamental issues of the very nature of the Universe. In addition to the four dimensions that we normally experience, there may be extra dimensions, as predicted by string theory. Space-time itself may be continuous, or may exhibit discreteness due to quantum gravitational effects. The space-time may be Lorentz-covariant, or there may be a breakdown in this symmetry and a preferred frame may exist. The topology of the Universe may be non-trivial, and the equations which govern the evolution of space-time may differ from those of Einstein gravity.
These all represent core questions about the fabric and behaviour of the space-time we inhabit. A variety of possible techniques can be used to try to help answer them. Global topology may be studied with detailed analysis of the microwave background radiation. Cosmological-scale extra dimensions, such as may exist in braneworld string models, may alter the effective gravitational law and change the growth rate of structure, which may be studied with gravitational lensing and large-scale redshift surveys of galaxies. The law of gravity can be studied via its potentially different effects on relativistic and non-relativistic particles or systems through the same observations. A breakdown of Lorentz covariance can be studied in various ways, such as by searching for preferred frames in cosmic ray experiments, or energy-dependent light speeds in gamma-ray observations. The linked question of CPT violation can be investigated in terrestrial colliders. The existence of other dimensions can be studied via precision observations seeking the breakdown of the inverse-square law on sub-mm scales, or by searching for the effects of Kaluza-Klein particles on interaction cross-sections as a function of energy in colliders, or in the appearance of mini-black holes. Evidence for superluminal particle speeds from precision timing would challenge the entire relativistic framework. Extra dimensions can also produce oscillatory modes in gravitational-wave signals. Finally, tests of gravitation theory and the structure of space-time can be made with high precision measurements of gravitational waves in regions of extreme gravitational fields.
At the energies currently probed by Particle Physics experiments, the effect of the gravitational force is tiny in comparison to the other two forces (electroweak, strong). The apparent weakness of gravity on the quantum scale is a mystery. One explanation why gravity is so weak in our 4-dimensional space-time world is that our Universe exists as part of a higher-dimensional structure.
There are additional spatial dimensions in which only gravity acts wherein the gravitational flux is dissipated. It may be possible to infer the presence of macroscopic extra dimensions from cosmological observations, or from precision measurements of gravity. If the extra dimensions are microscopic, high-energy particle accelerators and cosmic-ray experiments are the only ways to detect them e.g. via the production of rapidly evaporating microscopic black holes.
The unification of forces requires the identification of new symmetries that will explain the pattern of fundamental particles. Such symmetries can be crudely probed by the current experiments but more precise experiments will be necessary to understand any new physics discovered at the LHC.
Dark Matter is required to provide a large enough gravitational force to keep stars in orbit around galaxies, and to keep galaxies bound to galaxy clusters. Most of the Dark Matter cannot be in ordinary (baryonic) form, without destroying the agreement between light element abundances and predictions from Big Bang nucleosynthesis. Detailed study of the microwave background fluctuations shows that there is 5 times as much non-baryonic Dark Matter as baryonic, but little else is known about its nature. The search for Dark Matter can be via direct detection in laboratory experiments, indirect from decay or annihilation products in space, or from study of the matter profiles of astrophysical objects. More exotic models such as coupled Dark Matter-Dark Energy can be studied through their effect on the large-scale density fluctuations.
The accelerating expansion rate of the Universe requires gravity to be repulsive on large scales. This can be effected by Einstein’s cosmological constant, or by Dark Energy - a non-zero (but small) energy density of the vacuum, or a new field which evolves with time. More radically, the acceleration may be a manifestation of other modifications to Einstein gravity. Techniques focus on tests of the geometry of the Universe and the growth rate of fluctuations, both of which depend on the Dark Energy properties, and include weak gravitational lensing, large-scale galaxy redshift surveys and supernova searches.
Understanding the nature of strongly interacting nuclear matter requires a fundamental description of nucleons starting with the individual quarks and gluons and ending with a description of nuclei and their structure. Surprisingly, even the structures of the proton and neutron remain undetermined, and the challenge of describing nucleons within the framework of Quantum Chromodynamics complements that of understanding the nucleus from the interaction of the constituent nucleons. The most extreme test of nuclear matter occurs when nuclei collide at energies at which the energy density reaches five times normal nuclear matter density, providing a laboratory test of the nature of matter a short instant after the Big Bang. Extremes of matter also occur in violent stellar processes such as supernovae and X-ray bursters in binary stars, thought to give rise to the synthesis of many of the heavier elements that exist naturally on Earth.
There are very good reasons to believe that at the time of the Big Bang, some 13.7 billion years ago, matter and antimatter were created in equal amounts. But the Universe as we see it today is made almost entirely of matter. It is known that when the Universe was about 380,000 years old, matter and antimatter had annihilated producing what we now see as the cosmic microwave background radiation, but a small amount of the original matter was left over, and this makes up the matter of the Universe as we now see it. Thus the laws of nature are not symmetric between matter and antimatter; somehow, before the Universe was 380.000 years old, some of the antimatter had converted to matter, but this was not balanced by an equal conversion of matter to antimatter.
The Standard Model of particle physics includes a mechanism, known as CP-violation, that can induce the sort of matter-antimatter imbalance described above. This mechanism has been studied and verified in many particle physics experiments. The problem is that the effect of the Standard Model CP-violation is far too small to account for the original imbalance in the early Universe. There must be some other mechanism that induces matter-antimatter asymmetry, and particle physicists are searching for it. There could be something in the quark sector that is not yet understood, or the sought-after mechanism may be in the lepton sector and related to the behaviour of neutrinos.
The disciplines of Physics and Astronomy are based on the fundamental drive to understand the laws that govern the nature and behaviour of our Universe. Understanding breaks down when considering physics in action at the extremes of the Universe – at astrophysical energies, temperatures and pressures that can be impossible to simulate in the laboratory; and at the smallest distance scales, where the structure of space-time is unknown. By trying to understand the extremes of our Universe, we directly explore the limits of modern physics and expand the frontiers of our knowledge.
Investigations in these areas are becoming possible through a range of experiments. Astrophysical experiments can develop understanding in the phenomenally energetic acceleration mechanisms which result in the production of ultra-high-energy cosmic rays and cosmic neutrinos. Searches for gravitational waves and x-ray observations are used to study the limits of extreme gravity in the environment of black holes. Supernova and gamma ray burst observations determine the formation and behaviour of compact objects – black holes and neutron stars.
Particle physics experiments are complementary to those mentioned above, for example by allowing the structure of space-time to be probed to an unprecedented level in a search for evidence for extra dimensions. Particle and Nuclear Physics experiments also explore the regimes of high temperature and pressure which existed early in the evolution of the Universe allowing us to gain an understanding of the composition and interactions which were present in the primordial Universe.
We can learn an immense amount about physics in extreme conditions by studying astrophysical environments where such conditions are commonplace. These include compact objects such as neutron stars, pulsars, quasars and black holes (see Science Challenge D3), as well as transient phenomena such as supernovae and gamma-ray bursts. The early universe is also an ideal laboratory for studying such conditions, using observations of the Cosmic Microwave Background radiation.
High energy particles and gravitational waves from the cosmos can provide additional information about extreme astrophysical environments.
Cosmic ray particles such as gamma rays can be detected indirectly by their interaction with the earth’s atmosphere using arrays of detectors sensitive to the resulting Cerenkov radiation.
Cosmic neutrinos are harder to detect but progress is being made using radio wave and optical signatures. Although yet to be detected directly, gravitational waves are known to be produced in many extreme environments such as binary systems containing neutron stars or black holes. They therefore provide a complementary tool for studying such systems and their detection would tell us much about their sources in addition to confirming a key prediction of Einstein’s general theory of relativity. High energy particles generated in terrestrial particle accelerators provide a unique route to studying extreme conditions with high precision.
Although not directly observable, astronomers now believe that the existence of black holes in the Universe, including in our own Milky Way, is incontrovertible with evidence for black holes with masses from a few tens to a few millions of solar masses. While there is a plausible theory for the formation of stellar mass black holes through rapid gravitational collapse that leads to a supernova explosion, the formation of supermassive objects is highly uncertain but can be probed through their strong interaction with surrounding gas and stars which gives rise to the emission of gravitational waves and gamma rays. Astronomers are opening up these new windows to the Universe through the use of novel space and ground based facilities.