Other attributes
The universe encompasses everything, all of space and time, as well as all the matter and energy in its various forms. This includes galaxies, stars, planets, and moons. The universe, including how matter, energy, space, and time interact, is governed by physical laws that scientists on Earth infer through repeated experimentation and observation. These include electromagnetism, general relativity, quantum mechanics, and the conservation of energy/mass. Research shows the universe is governed by four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. Scientists are striving to synthesize existing theories and force interactions to produce a grand unified theory (GUT) or theory of everything (TOE) describing the universe. A GUT requires the unification of quantum theory and general relativity principles and the reconciliation of the electroweak force with the strong force and gravitational theory.
Science's best explanation for the start of the universe is the big bang theory. Observations of galaxies show they are, on average, moving away from every other galaxy, and the universe is expanding. Therefore, in the past, the universe was smaller, and in its earliest moment, the entire universe was compressed into an infinitely small point known as the singularity. Estimates based on a variety of observations, including cosmic microwave background (CMB) radiation and the abundance of light elements, suggest the universe is approximately 13.787 billion years old. Current scientific theories cannot explain what came before the big bang.
Astronomers do not know how large the universe is, and there is a limit as to how far existing instrumentation can probe. This volume is known as the observable universe. Given the universe's finite age and the maximum speed light can propagate, astronomers can only observe a portion of the universe from Earth. The observable universe is a sphere roughly 42 billion light-years across. Light reaching Earth from the most distant galaxies, at the edge of the observable universe, was released up to 13 billion years ago. If the universe were static, we would be able to observe more distant galaxies. However, the expansion of the universe means distant galaxies are traveling away from us faster than the light from them can travel back to us.
The majority of the universe's content is of a form currently unknown to modern physics. Roughly 68% of the total energy within the universe is composed of dark energy, a hypothetical form of energy that appears to reside in the vacuum of space-time itself. Physicists do not know the origin of dark energy or why it has the strength it does. About 27% of the matter and energy of the universe is composed of dark matter, believed to be an invisible form of matter that does not interact with light. While the majority of physicists think dark matter is some new kind of fundamental particle (or particles), it has not yet been directly detected.
The remaining 5% of the universe consists of observable matter described by the standard model. While this includes matter and antimatter, almost everything we observe is made of matter. Antimatter particles share the same mass as their matter counterparts, with the opposite electrical charge. Matter and antimatter pairs are produced as pairs and annihilate when they come into contact, leaving behind pure energy. Due to matter-antimatter asymmetry, shortly after the big bang, matter was left while all of the antimatter was annihilated. Understanding the matter-antimatter asymmetry remains an active area of scientific research. Much of the observable matter in the universe takes the form of individual atoms of hydrogen, the simplest atomic element made of only a proton and an electron (if the atom also contains a neutron, it is instead called deuterium). Observations show the milky way contains at least 100 billion stars, and the observable universe contains at least 100 billion galaxies.
Space is the three-dimensional representation of everything we observe where objects extend in the left/right, up/down, and forward/backward directions. Time is the fourth dimension that measures the procession of events in space. Einstein's theory of relativity showed space and time cannot be separated. The conceptual model of space-time explains the relativistic effects at speeds close to the speed of light, as well as the motion of massive objects in the universe. The conclusion that space-time is a single fabric came from German mathematician Hermann Minkowski, not Einstein. Minkowski stated in 1908:
Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.
Known as Minkowski space-time, the model serves as the backdrop of calculations in both relativity and quantum field theory. The fabric of space-time is often described as a sheet that is curved and distorted due to the general relativistic effects of massive objects. These curves affect how objects in the universe move, with motion due to gravity actually being a manifestation of distorted space-time.
Depiction of warped space-time due to the sun and Earth
Redshift is a physical phenomenon where a receiver measures the light emitted from a source moving away from it as stretched or shifted toward the red part of the spectrum. Although the name refers to red, the longer wavelength end of the visible spectrum, redshift occurs across the entire electromagnetic spectrum. Redshift is the opposite of blueshift, where light from a source moving toward a receiver is shifted toward the blue end of the spectrum. The phenomenon is similar to the Doppler effect, where the frequency of sound waves varies depending on the relative motion of the source and receiver. However, while the Doppler effect is due to the relative motion of the source and receiver, redshifts observed in distant objects are the result of the expansion of space itself. The redshift of distant light-emitting objects is measured by comparing its spectrum with a reference spectrum. Atomic emission and absorption lines occur at well-known wavelengths. By measuring the location of these lines in astronomical spectra, astronomers can determine the redshift of the receding sources.
In 1929, Edwin Hubble first formulated Hubble's law of cosmological expansion. By comparing the distances to galaxies to their redshift, Hubble found a linear relationship. He interpreted the redshift as being caused by the receding velocity of the galaxies. The recession-distance relationship is interpreted as an overall expansion of the universe. As light travels toward us from the distant galaxies, it is stretched over time by the expanding space it is traveling through.
Cosmic microwave background (CMB) radiation is the oldest light we can see. Most visible in the microwave region of the electromagnetic spectrum, it fills the universe in every direction with nearly uniform intensity. The CMB represents the heat leftover from the big bang, which has cooled due to expansion to 2.725 K (-459.67oF or -273.15 oC). CMB radiation is believed to have formed about 380,000 years after the Big Bang. Studying the CMB offers indications of how the first stars and galaxies formed. The CMB is central to the Big Bang Theory and modern cosmological models.
The early universe was filled with a uniform glow of high-energy plasma particles that consisted of protons, neutrons, electrons, and photons. Between 380,000 and 150 million years after the Big Bang, photons were constantly interacting with free electrons and could not travel long distances; this epoch is colloquially referred to as the Dark Ages. As the universe expanded, it cooled to the point that electrons were able to combine with protons to form hydrogen atoms, the Recombination Period. Without free electrons, photons were able to move unhindered through the universe. Due to the expansion of space, the wavelengths of these photons increased (redshifted) to roughly 1 millimeter, and their effective temperature decreased to just above absolute zero – 2.7 Kelvin (-270 °C; -454 °F).
Astronomers estimate the age of the universe using two methods:
- Looking for the oldest stars
- Measuring the rate of expansion of the universe and extrapolating back to the Big Bang
Studying globular clusters (a dense collection of roughly a million stars) puts a lower limit on the age of the universe. Stellar life cycles depend on mass; high mass stars burn through their supply of hydrogen fuel quicker than low mass stars. The sun has enough hydrogen to last approximately nine billion years at its current brightness. A two solar mass star burns through its fuel in only 800 million years, and a ten solar mass star (nearly a thousand times brighter than the sun) only burns hydrogen for roughly twenty million years.
Globular cluster M5 photographed by the Hubble Space Telescope.
Given all of the stars in a globular cluster form at roughly the same time, they can provide a lower limit to the universe's age. For example, a globular cluster more than twenty million years old must contain only main sequence (hydrogen-burning) stars with less than ten solar masses, implying no individual main sequence star is 1000 times brighter than the sun. The oldest globular clusters only contain low-mass, dim stars less massive than 0.7 solar masses, suggesting they are between 11 and 18 billion years old. The uncertainty in this estimate is due to the challenge of measuring the exact distance to the clusters and their relative brightness.
The Hubble constant (Ho) indicates the rate at which the universe is expanding. Scientists use the Hubble constant to estimate the size and age of the universe as well as determine the intrinsic brightness and masses of stars in nearby galaxies. By examining these properties for nearby galaxies, scientists infer the same properties in more distant galaxies and galaxy clusters.
The Hubble constant can be stated using the simple mathematical expression:
Where v is the galaxy's radial outward velocity (motion along observed line-of-sight), and d is the galaxy's distance from Earth. The units of the Hubble constant are "kilometers per second per megaparsec" (velocity divided by distance measured in megaparsecs).
Obtaining the Hubble constant's true value is complicated, with astronomers requiring two measurements:
- Spectroscopic observations to reveal the galaxy's redshift, indicating its radial velocity
- The galaxy's precise distance from Earth
Accurately measuring the Hubble constant allows cosmologists to extrapolate back to the big bang and discover the age of the universe. This extrapolation depends on the history of the expansion rate, which in turn depends on the current density of the universe and on the composition of the universe.
- If the universe is flat and composed mostly of matter, then the age of the universe is: 2/(3 Ho)
- If the universe has a very low density of matter, then its extrapolated age is larger: 1/Ho
- If the universe contains a form of matter similar to the cosmological constant, then the inferred age can be even larger.
The value of the cosmological constant, which describes the energy density of empty space, is a key point of discussion between scientists trying to reconcile general relativity and quantum field theory.
Astronomers measure the Hubble constant using a variety of different techniques. Until recently, the best estimates ranged from 65 km/sec/megaparsec to 80 km/sec/megaparsec, with the best value being about 72 km/sec/megaparsec. Using 1/Ho, astronomers age the universe to between 12 and 14 billion years.
The cosmic microwave background (CMB) is remarkably uniform over the entire sky, but tiny temperature fluctuations reveal the imprints of sound waves triggered by quantum fluctuations in the universe just moments after it was born. The detailed structures present in fluctuations of the cosmic microwave background depend on the current density of the universe, the composition of the universe, and its expansion rate. By accurately measuring the composition of matter and energy density in the universe, scientists can use general relativity to calculate how fast the universe was previously expanding and, therefore, the age of the universe. This calculation assumes the universe is flat.
By measuring the CMB fluctuations, NASA's WMAP satellite and ESA's Planck measured the age of the universe to be 13.772 billion years and 13.82 billion years, respectively.
Map of the CMB fluctuations (the oldest light detected, 370,000 years after the big bang), taken by ESA's Planck space telescope
Looking for the farthest observable point from Earth (and therefore the oldest given the speed of light), scientists can estimate the diameter of the observable universe. However, the size of the universe depends on a number of things, including its shape and expansion. As a result, scientists are restricted to estimates of the true size of the universe.
If the universe is 13.8 billion years old, then Earth is inside an observable sphere with a radius of 13.8 billion light-years. However, due to the universe expanding, the observable universe is significantly larger. Assuming a constant rate of expansion, the observable universe has a radius of 46 billion light-years (diameter of 92 billion light years).
Scientists measure the size of the universe using multiple methods, including measuring the waves from the early universe (known as baryonic acoustic oscillations) or through the use of standard candles, such as type 1A supernovae, to measure distances. Estimates for the size of the universe are complicated by the possibility that the universe isn't expanding in a uniform, linear manner. For example, a 2020 study using data from ESA’s XMM-Newton and NASA’s Chandra Space Telescope and Rosat X-ray observatory suggests the universe is not expanding at the same rate in all directions.
Instead of taking one measurement method, a team of scientists led by Mihran Vardanyan at the University of Oxford performed a statistical analysis of all the existing results. They used Bayesian model averaging, which focuses on how likely a model is to be correct given the data, rather than asking how well the model itself fits the data. The group estimated that the universe is at least 250 times larger than the observable universe, or at least 7 trillion light-years in diameter.
The size of the universe is dependent on the shape of the universe. Different scientists have predicted various possible shapes for the universe, including closed like a sphere, negatively curved like a saddle, or flat. These three possibilities satisfy the theory of general relativity. The shape of the universe also informs both whether it will expand forever or eventually collapse and whether it is finite or infinite.
Sixty-eight percent of the universe is dark energy, 27% is dark matter, and 5% is normal matter. The universe's density refers to how much of this matter is packed into a given volume of space. This is defined by the universe's density parameter (Ω0)—the ratio of the average density of matter and energy in the universe (ρ) to the critical density, the density at which the universe would stop expanding only after an infinite time (ρc). The density parameter (Ω0) is given by:
Possible shapes of the universe based on its density.
If the density parameter is greater than one, the universe is closed and will eventually halt its expansion and recollapse. This is known as the closed model, with positive curvature resembling a sphere. If the density parameter is less than one, the universe is open and will continue to expand forever. In this instance, space will warp in the opposite direction, forming an open universe with negative curvature resembling a saddle. If the density parameter is exactly equal to one, then the universe is flat and contains enough matter to halt the expansion but not enough to recollapse it. In this goldilocks scenario, equivalent to around six protons per 1.3 cubic yards, the universe is flat as it expands in every direction without curving positively or negatively.
The cosmological principle states that the spatial distribution of matter in the universe is both homogenous and isotropic on a large scale. Isotropic means the universe looks approximately the same in all directions (when we observe the universe matter is not more prevalent in a specific direction; i.e., there are no special directions in the universe), and homogenous means one large region of the universe is approximately the same as any other large region of the universe (the average density of matter is approximately the same in any region of the universe; i.e., no special regions of the universe).
There are local variations that exist; for example, some relatively small regions of space have more galaxies than others. The cosmological principle states that on average, over a large enough scale, you'll see the same number of galaxies, or matter, irrespective of where you look. The principle goes further to state any large region of the universe is effectively the same as another, containing the same number and same types of galaxies.
The cosmological principle is derived from the Copernican principle, but it does not have any foundation in any particular physical model or theory (i.e., it can not be proved in a mathematical sense). While the cosmological principle is supported by observations of the universe, it has also received criticism, with many arguing observations calling into question the universe's isotropy and homogeneity.
The big bang theory is the leading cosmological model explaining how the universe began. The theory states the universe was an infinitely hot and dense single point 13.8 billion years ago that inflated and stretched to produce the still-expanding universe we observe today. Cosmologists believe the big bang not only created the majority of matter but also the physical laws that govern the universe. Much of the big bang theory is based on models, observations of the universe expanding, and studies of cosmic microwave background radiation.
Based on theory and observation, scientists make three assumptions about the universe:
- The laws of physics hold across the universe irrespective of time or location in space.
- The universe is homogeneous, or roughly the same in every direction or isotropic (though not necessarily for all of time).
- Humans do not observe the universe from a privileged location (such as at its very center).
Applying these assumptions to Einstein's theory of relativity indicates the universe has the following properties:
- The universe expands (astronomers measure light from the universe's distant regions redshifted by the expansion of the space between).
- The universe emerged from a hot, dense state at some finite time in the past.
- The lightest elements were created in the first moments of time.
- A background of microwave radiation fills the universe due to the transition that occurred when the hot, early universe cooled enough for atoms to form.
Given the current observations of the expanding universe in all directions, scientists theorize it must have started at a single point of infinite density at a finite period of time in the past. After initial rapid expansion, the big bang theory maintains that the universe cooled enough to allow the formation of subatomic particles and later simple atoms. Giant clouds of these early elements later coalesced through gravity to form stars and galaxies.
Scientists cannot observe the big bang directly; therefore, the timeline and circumstances of the early universe are the subjects of much speculation and competing ideas. However, the big bang theory can be broken down into a series of stages:
- Singularity
- Inflation epoch
- Cooling epoch
- Structure epoch
The big bang theory states that the universe was condensed in an infinitesimally small singularity (also known as the Planck Epoch) of infinite denseness and heat. This period extends from point 0 to approximately 10-43 seconds. From 10-43 to 10-36, the universe temperature transitioned such that the fundamental forces of the universe began to separate.
The singularity began expanding faster than the speed of light (known as cosmic inflation). This period was incredibly brief, lasting roughly 10-32 seconds, according to Alan Guth's theory. Most cosmological models suggest the universe was filled homogeneously with a high-energy density that rapidly expanded and cooled. Baryogenesis occurred with particles moving at relativistic speeds continuously creating particle-antiparticle pairs.
As the universe further cooled, it decreased in density and temperature particles reached energies that particle physics experiments can obtain. Since temperatures were no longer high enough to produce new matter-antimatter pairs, mass annihilation immediately followed, leaving just one in 1010 of the original protons and neutrons and none of their antiparticles.
A few minutes into the expansion, Big Bang nucleosynthesis began. With temperatures dropping to 1 billion kelvin and energy densities dropping to about the equivalent of air, neutrons and protons began to combine to form the universe's first deuterium (a stable isotope of hydrogen) and helium atoms. However, most of the universe's protons remained uncombined as hydrogen nuclei.
After roughly 379,000 years, electrons combined with nuclei to form atoms, primarily hydrogen, while the radiation decoupled from matter and continued to expand through space, largely unimpeded. This radiation is now known to be what constitutes the Cosmic Microwave Background (CMB).
Over several billion years, small differences in density in the almost uniformly distributed matter of the universe became gravitationally attracted to each other—producing gas clouds, stars, galaxies, and the other astronomical structures that we regularly observe today.
At the start of the universe, the four fundamental forces of the universe were unified. As the universe cooled, the forces began to separate. Below is an attempt to illustrate this "spontaneous symmetry breaking," which is theorized to have separated the original force into the four forces that we observe today. Proposed energies and temperatures associated with each of the symmetry breaks shown along with a modeling of the time elapsed in the big bang model.
Diagram showing the separation of the four fundamental forces as the universe expanded and cooled with time.
Spontaneous symmetry breaking began during the very early universe in what is known as the grand unification epoch, between 10-43 and 10-36 seconds after the big bang. Although the universe was still extremely hot and small, it had cooled enough for gravity to separate from the other three fundamental forces. The remaining unification of the strong nuclear, weak nuclear, and electromagnetic forces during this period of time is referred to as the electronuclear force.
Near the end of this epoch, theories predict that the universe had cooled to the point that the nuclear strong force began to "freeze out," leaving three fundamental forces: gravity, the strong force, and the still combined electroweak force. The phase transition of the strong nuclear separating from the electroweak force released a huge amount of energy, causing space to undergo rapid inflation, significantly faster than the speed of light. As it is space itself expanding, not particles moving through space, this inflation does not break the hard limit of the speed of light.
In the present low-temperature universe, we observe the weak and electromagnetic forces and separate, distinct mechanisms. But in the early universe, when it was hot enough such that the equilibrium thermal energy was on the order of 100 GeV, these forces were essentially identical—part of the same unified "electroweak" force. The temperature of the universe at this stage was greater than 1015 K.
Roughly 10-10s after the start of the universe, the temperature cooled to below 1015 K, and the last of the fundamental forces, electromagnetic and nuclear weak forces, became distinct. Scientists have performed particle physics experiments at energies corresponding to these temperatures (1015 K), allowing us to probe these conditions.
The exchange particle for electromagnetic interaction is the massless photon while the exchange particles for the weak interaction are the massive W and Z particles, with masses around 80 and 90 GeV, respectively. The difference in masses is attributed to spontaneous symmetry breaking as the universe cooled. Theories suggest that at very high temperatures, when the equilibrium energies are in excess of 100 GeV, these particles are essentially identical, and the weak and electromagnetic interactions are manifestations of a single force.
The big bang should have created equal amounts of matter and antimatter in the early universe. However, everything that now constitutes the universe is made almost entirely of matter. Understanding this matter-antimatter asymmetry is one of the greatest challenges in physics.
Antimatter was first postulated by Arthur Schuster in 1896 and was given a theoretical footing by Paul Dirac in 1928. In 1932, Carl Anderson discovered the first antimatter in the form of anti-electrons, dubbed positrons. Positrons occur in natural radioactive processes, such as in the decay of potassium-40. This means the average banana, containing potassium, emits a positron every 75 minutes. These then annihilate with matter electrons to produce light.
Antimatter particles share the same mass as matter counterparts, but other qualities, such as electric charge, are opposite. The positively charged positron, for example, is the antiparticle to the negatively charged electron. Matter and antimatter particles are always produced as pairs, and if they come in contact, they annihilate one another, leaving behind pure energy. During the very early universe, the hot and dense environment was filled with particle-antiparticle pairs popping in and out of existence. If matter and antimatter are created and destroyed together, then the universe should contain nothing but leftover energy.
A tiny portion of matter, roughly one particle per billion, managed to survive, and this is what we see today. Particle-physics research has now shown that the laws of nature do not apply equally to matter and antimatter, but scientists are yet to discover the reasons why. Experiments have observed spontaneous transformations between particles and their antiparticles, occurring millions of times per second before they decay. Yet, some unknown mechanism that intervened in this process in the early universe has caused these "oscillating" particles to decay as matter more often than they decayed as antimatter. Physicists attempt to find hints as to why this process may create subtle differences in the behavior of matter and antimatter particles by studying high-energy particle collisions.
Dark energy is an unidentified component of the universe that exerts a negative, repulsive pressure, to behave in a manner opposite to gravity, accelerating the expansion. Dark Energy is not directly observed, but rather inferred from observations of gravitational interactions between astronomical objects. Observations suggest it overwhelms other components of matter and energy. ESA's Planck mission estimates dark energy contributes 68% of the matter-energy density of the universe.
Dark energy is the name given to the force causing the rate of expansion of our universe to accelerate over time rather than slow down. The universe is seen as expanding faster today than billions of years ago. This phenomenon is not currently explained. Dark energy seems to be linked to the vacuum of space, perhaps an intrinsic property of the vacuum. Therefore, the larger the volume of space, the more dark energy is present and the greater its effect.
Diagram showing the rate of the universe's expansion since the big bang.
Evidence for dark energy comes from the observation of supernovae. Type Ia supernovae produce roughly the same amount of energy, and astronomers can use them to gauge cosmological distances and, therefore, the rate of expansion in the past compared to now. In 1988, American astronomer Saul Perlmutter launched a group to make these supernovae Ia measurements, called The Supernova Cosmology Project. In 1994, they were joined by an independent group called The High-z Supernova Search Team, led by American Australian astronomer Brian Schmidt and with American astronomer Adam Riess playing a crucial role. The two teams were expecting to observe the universe's expansion decelerating due to the gravitational force of celestial objects. However, they both found the expansion was accelerating. This was unexpected with nothing in known physics capable of producing this effect. In keeping with the naming of the mysterious dark matter, astronomers began referring to whatever was causing the acceleration as dark energy.
The most accurate measurements defining the amount of dark energy come from Planck, an ESA mission with contributions from NASA. Planck observed the oldest light in the universe and found the universe is expanding slower than previously thought. Planck estimates the expansion rate of the universe with a Hubble constant of 67.15 +/- 1.2 kilometers/second/megaparsec. Previous missions, such as NASA's Spitzer and Hubble, returned slightly higher estimates using a different technique. Using Planck's data suggests dark energy makes up 68.3% of the universe; the previous best estimate was 71.4%.
There are many theories attempting to explain dark energy. One of the most compelling goes back to Einstein and his introduction of the cosmological constant. The cosmological constant is an energy field present across the entire universe. Initially, Einstein introduced it to resist the pull of gravity from celestial objects and hold the universe stable and unmoving. The discovery that the universe was expanding made Einstein's use of the cosmological constant redundant. He removed it from his equations and is reported to have called it his biggest "blunder." However, cosmologists have re-introduced the cosmological constant, using it as the simplest way to explain dark energy observations.
Other theories include the acceleration being produced by a new force of nature or due to a misunderstanding of the way general relativity works. Each explanation alters the way the acceleration develops across cosmic time, but no experiment has been capable of measuring the acceleration in sufficient detail to distinguish between the possible solutions.
Dark matter emits no light or energy and, therefore, cannot be detected by conventional sensors and detectors. The same Planck measurements from ESA that decreased the dark energy estimates increased the estimate of dark matter present in the universe up to 26.8%, from the previous value of 24%.
Since the 1920s, astronomers have hypothesized that the universe contains more matter than we currently observe. This is because the gravitational forces in the universe appear stronger than the visible matter alone could account for. Observations of spiral galaxies in the 1970s expected to see the material in the center moving faster than at the outer edges. Instead, they found the stars in both locations traveled at the same velocity, indicating the galaxies contained more mass than could be seen. Studies of gas in elliptical galaxies also indicate there is more mass than found in visible objects. Clusters of galaxies would fly apart if the gravitational forces present only came from the mass visible to conventional astronomical measurements. Different galaxies seem to contain differing levels of dark matter. In 2016, a galaxy called Dragonfly 44 was observed to be composed almost entirely of dark matter. On the other hand, since 2018, astronomers have found several galaxies that seem to lack dark matter altogether.
Given gravity also affects the trajectory of light (due to the bending of space-time described by general relativity), it is possible to map dark matter in the universe by studying gravitational lensing effects. Dark matter appears to be present across the cosmos in a network-like pattern, with galaxy clusters forming at the nodes where fibers intersect.
Dark matter forming a network-like pattern across the universe.
Although evidence points toward the existence of dark matter, there is also the possibility that instead of additional mass being present in the universe, our current understanding of gravity needs revision.
Visible matter that we observe using conventional sensors and detectors is also referred to as baryonic matter. Baryons are a type of hadron (particles made from quarks that experience strong nuclear force) made up of three quarks, such as protons and neutrons. In astronomy, the term baryonic matter is used more broadly to include all objects made of normal atomic matter, including electrons.
Most scientists think that dark matter is composed of non-baryonic matter. The lead candidates, WIMPS (weakly interacting massive particles), are believed to have ten to a hundred times the mass of a proton, but their weak interactions with "normal" matter make them difficult to detect. Neutralinos, massive hypothetical particles heavier and slower than neutrinos, are the foremost candidate, though they have not been observed. Sterile neutrinos are another candidate. There are three known types of neutrinos; the sterile neutrino would be a fourth, proposed as a dark matter candidate that only interacts with regular matter through gravity. Other potential placeholders for dark matter include the smaller neutral axion and the uncharged photinos—both theoretical particles.
The standard model of particle physics is currently the best theory scientists have to describe the most basic building blocks of the universe. Developed in the early 1970s, the standard model successfully explains almost all experimental results and has precisely predicted a wide variety of phenomena. It lists fundamental particles that make up the known universe, including matter particles known as Fermions and force-carrying particles known as bosons. The standard model explains three of the four fundamental forces that govern the universe: electromagnetism, the strong force, and the weak force. The fourth fundamental force, gravity, is not currently explained by the Standard Model. Other limitations of the standard model include whether the Higgs boson gives mass to neutrinos and the incorporation of dark energy and dark matter.
Diagram showing the particles of the standard model.
Also called matter particles, fermions are particles that make up the physical matter of the universe that we can observe. This includes leptons, quarks, and composite particles made up of quarks, such as protons and neutrons. Fermions obey the rules of Fermi-Dirac statistics, namely the Pauli Exclusion Principle, and have an odd half-integer (like 1/2, 3/2, and so forth) spin. Spin describes a particle's intrinsic angular momentum. It is a purely quantum mechanical phenomenon without an analog in classical physics, i.e., it is not associated with rotating internal parts but intrinsic to the particle itself.
Fermions were first predicted in 1925 by the physicist Wolfgang Pauli while working with the atomic structure proposed in 1922 by Niels Bohr. Bohr used experimental evidence to build an atomic model containing electron shells, creating stable orbits for electrons to move around the atomic nucleus. Though this model matched well with experimental evidence, there was no reasoning as to why this structure would be stable. Pauli realized that if you assigned quantum numbers (later named quantum spin) to these electrons, then there seemed to be some form of principle meaning that no two electrons could be in exactly the same state. This rule became known as the Pauli Exclusion Principle.
In 1926, Enrico Fermi and Paul Dirac both independently developed a statistical method explaining electron behavior. Although Fermi developed the system first, they were close, and both did important work, such that their statistical method was named Fermi-Dirac statistics. The particles themselves were named after Fermi.
Fermions can be separated into two basic types: quarks and leptons, each consisting of six particles separated into three generations of increasing mass. A table of the twelve fermion matter particles is shown below; each particle has a spin equal to a half:
Fermion properties
The stable visible matter in the universe is made from the particle of the lightest, first generation. The heavier 2nd and 3rd generation fermions are less stable, quickly decaying into more stable ones.
Bosons are particles with an integer spin (i.e., 0, 1, 2) that includes all the force carrier particles and composite particles with an even number of fermion particles, such as mesons. Unlike fermions, that obey the Pauli exclusion principle. Bosons can occupy the same place at the same time (i.e, two or more bosons may be described by the same quantum numbers). The statistical rules describing bosons were first described by Satyendra Bose and Albert Einstein.
There are four fundamental forces of the universe: the strong force, the weak force, the electromagnetic force, and the gravitational force. Three of the fundamental forces are explained by the standard through the exchange of force-carrier particles. Fermions transfer discrete amounts of energy by exchanging bosons with each other, and each fundamental force has corresponding bosons. Electromagnetism is carried by photons and involves the interaction of electric fields and magnetic fields. The strong force is carried by gluons that bind together the quarks that make up nucleons (protons and neutrons), making nuclei stable, even with only neutral and positive charged particles. The weak force, carried by W and Z bosons, causes nuclear reactions. These four force-carrying bosons have a spin of 1 and represent vector fields.
Gravity is not a part of the Standard Model, as the quantum theory used to describe the micro world, and the general theory of relativity used to describe the macro world, cannot be unified by a single mathematical framework. However, for particle physics, when it comes to the scale of individual particles, the effect of gravity is so weak as to be negligible. Only when matter is in bulk, for example at the scale of the human body or of the planets, does the effect of gravity dominate. So the Standard Model still works well despite its reluctant exclusion of one of the fundamental forces. Although as yet undiscovered, the standard model predicts a gravity force-carrying particle known as the "graviton." If the particle of the gravitational field is ever discovered, it is predicted to have a spin of 2, corresponding to a tensor field.
The particle now known as the Higgs boson was first described in a 1964 scientific paper written by Peter Higgs. At the time, physicists were working on describing the weak force using a framework called quantum field theory. Quantum field theory describes the microscopic world of particles with fundamental “quantum fields” filling the universe and dictating what nature can and cannot do. In this description, every particle can be represented by a wave in a “field."
Studying the weak nuclear force showed its carriers (W and Z bosons) had to be massless to not break the fundamental symmetry of the theory. However, the weak force bosons also had to be massive to explain the short range of its interactions. A solution was found with the Brout-Englert-Higgs (BEH) mechanism that consists of two main components:
- An entirely new quantum field known as the Higgs field
- Spontaneous symmetry breaking
A spontaneously broken symmetry is one that is present in the equations of a theory but broken in the physical system. Since it is possible to represent every particle as a wave in a quantum field, introducing a new field into the theory means that a particle associated with this field should also exist. Most properties of this particle are predicted by the theory, so observations of a particle matching theory provide strong evidence for the BEH mechanism. The Higgs boson was discovered in 2012, confirming the existence of the BEH mechanism and the Higgs field. The Higgs boson corresponds to a scalar field and therefore has a spin of 0.
A table of the five known force-carrying bosons and their associated properties is shown below:
