The High Energy Universe
Ultra-High Energy Events in Astrophysics and Cosmology
In the last two decades, cosmology, particle physics, high energy astrophysics, and gravitational physics have become increasingly interwoven. The intense activity taking place at the intersection of these disciplines is constantly progressing, with the advent of major cosmic ray, neutrino, gamma-ray, and gravitational wave observatories for studying cosmic sources, along with the construction of particle physics experiments using beams and signals of cosmic origin.
This book provides an up-to-date overview of the recent advances and potential future developments in this area, discussing both the main theoretical ideas and experimental results. It conveys the challenges, but also the excitement associated with this field. Written in a concise yet accessible style, explaining technical details with examples drawn from everyday life, it will be suitable for undergraduate and graduate students, as well as for other readers interested in the subject. Color versions of a selection of the figures are available at www.cambridge.org/9780521517003.
PETER MESZAROS is Eberly Chair of Astronomy & Astrophysics and Professor of Physics at the Pennsylvania State University, where he is also Director of the Center of Particle Astrophysics. His main research interests are high energy astrophysics and cosmology. He has been a co-recipient of the Rossi Prize of the American Astronomical Society and the First Prize of the Gravity Research Foundation. He is a member of the American Academy of Arts and Sciences and the Hungarian Academy of Sciences.
The High Energy Universe
Ultra-High Energy Events in Astrophysics and Cosmology
Peter MeszAros
Pennsylvania State University
Cambridge
UNIVERSITY PRESS
Deborahnak, Andornak
Introduction
1.1 The dark and the light
The Universe, as we gaze at it at night, is a vast, predominantly dark and for the most part unknown expanse, interspersed with myriads of pinpricks of light. When we consider that these light spots are at enormously large distances, we realize that they must be incredibly bright in order to be visible at all from so far away. Occasionally, some of these specks of light get much brighter, and some of them which were not even seen with the naked eye before become in a few days the brightest spot in the entire night sky, their brightness having increased a billion-fold or more against the immutable-looking dark background. Thus, we have come to realize that the Universe is characterized by what Renaissance artists called chiaroscuro, referring to the contrast between light and dark, which is both stark and subtle at the same time. In the case of the Universe, the contrasts can be enormous and surprisingly violent, as well as of a subtlety which beggars the imagination. In this book we will focus on these contrasts between the vast, unknown properties ofthe dark Universe and its most violent outpourings of energy, light and particles.
According to current observations and our best theoretical understanding, the Universe is made up of different forms of mass, or rather of mass-energies, since as we know from special relativity, to every mass there corresponds an energy E = mc2 and vice versa, where E is energy, m is mass and c is the speed of light. About 74% of the Universe's total energy content is in the form of dark energy, a very strange component whose true nature we are completely ignorant of. All we know about it at present is what it appears to do to us and to the rest of the massive objects in the Universe: it affects the rate of the expansion of the Universe. The next most prominent mass-energy component in the
Universe, amounting to about 22% of the total, is in the form of dark matter (another "dark" constituent!), of whose nature we are only slightly less ignorant than we are about dark energy. Despite 30 years of pondering it, all we know for sure about dark matter is how it affects the gravitational attraction felt by the "normal" matter of galaxies, we know roughly how it is distributed in space, and we can rule out some classes of objects as being responsible for it. The remaining fraction of the mass-energy of the Universe amounts to 4%, which is in the form of "normal" everyday baryons, or atoms (Fig. 1.1), although only about one in 10 of these (~ 0.5%) emit light or detectable radiation, a very modest-looking contribution indeed. Physicists have taken to describing these two types of components as the dark and the light sectors of the Universe.
Figure 1.1 Relative amount of different forms of mass-energy densities in the Universe.
Source: SNAP project website.
1.2 Where the fires burn
In the deep dark night of the Universe, the tiny bright specks of light shine as reassuring outposts, or so it would seem. These small corners of the Universe where we feel warm and at home form that portion which we can probe with our various instruments, telescopes, satellites, accelerators and
laboratory experiments. In fact, this portion of the Universe makes up for its relatively small size with its sheer brilliance, and upon closer inspection, with its concentrated violence.
The most obvious denizens of the light sector, just from their sheer numbers, are the so-called main sequence stars, of which the Sun is a very ordinary example. The Sun's luminosity, that is its energy output per unit time, is L ~ 4 x 1033 ergs-1 = 4 x 1027 watts, which can also be expressed as 5 x 1023 horsepower.1 Most of this energy, in the case of the Sun, is in the form of "optical" light, to which our eyes are sensitive, with smaller fractions in the infrared and in the ultraviolet parts of the electromagnetic spectrum. There are other stars which emit most of their electromagnetic radiation outside the optical range, either at shorter or longer wavelengths. Like the Sun, all stars shine because of nuclear reactions going on in their core, which results in their emitting copious amounts of neutrinos, a type of elementary particle, the stellar neutrino luminosity being in general comparable to the electromagnetic luminosity.
Despite their huge power, stars are just the lumpen proletariat of the Universe, humble light-bugs compared to some of the rare, lavish energy plutocrats which arise occasionally here and there. When they occur, the sky is pierced by extremely concentrated outbursts of high energy radiation pouring out from them, which make the normal stars pale by comparison, outshining them by a factor of a billion or more over periods of weeks. These outlandish events are called supernovae, and besides their optical and other forms of electromagnetic radiation, we have managed to measure on at least one occasion their neutrino luminosity as well. They are also thought to be powerful sources of other forms of cosmic rays, and to a lesser degree of gravitational waves, which however have not so far been detected. Some of these supernovae occur as a consequence of the collapse of the inner core of massive stars, while others are due to smaller stars slowly gaining mass until a nuclear deflagration occurs. In many cases, the collapse leaves behind an extremely compact remnant called a neutron star, composed of matter whose density is extremely high, comparable to that of atomic nuclei.
The most extreme stellar outbursts, however, appear to occur as a result of the core collapse of the most massive stars leading to the formation of a black
1 We use the common scientific notation where a quantity written as, say, 6 x 10x is equivalent, in the usual decimal notation, to 6 followed by X zeros before the decimal point, for instance, 6 x 103 = 6000, or in general, the first number followed by X figures, with zeros added after the significant figures to make up X figures after the first one, for instance, 1.56 x 104 = 15 600.
hole, or as a result of the merger of two compact stars leading to a black hole. The black hole formation may perhaps proceed through an intermediate stage as a neutron star with an extremely high magnetic field. These cataclysmic events are called "gamma-ray bursts", or GRBs. They flare up very fast, and for short periods of time (seconds or minutes), their brightness can exceed the total luminosity of the rest of the observable Universe.
Slower flares of even higher total energy occur in some galaxies, made up of billions or trillions of stars. These are related to massive black holes which lurk at the center of most galaxies, millions to billions of times more massive than the stellar mass black holes. As gas or stars fall in and are stretched and ripped apart by the enormous gravitational fields of these black holes, the resulting heated gas leads to correspondingly brighter electromagnetic flaring episodes, spread out over longer times, and recurring fitfully. These flaring episodes on the galactic scale have brightnesses which exceed thousands or tens of thousands of times the luminosity of the more peaceful steady-state emission produced by their stars or by the low and steady accretion of gas onto the black hole. Yet, bright as these electromagnetic galactic flares are, observations as well as simple physical arguments tell us that many of them must be accompanied by comparable or even larger outpours of energy in the form of cosmic rays, neutrinos and gravitational waves (Fig. 1.2).
Figure 1.2 A relativistic jet shooting out from the massive black hole at the center of the active galaxy M87, which is an incredibly energetic source of photons and particles.
Source: NASA Hubble Space Telescope.
1.3 The vast dark sea
The looming bulk of the dark Universe, alas, provides the greatest and least tractable mysteries. What are the dark energy and the dark matter, and what can we do to find out what they are, and how they operate?
Of these, dark matter appears to offer somewhat more promising or at least straightforward approaches for its investigation. For more than three decades, it has been studied indirectly through its gravitational effects on normal, visible matter. However, direct methods of investigation, such as capturing or analyzing the effects of dark matter interacting within laboratory detectors, appear at least possible as well. If the dark matter is not made up of hard-to-detect macroscopic objects, as seems to be the case after long and fruitless searches, it should consist of hard-to-detect elementary particles, for which there are some possible candidates. Those in the known arsenal of the Standard Model of particle physics, such as electromagnetic radiation at hard-to-detect frequencies, or neutrinos, appear to be ruled out. But there are many plausible extensions of the Standard Model which predict particles that could fit the bill, such as various types of weakly interacting massive particles (graced with the acronym WIMPS), or another type of hypothetical wimpy particle called axions. WIMPS are thought to be able to annihilate each other to produce neutrinos, which are in principle detectable with large neutrino detectors such as IceCube under the Antarctic ice or KM3NeT under the Mediterranean sea. In deep underground laboratories, WIMPS are also being searched for through the weak recoil they would impart to nuclei with which they (very rarely) interact. And one of the prime targets of large particle accelerators such as the new Large Hadron Collider (LHC) near Geneva, in Switzerland, is the detection of "something missing" when accounting for the energy budget of colliding high energy particles, which could indicate the creation of WIMPS. The latter, being weakly interacting, would leave the detector unnoticed, without paying their bill, so to speak, but leaving a noticeable gap in the collision energy balance.
Dark matter WIMPS can also annihilate by interacting with each other, leading to distinctive gamma-ray signatures which are being searched for with, among others, the recently launched Fermi Gamma-ray Space Telescope (formerly known as GLAST), and also with ground-based devices called imaging air Cherenkov telescopes (IACTs), such as HESS, VERITAS, MAGIC and CANGA-ROO. Besides their more spectacular and speculative task of probing the dark matter sector of the Universe, these space and ground instruments earn a hard living through honest, untiring and only slightly less spectacular studies of the more extreme forms of "normal" matter, such as black holes, gamma-ray bursts, supernovae, active galaxies, etc.
Dark energy is even harder to grasp, both experimentally and conceptually, than dark matter. The experimental study of dark energy is, for now, mainly
confined to indirect methods. As in the case for dark matter, dark energy manifests itself most blatantly through its dynamical effects on the large scale behavior of the normal visible matter, in particular on the apparent acceleration of the expansion rate of the Universe. This is being studied by a variety of large scale optical surveys of distant objects, with new and proposed ground-and space-based experiments.
However, a theoretical understanding of the nature of dark energy, of what it is and how it fits in with the fundamental forces and other constituents of the Universe, remains perhaps the most challenging task of theoretical physics and astrophysics. If it is indeed a fundamental physical property, the answer is likely to lie at the interface between gravitation and quantum mechanics.
1.4 The great beyond
The study of both dark matter and dark energy pushes at the boundaries of particle physics and appears to require a unification of quantum mechanics and gravity, which is currently the most ambitious goal of theoretical physics. A major and very active component of this quest is the exploration of particle theories "beyond the Standard Model" (BSM). There are two major arenas where this is being played out. First, terrestrial experiments on very large particle accelerators such as the LHC or deep underground detectors such as Super-Kamiokande in Kamioka, Japan; experiments underway at Gran Sasso Laboratory in Italy and at the planned Deep Underground Science and Engineering Laboratory (DUSEL) in the USA, among others (Fig. 1.3). Second, theoretical models of processes in the very early Universe and related cosmological observations.
One critical epoch in the early history of the Universe is the so-called electroweak transition epoch, when the thermal energies of particles in the Universe had values comparable to those that are achievable in the LHC. There is also an even earlier epoch, during which an episode of greatly accelerated expansion is thought to have occurred. This is called the epoch of inflation, at a time when the Universe would have been so dense and hot that so-called Grand Unified Theories (GUTs) of particle physics hypothesize that three of the known forces of nature, the strong, the weak and the electromagnetic forces, would have been unified into a single interaction (e.g. [1]). And even earlier than that, at the so-called Planck epoch, the fourth force, gravity, would also have become comparable in strength to the other three forces, and the structure of space-time itself would have been a jumble of random quantum fluctuations. Somewhere in this imposing, chaotic landscape may lie the clues to unravel the nature of dark energy and its connection to the rest of physics, or at least that is the hope.
Figure 1.3 Aerial view of CERN, the European Center for Nuclear Research in Geneva, and the surrounding region. Three rings are visible, the largest of which (27 km in circumference) is the Large Hadron Collider (LHC). One of the goals of the LHC is the investigation of dark matter, within the broader context of physics beyond the Standard Model.
Source: CERN.
Another area where the microcosmos and the macrocosmos are intermeshed is the cross-fertilization between high energy physics and black hole astrophysics. One potentially interesting and exotic aspect of this arises in so-called low energy extra-dimensional theories (which are beyond the Standard Model, since they involve more dimensions than the usual four of space-time, e.g. [2]), where there is a possibility that proton collisions in the LHC at teraelectron-volt (TeV) energies could produce very small black holes. While the probability of this is acknowledged to be extremely low, even upper limits on it would provide useful constraints on possible non-standard models. Incidentally, it is worth noting that concerns that such microscopic laboratory black holes could pose a danger have been shown to be groundless [3, 4]. On a more abstract plane, black holes and particle physics mingle intimately in theories of quantum gravity. Both string theories and quantum loop gravity have made advances in describing the quantum properties of black holes, and have derived more or less self-consistent descriptions of black hole quantities such as the mass, spin, charge, information content, entropy, etc. [5-7]. However, these pursuits are still in their early stages, and the road ahead remains largely unfathomable.
It has also been suspected for a long time that black holes may play a role in the evolution of the early Universe. Some of the speculations include, for example, that black hole formation could lead to the currently observed photon-to-baryon ratio; that black holes could hide baryons which might otherwise have caused departures from the observed nuclear abundances of the chemical elements; that black holes might act as dark matter, or as a catalyst for nucleating galaxies, etc. Another speculation is that black holes could provide a feedback mechanism which, out of many possible Universes (the so-called multiverse [8]), selects the one where we happen to live [6]. And of course, the rate at which small and large black holes form in the more recent Universe, which is susceptible to direct observation, would provide a very powerful tool for tracing the dynamics and the evolution of star, galaxy and large scale structure formation. Ultra-high energy cosmic rays, neutrinos and gravitational waves, whether associated with these black holes, or perhaps other more exotic phenomena, will certainly provide unique probes to extend our current reach into the depths of the Universe.
1.5 The next steps
Mountaineers are familiar with the feeling of straining to climb a mountain range whose summit they can see and which apparently has only blue sky beyond, only to reach the presumed summit and discover that the view from there now opens new vistas of another, even higher mountain ridge. The process then repeats itself time after time, until (at least in earthly mountaineering) a final top is reached. The same is known from everyday hard work at an apparently impossibly large task; we know that the only way to accomplish it is to do it one step at a time, one day at a time, and just concentrate on the immediate task ahead, until we reach our goal.
What are some of the direction signposts and the first steps we can take towards these vast unknown territories of the Universe? Starting with the visible sector, the greatest challenges in the astrophysical arena are twofold: understanding the nature and dynamics of the expanding dark Universe, and unraveling the inner workings of its brightest concentrated high energy sources, such as supernovae, gamma-ray sources, super-massive black holes and their related objects. Due to their extreme brightness, which makes it possible to detect them out to the farthest reaches of the Universe, another crucial role of these sources and their messengers may be their acting as tracers of the development and dynamics of the Universe at the dawn of the stellar and galaxy formation epochs. Our horizons could be extended to even larger distances than now being reached if we were to detect from them ultra-high energy
Figure 1.4 Artist's view of the Fermi Gamma Ray Space Telescope, launched in 2008, which is observing distant gamma-ray bursts, active galactic nuclei, pulsars and other objects, as well as providing limits on cosmic rays and setting constraints on dark matter models.
Source: NASA.
neutrinos resulting from ultra-high energy cosmic rays. Gravitational waves arising in these objects would also be able to reach us without any absorption from the largest distances, and these are the target of large gravitational wave observatories such as the Laser Interferometric Gravitational Wave Observatory (LIGO) in the USA, a similar observatory called VIRGO near Pisa in Italy, and a planned European spacecraft called the Laser Interferometer Space Antenna (LISA). Together with the more obvious visible tracers, these may help to track the "bulk" properties of the dark energy, as well as the details of the dark matter distribution (Fig. 1.4).
The most energetic type of radiation known so far, either from the laboratory or from the cosmos, are the ultra-high energy cosmic rays, and a major question is their possible relation to black holes, either massive or stellar. Are these cosmic rays astrophysical in origin, and related to active galactic nuclei, to gamma-ray bursts, or to supernovae? If so, they may shed light on the origin and nature of these objects. Or, alternatively, could they be the product of exotic processes beyond the Standard Model of particle physics in the early Universe? For their part, independently of any relation to ultra-high energy cosmic rays, the physics of black holes in active galactic nuclei and in stellar systems, gamma-ray bursts and supernovae involves extraordinary mass and energy densities which probe states of matter beyond anything which the laboratory can provide. And, as a population, they may play a very significant role in the development of large scale structure in the Universe.
Whatever their origin, at the enormous energies of 1020 eV the ultra-high energy cosmic rays surpass anything achievable in earthly accelerators, and
provide an intimate link between the cosmological macrocosmos and the microscopic world of particle physics, at energies which may disclose features beyond the Standard Model of particle physics. This possibility remains even if, as it increasingly appears, they are not the product of the decay of exotic particles, but rather result from astrophysical acceleration in active galactic nuclei or in gamma-ray bursts. In all cases, the center of mass-energies in the collision of such cosmic rays with protons in the Earth's upper atmosphere is hundreds or thousands of times larger than the highest energies in the LHC.
The neutrinos arising from the interactions of cosmic rays at these energies also surpass by orders of magnitude any neutrino energies achievable in laboratories. Neutrino interactions, both at these terrestrially unachievable energies and at lower energies, are especially interesting, because neutrinos provide to date the only clear experimental evidence for physics beyond the Standard Model, through the phenomenon known as neutrino oscillations. This is related to the (non-Standard Model) phenomenon of the neutrinos having a small mass, which leads to neutrinos of different types changing identities as they travel over very large distances. The best known example of this is electron-type neutrinos from the interior of the Sun changing into muon-type neutrinos, as they make their way to the Earth. These "neutrino-flavor" changes and related phenomena are the subject of numerous laboratory, reactor, accelerator and underground experiments, using both terrestrially generated and cosmic neutrinos.
Such neutrino properties could have a direct bearing on the reason why the Universe consists mainly of matter (as opposed to anti-matter), instead of being a symmetric mixture of both. While the Universe may have started out with a uniform mixture, at some early point an imbalance must have set in leading to the survival mainly of matter, or baryons, a process called baryogenesis. Some of the leading theories attempting to address baryogenesis start out from lep-togenesis, a process where leptons (which include neutrinos and other lighter particles such as electrons, etc.) become asymmetrical, which later through the weak interactions of baryons could lead to a baryon asymmetry.
The nuts and bolts of the Universe
2.1 The building blocks: elementary particles
2.1.1 Atoms and quanta
We are all familiar with the concept of atoms. We are made out of them, our surroundings are made out of them, and our Universe is made out of them. The name derives from the Greek, meaning "indivisible", which conveys the idea that these are the smallest building blocks out of which the Universe is built. In the early 1900s the smallest units were indeed considered to be the atoms, consisting of a central more massive kernel, the nucleus, surrounded by a cloud of orbiting, much smaller and lighter particles called the electrons. The electrons were found to have negative electrical charge, while the much heavier nucleus had an equal amount of positive electrical charge, which was attributed to heavy particles called protons. Later, in the early 1930s, it was found that the nucleus contained other particles as well, slightly heavier than the protons but electrically neutral, which were consequently given the name of neutrons.
For a while these appeared to be all of the basic building blocks of matter. Different atoms, such as hydrogen, helium, carbon, iron, etc., consisted of a nucleus which differed by containing increasing amounts of protons, and except for hydrogen, a comparable or slightly larger number of neutrons, and around the nucleus a number of electrons matching the number of protons, so as to ensure electrical neutrality. This was thought to be what ordinary matter consists of, and in fact this picture continues to be basically correct to this day, except for the fact that it is not the complete picture. First, the nuclear particles have since turned out not to be elementary at all but to have sub-constituents, and second, a new theory had to be developed to correctly describe
the mechanics of the atomic and sub-nuclear world, which differed greatly from the old Newtonian mechanics describing the classical world of planets, pulleys, inclined planes, cars, etc.
This new theory of atomic and sub-nuclear physics is called quantum mechanics, where the word "quantum" means that the quantities involved come in discrete chunks, or quanta. The energy, the impulse, the angular momentum and most of the other properties of the electrons, protons, etc. are "quantized", i.e. they come in discrete multiples of a small number. Previously, in classical Newtonian mechanics and Maxwellian electrodynamics, it was thought that the various physical quantities associated with a system, such as its energy, momentum, etc., could adopt any of a continuum of possible values. There was no obvious reason why any possible value could not be mentally halved and give an equally possible value. The need for a discretization of physical quantities originated with Max Planck in 1900, who showed that electromagnetic radiation had to be quantized, i.e., it did not consist of continuous infinite waves of arbitrary frequencies but of discrete "wave packets" or "photons", carrying a discrete amount of energy given by the product of the wave frequency times a small constant number now denoted h (h-bar) and known as Planck's constant.
The concept of quanta was extended to material particles by Einstein and later to atoms by Bohr, in the first two decades of the 1900s, and quantum mechanics in its basic current form was laid down in the mid-1920s by Heisenberg, Schrodinger and Dirac. In quantum mechanics, all dynamical quantities are discrete multiples of some smallest unit involving Planck's constant h10-27 ergs. These discrete quantities characterizing the particles are called the quantum numbers. Quantum mechanics differs from ordinary mechanics also in that it deals not with deterministic predictions of the future position and the dynamical quantities, but with the probabilities of being at some later time at some position with some particular values of the quantum numbers. One aspect of this is that we cannot determine all the relevant variables of a particle with high precision. For instance, if the position x of a particle is measured to within an error, its momentum p cannot be determined to an accuracy better than , i.e., the uncertainties in the two quantities satisfy in general the relation
which is a statement of the Heisenberg Uncertainty Principle.
Another development around this time was the realization that all particles have a spin, which can be thought of as the particles spinning about some
axis like a top, or like a tennis ball, in addition to their motion through space. According to quantum mechanics and also to experiments, the amount ofspin comes in integer or half-integer multiples of Planck's constant h. Thus, protons and electrons have, in quantum mechanics, a probability density describing an orbital motion, somewhat like the Earth around the Sun, and describing also their spin, somewhat like the Earth and the Sun spin around their own axes. All particles in quantum mechanics can have a spin, just like a thrown tennis ball or a football can be imparted a spin. According to experiments and their quantum interpretation, particles like the electron, the proton and the neutron have a half-integer spin, which means that its value is (1/2)h, and the spin can be either right-handed or left-handed along the direction ofmotion. Other particles, such as photons, however, have an integer spin; in the specific case of photons this is h, while there are other particles whose spin is 2h,3h, etc. The spin is another way of describing the polarization of the electromagnetic waves (e.g., as seen through polarized sunglasses).
Interestingly, when describing the statistical properties of particles of half-integer or integer spin, it is found that they obey different statistical laws [9]. That is, when describing the probabilities of finding x amount of a certain type of particle at a certain location with certain sets of quantum numbers, these probabilities are drastically different for half-integer or integer spin particles. Half-integer spin particles cannot be at the same location and have the exact same quantum numbers (energy, spin, etc.). This is an experimental fact, also called Pauli's Exclusion Principle, and the type of statistics obeyed by such half-integer spin particles is called Fermi-Dirac statistics. This is very important, as we will see later, and such half-integer spin particles are called fermions. Most of the known massive particles, such as protons, neutrons, electrons, etc., are fermions. On the other hand, integer spin particles, such as photons, obey a different type of statistics, called Bose-Einstein statistics, and for this reason integer spin particles in general are called bosons. Unlike fermions, bosons can coexist in the same location with the same quantum numbers in any amounts. Unlike fermions, which may be considered individualistic or stand-offish, bosons may be termed gregarious. This is what makes possible devices such as the laser, where a great many photons of exactly the same frequency and polarization can bunch up together, thus greatly multiplying their collective effects.
2.1.2 Anti-matter, neutrinos and the particle explosion
Starting in the early 1930s, it was found that besides ordinary matter there existed other types of matter, far from ordinary. For several decades it had been known that cosmic rays, which are mainly charged particles such as electrons, protons and heavier nuclei, arrived at the top of the Earth's atmosphere
from outer space with extremely large energies. When these interacted with a detector they produced secondary particles, among which were found particles with the same mass as electrons but with an electric charge of the same value but opposite, positive sign. Such anti-electrons, or positrons, had been predicted theoretically by Dirac in the late 1920s, and this was the first example of what has come to be known as anti-matter.
Also in the 1930s another new type of particle, even more mysterious, made itself increasingly more evident. These particles occurred in some nuclear reactions and radioactive decays, and appeared extremely hard to detect directly. However, their presence became increasingly obvious due to the fact that in the nuclear reactions, when measuring the energy and the momentum of the initial and final particles, which are thought to be subject to an overall conservation law, the accounting fell short, unless one postulated the existence of such undetected particles. They had to have zero electric charge, otherwise they would have been easier to detect, and they had to be either massless or have extremely small masses. These particles, whose existence was first postulated by Pauli, were given the name of neutrinos by Fermi [10].
During the 1940s and 1950s other new, very short-lived particles were found in cosmic-ray interactions as well as in particle collisions produced in laboratory accelerators. These were heavier than the electron but lighter than the proton, with names such as pion, muon, etc., some being negatively or positively charged, while others were neutral. Other types of anti-matter started being found as well, such as anti-protons (labeled p), which have the same mass and other properties as the usual protons, but with a negative electric charge. However, anti-matter was found to be extremely short-lived in the presence of ordinary matter, since the anti-particle quickly annihilates itself with one of its ubiquitous (ordinary matter) partners, emitting two photons. By the late 1950s and 1960s, unstable particles and anti-particles even heavier than protons and neutrons were being found in increasing numbers, in what came to be called the particle zoo. Being unstable, all of these exotic particles decayed in a very short time into other, more normal, stable particles.
Some of the more common particles and their properties are listed in Table 2.1. The masses are measured in energy units of megaelectronvolts (MeV) (divided by the speed of light squared). This is because energies are easier to measure in particle physics, and the mass follows from the well-known E = mc2 relation. The MeV is a natural energy unit in nuclear physics, but it is extremely small compared to everyday quantities. For example, one calorie is equivalent to 2.6 x 1013 (26 trillion!) MeV, and an average human eats a few thousand calories per day, which is about 5 x 1016 MeV (fifty thousand million million megaelec-tronvolts! For some other common units and their equivalents, see Table A.1 in
Table 2.1. Properties of some of the more common particles
Type | Name | Symbol | Mass (MeV/c2) | Mean life (s) |
Baryons | proton (anti-proton) neutron | p, p n | 938.2773 939.5656 | > 1032 year 887 |
Mesons | pion (charged) pion (neutral) | n ± n 0 | 139.57 134.98 | 2.6 x 10-8 8.4 x 10-17 |
Leptons | electron (positron) muon | e± *± | 0.511 105.658 | stable 2.197 x 10-6 |
the Glossary). The mean lifetimes of the particles, when they are unstable, are indicated in seconds.
For a long time it was thought that all of these particles, both those making up the ordinary stable matter and the exotic unstable ones, were "elementary" particles. That is, particles which have no sub-units, they are just themselves, period, the only qualifiers being their quantum numbers. The problem was that there were so many particles that any sort of classification and categorization of properties which could lead to a comprehensive theory was extremely difficult, and indeed frustrating.
2.1.3 Elementary, dear Watson
In the late 1960s and early 1970s, however, it was realized that protons and neutrons, and indeed many of the unstable particles arising in high energy collisions, were not elementary after all. They turned out to be made up of sub-units which came to be known as quarks, most being made up of different combinations of the two commonest quarks, called the "up" and "down" quarks [9]. However, electrons and positrons, as well as photons, still remain as elementary particles, with no known sub-structure. The electrons have unit (negative) electric charge (the unit is labeled e), whereas the quarks have fractional electric charges, the up quarks having +(2/3)e and the down quarks having -(1/3)e. Protons, neutrons and most of the heavier unstable particles (collectively labeled baryons, meaning heavy) consist of three quarks, in combinations such that their total charge gives the observed electrical charge. That is, the proton is a combination uud, of charge (+2/3 + 2/3 - 1/3)e = +e, while the neutron is a combination udd, of charge (+2/3 -1/3 -1 /3)e = 0. The quarks are of course fermions with half-integer spin, their combination giving the resulting spin of the protons and the neutron. Also, being charged, a quark q has a corresponding anti-quark (labeled q) which has the opposite charge sign, so that anti-protons are made up of anti-quarks, etc.
Table 2.2. The elementary fermions
Sector | 1st family 2nd family 3rd family | Q /|e| |
Leptons | e iix | -1 0 |
Quarks | uc t ds b | +2/3 -1/3 |
Other combinations of quarks give rise to most of the various unstable particles which are encountered for brief times in high energy collisions. There are two such groups ofunstable particles consisting ofquarks [11]. One group consists of medium-weight particles (compared to the proton), called mesons (from the Greek word for "middle"), which are made up of a quark and an anti-quark. These include particles with names such as the pion, the K-meson, the D-meson, etc. The other group contains the aforementioned baryons, heavier than the mesons, which consist ofcombinations ofthree quarks, and includes the protons and neutrons, as well as large numbers of different unstable particles heavier than the proton. Most of these unstable particles, aside from the pions, however, include two additional families of quarks, besides the first family of up and down quarks which make up the ordinary stable matter. The second quark family consists of the strange (s) and charm (c) quarks, which are heavier than the u and d, and the third family consists of the bottom (b) and top (t) quarks, which are even heavier than the others (the t is a hefty 180 times the mass of the proton).
Not all unstable particles are, however, made up of quarks. The leptons are another group of elementary particles, which share with the quarks the property of being fermions, but which are lighter than the quarks, mesons and baryons. The leptons consist of elementary particles, without sub-structure, some of which are stable (such as the electrons) while others are not. The lep-tons are again divided into three families, or flavors, coming in pairs consisting of one electrically charged and one neutral fermion. The first flavor or family consists of the electron e- and the electron neutrino ve, the second flavor consists of the muonand the, and the third flavor consists of the tauon and the. These also have their corresponding anti-particles (,
etc.). These families parallel the quark families. The whole set of elementary fermions is shown in Table 2.2, where Q indicates the electrical charge in units of the elementary charge e.
These particles interact through various types of forces, the interaction occurring through the exchange of an intermediary particle which is a boson,
more technically called a gauge boson. These forces and the corresponding exchange bosons are described in the next section.
2.2 The forces: three easy pieces and a harder one
Four basic types of forces, or types of interactions, are known so far in Nature. These are the electromagnetic, the gravitational, the weak and the strong interactions. The effects of the first two are felt in everyday life, while the second two appear mainly in nuclear and particle physics processes. These forces appear to emanate from individual sources (masses, charges, etc.) which are either particles or are made up of particles. In the case of electromagnetism and gravity, these forces are most readily apparent from large macroscopic amounts of matter, but electromagnetism plays a significant role also when considering the smallest indivisible amounts of matter, elementary particles such as electrons and quarks. In the case of the weak and the strong forces, these first become apparent when considering elementary particles, or small groups of them, although large amounts of them can transcend the sub-nuclear realm and lead to wondrous large scale manifestations, such as nuclear reactors, explosions and stellar energy generation. Whereas electromagnetism and gravity "in bulk" have very good classical (macroscopic) descriptions given by classical mechanics and Maxwellian electrodynamics, similar "bulk" descriptions are not adequate in the case of the weak and the strong interactions. The latter two can only be described adequately by means of a new type of description, quantum mechanics, which as mentioned is based on the postulate that all physical quantities associated with elementary particles come in small discrete chunks, or quanta. Electromagnetism has, in addition to a successful macroscopic Maxwellian description, also a quantum description, called quantum electrodynamics, which is important in the atomic and nuclear world. For gravity, however, the search for a quantum extension of the macroscopic theory is still on.
To each of these four forces there are, in the language of quantum mechanics, associated messenger particles, which mediate the interaction between the sources susceptible to that particular interaction. These messengers act like springs between masses, or like balls bouncing back and forth between the sources, transferring energy and momentum between them. These messenger particles are themselves also quanta, with discrete energies, momenta, etc., which under their technical name are called gauge bosons. While the "sources", that is the particles which interact through the forces are fermions, the messengers carrying the force between them are bosons. The names ofthese messenger bosons for the electromagnetic, the gravitational, the weak and the strong
forces are, respectively, the photon, the graviton, the W and Z bosons, and the gluon [11,12]. We discuss these four forces and their messenger bosons in turn.
2.2.1 The electromagnetic force
While on the large astronomical scales of planets, stars, galaxies, etc. the dominant force is gravity, almost all the information we have about these objects comes to us through their light [13]. Classically, light is a form of electromagnetic radiation, which in its quantum description comes in discrete quanta called photons. When we observe galaxies, or anything else for that matter, we do it by collecting myriads of photons, which enter our eyes or our telescopes, and are analyzed there by various physiological or electronic devices. The basic sources of these electromagnetic quanta are elementary particles endowed with electric charge, such as electrons, quarks, or smaller groups of them. The electromagnetic force is attractive between electrical charges of opposite sign, and repulsive between charges of the same sign.
The photons are in fact the messenger, or gauge bosons, which mediate the electromagnetic interactions between charged elementary particles, and they are described by a few basic quantities, such as their energy (or their frequency or wavelength, which are also present in the classical description), their momentum, and their spin or polarization. Photons, however, do not have any mass, as far as we know, and they travel (in vacuum) at the speed of light, c = 300 000 km s-1, which according to special relativity is the maximum physical speed achievable by any object.1 The fact that photons transmit an electromagnetic force can be appreciated also at the mundane level, by the fact that sunlight impinging on our skin causes a sensation ofheat. This results from the photons giving energy to electrons and molecules in our skin, whose energy of motion is dissipated and absorbed, resulting in a sensation of warmth. The electromagnetic waves, consisting of many photons traveling together and behaving similarly, can be described through Maxwell's equations as a traveling set of forces exerted in a direction perpendicular to the direction of travel. These forces reverse their sense (say from left to right, or from up to down) at regular intervals in space (if observed at a given time) or with a certain frequency (if observed at a fixed point in space). These alternately reversing forces act on electrically charged particles, such as electrons, and make them wiggle in response (Fig. 2.1). A more macroscopic manifestation of the electromagnetic force is
1 The fact that the speed of light cannot be exceeded by any kind of physical object or signal, a fact amply verified by experiment, is the basis of the special and the general theory of relativity, which is an integral part of both Maxwellian and quantum electrodynamics.
Figure 2.1 Electromagnetic interaction between two electrons, mediated by the electromagnetic gauge boson (the photon, represented by the wavy line). This figure, an example of what are called Feynman diagrams, represents electron-electron scattering via photon exchange. Time increases to the right.
illustrated by the reverse effect, when electrical currents, that is bunches of electrical charges, are made to circulate in a circuit around magnets such as in electrical motors, resulting in the bodily motion of the rotor which energizes diverse types of machinery. This is because the electric and magnetic fields of force are intimately linked and act on each other, hence the unified name of electromagnetic force.
Photons or electromagnetic waves whose wavelengths are of order 10-5 cm are called optical photons (or light), these being the photons to which our eye is sensitive. At longer wavelengths, the electromagnetic radiation consists of, successively, infrared, sub-millimeter and radio photons, while at shorter wavelengths we have ultraviolet, X-ray and gamma-ray photons. The Sun emits most of its electromagnetic energy in the form of optical photons (that is why our eyes developed to be sensitive to optical photons), but it also emits smaller fractions of energy at practically all other wavelengths. However, other types of cosmic sources are found which emit most of their electromagnetic energy, or are primarily detected, at different wavelengths, such as gamma-ray burst sources, X-ray pulsars, or radio-galaxies.
The electromagnetic force is the best understood force in nature, and it plays a major role in everyday life, from controlling molecular structures in our and in other bodies, animate and inanimate, to being the basis of countless industrial applications such as motors, lighting, radio, television, telephony, wireless, etc. An important property of this force is that it is long-range: the electric field of a single charge, that is the force experienced by another electric charge located at a distance r away from the first charge, falls off as the inverse square of
the distance,. This is what makes radio and other electromagnetic
signals, consisting of individual photons, propagate not just from some station to our home, but also over astronomical distances. It is thanks to this long-range property that almost all of what we know about the Universe has been learned through analyzing the photons emitted by various astronomical objects.
An important factor in the electromagnetic interactions is that there are two types of electrical charges, positive and negative. The forces binding the electrons to the nucleus in atoms are electromagnetic in nature, and so are the intermolecular forces. However, in molecules there are so many (negatively charged) electrons and so many (positively charged) protons that at some small distance away from the molecule the two signs of the charges cancel out, making the electromagnetic interaction between molecules become effectively a short-range one. This phenomenon is called shielding. Shielding is however not perfect, and it is the residual electromagnetic force which keeps together the molecules of fluids and solids, or the molecules in our body as well as the molecules in a wall, which we can push with our hand without one penetrating the other.
In the 20th century, the 19th-century classical "macroscopic" description of Maxwellian electromagnetism was successfully translated into the language of quantum mechanics. This is the theory of quantum electrodynamics which takes fully into account the fact that the electromagnetic field consists of individual photons, the quanta of this field, and that these interact with particles whose properties are also quantized and obey quantum mechanics. One of the great successes of quantum electrodynamics, due to Dirac, was the prediction of the existence of anti-matter, or anti-particles. The quantization of the electromagnetic field, besides providing a far deeper understanding of the basic nature of this interaction, has had an enormous practical impact on various industrial applications, such as lasers, optical fiber communications, data encryption and quantum computing, etc., which in turn have greatly impacted the development of detectors for astronomical as well as laboratory measurements.
2.2.2 The weak force
The weak and the strong forces occur in nuclear physics and in high energy interactions between elementary particles, such as in large laboratory accelerators, in stars or in cosmic rays accelerated by cosmic sources. In contrast to the electromagnetic and the gravitational forces, the weak and the strong forces are felt only at short range, over dimensions comparable to the sizes of nuclei and elementary particles. Also, in contrast to electromagnetism and gravity, there are no "classical" or macroscopic descriptions of these nuclear
forces. Quantum mechanics is needed to describe them even at the simplest level, when individual nuclei or particles are considered, or small assemblies of them.2
The quantum mechanical description of the weak force is modeled after quantum electrodynamics. The latter is a theory which has been fantastically successful, allowing incredibly precise calculations which agree with experiment to within 10 digits and more accuracy. The weak interaction has, after electromagnetism, the next best developed quantum theory, although the level of complexity is significantly higher and the level of understanding is much more approximate. In its modern form the weak interactions have in fact come to be described in a completely similar manner as electromagnetism, in a joint quantum formulation called the electroweak theory. In this joint theory, these two interactions and their experimental phenomenology differ substantially from each other at energies below the so-called electroweak energy scale, which is about 100 GeV, but above this energy the two sets of phenomena start to become increasingly similar. At energies somewhat below the electroweak scale this has been verified experimentally, and a study of these phenomena at the electroweak scale and above is one of the major goals of modern accelerators such as the LHC at CERN in Geneva.
The weak interactions were first observed in radioactive nuclear decays, and more generally they involve elementary particles such as leptons and the quarks making up nucleons or other unstable particles. They are characterized by always involving neutrinos, which as mentioned are extremely light, electrically neutral elementary particles. According to the Standard Model of particle physics, neutrinos would actually be massless, and consequently they would be expected to travel at the speed of light in vacuum. But one of the reasons why we know that physics beyond the Standard Model is needed is that now we know that neutrinos do have a very small mass, as discussed below. Also unlike the photon, of which there is only one kind, there are three kinds or "flavors" of neutrinos: the electron neutrino, the muon neutrino and the tau neutrino, which participate in different types of weak processes. The sources or particles producing the weak interaction are endowed with a "weak" charge, which is related to their electrical charge.
There are two major reasons why these interactions are called "weak". One of them is that the neutrino, which is characteristic of such interactions, is extremely hard to detect, unlike the photon - the neutrinos interact extremely
2 However, when very large numbers of particles are considered, a suitable averaging of the quantum mechanical equations leads to the usual macroscopic classical description of matter in bulk.
weakly with any detector. The interaction rate is so minuscule that more than 5 x 1013 (50 trillion) neutrinos emanating from nuclear reactions in the Sun pass through our bodies every second, without causing any harm. Unlike the photons from the Sun, they don't stir up the electrons in our body molecules, they just go right through them (except so rarely as not to make any difference). The other main reason why their name is appropriate is that the weak interactions occur extremely slowly. For instance, the chain of nuclear reactions in the interior of the Sun, which generate the energy (and the photons) ultimately giving rise to life on Earth, involve both weak and strong processes, but it is the weak interactions which take the longest to occur. They set the slow pace of evolution of the Sun, and in fact if they had been any faster, biological evolution and life on Earth would not have had the billions of years necessary to reach its current state.
The messenger particles of the weak interactions are of three types: the W +, W- and Z° bosons. These, unlike the photons, are massive particles - in fact, quite massive, about 80 and 90 times heavier than protons. The W bosons are endowed with electrical charges indicated by the +, - superscripts, while the Z bosons are electrically neutral. The W and Z bosons mediate between particles carrying a weak charge, just as the photons mediate between particles carrying electrical charges. The fact that the messenger particles are so heavy is the basic reason why the interaction is of short range. The messengers are so heavy and sluggish that they can't travel very far, unlike the massless, infinitely nimble photons.
One of the aspects of the unified electroweak theory is that at energies above that of the W and Z boson mass-energy (which is roughly the "electroweak" energy) it considers the weak bosons as massless, just as the photons are (the latter are massless however at lower energies as well). Below the electroweak scale, however, the weak bosons acquire a mass. This is part of the more general theory of the Standard Model, which generates the mass of these and other particles below the electroweak scale through the intermediary of a new complex quantum field, called the Higgs field. This field behaves as a scalar (instead of as a vector, such as the electric field), and it has the property of allowing at high energies a description of the electroweak theory where all particles are mass-less (fermions and bosons), while below the electroweak energy the fermions and some of the bosons acquire masses, while leaving the photons massless and predicting the existence of a massive scalar particle called the Higgs boson. The electroweak theory has had numerous successes, such as predicting the mass of the W and Z bosons, and explaining various other aspects of the weak interactions. This success has motivated the consideration of other types of scalar fields at high energies, such as those invoked to explain inflation and dark energy (discussed in Chapter 3). The mechanism for generating the masses of
the particles is called the Higgs mechanism, and the mass of the predicted Higgs boson is expected to be in the range of > 100 GeV. Discovering the Higgs particle is one of the prime targets of the LHC and similar machines.
2.2.3 The strong force
The strong force is, together with the weak force, the other type of interaction which acts only over a limited range of distances, of the order of the size of nuclei or smaller. As in the case of the weak interactions, the strong interactions can only be described with any degree of success in a quantum formulation, the modern version of which is called quantum chromodynamics (QCD). As the name implies, the forces binding the quarks into nucleons (protons and neutrons) and binding the nucleons inside the nuclei are extremely strong. This enormous strength is what causes the splitting of a nucleus (fission) or the creation of more complex nuclei (fusion) to release the huge amounts of energy locked inside the nuclei in nuclear bombs and in nuclear reactors.3 As a rough comparison, the fission of one kilogram (kg) of fissile material can deliver, undergoing strong nuclear reactions, an energy comparable to that which 1 kilo-tons (one million kilograms) of TNT would deliver through chemical reactions. Thus nuclear reactions are roughly a million times more efficient at delivering energy than the most energetic chemical reactions, which essentially depend on electromagnetic interactions.
The strong force acts between nucleons in nuclei, or rather between the quarks that make up the nucleons or other unstable particles. The quarks are the sources of the strong force, and as mentioned there are six types of quarks. Of these, the up and down quarks are the most common ones, making up the stable nucleons, the proton and the neutron. The other four types of quarks, the strange, charm, bottom and top quarks, are heavier and appear in the much rarer fleeting particles produced in very high energy particle collisions, and the six quarks are arranged in three families, or generations (Fig. 2.2). Each of the quarks in each family is endowed with three possible types of strong charge, called "color" charges, hence the name of quantum chromodynamics for the theory describing them. These colors are generally called red, blue and green (r, b, g). All particles made up of quarks, which are subject to the strong force, are posited to be color blind, or color neutral, i.e., they have quarks whose colors neutralize each other. The combination of r, b and g is neutral. There are also
3 This might at first sight seem at odds with the short-range nature of the strong force. The long-range macroscopic effects ofthe strong nuclear force can be, and are, produced by bringing close together large amounts of strongly interacting particles so that their collective energy generation irradiates and heats up the neighboring matter, leading to electrical currents or large scale shock waves.
Figure 2.2 A particle collision event display in the CDF detector at FermiLab showing a single top quark event. Such collisions lead to jets of quarks and gluons, here showing two jets plus a neutrino ("missing energy") track and a track in the direction of a muon escaping from the decay of a top quark.
Source: Courtesy of the CDF Collaboration.
anti-colors (r, b, g) for the anti-particles, the sum of which is also neutral. Other unstable particles made up of only two quarks, i.e., mesons such as the pion, must consist of quarks with one color and the same anti-color.
The messenger particles or gauge bosons mediating the strong color forces are the gluons. The gluons are massless and electrically neutral. Since they have to mediate between six different types of quarks with three different colors, there are eight different kinds of gluons, each of which carries a color charge and a different anti-color charge, to ensure that the particles between which they mediate remain color neutral after the interaction. The fact that the gluons, i.e., the messenger particles, carry colors means that they themselves can act as color charges, i.e., they are subject to strong interactions among themselves. This is unique to the strong force: none of the other three interactions have messengers carrying the charge corresponding to the interaction, only the sources do. This means that quantum chromodynamics is a more complicated theory in this respect as well: not only is the number of charges larger, but the messengers can interact among themselves.
The fact that the gluons are massless might suggest that the range of the interaction is infinite, as in the case of the photons. However, since there are
Table 2.3. Approximate comparison ofthe relative strengths of the four basic interactions
Strong Electromagnetic Weak Gravitational |
110-2 10-7 10-39 |
three types of color charges and since the particles must be color neutral, this leads to a cancellation of the color forces beyond nuclear distances, making the strong force essentially short range. The fact that the gluons can interact with themselves, unlike the photons in electromagnetism, leads to a phenomenon of anti-shielding of the color charges, which has important implications for the dynamics and kinematics of particle interactions at very high energies [1]. It appears rather complicated, but it all works out, and quantum chromodynamics is a very successful theory, which has allowed much progress to be made in the understanding of the strong interactions and particle physics in general.
2.2.4 The gravitational force
The gravitational force is by far the weakest of the four forces, much weaker than the nuclear "weak" force. If one compares the forces due to the four types of interactions between two particles of equal masses and charges across the same distance, the relative strengths are shown in Table 2.3.
Despite its extreme weakness, gravity is the most obvious of all forces: we see apples falling, we feel the weight of heavy objects, etc., and this is because across moderate to large distances gravity can overwhelm the other forces. One reason for this is that it shares with electromagnetism the property of being a long-range force, which also drops off as the inverse square of the distance,.It
is because of this long-range property that the effects of these two forces can be felt over macroscopic distances. This makes their effects more palpable to the human senses, so they have been known and studied from much earlier times than the nuclear strong and weak forces, which are microscopic-scale short-range effects, requiring special instrumentation for their study. The gravitational force is the one that has been known and studied for the longest time, and in its classical Galilean and Newtonian form is perhaps the best understood. The motions of the planets, the fact that our bodies are "weighted down" by the gravitational attraction of the Earth, etc., are phenomena which can be appreciated with the eyes and with the senses, even without instruments, and this study underwent enormous development from the 17th through the 19th centuries.
However, the all-pervading obtrusiveness of the gravitational interaction relies on an additional important reason. This is that, unlike the electromagnetic interaction, it has only one type, or sign, of gravitational "charge", namely the mass. There are no negative or positive masses, just masses. This means that they cannot cancel each other out at a large distance. All masses produce attractive forces proportional to their mass, falling off with distance as 1 /r2, no matter what other matter is between the source and the point of observation. And because there is only one sign of the mass, there is no screening of the gravitational force (whereas screening of the electromagnetic force can reduce the latter, in bulk matter, to an effectively short-range force). This is the reason why our bodies are attracted to the Earth (our "weight") by the gravitational force, since the other three forces, although microscopically stronger than gravity, have their effects canceled out by virtue of their effective ranges being much smaller than our body size. In fact, gravity can "coop up" the other interactions (Fig. 2.3).
The understanding of gravity underwent a major qualitative jump starting in the early 20th century. This was triggered by the fact that when one looks at the finer details of the motion of planets very near the Sun (Fig. 2.3), or when one considers very large masses such as contained in large expanses of the Universe, Newtonian mechanics leads to small but noticeable inconsistencies (and this gets worse, nowadays, with extremely dense and compact objects such as neutron stars or black holes). To treat these phenomena, one has to use a more complicated description, based on Einstein's general relativity. In this latter form, gravity is not even described as a force anymore, but rather as a distortion of space-time which results in the observed dynamics of the massive bodies being considered. Thus, for instance, the larger mass of the Earth distorts the structure of the space-time around it, and the Moon simply follows, or freely falls along, the natural curvature of this space-time in which it finds itself, resulting in its motion around the Earth. This description of celestial mechanics is somewhat harder to grasp, but as long as one is considering large macroscopic masses it provides a rather detailed and accurate mathematical machinery to describe the behavior of matter. Gravitation, in this description, still acts on the macroscopic scales, and one does not need to consider discrete chunks or quanta of the gravitational field: its bulk properties describe essentially everything that one observes macroscopically. General relativity even describes the large-scale gravitational equivalent of Maxwell's electromagnetic waves, namely gravitational waves. These are ripples in space-time, which travel at the speed of light, and which are being searched for with large experiments such as LIGO and VIRGO (discussed in Chapter 9).
However, even with this version of (general relativistic) gravity, conceptual problems arise if we consider extremely large mass or energy densities, that is,
Figure 2.3 Our Sun, as seen by the SOHO satellite in ultraviolet light. The Sun is the source of all life on Earth, thanks to the light and heat that it delivers. The gravitational, electromagnetic, weak and strong forces all play important, concerted roles in the Sun, which is the ultimate environmentally friendly nuclear reactor in our cosmic backyard. The nuclear reactions in its core produce copious neutrinos, which escape but do not harm us; they produce nuclear waste, which is trapped by gravity in the core; and they produce gamma-rays, which multiply and lose energy as they slowly diffuse out, until they emerge mainly as beneficial optical sunlight.
Source: NASA.
extremely large amounts of matter or energy compressed into extremely small regions of space-time. In this case quantum effects are expected to become important, and the quanta of the gravitational field, the gravitons, need to be considered as separate discrete entities. This regime is encountered near the central "singularity" which would appear close to the geometrical center of black holes, or in the very early Universe, very close to the instant described as the Big Bang. This would require a quantum theory of gravity, which however is so far non-existent. There are candidate theories which have the elements of a quantum gravity theory, such as string theory, or quantum loop gravity, which attack this problem and absorb huge amounts of effort by some of the sharpest researchers, but so far with only suggestive results pointing towards an ultimately workable theory [7].
Thus, despite being the oldest and perhaps the best understood force in its macroscopic form, in its wished-for microscopic quantum formulation gravity
remains the most recalcitrant among the four forces. The chamber concert of modern physics consists, at the moment, of three somewhat easier pieces, and a fourth one which apparently is simpler but upon closer inspection turns out to be more puzzling than the other three.
2.3 Beyond the Standard Model
The above-mentioned incompleteness of the gravitational theory is just one of the signs that some important pieces are missing from the jigsaw puzzle.
Another sign is that neutrinos, which are key participants in the weak interactions, were for a long time happily considered to be massless, even in the unified electroweak theory. However, according to experiments in the last one and a half decades, it appears that neutrinos, unlike what is postulated in the Standard Model of particle physics, do have very small masses. This means that they must travel at speeds extremely close but not quite equal to the speed of light. Their very slight sluggishness is caused by their tiny mass, which slows them down ever so slightly. This mass is also tied to the fact that neutrinos of different flavors can, as they propagate, switch from one flavor to another ("oscillate" between flavors). These are phenomena which are definitely beyond the Standard Model, and the study of such BSM phenomena is one of the major frontier areas of physics. These also have interesting astrophysical implications, which are discussed in some of the subsequent chapters.
Yet another indication that skeletons remain lurking in the closet is that the electroweak theory can explain the masses of fermions and bosons, but it requires a large number of ad-hoc parameters to do so, including the coupling strengths [14].
Looking ahead, theorists guess that if the electromagnetic and the weak forces become united at energies100 GeV, at even larger energies one would expect that the strong force should also become unified to the other two. There is in fact experimental evidence indicating that the strengths of the electromagnetic, the weak and the strong forces tend towards a convergence at energies of order 1016 GeV, an energy which is however well beyond the reach of even planned accelerators. The search for such "Grand Unified Theories" (GUT, for short) is a major field of ongoing activity [1].
A major group of such GUT theories is based on a new type of symmetry between particles called supersymmetry (abbreviated SUSY GUTs). This considers the possibility of bosons and fermions inter-converting, and posits the existence of "superpartners" for each particle. To each fermion corresponds a boson, given the name "sfermion", and to each boson corresponds a "bosino" superpartner. For example, each "quark" (a fermion) has a "squark"
superpartner, which is a boson; while the Z-particle (a boson) has a "zino" superpartner which is a fermion.
While electroweak theory, GUTs, SUSY GUTs, etc. were originally motivated by laboratory experiments and particle physics theory, these ideas soon spilled over into cosmology. In the very early Universe, of course, the Big Bang model predicts energy densities which are so high as to exceed anything in the laboratory, providing a likely arena where these ideas can play themselves out. One such scenario involving scalar fields modeled after the Higgs field soon swept through with models for an inflationary expansion phase at epochs characterized by the GUT energy scale. Other BSM ideas were developed to address the presence of the dark matter (see Chapter 3), which is expected to be a new form of extremely weakly interacting matter. The apparent acceleration of the expansion of the Universe at the most recent epochs has, after exhaustion of the more plausible astrophysical explanations, led to the consideration of a different type of scalar fields leading to forms of dark energy as an explanation for this dynamic bulk behavior.
Then, if we ratchet up the energies to even much larger levels than GUT energies, simple dimensional arguments strongly suggest that quantum effects will become comparable to gravitational ones. This occurs at the Planck energy scale,, where one might expect all four of
the known forces to become unified. This leads to the need to formulate a (so far unfinished) quantum theory of gravity. String theory is the most widely considered approach towards achieving this goal, while quantum loop gravity is a different approach which is also being considered (e.g. [5-7]). These theories address what happens at the earliest conceivable instants in the Universe, as well as what happens inside black holes near the classical central singularity, which in a quantum theory of gravity is expected to be avoided due to the Uncertainty Principle which introduces an unavoidable fuzziness over energies and times of order.
2.4 Into the soup
All of the previous ingredients, quarks, leptons, bosons, atoms, etc., go into making our Universe, as so many ingredients of a Cosmic Soup. The current mainstream scenario is that initially, at times extremely close to the initial instant of the Big Bang, the Universe would have been extremely hot, with temperatures of the order of the Planck energy scale, and it would have been permeated with chaotic space-time fluctuations of the quantum vacuum. These might have already coexisted with, or later transitioned into, quantum fields containing the seeds of what later would become the separate strong,
weak, electromagnetic and gravitational fields, the super-symmetric quanta of which inter-converted between fermionic and bosonic states. The gravitational fields would have decoupled very soon after this instant from the rest of the fields. By the time the Universe cooled to temperatures comparable to the GUT energy scale, the strong force would have in turn decoupled from the rest, and by the time temperatures comparable to the electroweak energy scale were reached, the weak and the electromagnetic forces would have decoupled from each other. The Universe would still have been made up of a quark, gluon, lepton and boson soup, which only when QCD-scale temperatures (GeV and above) were reached would have jelled into the recognizable baryons, protons and neutrons that we recognize today. This journey and its aftermath, from the Planck Era to today, is discussed in the next chapter.
To get Idea gprs settings visithttp://ideagprssettings.blogspot.com/
To get aircel gprs settings visit
http://aircelgprssetting.blogspot.com/
To get uninor gprs settings visit
http://uninorgprssettings.blogspot.com/
To get tatadocomo gprs settings visit
http://tatadocomogprssettings.blogspot.com/
To get videocon gprs settings visit
http://videocongprssettings.blogspot.com/
To get vodafone gprs settings visit
http://vodafonegprssettings.blogspot.com/
To get airtel gprs settings visit
http://airtelgprssettingsforall.blogspot.com/
To get details of High Energy universe Please Visit
http://thrissurinformations.blogspot.com/
