Introduction

Parker (1958) showed that the natural state of a hot and extended stellar corona is one of supersonic, super-Alfvénic expansion. In the case of the Sun, the evolution of the strong magnetic field that permeates the corona modulates this expansion (e.g. Pneuman and Kopp, 1971). Indeed, it is the interplay between the coronal magnetic field and the expansion that produces both a highly structured solar corona and a spatially variable solar wind (see Vol. I, Chapter 9). For example, the combed-out appearance of the outer solar corona is a product of the coronal expansion. Because the solar wind plasma is an excellent electrical conductor, the coronal magnetic field is “frozen” into the solar wind flow (Vol. I, Chapter 3) as it expands away from the Sun, forming what is now commonly called the heliospheric magnetic field, HMF. A simple model of the HMF (Parker, 1958) predicts that solar rotation causes the HMF in the solar equatorial plane to be bent into Archimedean spirals (Vol. I, Section 9.2) whose inclinations relative to the radial direction depend on heliocentric distance and the speed of the wind.

The Sun's magnetic field evolves continually, the most pronounced changes being those associated with the advance of the ˜11-year solar activity (sunspot) cycle and the ˜22-year magnetic cycle.

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.

This book provides an overview of topics in high energy, particle and gravitational astrophysics, aimed mainly at interested undergraduates and other readers with only a modest science background. Mathematics and equations have been kept to a minimum, emphasizing instead the main concepts by means of everyday examples where possible. I have tried to cover and discuss in some detail all the major areas in these topics where significant advances are being made or are expected in the near future, with discussions of the main theoretical ideas and descriptions of the principal experimental techniques and their results.

Cosmology, particle physics, high energy astrophysics and gravitational physics have, in the last two decades, become increasingly closely meshed, and it has become clear that thinking and experimenting within the isolated confines of each of these disciplines is no longer possible. The multi-channel approach to investigating nature has long been practiced in high energy accelerators involving the strong, the weak and the electromagnetic interactions, whereas astrophysics has long been possible only using electromagnetic signals. This situation, however, is rapidly changing, with the advent of major cosmic-ray, neutrino and gravitational wave observatories for studying cosmic sources, and the building of particle physics experiments using beams and signals of cosmic origin. At the same time, theoretical physics has increasingly concentrated efforts in attempts to unify gravity with the other three forces into an ultimate theory involving all four. The intense activity in these fields is beginning to open new vistas onto the Universe and our understanding of Nature's working on the very small and very large scales. In this book I have sought to convey not only the facts but also the challenges and the excitement in this quest.

Particles from Heaven

Cosmic rays are energetic particles that reach us from outer space, arriving from all directions. They are generally electrically charged particles, such as protons, heavy nuclei, electrons and positrons, but more broadly one includes among them also electrically neutral particles such as neutrons and neutrinos from outer space. If one subtracts those that arrive from the Sun, the rest arrive essentially isotropically, constituting a uniform background of cosmic-ray radiation, made up of particles with a finite mass. In addition to these, there is also a separate photon background, which includes the cosmic microwave background, the diffuse starlight optical-infrared background, and X-ray and gamma-ray backgrounds, all of which are also essentially isotropic, after subtraction of individual resolved sources.

A major difference between the cosmic-ray background and the photon background is that photons are massless and electrically neutral, so they travel essentially in straight lines from their sources, making it (at least at some wavelengths) easier to identify where they ultimately came from. The vast majority of cosmic rays, however, are electrically charged, and this makes it far harder to discern where they came from. This is because the interstellar and intergalactic space is woven through by random magnetic fields, and the Earth's atmosphere is permeated by an ordered magnetic field, so that as a result of propagating through these magnetic fields the cosmic ray path has little to do with the direction of whatever source they originated from [57].

The dynamics of the Universe

The present-day Universe appears to be expanding in all directions, as shown by the fact that all distant galaxies and clusters of galaxies appear to be receding from us. This was the first and most obvious piece of evidence indicating that our Universe was initially much denser, leading to the hypothesis of an origin in an initial “Big Bang”.

The recession velocities of the galaxies are measured by analyzing the light they emit, which in a spectrograph is seen to contain not only a continuum of frequencies but also discrete frequencies, due to electronic transitions between energy levels of atoms in these galaxies. Such lines have a well-determined laboratory frequency, and when we observe such well-known atomic lines but we see that their frequency is lower (or their wavelength is longer, since wavelength equals speed of light divided by frequency), we infer that the atoms and the galaxy are moving away from us. This effect is called the Doppler shift. A simple everyday acoustic analogy of this Doppler shift is provided by the pitch of an ambulance's siren, which gets lower as the ambulance speeds away from us: the motion away from us “stretches” out the wavelength.

The expansion velocities increase with the distance away from us at a rate which is proportional to the distance, as long as the galaxies are not too far away.

What are gamma-ray bursts?

Gamma-ray bursts are sudden, intense flashes of gamma-rays, detected mainly in the MeV gamma-ray band. When they occur, for a few seconds they completely overwhelm every other gamma-ray source in the sky, including the Sun.

GRBs were first discovered in 1967 by the Vela military satellites, although a public announcement was only made in 1973. These spacecraft carried omni-directional gamma-ray detectors, and were flown by the US Department of Defense to monitor for nuclear explosions which might violate the Nuclear Test Ban Treaty. When these mysterious gamma-ray flashes were first detected, it was determined that they did not come from the Earth's direction, and the first, quickly abandoned suspicion was that they might be the product of an advanced extraterrestrial civilization. However, it was soon realized that this was a new and extremely puzzling cosmic phenomenon. For the next 25 years, only these brief gamma-ray flashes were observed, which vanished quickly and left no traces, or so it seemed. Gamma-rays are notoriously hard to focus, so no sharp gamma-ray “images” exist to this day: the angular “error circle” within which the gamma-ray detectors can localize them is at best several degrees, which contains thousands of possible culprits. This mysterious phenomenon led to huge interest and to numerous conferences and publications, as well as to a proliferation of theories. In one famous review article at the 1975 Texas Symposium on Relativistic Astrophysics, no fewer than 100 different possible theoretical models of GRBs were listed, most of which could not be ruled out by the observations then available.

Dark matter

As we discussed in Chapter 3, about 26% of the mass-energy density of the Universe at present is in the form of non-relativistic gravitating “matter”, and the rest is in some form of “dark energy”. Considering for now this 26% non-relativistic matter component, about 1/7th of this, or 4% of the grand total, is in the form of known particles, mostly baryons (the leptons, neutrinos and photons represent much less mass than the baryons). The other 6/7th of the non-relativistic matter, or 22% of the grand total, is something which we call “dark matter”, colloquially referred to as DM.

What we know about the dark matter is frustratingly little, but enough to convince us of its existence and to give us an idea of some of its overall properties, as outlined in Chapters 3 and 4. We know that it is there, because its gravity makes itself felt in the dynamics of our galaxy and that of other galaxies, as well as in the dynamics of the expansion of the Universe, and its presence is directly mapped via the “gravitational lensing” effect which distorts the paths of the light rays coming to us from distant objects through foreground dark matter-dominated clusters of galaxies. The DM also plays an important role in determining at what epoch in the expansion of the Universe proto-galaxies start to form and assemble. We also know that it is extremely weakly interacting, if not downright inert. It does not emit, block or reflect light or electromagnetic waves, nor does it seem to interact with other particles, at least not enough to have been detectable so far.

Importance of the GeV–TeV range

The GeV–TeV gamma-ray range holds a strategically important role in astrophysics, by providing the first high quality surveys of most classes of very high and ultra-high energy sources, including sufficiently large numbers of objects in each class to be able to start doing statistical classifications of their properties. The number of photons collected for individual sources in this energy range extends in some cases into the tens of thousands, leading in many cases to quite high signal-to-noise ratios.

The GeV–TeV photon emission provides not only important information about the photon emission mechanisms and the source physical properties, but also clues for the importance of the corresponding very high energy (TeV and up) neutrinos and even higher energy cosmic rays which may be emitted from such sources [44]. In addition to the discrete astrophysical sources, instruments in this energy range also provide information about the diffuse gamma-ray emission, such as that associated with cosmic rays interacting with the gas in the plane of our galaxy, the diffuse emission from our galactic center, and the extragalactic emission component, all of which could yield information or constraints about possible dark matter annihilation processes, in addition to the astrophysical processes and the sources involved.

Ripples in space-time

The gravitational force field, as discussed in Chapter 2, is described in General Relativity as a distortion of space-time caused by the masses in it, which results in any small test mass in this space-time moving along the curvature of the space-time. If the position of the large source mass (or masses) which dominate a certain region of space-time is varying, the space-time structure readjusts itself to reflect the changed positions of the source masses, after a delay caused by the fact that the information about this change of position of the source masses cannot be communicated faster than the speed of light. That is, the space-time at some location r away from the source mass which has moved can respond to this change only after a time t = r/c. This traveling information about changes in the space-time structure is the basis of the phenomenon of gravitational waves, which can be thought of as ripples in the texture of spacetime that travel at the speed of light.

One can visualize this also in a simpler quasi-Newtonian picture, provided one accepts the relativistic principle that information travels at most at the speed of light. Imagine two equal masses M in a circular orbit of radius d around each other, in a plane parallel to the line of sight to the observer, with the center of mass of the orbit (the mid-point of the line separating the two) being a fixed point in space at a distance D from the observer (see Fig. 9.1).