Fireworks in a Dark Universe

The following sections are included:

• Preface
• Introduction
  • Dramatic Developments in the 20^th Century
• Acknowledgements
• Contents

The universe is home to stars and galaxies, as well as dark matter, whose nature is still unknown, but whose effects are apparent. A divine spirit dwells in the universe, in the form of dark energy, whose properties are even less understood than those of the dark matter. And all of this splendor and glory basks in a huge bath of microwave radiation called the cosmic background radiation, an ancient remnant of the primordial universe. The exact proportions of the various components, as measured in astronomical experiments conducted over the past two decades, is about 4% visible matter, namely stars and galaxies as well as gas and radiation permeating the space between the galaxies; around 23% dark matter, and the rest — about 73% — is dark energy…

The following sections are included:

• Galaxies — The Natural Habitats of Stars
• Giant Black Holes in the Centers of Galaxies
• The Milky Way Galaxy and the Black Hole in its Center

Like all living creatures, stars are born, they live, and they die. They are created within dense dust and gas clouds like that presented in Figure 1. This happens when a certain region of a cloud is slightly compressed and cooled. As a result of the gravitational force acting on it, that region becomes even more compressed and a protostar, which will later become a star, is formed. This stage is relatively short and lasts only several million years. When the density and temperature at the center of the protostar reach sufficiently high values, nuclear reactions at the center of the protostar “ignite”, and a process of nuclear fusion occurs, like the process that takes place in a nuclear fusion reactor, turning hydrogen into helium. The instant at which the fusion reaction ignites marks the star’s moment of birth. A prodigious amount of energy is released during the nuclear fusion process, most of which turns into heat and radiation…

The following sections are included:

• The Structure of Matter
• Quantum Theory and the State of Matter
• Bundles of Light Quanta
• Schizophrenia and Uncertainty in the Quantum World
• The Exclusion Principle and the Periodic Table
• Antimatter
• The Four Forces
• Force Mediators
• Spin and the Social Customs of the Elementary Particles
• Symmetry and Conservation Laws
• Symmetry Breaking
• Unification of Forces and the God Particle

Electromagnetic radiation is emitted from accelerating electric charges and is transmitted through various media (including absolute vacuum) by means of changes in the electric and magnetic fields in space. This is the most well-known kind of radiation, and our life without it is hardly imaginable. Radio and television broadcasts, cellular phones, sunlight, electric lights, X-ray imaging, infrared devices, microwave ovens, and radar are all just a few examples of electromagnetic radiation and its applications. And as we mentioned earlier, the discovery of electromagnetic radiation also led to the development of Einstein’s theory of special relativity. The velocity of an electromagnetic wave in vacuum is the highest in the universe and one of the basic constants of nature. Its value is 300,000 km/sec (or, to be precise, 299,792 km/sec), and its common symbol in scientific literature is c. This velocity, which is called the speed of light, is identical for all kinds of electromagnetic radiation — radio waves, microwaves, visible light, ultraviolet radiation, X-rays, and gamma rays. The parameter that distinguishes between these types of radiation is the wavelength, or alternatively, its frequency. Other than this difference, all kinds of radiation mentioned are identical. Visible light, the range of wavelengths to which the human eye is sensitive, constitutes only a very small portion of the electromagnetic spectrum. Radio waves have the longest wavelength, reaching several kilometers, while gamma rays have the shortest wavelength — less than one millionth of a nanometer (one nanometer equals one billionth of a meter). The possible existence of electromagnetic waves was first predicted in the late 19^th century by British physicist James Clerk Maxwell, after formulating the laws of electromagnetism as four basic equations called Maxwell’s equations. These equations describe every phenomenon related to electric and magnetic fields, including phenomena related to electromagnetic waves. In 1887, Heinrich Rudolf Hertz built the first radio wave transmitter and receiver, demonstrating the existence of electromagnetic waves. Indeed, Hertz’s experiments led to the development of the telegraph and radio devices, and the frequency unit, which is measured in cycles per second, is named after him. A frequency of one megahertz (one million hertz) is equivalent to a wavelength of 300 m…

Neutrinos are the strangest, most elusive particles of all particles in nature, and at the same time they are among the most important of them. Neutrinos play a key role in nuclear fusion and fission processes, and can edify us regarding the properties of matter, the elementary forces, the structure of stars, the evolution of the universe, and perhaps also, so the scientists hope, about black holes. The accepted symbol for neutrinos in the scientific literature is the Greek letter v and, as its name implies, it is neutral. In other words, neutrinos carry no electrical charge. The idea of the existence of this particle was born as a wild theoretical hypothesis aimed at solving a contradiction between one of the most basic laws of physics — the energy conservation law — and the results of beta-decay experiments conducted in the 1920s. Only in 1956, over a quarter of a century after the concept of the neutrino was first proposed, did a physicist by the name of Frederick Reines succeed in providing empirical evidence of the existence of this particle, work for which he was awarded the Nobel Prize in 1995…

So far, we have addressed electromagnetic radiation and its discovery as well as the properties of neutrino particles. Nature, however, provides us with another, completely different kind of radiation that is related to the gravitational force. One of the most important predictions of the theory of general relativity is the existence of gravitational waves, or gravitational radiation. Such radiation is not possible according to Newton’s gravitational theory, which is an essentially static theory. The basic reason that necessitates the existence of this kind of radiation in the relativity theory is the fact that no information can travel faster than the speed of light. To understand the meaning underlying this last sentence, try to imagine the following thought experiment: Suppose two small masses are located at a given distance from one another. We hold one of the masses in our hand while the other is attached to a spring scale. Since the force of gravity acts between the two masses, they have a tendency to pull closer to one another, but the reactive forces applied by our hand and by the scale cause the masses to remain in place. The scale will, therefore, give a reading that corresponds to the gravitational force acting between the two masses. Since the gravitational force depends on the distance between the masses, the reading on the scale will change if we change the distance between them. Let us now suppose that we suddenly draw our hand back, increasing the distance between the two masses. What will the scale reading be now? Solving Newton’s equations yields an instantaneous change in the scale reading. In other words, the information regarding the movement of the mass that is in our hand was transmitted to the other mass, which is attached to the scale, at an infinitely high speed. And here lies the problem, since according to the principles of the relativity theory, no entity in nature can move faster than light. The equations of general relativity contain this principle within them. Indeed, solving these equations instead of Newton’s, one finds that the scale does not in fact react immediately, but only after a time that is equal to the time it takes light to travel the distance between the two masses. In other words, the disturbance we created propagated in space at a finite velocity, like a sound wave that propagates through the air when we clap our hands or like a light wave emitted from a flashlight…

Cosmic rays, as distinguished from cosmic background radiation, is the name given to a collection of very high-energy particles that reaches Earth from the expanses of the universe. Cosmic rays contain mainly hydrogen nuclei (protons) and some helium, as well as heavier nuclei. In addition to all of these particles, cosmic rays also contain electrons and a minute amount of antimatter. The slightly misleading term, cosmic rays, was coined in the 1920s by Robert Millikan who believed that this radiation was a kind of gamma rays arriving from outer space. Only later did the true nature of this cosmic radiation become clear, but for some reason, the name stuck and is still in use today…

We have seen that a white dwarf is created when a star, whose mass is smaller than about 10 times the mass of the Sun, dies after exhausting the nuclear fuel in its core. White dwarfs are a million times denser than the average density of the Sun (which is approximately equal to that of Earth), so that even though their mass is similar to that of the Sun, their radius is only just one hundredth the radius of the Sun. If the Sun were the size of a tennis ball, a white dwarf will be smaller than a pinhead (see Figure 1), hence its name dwarf. Due to their small dimensions and high density, the gravitational force on the face of white dwarfs is a million times stronger than the force of gravity on Earth. Their color, white, results from their high surface temperature, which is caused by the considerable heating of the core during its contraction. Over time, the dwarf cools down and grows pale. Most white dwarfs consist mainly of carbon and oxygen; some, those that were created from especially low-mass stars, are made of helium…

As mentioned earlier, objects whose mass is greater than the Chandrasekhar mass cannot exist as white dwarfs and must collapse. If their mass is not too great, their collapse will be halted by the neutron degeneracy pressure, and they will become neutron stars. The existence of neutron stars was predicted by astrophysicists Walter Baade and Fritz Zwicky in 1933 based on Chandrasekhar’s work, about one year after the neutron was discovered. Only in 1967, however, did a group of British astronomers discover actual evidence of the existence of neutron stars, in the form of a celestial object referred to as a pulsar, a discovery discussed extensively in Chapter 13.

Quark stars and strange stars are still considered to be hypothetical objects that are made of matter referred to as quark–gluon plasma, which is, according to conjecture, created in the centers of neutron stars of especially high density. To date, no decisive evidence of the existence of such stars has been brought forward. Recent observations made by the Chandra X-Ray Observatory form the basis of a claim that two objects, which until recently were considered to be neutron stars, are in fact quark stars. The discovery of several especially bright supernovae in the past decade has also led to a suggestion that quark stars, rather than neutron stars, were possibly created in these systems. These claims are controversial and, as of today, no concrete evidence of the existence of quark stars was found, and the search for them is still underway.

The following sections are included:

• Little Green Men
• The Lighthouse Effect
• Magnetic Tops
• The Crab Nebula and the Crab Pulsar
• James Bond and the World’s Largest Pulsar Observatory
• New Discoveries — Gamma Radiation from the Magnetosphere
• Neutron Star Tango and the Theory of General Relativity
• X-Ray Pulsars
• Cosmic Starquakes

On 27 December 2004, several research satellites launched by NASA and other space agencies detected a huge burst of gamma radiation. The event, which lasted less than one second, was the result of a sudden magnetic field reconfiguration in a highly magnetized neutron star located in a distant region of our galaxy, about 50,000 light years from Earth. It was the strongest explosion documented in the Milky Way galaxy in the past 1000 years (much stronger explosions occur throughout the entire universe, as will be explained later on). Since most of the explosion energy was emitted in the form of gamma rays, it was not visible to the naked eye, but the luminosity released during the explosion was so great that it “blinded” the gamma ray detectors installed on the various satellites and caused deviant oscillations in the Earth’s atmosphere. Had this explosion taken place closer to Earth, its effect would have been similar to that of a large nuclear bomb, and it might have possibly annihilated all life on Earth…

Since the possible existence of black holes was predicted in the early 20^th century, black holes have captured human imagination. This mysterious object has been the inspiration for dozens of science fiction books and movies and for countless television programs. Hyperspace leaps used by protagonists in Star Wars and in other science fiction series, time travel, and even just an endless, unrestrained fall into a galactic hole — all of these phenomena are based on the physics of black holes. Black holes are also associated with information theory and with the possibility that the universe in which we live is a kind of holographic image. So what then is a black hole?

The rapid development of radio astronomy following World War II led to the discovery of many radio sources in the universe. The Third Cambridge Catalogue of Radio Sources, published in 1959, contained several hundreds of objects. The nature of these objects was unclear, and efforts were made to identify them with known sources of visible light discovered by ordinary telescopes; such efforts did not, however, meet with much success. The first identification took place in 1960, when a star-like object was discovered in the direction of one of the radio sources designated in the Cambridge catalog as 3C 48. To the researchers’ surprise, however, the spectrum measured did not match that of the ordinary stars that were known then, and the nature of the source remained a mystery. In 1963, a breakthrough was made when an American astronomer of Dutch origin, Martin Schmidt, succeeded in identifying another radio source, 3C 273, in visible light, and measured its spectrum accurately using a telescope positioned on Palomar Mountain, California. Like in the first case, here too it turned out that the spectrum had different characteristics than those familiar theretofore. Schmidt did not give up, and after a systematic analysis of the spectral components, he reached the conclusion that it was not a new kind of matter, but rather a radiation source that was moving away from us at a stupendous velocity of 47,000 km/sec, about 15% of the speed of light. The irregular properties of the spectrum could be explained by the Doppler effect, which is caused by the motion of the source, and after taking this effect into consideration, Schmidt discovered that the light was being emitted from ordinary matter that primarily contained hydrogen. It was then suggested that the source was a cosmological source located very far from our Milky Way galaxy, and that its high speed actually stems from the expansion of the universe. Hubble’s law led to the conclusion that the source was located about two billion light years away from Earth. Schmidt’s discovery led to a reexamination of the previously discovered 3C 48 source, and indeed it turned out to be a manifestation of the same phenomenon…

The following sections are included:

• Was a Star Born?
• Cosmic Explosions
• A Neutrino Signal from a Collapsing Core
• Shock Breakout and Light Emission
• The Luminescent Demise of White Dwarfs as Cosmic Probes
• The Remains of the Explosion

The following sections are included:

• The Cold War and One Accidental Discovery
• Gamma Ray Bursts
• Long Bursts and Short Bursts, Two Distinct Populations
• Long Bursts from Massive Stars
• Short Bursts: Fatal Attraction of Dense Stars

The following sections are included:

• Epilogue
  • Crossroads
• Bibliography