Advances in Science And Technology
In this chapter I will go over a number of scientific advances, in their chronological order when possible, that when taken together are responsible for shaping our modern view of the universe. They are presented first, as background information, without any attempt at systematization or cohesion. I hope this information will aid in your understanding of the theories to be introduced in the following chapter.
Energy in space manifests itself in the form of electromagnetic radiations or waves, and can be detected by various means. For example, visible light is emitted by a light source such as the incandescent filament of an electric lamp or a star, and can be detected by our unaided eyes or a telescope, and recorded on photographic film. Gamma rays, released by the radioactive decay of uranium, are detected a by a Geiger counter. X-rays, emitted when high-speed electrons are shot against metals, can be detected by special photographic plates, as are infrared and ultraviolet rays. Night-vision scopes and some modern video cameras and camcorders can also detect infrared rays, which are emitted by a source of heat, such as a human body or a star. And radio waves are received by radio receivers and transformed into sound, or as spectra that the computer can digitize for analysis.
Astrophysics made its humble debut in Munich in 1814, when the German physicist and lens-maker Joseph von Fraunhofer (1787-1826) passed the light from the Sun and stars through a triangular glass prism, which breaks the white light down to a spectrum of colors arrayed according to their wavelengths, with red having the longest wavelength and violet the shortest, and orange, yellow, green, blue, indigo in between. The glass prism, mounted between a collimator that produces parallel rays of light, and a small view telescope to view the rays dispersed or refracted by the prism, becomes the spectroscope. If a photographic plate replaces the eyepiece, the spectroscope becomes the spectrograph. This instrument plays a vital role in astrophysics. Although visible light is only a small part of the entire spectrum of electromagnetic radiation in the universe, ranging in descending order of wavelengths from radio waves (hundreds of feet in length), heat or infra-red radiation through visible light, ultra-violet radiation, x-rays, to gamma rays (less than one hundred-millionth of an inch) and cosmic rays, a study of this tiny visible window along with observations of the other segments of the electromagnetic spectrum reveals a great deal about the chemical composition, distance, speed, energetic radiation, magnitude, and movement of celestial bodies, all being essential in the construction of a theory of the universe's origin.
Fraunhofer recorded the spectrum of various elements, and found that each of the bands had hundreds of distinctive dark lines across its width. The spectroscope gives three spectra. The continuous spectrum is an unbroken band of colors coming from a source of light that emits all visible wavelengths. The bright-line spectrum, which consists of an unevenly spaced array of lines of different colors and brightness, shows that its source is sending out rays of specific wavelengths. The dark-line spectrum is a continuous spectrum with dark lines in exactly the same positions as in the bright-line spectrum for the same element. No one understood these lines until in 1859 the German physicist Gustav Kirchhoff (1824-1887) showed that each chemical element produced a distinct pattern of lines in the spectrum of the Sun. He correctly deduced that the dark lines are caused by the absorption of light by the same element, as is the case when the Sun's rays pass through its cooler outer gases and Earth's atmosphere. We now know that the bright lines are emitted by a gaseous element such as neon or sodium, and the corresponding dark lines are absorbed by the same element when its light passes through cooler gas.
The thousands of dark lines of the sun's spectrum tell us what elements it contains. By matching the dark lines of the sun's spectrum with the white lines of the bright-line comparison spectrum of say, glowing iron vapor, we know that the sun has iron vapor. Other elements can similarly be identified.
Spectroscopy is the analysis of light spectrum to determine the object's chemical composition, temperature, pressure, movement and velocity. By 1863 spectroscopy had allowed the English astronomer William Huggins to compile a catalog of stellar spectral lines. He also found that the Great Nebula in Orion was gaseous and contained hydrogen, which is found in the sun and in the spectrum of a nova. This is the first evidence that all stars contain hydrogen.
A more important application of spectroscopy is in determining whether an object is moving away from or toward Earth. In 1868 Huggins found that the dark lines of the star Sirius had shifted toward the red end of the spectrum; it had undergone a decrease in frequency, i.e., a lengthening of wavelength. In 1843 the Austrian physicist Christian Doppler (1803-1853) observed that when a sound (such as a train siren) moved toward an observer it increased in pitch, and trailed off as it moved away from her. This is known as the Doppler effect. The French physicist Hippolyte Louis Fizeau (1819-1896) independently discovered a similar change in frequency for light phenomena in 1848, and effected the first direct measurement of the speed of light in 1849. The shifting of spectral lines toward the red end, known as redshift, that Huggins found meant by the Doppler effect that Sirius was receding from Earth along the line of sight. Conversely a shift toward the violet end (a blueshift) means motion toward Earth along the line of sight. If the star moves at right angles to the line of sight no shifting of spectral lines occurs.
Studies begun in 1912 showed that most spiral nebulae were moving away from Earth, i.e., they were redshifted. In 1929 the American astrophysicist Edwin Hubble (1889-1953), using spectroscopy and the Doppler effect, determined that the farther away a galaxy is, the faster its velocity of recession. He thus formulated the empirical law, known as Hubble's Law, that galaxies move away from each other at the speed proportional to their distance, thereby giving empirical evidence that the universe was expanding, and laying the foundation for the Big Bang theory of the origin of the universe. The current expansion rate of the universe (ratio of velocity to distance) is referred to as the Hubble Constant.
2.0 The Telescope
Advances in the design and construction of the telescope, from the optical reflecting model to the refracting lenses, the catadioptric telescope, and the radio telescope, and the invention of photography and motion pictures, digital imaging, the computer, and satellites extend our capabilities to see farther and experiment more with our observations of the cosmos.
Refracting telescopes use a precisely ground lens at the top of the tube (the objective) to gather the star's light rays which are then bent (refracted) to a focus, and sent on to be seen through an eyepiece at the opposite end. Because of this construction refracting telescopes necessitate long tubes. The largest of refractors is the 40-inch telescope at the Yerkes Observatory at Williams Bay, Wisconsin. Reflecting telescopes reflect light collected by an objective mirror situated at the bottom of the tube, and send the light rays to an angled mirror farther up, where they are reflected to the eyepiece. In 1917 the 100-inch Hooker telescope was built on Mount Wilson, California. Other important reflectors are the 84-inch telescope of Kitt Peak Observatory, Arizona, and the 82-inch of the McDonald Observatory on Mount Locke, Texas. The 200-inch Hale Telescope on Mount Palomar, California, completed in 1948, was the world's largest until the Russians finished construction of their 240-inch Bolshoi Telescope in the Caucasus Mountains in 1977. But the current world's biggest are the Keck I Telescope (completed in 1992) and the Keck II Telescope (completed in 1995), both 400 inches in diameter, on Mauna Kea, Hawaii. The catadioptric telescope is a hybrid between the refractor and the reflector, in which light is reflected to a corrector lens near the top by a mirror mounted at the bottom of the tube, and transmitted to the eyepiece at the bottom. With proper accessories the hybrid Schmidt-Cassegrain is suitable for astrophotography, and the Maksutov is a perfect instrument for planetary observation.
All optical telescopes may be equipped with photographic equipment using fast, high-resolution film. In the 1970's light-sensitive electronic imaging cameras called charge-coupled devices (CCD's), which record starlight in pixels with a chip similar to the ones used in home camcorders. CCD cameras are available for the amateur astronomer. The lowest resolution of an amateur CCD camera is 196 x 165 pixels with the top-end camera reaching 4,000 x 4,000 pixels. And chips are getting larger, just like the microprocessor in your personal computer is getting faster and more powerful. With a top-end CCD camera, coupled to a telescope with a focal length of a least 1,000 mm, and image-processing software, today's amateurs can produce pictures of stars that rival the best ones achieved by the 200-inch Hale Telescope.
To minimize the effects of atmospheric conditions on the resolution and sharpness of ground-based telescopes, some telescopes are put into Earth's orbit. The most famous is the 95-inch Hubble Space Telescope launched in April 1990, which is controlled by a team of 400 astronomers and computer scientists at the Space Telescope Science Institute (STScI). It has provided spectacular photographs in visual, near-infrared, and ultraviolet wavelengths, and spectroscopic observations of nebulae, galaxies, and interstellar gas with the sharpness and resolution never before achieved. A new-generation space telescope is being readied for operation this year. Progress in computer-controlled adaptive optics system can now minimize the effects of turbulent atmospheric conditions on ground-based telescopes, and the technology is extended to all newer large telescopes.
While optical telescopes with the aid of spectroscopy focus on light radiation, radio telescopes peer deeper into space to uncover objects that optical telescopes cannot see, such as very distant galaxies, black holes, and cosmic radiation. Radio telescopes have huge saucer-shaped dishes as antennas to collect radio waves and feed them to special receivers, which record the strength and wavelengths of the signals and the direction from which they came. Among the largest radio telescopes are the 250-feet dish antenna of the University of Manchester steerable radio telescope at Jodrell Bank, England, and the world's largest antenna, 1,000-feet in diameter, of the non-steerable radio telescope of Cornell University located at Arecibo, Puerto Rico. The world's largest array of radio telescopes is the Very Large Array (VLA) near Socorro, New Mexico, consisting of 27 dish antennas, each 80-feet in diameter, arranged in a giant Y-shape, and connected together using a technique known as interferometry. This ingenious technique enhances resolution so much it now applies to some giant newer telescopes. Among the new interferometers are the twin 336-inch mirrors of the Large Binocular Telescope (LBT) in Arizona, the Keck telescopes in Hawaii, and the four 328-inch telescopes of the European Southern Observatory's Very Large Telescope (VLT) in Chile.
3.0 The Periodic Table of The Elements
In the late 1860's the Russian chemist Dmitri Mendeleev (1834-1907) arranged the 63 known elements (up from the Greeks' four elements of earth, water, fire, and air) into the first periodic table of the elements based on their atomic weights, and sorted them into groups with similar chemical properties. Thus sodium and potassium share similar properties and belong to one family whereas carbon, silicon, and titanium, members of another family, share a different set of properties Where gaps existed in the table, he was able to deduce their properties, and thus predicted their discoveries. The periodic table became essential in our understanding of the elements that make up the universe. But Mendeleev had no idea that the atom's weights and properties were indicative of its internal structure. Since Mendeleev's time, more elements have been discovered or created artificially, so that the number of known elements now stands at 115.
4.0 The Internal Structure of the Atom
In the Greek antiquity days, Democritus (c. 460-370 B.C.) had advanced the notion of the atom as the smallest indivisible element of matter. When scientists investigated the atom in the 19th century, they found evidence that the atom had structure, i.e., they are not fundamental. In 1895 the discovery of X-rays by the German physicist Wilhelm Roentgen (1845-1923) was followed by the French physicist Henri Becquerel's (1852-1908) discovery of radioactivity, the process by which one element is transformed into another. By this time scientists had found that not all elements are stable. The nuclei of heavy elements such as thorium, radium, uranium, and plutonium are radioactive, i.e., they decay into nuclei of lighter elements, releasing energy in the process. The Polish scientist Marie Curie née Sklodowska (1867-1934) and her French husband Pierre Curie (1859-1906) devoted their lives to studying radioactivity, and discovered two new elements polonium and radium. In 1897 the British physicist Joseph John Thomson (1856-1940) discovered the electron, the negatively charged particle with a mass of only 1/1837 that of hydrogen, which is the lightest element. Taken together, these discoveries and observations show that elements had smaller constituents.
The New-Zealander Ernest Rutherford (1871-1937), while studying radioactivity, showed that radioactive decay took three forms. He called the first form of radiation alpha particle, which turns out to be the nucleus of a helium atom, consisting of two protons and two neutrons, and is thus positively charged. The second type of rays was the energetic beta particle consisting of one electron or its antiparticle positron. The third are the gamma rays, which are the highest energy of light, the neutral photon. The amount of energy emitted by radioactive decay is calculated by Einstein's equation E = mc², i.e., the energy generated is the difference in mass between the original and final nuclei multiplied by the square of the speed of light. Note that this equation implies first that mass can be converted into energy and vice versa, and second that a small amount of mass can generate a tremendous amount of energy given that the conversion factor, the speed of light squared, is an enormous quantity. This is the secret of stars, whose centers are superhot furnaces where four lighter atoms of hydrogen combine into one heavier helium atom with a smaller mass than the total masses of the hydrogen atoms. The lost mass is then converted to the prodigious energy that makes the stars shine. The Sun, for example, has such an enormous mass that its 15-million degrees Celsius core converts 4 million tons of matter into energy every second for the past 4.5 billion years, with enough remaining mass for a further 5 billion years.
Most important was Rutherford discovery of the atom's structure. In 1909 by shooting alpha particles at an extra thin zinc sulfide coated gold foil several hundred atoms thick placed in front of a curved screen, he noticed that while most of the particles went through the foil and struck the screen with flashes of light, some occasionally bounced back, some even hit the part of the screen that was in front of the foil. He hypothesized then that atoms are mostly made up of empty space, with positive nuclei repelling the positive alpha particles that hit them.
Quantum mechanics tells us that atoms are made up of subatomic particles: electrons (negatively charged), and nuclei consisting of protons (positively charged) and neutrons (no electrical charge). Further, each particle of matter has its antiparticle counterpart with the opposite electrical charge. Thus the electron has the positron as its antiparticle, the proton has its antiproton (with a negative charge), and the neutron has its antineutron (neutral). In the 1960's the American physicist Murray Gel-Mann showed that protons and neutrons have internal structures, and are made up of smaller particles that he dubbed quarks. Physicists now know there are six quarks, 6 leptons, and force carrier particles. Quarks come in six types or flavors: up, down, strange, charmed, top and bottom. Quarks exist only in combinations called hadrons, which fall into two classes: baryons, consisting of three quarks, and mesons, composed of a quark and an antiquark. A neutron has two down quarks and one up quark (udd) whereas the proton consists of two ups and one down (uud). They are therefore baryons. Other combinations are possible although unstable, and quickly decay to protons or neutrons. The other matter particle is the lepton. Of the six leptons, three have electrical charge, and three do not. The best known lepton is the negatively charged electron. The other two charged leptons are the muon and the tau both having a lot more mass than the electron. The remaining three leptons are the hypothesized neutrinos, which have no electrical charge, very little mass, and are hard to find.
Finally, to account for the interactions among matter particles, there are the force carrier particles. The photon is the electromagnetic force carrier particle. Photons have no mass, travel at the speed of light, and carry different energies for the entire electromagnetic spectrum from gamma rays to radio waves. It is the electromagnetic force that keeps atomic particles from flying away from one another, and thus allows atoms to bond and form molecules that make life possible. Think about it. The structure of the world exists (and that includes you and me) because protons and electrons have opposite electrical charges. That still does not explain what keeps the particles inside the nucleus together. Gravity is too weak to do the job. Physicists found that protons in the nucleus are bound by the strong carrier particle gluon, which "glues" quarks tightly together to form hadrons. The residual strong interaction between the quarks in one proton and those in another proton is strong enough to overcome the repulsive electromagnetic force. Another observation to account for is the fact that all the stable matter seems to be composed of the two least massive quarks (up quark and down quark), the least massive lepton (the electron), and the neutrinos. Unstable matter is heavier, more massive, and tends to decay spontaneously to less massive matter. Decay occurs when the electroweak force causes massive quarks and leptons to become lighter, less massive quarks and leptons, with the lost mass converted to energy. When a quark or lepton decays due to weak interaction, it is replaced by two or more quarks or leptons with a different flavor. The weak carrier particles are the electrically charged W+, W- particles, and the neutral Z particle. Electroweak interactions and electromagnetism have been combined into a unified electroweak theory, which so correctly predicted weak neutral currents and observed properties of the W and Z bosons, that it has been widely accepted as the Standard Model. Bosons are particles in the same quantum state that can exist together in the same place at the same time. They do not follow the Pauli Exclusion Principle. Particles that obey this principle, i.e., that cannot coexist in the same place at the same time, are called fermions.
Now we know that matter is composed of quarks and leptons, they are the ultimate building blocks of matter, and the early universe must have been a dense cosmic soup of quarks and antiquarks.
Predicted in 1928 by the British physicist Paul Dirac (1902-1984), one of the pioneers in quantum mechanics, antiparticles were discovered in 1932. When particles and antiparticles collide, they cause devastating results, annihilating each other, releasing enormous amounts of electromagnetic radiation, and creating neutral photons that our telescopes can see. Matter and antimatter exist, according to the laws of particle physics, in equal amounts in the universe. Yet observational evidence shows that at least in our galaxy and well beyond, there is more matter than antimatter. It is this bias toward matter, which still remains a mystery, that explains the existence of matter in the universe and eventually ourselves.
Before leaving this background section, let me mention one more name, Albert Einstein (1879-1955). Born in Ulm, Germany, Einstein went to Munich then studied mathematics and physics in Zurich, Switzerland, for a teaching career. Unable to find a teaching position, he worked as technical assistant at the Patent Office in Berne. In 1905 he published three significant papers. The first mathematically described random motions of tiny particles. The second described the photoelectric effect of light impact on certain metals resulting in electron emission. This work won him the Nobel Prize in physics in 1921. And the third paper was on special relativity in which he described the physics of motion at constant speeds, and discovered that mass is a form of energy as expressed by the equation E = mc². In 1916 he published his general theory of relativity, which lays the foundation for much of our understanding of the structure of the universe. It describes from planets orbiting the Sun to light bending as it passes massive objects, and predicts that the universe is expanding.
The origin of the universe has always been a vexing question for theoretical astrophysicists. Painstakingly and over three thousand years we as a species have pieced together one of the most fascinating stories ever told, the story of the cosmos, and of ourselves.
Within the last one hundred years, our understanding of the universe has exploded, and although a great deal remains to be learned, we now have a clearer picture of its origin and the tools to refine our theories.
We have gone from a belief in a flat earth to a that in a round earth, from placing Earth at the center of the universe to replacing it with the Sun, and finally to see our Milky Way galaxy as just another galaxy in billions of galaxies.
Most astronomers in the early twentieth century believed the universe never had a beginning, and would extend indefinitely into the future. This belief was intuitive and philosophically satisfying, and had a number of adherents through the 1950's and 1960's during a time when the Big Bang theory had already been gathering an increasing number of converts.
Keep in mind that theories in cosmology cannot be proved scientifically, but should be supported by, or be consistent with, observational evidence and known laws of physics, and have predictive power.
1.0 The Steady-State Theory
The Steady-State Theory was proposed in 1948 by the British astrophysicist Fred Hoyle, and the Austrian-born Thomas Bondi and Thomas Gold as an extension of the perfect cosmological principle, first introduced by the English astronomer Arthur Milne (1896-1950) in 1933. According to this view, the universe is homogeneous (the same in all places) and isotropic (the same in all directions). There is no beginning and no ending. Fred Hoyle (1915-2001) with his colleague William Fowler (1911-1995) showed that all elements from helium to heavy elements are made in the nuclear fusion taking place in the cores of stars. He also hypothesized that elements heavier than iron could form in the cataclysmic explosion of a supernova when the nuclear fuel in its core had been exhausted. Most scientists now accept this idea. However, he failed to show that enough helium was made in the stellar furnaces to account for large amounts of helium detected in the universe. Twenty-five percent of matter in the universe is made up of helium, and the steady-state theory cannot account for it.
Given that the universe was expanding (as shown convincingly by Edwin Hubble in 1929), and the critical density of the universe (density at which the expansion rate is just sufficient to prevent a recollapse) remains the same, supporters of the theory hypothesized that matter is being continuously created out of nothing.
In the early 1960's scientists discovered very distant sources of strong radio emission, called quasars, quasi-stellar radio sources. These sources were highly redshifted, and by Hubble's Law were found to be very distant, and moving away from Earth. The distant and ancient universe is therefore not the same as the younger, nearer one. The steady-state theory offers no explanation for this evolution.
The demise of the theory came when Arno Penzias (1933-) and Robert Wilson (1936-) of Bell Laboratories discovered cosmic background microwave radiation (CMB or CMBR) in the 1960's. With an antenna designed to track signals from the Echo satellite, they inadvertently stumbled on a radio hiss coming from all directions that they could not account for after considering all possible sources of error. This radio emission was identified, after consultation with Princeton physicist Robert Dicke, as the faint echo of the Big Bang glowing softly and fairly uniformly across the sky at about 3 degrees C above absolute zero (which is -273 degrees C), the lowest possible temperature, at which particles of matter that produce heat are at rest. This discovery earned Penzias and Wilson the Nobel Prize for physics in 1978.
The steady-state theorists could not explain this cosmic microwave background radiation. Consequently, the theory lost all credibility.
2.0 The Big Bang Theory
At this time there is nothing out there that has more currency about of the origin of the universe than the Big Bang Theory. To be more precise, the Big Bang Theory is not a theory of the origin of the universe, but of the aftermath of Big Bang. While it is not perfect, as will be discussed, it has no serious rivals.
It all began in 1915 when Albert Einstein introduced the general theory of relativity. In 1931 the Belgian cosmologist George Lemaitre (1894-1966) reasoned from the idea of an expanding universe, that if the process is reversed back in time, the pieces of the universe must have come closer together until they were crushed into a very small object which he termed "the primeval atom," a few dozen times the size of the Sun. This "primeval atom" would then explode and eject all constituent parts in all directions, which kept splitting until the atoms of the universe formed. This basic idea was later refined as the Big Bang hypothesis, which is far different from what Lemaitre envisaged.
Einstein's general relativity predicted an expanding universe, but the weight of tradition was so powerful that he had to introduce a fudge factor called the cosmological constant, a hypothetical repulsive force to balance the force of gravity, anti-gravity, as it was called, to bring the universe back to a stable condition, which he believed the universe ought to be in. It was not until 1929, when Edwin Hubble showed by their redshifts that all galaxies were moving away from each other at a speed proportional to their distances, that Einstein realized his error, and termed the cosmological constant "the biggest blunder of my life." He later removed the cosmological constant from his equations, but it still remains in use for various purposes.
Besides, the cosmological constant would result in a very unstable model. If the universe grew slightly, the vacuum energy density stays the same. Recall that space vacuum is filled with virtual particle-antiparticle pairs, which have density in their very brief life before their mutual annihilation. At the same time, the matter energy density decreases slightly, and the resulting net negative gravitational acceleration would make the universe grow even more. Conversely, if the universe shrank slightly, the net positive gravitational acceleration would make the universe shrink further.
In the early 1920's the Russian physicist Alexander Friedmann was the first to accept Einstein's general relativity, which called for a universe in motion. Friedmann envisioned two scenarios. (1) A Closed Universe, which at large enough scales, is homogeneous and isotropic (by the cosmological principle) and in motion (by using Einstein's equations). The universe begins with a Big Bang, and continues its expansion for billions of years. This is the stage we are in now. After long enough periods of time the gravitational pull of all matter will stop the expansion, and the universe will pull back upon itself, reversing the expansion. Eventually the universe collapses into a singularity, and the result is the Big Crunch. (2) The second scenario is the Open Universe, in which there is not enough matter to effect the collapse, and the universe expands forever, albeit at a slower rate of expansion as time goes by. Eventually, all the stars exhaust their fuel, and become cold and dark. Intermediate between these two possibilities lies the Flat Universe, which expands forever until the expansion rate approaches zero.
Whether the universe will expand forever or recollapse depends on the ratio of the density of the universe to its critical density. Recall that critical density is the density sufficient and necessary to keep the expanding universe from a recollapse into a Big Crunch. Current data suggests that the universe's density is less than or equal to the critical density, and hence, the universe will expand forever.
2.1 A Brief History of Big Bang
The Big Bang begins not as a black hole, but as a singularity. A black hole, too, is a singularity, but one extending through all time at a single point whereas the Big Bang singularity extends through all space from a single instant. Before the Big Bang, there was nothing. Recall that at a singularity all known laws of physics do not apply. A zero second, there is only a tiny point of zero volume with infinite density and temperature. Time and space come into existence. A region about 10E-33 cm across is homogeneous and isotropic. There is perfect symmetry. The temperature stands at 10E32 K. The quantum wavelength of the universe is greater than the universe itself. All four fundamental forces of nature, gravity, electromagnetism, the strong nuclear force, and the weak nuclear force are united into one.
At 10E-43; second, also called the Planck time, the symmetry of the Grand Unified Theory (GUT), the theory that describes the unification of the fundamental forces, is broken, and gravity separates from the other forces. The temperature drops to 10E27 K to 10E28 K at 10E-35 seconds after the Big Bang during the period called false vacuum. The inflation era begins at 10E-34 second, and ends at 10E-32 second. The universe expands rapidly, doubling its size every 10E-37 second until the false vacuum decays, and the cosmological principle exists temporarily. The large vacuum energy density that drives inflation, around 10E71 gm/cc., is converted into heat. At the end of inflation the expansion rate is extremely fast so that the apparent age of the universe is only 10E-35 seconds. This inflation, acting on the cosmological principle, explains the uniformity of temperature throughout the universe as we observe it today.
At 10E-32 second the electroweak era begins, and between 10E-32 second and 10E-11 second, the electroweak force breaks down to the weak nuclear force (responsible for radioactivity), and the electromagnetic force, which holds the subatomic particles together. The temperature is at 10E15 K, and the universe is 2 light-minutes across. We are entering the era of the quark soup.
The next period begins at 2x10E-7 second, at a temperature between 2x10E13 K and 1x10E13 K, with the universe about the size of the solar system. Tau and antitau annihilate, and the matter energy density is no longer sufficient to create quarks.
At 1x10E-6 second, the universe now at 1.4 light-days enters baryogenesis, or creation of excess baryons. The temperature is 12x10E12 K, the energy density is no longer sufficient to create protons, so annihilation of baryons takes place. Yet there is evidence of bias toward matter, 100,000,001 protons for every 100,000,000 antiprotons (and 100,000,000 photons). Annihilation wipes out all baryons and antibaryons leaving 1 baryon per hundred million. Small as it is, this bias toward matter proves to be crucial to the dominance of matter.
As the universe cools, at 7x10E-5 seconds and a temperature of 3x10E12 K, muons and antimuons annihilate.
The universe grows to 4 light-years, and cools to 10E10 K, one second after the Big Bang. The weak interaction freezes out with a proton/neutron ratio of about 6.
The universe continues to grow and cool until 100 seconds after the Big Bang, when the temperature drops to 1 billion degrees, 10E9 K. Electrons and positrons annihilate to create more photons while protons and neutrons combine to make deuterons. Nucleosynthesis (helium making) begins when almost all of the deuterons combine to make helium. The final result: the universe is 3/4 hydrogen, 1/4 helium by mass; with a deuteron/proton ratio of 30 parts per million. There are about 2 billion photons per proton or neutron.
Two hundred seconds after the Big Bang the universe grows to 55 light-years and cools to 8.4x10E8 K.
After one thousand seconds nucleosynthesis stops, no more helium is produced, and the universe is a cool 4x10E8 K.
One month after the Big Bang the universe expands faster than the conversion of radiation field to a blackbody spectrum, so the spectrum of the Cosmic Microwave Background (CMB) preserves information back to this time. A blackbody is an object with a constant temperature that absorbs all radiation that hits it
Matter density equals radiation density 3,000 years after the Big Bang. The temperature is 60,000 K. Dark matter, which does not emit, absorb, or propagate light, but has gravity, starts to collapse.
Three hundred thousand years after the Big Bang, protons and electrons combine to form neutral hydrogen, and the universe becomes transparent. The temperature is 3,000 K. Matter predominates. Ordinary matter can now fall into the dark matter clumps. The CMB travels freely from this time until now.
The first stars form 100-200 million years after the Big Bang. One billion years go by before the first galaxy form. Supernovae explode and spread carbon, nitrogen, oxygen, silicon, magnesium, iron, all the way up through uranium throughout the universe. Galaxies continue to form as many clumps of dark matter, as stars and gas merge together. Finally clusters of galaxies form. And by about two billion years most elements have formed.
The solar system and Sun formed 4.6 billion years ago. And life began to emerge on Earth about 8.5 billion years after the Big Bang.
We are now 12-15 Gyr (gigayear or billion years) after the Big Bang, and the temperature of the universe is uniformly cold at 2.726 K. Some accounts put our universe's age at 13.7 billion years.
2.2 Evidence for the Big Bang
Recall that Hubble's Law shows the universe is expanding. Furthermore, it is expanding at an accelerating pace. This evidence comes from observing the brightness of distant supernovae. The redshift tells us by what factor the supernova has receded since its explosion.
The existence of the blackbody CMB, cosmic microwave background, indicates that the universe has evolved from a dense, isothermal state.
The existence of quasars, which are very distant sources of radio emission, shows that the ancient universe is far away whereas the more recent universe is closer.
2.3 Questions and Elucidations on the Big Bang
Cosmology is a branch of physics that studies the origin, structure, and evolution of the universe. Since nothing exists outside the universe, the observer must also be inside the universe. Unlike in other sciences where the observer maintains her objectivity by staying outside looking in, the cosmologist is inside trying to describe the environment. It is just like she is in one tiny corner of a huge building trying to understand the structure of the entire building without leaving her corner. Anything she knows about the building comes from observations, experiments, and a set of rules (laws) that have proven to have predictive power. So it is with theories about the universe, including the Big Bang Theory.
It is assumed that there was nothing before the Big Bang. In other words, everything begins with the Big Bang. At time zero the Big Bang is a singularity. This means nothing can be defined, not even time since spacetime is singular. Although it is hard to conceive that an infinite universe expanding forever should have begun all by itself along with the spacetime curvature, there is evidence for the concept of self-organization of complex systems, the Darwinian theory of evolution being one. Recently the same concept has spread to various disciplines from physics to biology and economics. The theoretical physicist Lee Smolin at Penn State University advanced the idea that the most important principle of 20th century physics is that all observable properties of things are about relationships. The provocative conception is to extend the Darwinian idea that the structure of a system must be formed from within by natural processes of self-organization, including the properties of space and time. Self-organized systems turn out to be complex systems in terms of the interactions of the parts within the systems. Space and time too are defined in terms of relationships. Since everything that exists, exists inside the universe, there can be nothing outside the universe to create it.
At the time of the Big Bang the universe is a singularity, where no known laws of physics apply. Both quantum mechanics and the general relativity break down. That is a loose end that occupies physicists for the rest of the twentieth century until now. Physicists are hard at work trying to find a single and elegant set of laws that describe all the fundamental forces of nature. Theoretically, they run into a thorny problem, the mathematical incompatibility of quantum mechanics and the general theory of relativity. Quantum mechanics provides a superb description of the smallest structures such as electrons and quarks. And general relativity does equally well for the force of gravity that applies to large structures such as stars, galaxies, black holes, and ultimately, the universe. But in extreme circumstances which combine enormous large-scale structures (requiring general relativity) and tiny structures (requiring quantum mechanics), such as a spacetime singularity, in the center of a black hole, or the state of the universe just before the big bang, the incompatibility of quantum mechanics and general relativity yield nonsensical answers.
We obviously need a theory that combines the strengths of both. Einstein had spent the last 30 years of his life on that effort, unsuccessfully trying to develop a "theory of everything." At this time there are several candidates, all varieties of the string theory. The problem to be resolved lies in the bad behavior of equations when particles interact with each other across minute distances on the order of 10E-33 cm, called the Planck length. String theory is based on the premise that on extremely tiny scales particles do not behave as points but rather as closed loops of string with radii approximately of Planck length. This view provides a way to harmonize gravity and quantum mechanics into a unified theory. Unfortunately, the string hypothesis requires that the universe contain ten spatial dimensions. Instead of going too far into an esoteric hypothesis that keeps scientists up at night, suffice it to say that we are still nowhere near a solution. This leaves room for all of us with imagination to plunge in and have some fun probing, hypothesizing, and agonizing in the process. As an aside, modern particle accelerators, such as the 27-km circular one at the Centre Européen de Recherches Nucléaires (CERN) near Geneva, can probe particles down to distance scales around 10E-16 cm only.
If the universe is expanding, what is it expanding into? This is a misconception fostered by the balloon analogy used to explain the universe's expansion. Imagine the universe is like a balloon being blown. Then any two dots on the balloon surface will move away from each other as the balloon expands. Unfortunately this analogy is a 2-D spherical object (the balloon's surface) expanding into a 3-D space. Nothing that we can measure about the universe shows anything about the larger 3-D space. Everything we measure is within the universe, and the universe has no boundary, no edge, or center of expansion. Thus the universe is not expanding into anything. Keep in mind that what we can observe is only a tiny part of the universe.
Is there a possibility of a Big Crunch, where the whole universe throws itself into reverse expansion and implodes? That depends, as seen above, on the density of the universe and the critical density. If the universe's density is less than or equal to the critical density, there will not be a Big Crunch, and the universe will expand forever. However, some scientists speculate that a Big Crunch may not be the end. Perhaps another Big Bang will follow this Big Crunch, and thus bangs follow crunches which follow bangs in an endless cycle known as an oscillating universe. The only problem is no one has yet developed a theory to explain how this universe works.
The inflation era is introduced very early after the explosion to bolster the standard Big Bang theory, which sees the universe as flat, to explain why the temperature of the universe is the same everywhere, now standing at 2.726 K. While the universe is still very small it is a lot easier for it to go through phase transitions and reach uniform temperatures than when it is larger. Phase transitions are various states of matter as its temperature changes, as observed in water when it changes from solid to liquid, and gaseous states, and reaches thermal equilibrium. But the universe must be out of equilibrium from the very beginning in order for matter to form.
Inflation relies on the modern particle physics findings that extremely high temperatures lead to a form of matter called false vacuum, which causes gravity to be repulsive rather than attractive, and allows the expansion of the universe to accelerate exponentially, instead of decelerate, in a very short time. Where does matter come from? Recall that during baryogenesis, matter forms, 100,000,001 protons for every 100,000,000 antiprotons (and 100,000,000 photons). Annihilation wipes out all baryons and antibaryons leaving 1 baryon per hundred million. If the universe contains matter and antimatter in equal amounts, they annihilate one another completely, release energy, destroy all mass, and there is nothing left to make stars, you and me. In 1964, J. H. Christenson found evidence of this bias toward baryons during interactions among certain baryons in laboratory experiments. It is this fundamental matter-antimatter asymmetry (also known as charge parity violation) that insures dominance of matter in the universe.
We know from thermodynamics, the physics of hot systems, that matter "loses its memory" at high temperatures, meaning it does not remember its former state, and takes on the ambient temperature, eventually to end up at the same temperature as its environment. This is called thermal equilibrium. But in order for matter to pass from one state to another, it must be out of equilibrium. Fortunately, this is precisely what happens every day: when water vapor condenses into liquid, it passes from one phase to another, a phenomenon called phase transition. The early universe went through phase transitions as well. However, knowing that nature exhibits a bias toward matter is not the same as explaining the mechanism of this bias. We have yet to come up with a particle physics model to account for this mechanism.
Finally, does dark matter exist? What is made of? Nucleosynthesis suggests that dark matter is non-baryonic, is not ordinary matter, i.e., not proton, electron or neutron, and that the density of ordinary matter (made from atoms) is at most 10% of the critical density. Thus most of the universe does not emit, scatter, or absorb light, and is not even made out of atoms. This invisible matter manifests itself only by gravitational interaction.
One piece of evidence for dark matter is the velocities of galaxies and galaxy clusters. In one 1930's study made of the Coma cluster and the Virgo cluster revealed that their velocities were ten to a hundred times larger than expected. One explanation could be that some invisible matter exerts gravitational pull that accelerates the clusters' motion.
Another piece of evidence for dark matter relates to Kepler's laws of motion, that the speed of rotation of a galaxy is a function of its distance from its center and the galaxy's total mass within the orbit. By finding the rotation velocities along a galaxy, we can determine the mass of the galaxy inside that orbit. Along the edge of a galaxy the amount of light quickly starts falling off, and we would expect the rotation speeds to diminish. Instead, the rotation speeds remain unexpectedly high, which indicates that there is a great deal of invisible mass in the galaxy.
When the masses and luminosities of the stars near the Sun are added up, astronomers find that there are about 3 solar masses for every 1 solar luminosity. A comparison of the total mass of clusters of galaxies and the total luminosity of the clusters shows about 300 solar masses for every solar luminosity. Thus most of the mass in the universe is dark.
Vera Rubin (1928-) observed that the velocities of stars revolving around the galaxy's center remained fairly constant regardless of their distances from the center, unlike in the solar system, where planets closer to the Sun move faster than those farther out. This observation can be accounted for only if vast invisible halos exist around the galaxies and galaxy clusters to provide gravitational pull. Calculations estimate that dark matter adds up to ten times more mass than the visible stars, dust and gas.
The amount of matter in the universe is expressed in terms of a parameter called Omega. A closed universe, massive enough that it eventually collapses back onto itself, has Omega larger than 1; an open universe, one that expands forever, has Omega less than 1; and a flat universe, balanced between the two, has Omega of 1. The amount of visible matter in the universe is about Omega equals 0.05. If the total of all of the mass is Omega equals 1, then dark matter makes up 95% of the universe. Even at a more realistic Omega no larger than about 0.4, the amount of dark matter still makes up about 35% of the universe.
Candidates for dark matter include the ubiquitous neutrinos (hot dark matter), which are particles with little mass but exist in huge quantities, one billion for every proton or electron; cold dark matter WIMPs, or weakly interacting massive particles, heavy particles that only interact weakly with other matter. Predicted by theory, WIMPs have so far remained undiscovered. Another candidate goes by the name of MACHO (massive compact halo objects) that could exist in vast halos around galaxies. Brown dwarfs, ranging in size from a normal star to a planet, could be a type of MACHO. They form like stars but have so little mass that nuclear fusion reactions did not occur.
Given the huge mass of dark matter, the fate of the universe may well depend on it. More mass means the universe will eventually collapse, and less mass means the universe will expand forever.
The universe is a fascinating subject of inquiry. Within the twentieth century, we have gained an unprecedented understanding of matter, and how it forms and makes up everything in it. The elements on which life is based, carbon, oxygen, hydrogen, nitrogen, and more were all made from matter that began forming by baryogenesis at 1x10-6 second after the Big Bang. Since then the universe has expanded its reach, and keeps going.
Out of this evolution, life appeared 8.5 billions after the momentous Big Bang. Our species began its fantastic career on Earth only about 3 million years ago, a tiny fraction in the long timeline of the universe. Yet it is our species that has pushed its curiosity beyond daily concerns to ask inquisitive questions about how we came about, and where we go from here.
A great deal more remains to be learned, but just like the universe, our inquiry and knowledge keep expanding. It is with optimism, tempered by humility, that we face our future, in which the survival of our species may just depend on how much we understand the universe, and dare to venture forth into other worlds in our galaxy and beyond.
Burnham, Robert, Dyer, A., Garfinkle, R. A., George, M., Kanipe, J., Levy, D. H. 2002. Practical Skywatching. San Francisco: Fog City Press
Sagan, Carl. 1983. Cosmos. New York: Random House
Stephen Hawking's Universe
Hubble Space Telescope
Phase Transitions in Early Universe
Relativity, General and Special
Space Telescope Science Institute (for Hubble Space Telescope)
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