Universe formation

Universe formation

We stand on a planet, in orbit around a star, speeding through space on the arm of a galaxy. Beyond our galaxy lie hundreds of billions of other galaxies. Where did all this “stuff” the matter of the Universe come from, and when did it first form? For most of human history, a scientific solution to these questions seemed intractable. But in the 1920s, unexpected observations about the nature of light from distant galaxies set astronomers on a path of discovery that ultimately led to a model of Universe formation known as the Big Bang theory. To explain these observations, we must first introduce an important phenomenon called the Doppler effect. We then show how this understanding leads to the recognition that the Universe is expanding, and finally, to the conclusion that this expansion began during the Big Bang, 13.7 billion years ago.

Waves and the Doppler Effect 

When a train whistle screams, the sound you hear moved through the air from the whistle to your ear in the form of sound waves. Waves are disturbances that transmit energy from one point to another in the form of periodic motions. As each sound wave passes, air alternately compresses, then expands. We refer to the distance between successive waves as the wavelength, and the number of waves that pass a point in a given time interval as the frequency. If the wavelength decreases, more waves pass a point in a given time interval, so the frequency increases. The pitch of a sound, meaning its note on the musical scale, depends on the frequency of the sound waves. 

Manifestations of the Doppler effect for sound and for light.
Imagine that you are standing on a station platform while a train moves toward you. The train whistle’s sound gets louder as the train approaches, but its pitch remains the same. The instant the train passes, the pitch abruptly changes it sounds like a lower note in the musical scale. Why? When the train moves toward you, the sound has a higher frequency (the waves are closer together so the  wavelength is smaller) because the sound source, the whistle, has moved slightly closer to you between the instant that it emits one wave and the instant that it emits the next  (figure above a, b). When the train moves away from you, the sound has a lower frequency (the waves are farther apart), because the whistle has moved slightly farther from you between the instant it emits one wave and the instant it emits the next. An Austrian physicist, C. J. Doppler (1803–1853), first interpreted this phenomenon, and thus the change in frequency that happens when a wave source moves is now known as the Doppler effect. 
Light energy also moves in the form of waves. We can represent light waves symbolically by a periodic succession of crests and troughs (figure above c). Visible light comes in many colours the colours of the rainbow. The colour you see depends on the frequency of the light waves, just as the pitch of a sound you hear depends on the frequency of sound waves. Red light has a longer wavelength (lower frequency) than does blue light. The Doppler effect also applies to light but can be noticed only if the light source moves very fast, at least a few percent of the speed of light. If a light source moves away from you, the light you see becomes redder, as the light shifts to longer wavelength or lower frequency. If the source moves toward you, the light you see becomes bluer, as the light shifts to higher frequency. We call these changes the red shift and the blue shift, respectively.

Does the Size of the Universe Change? 

In the 1920s, astronomers such as Edwin Hubble, after whom the Hubble Space Telescope was named, braved many a frosty night beneath the open dome of a mountaintop observatory in order to aim telescopes into deep space. These researchers were searching for distant galaxies. At first, they documented only the location and shape of newly discovered galaxies, but eventually they also began to study the wavelength of light produced by the distant galaxies. The results yielded a surprise that would forever change humanity’s perception of the Universe. To their amazement, the astronomers found that the light of distant galaxies display a red shift relative to the light of a nearby star (figure above d). 
Hubble pondered this mystery and, around 1929, attributed the red shift to the Doppler effect, and concluded that the distant galaxies must be moving away from Earth at an immense velocity. At the time, astronomers thought the Universe had a fixed size, so Hubble initially assumed that if some galaxies were moving away from Earth, others must be moving toward Earth. But this was not the case. On further examination, Hubble concluded that the light from all distant galaxies, regardless of their direction from Earth, exhibits a red shift. In other words, all distant galaxies are moving rapidly away from us. 

 The concept of the expanding Universe and the Big Bang.
How can all galaxies be moving away from us, regardless of which direction we look? Hubble puzzled over this question and finally recognized the solution: the whole Universe must be expanding! To picture the expanding Universe, imagine a ball of bread dough with raisins scattered throughout. As the dough bakes and expands into a loaf, each raisin moves away from its neighbours, in every direction;  figure above a. This idea came to be known as the expanding Universe theory.

The Big Bang 

Hubble’s ideas marked a revolution in cosmological thinking. Now we picture the Universe as an expanding bubble, in which galaxies race away from each other at incredible speeds. This image immediately triggers the key question of cosmology: did the expansion begin at some specific time in the past? If it did, then that instant would mark the physical beginning of the Universe. 
Most astronomers have concluded that expansion did indeed begin at a specific time, with a cataclysmic explosion called the Big Bang. According to the Big Bang theory, all matter and energy everything that now constitutes the  Universe was initially packed into an infinitesimally small point. The point “exploded” and the Universe began, according to current estimates, 13.7 (+-1%) billion years ago.
Of course, no one was present at the instant of the Big Bang, so no one actually saw it happen. But by combining clever calculations with careful observations, researchers have developed a consistent model of how the Universe evolved, beginning an instant after the explosion (figure above b). According to this model of the Big Bang, profound change happened at a fast and furious rate at the outset. During the first instant of existence, the Universe was so small, so dense, and so hot that it consisted entirely of energy atoms, or even the smallest subatomic particles that make up atoms, could not even exist. (See nature of matter for a review of atomic structure.) Within a few seconds, however, hydrogen atoms could begin to form. And by the time the Universe reached an age of 3 minutes, when its temperature had fallen below 1 billion degrees, and its diameter had grown to about 53 million km (35 million miles), hydrogen atoms could fuse together to form helium atoms. Formation of new nuclei in the first few minutes of time is called Big Bang nucleosynthesis because it happened before any stars existed. This process could produce only light atoms, meaning ones containing a small number of protons (an atomic number less than 5), and it happened very rapidly. In fact, virtually all of the new atomic nuclei that would form by Big Bang n ucleosynthesis existed by the end of the first 5 minutes. 
Eventually, the Universe became cool enough for chemical bonds to bind atoms of certain elements together in molecules. Most notably, two hydrogen atoms could join to form molecules of H2. As the Universe continued to expand and cool further, atoms and molecules slowed down and accumulated into patchy clouds called nebulae. The earliest nebulae of the Universe consisted almost entirely of hydrogen (74%, by volume) and helium (24%) gas.

The Nature of Matter

The nature of atoms and nuclear reactions.
What does matter consist of? A Greek philosopher named Democritus (ca. 460–370 B.C.E.) argued that if you kept dividing matter into progressively smaller pieces, you would eventually end up with nothing; but since it’s not possible to make something out of nothing, there must be a smallest piece of matter that can’t be subdivided further. He proposed the name “atom” for these smallest pieces, based on the Greek word atomos, which means indivisible. Our modern understanding of matter developed in the 17th century, when chemists recognized that certain substances (such as hydrogen and oxygen) cannot break down into other substances, whereas others (such as water and salt) can break down. The former came to be known as elements, and the latter came to be known as compounds. John Dalton (1766–1844) adopted the word atom for the smallest piece of an element that has the property of the element; the smallest piece of a compound that has the properties of the compound is a molecule. Separate atoms are held together to form molecules by chemical bonds, which we discuss more fully later in the book. As an example, chemical bonds hold two hydrogen atoms to form an H2 molecule. Chemists in the 17th and 18th centuries identified 92 naturally occurring elements on Earth; modern physicists have created more than a dozen new ones. Each element has a name and a symbol (e.g., N = nitrogen; H = hydrogen; Fe = iron; Ag = silver). Atoms are so small that over five trillion (5,000,000,000,000) can fit on the head of a pin. Nevertheless, in 1910, Ernest Rutherford, a British physicist, proved that, contrary to the view of Democritus, atoms actually can be divided into smaller pieces. Most of the mass in an atom clusters in a dense ball, called the nucleus, at the atom’s centre. The nucleus contains two types of subatomic particles: neutrons, which have a neutral electrical charge, and protons, which have a positive charge. A cloud of electrons surrounds the nucleus (figure above a); an electron has a negative charge and contains only 1/1,836 as much mass as a proton. (“Charge,” simplistically, refers to the way in which a particle responds to a magnet or an electric current.) Roughly speaking, the diameter of an electron cloud is 10,000 times greater than that of the nucleus, yet the cloud contains only 0.05% of an atom’s mass thus, atoms are mostly empty space! We distinguish atoms of different elements from one another by their atomic number, the number of protons in their nucleus. Smaller atoms have smaller atomic numbers, and larger ones have larger atomic numbers. The lightest atom, hydrogen, has an atomic number of 1, and the heaviest naturally occurring atom, uranium, has an atomic number of 92. Except for hydrogen nuclei, all nuclei also contain neutrons. In smaller atoms, the number of neutrons roughly equals the number of protons, but in larger atoms the number of neutrons exceeds the number of protons. The atomic mass of an atom is roughly the sum of the number of neutrons and the number of  protons. For example, an oxygen nucleus contains 8 protons and 8 neutrons, and thus has an atomic mass of 16. In 1869, a Russian chemist named Dmitri Mendelév (1834–1907) recognized that groups of elements share similar characteristics, and he organized the elements into a chart that we now call the periodic table of the elements. With modern understanding of the periodic table, it became clear that the ordering of the elements reflects their atomic number and the stream of the electron cloud. Nuclear bonds serve as the “glue” that holds together subatomic particles in a nucleus. Atoms can change only during nuclear reactions, when nuclear bonds break or form. Physicists recognize several types of nuclear reactions. For example, during “radioactive decay” reactions, a nucleus either emits a subatomic particle or undergoes fission. As a result of fission, a large nucleus breaks apart to form two smaller atoms (figure above b). Radioactive decay transforms an atom of one element into an atom of another and produces energy. For example, fission reactions provide the energy of atomic bombs and nuclear power plants. Atoms that spontaneously undergo the process are known as radioactive elements. During fusion, smaller atoms collide and stick together to form a larger atom. For example, successive fusion reactions produce a helium atom out of four hydrogen atoms. Fusion reactions power the Sun and occur during the explosion of a hydrogen bomb (figure above c).

Birth of the First Stars 

When the Universe reached its 200 millionth birthday, it contained immense, slowly swirling, dark nebulae separated by vast voids of empty space. The Universe could not remain this way forever, though, because of the invisible but persistent pull of gravity. Eventually, gravity began to remold the Universe pervasively and permanently. 
All matter exerts gravitational pull a type of force on its surroundings, and as Isaac Newton first pointed out, the amount of pull depends on the amount of mass; the larger the mass, the greater its pull. Somewhere in the young Universe, the gravitational pull of an initially more massive region of a nebula began to suck in surrounding gas and, in a grand example of the rich getting richer, grew in mass and, therefore, density. As this denser region attracted progressively more gas, the gas compacted into a smaller region, and the initial swirling movement of gas transformed into a rotation around an axis. As gas continued to move inward, cramming into a progressively smaller volume, the rotation rate became faster and faster. (A similar phenomenon happens when a spinning ice skater pulls her arms inward.) Because of its increased rotation, the nebula evolved into a disk shape (see Geology at a Glance, pp. 22–23). As more and more matter rained down onto the disk, it continued to grow, until eventually, gravity collapsed the inner portion of the disk into a dense ball. As the gas squeezed into a smaller and smaller space, its temperature increased dramatically. Eventually, the central ball of the disk became hot enough to glow, and at this point it became a protostar. The remaining mass of the disk, as we will see, eventually clumped into smaller spheres, the planets. 
A protostar continues to grow, by pulling in successively more mass, until its core becomes extremely dense and its temperature reaches about 10 million degrees. Under such conditions, hydrogen nuclei slam together so forcefully that they join or “fuse,” in a series of steps, to form helium nuclei (see the nature of matter).  Such fusion reactions produce huge amounts of energy, and the mass becomes a fearsome furnace. When the first nuclear fusion reactions began in the first protostar, the body “ignited” and the first true star formed. When this happened, perhaps 800 million years after the Big Bang, the first starlight pierced the newborn Universe. This process would soon happen again and again, and many first-generation stars came into existence. 
First-generation stars tended to be very massive, perhaps 100 times the mass of the Sun. Astronomers have shown that the larger the star, the hotter it burns and the faster it runs out of fuel and dies. A huge star may survive only a few million years to a few tens of millions of years before it becomes a supernova, a giant explosion that blasts much of the star’s matter back into space. Thus, not long after the first generation of stars formed, the Universe began to be peppered with the first generation of supernovas.
Credit: Stephen Marshak (Essentials of Geology)
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