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Science has advanced to the point where we can infer something about the entire universe. This has been a great challenge considering how unimaginably vast the universe is. The countless stars you see in the darkest sky constitute merely 3000 neighbors out of about 300,000,000,000 stars in our galaxy, and as many as 100,000,000,000 galaxies exist in the universe. Humans have always wondered: Has the universe always existed like we see it now, or did it somehow start all of a sudden? In the beginning of this past century, we found out in amazement that the entire universe is expanding. This led physicists to deduce that the universe started out in the finite past with a minuscule size. Realizing that the universe had a beginning, and awed by its vastness and its creations, people have asked: How did the universe begin? After all, we are here to be amazed by it because the universe eventually created lives like us. Now, after decades of observing and thinking, we have come to answer confidently the question of the origin of our universe... with what is known as the "big bang".
The APM Galaxy Survey containing about 3 million galaxies, covering about 15% of the whole sky. Small bright spots are galaxy clusters; these group together to form superclusters. Superclusters in turn are arranged like filaments surrounding darker voids.
According to the big bang theory, the universe began by expanding from an infinitesimal volume with extremely high density and temperature. The universe was initially significantly smaller than even a pore on your skin. With the big bang, the fabric of space itself began expanding like the surface of an inflating balloon – matter simply rode along the stretching space like dust on the balloon's surface. The big bang is not like an explosion of matter in otherwise empty space; rather, space itself began with the big bang and carried matter with it as it expanded. Physicists think that even time began with the big bang. Today, just about every scientist believes in the big bang model. The evidence is overwhelming enough that in 1951, the Catholic Church officially pronounced the big bang model to be in accordance with the Bible.
Until the early 1900s, most people had assumed that the universe was fixed in size. New possibilities opened up in 1915, when Einstein formulated his famous general relativity theory that describes the nature of space, time, and gravity. This theory allows for expansion or contraction of the fabric of space. In 1917, astronomer Willem de Sitter applied this theory to the entire universe and boldly went on to show that the universe could be expanding. Aleksandr Friedmann, a mathematician, reached the same conclusion in a more general way in 1922, as did Georges Lemaître, a cosmologist and a Jesuit, in 1927. This step was revolutionary since the accepted view at the time was that the universe was static in size. Tracing back this expanding universe, Lemaître imagined all matter initially contained in a tiny universe and then exploding. These thoughts introduced amazing new possibilities for the universe, but were independent of observation at that time.
Three main observational results over the past century led astronomers to become certain that the universe began with the big bang. First, they found out that the universe is expanding—meaning that the separations between galaxies are becoming larger and larger. This led them to deduce that everything used to be extremely close together before some kind of explosion. Second, the big bang perfectly explains the abundance of helium and other nuclei like deuterium (an isotope of hydrogen) in the universe. A hot, dense, and expanding environment at the beginning could produce these nuclei in the abundance we observe today. Third, astronomers could actually observe the cosmic background radiation—the afterglow of the explosion—from every direction in the universe. This last evidence so conclusively confirmed the theory of the universe's beginning that Stephen Hawking said, "It is the discovery of the century, if not of all time."
Around the same time that people began to come up with the idea of an expanding universe, astronomer Vesto Slipher noticed that there are more galaxies going away from us than approaching us. Astronomers know that a galaxy is approaching or receding by looking at the spectrum of its light. If the spectrum is shifted toward shorter wavelength (blueshift), then the galaxy must be approaching, just like the sound of an approaching racing car has a higher pitch (shorter sound wavelength). If the spectrum is shifted toward longer wavelength (redshift), then the galaxy must be receding, just like the sound of a racing car that has passed us has a lower pitch (longer sound wavelength). The degree of the shift depends on the speed of approach or recession. So in other words, Slipher observed more galaxies whose spectrum was redshifted than those whose spectrum was blueshifted.
In 1929, Edwin Hubble discovered that farther galaxies are going away from us at higher speeds, proportional to their distance. In other words, the spectra of more distant galaxies had higher redshifts. From distant galaxies, light takes millions or even billions of years to reach us. This means we are seeing an image from millions or billions of years ago. In redshift, the spectrum is shifted from shorter wavelength to longer wavelength as the light travels from the galaxy to us. This increase in wavelength is due to expansion of the very fabric of space itself over the years that the light was traveling. If the wavelength had doubled, space must have expanded by a factor of two. Thus, Hubble's discovery was that this expansion factor was roughly proportional to the distance light traveled, or equivalently, to how far back in time you looked. This means that the universe was smaller and smaller earlier and earlier. The universe has been expanding.
Radial velocities, corrected for solar motion, of galaxies in a cluster are plotted against distances estimated from involved stars and mean luminosities.
Tracing back this expanding universe, we see that the separations between galaxies become smaller while the density becomes higher. This continues until all matter is compacted into a completely shrunk volume of the universe with an incredible density—the moment of the big bang. We can estimate how long ago this was by dividing the distance to a galaxy by its recessional velocity. This way we estimate how long ago the distance between that galaxy and ours was essentially zero. Calculation shows that the big bang occurred as long as 10-15 billion years ago, which is about three times the age of the Earth.
As a way of checking this age estimate, we can examine the oldest things we find in the universe to verify that they are 10-15 billion years old, but definitely not older. From radioactive dating of uranium isotopes, we know that the oldest isotopes were created (through nuclear reactions in supernovae) about 10 billion years ago. From our current model of star evolution, we know that the oldest stars in our Galaxy are about 12 billion years old. These ages are consistent with the age estimated from the observed expansion of the universe. This agreement suggests that the universe really began a finite time ago, providing an encouraging reason to believe in the big bang model of the universe.
The notion that the expanding universe was extremely hot in the beginning provides a reasonable explanation for why helium and deuterium seem to have existed even before star formation. Both these species are created by nuclear fusion. Fusion of a proton and a neutron produces deuterium (also known as heavy hydrogen), while fusion of two deuterium nuclei produces helium. These reactions can occur only at very high temperatures, such as in the interiors of stars. In 1946, George Gamow, once a student of Friedmann, suggested that nuclear fusion must have taken place when the universe was so hot in the beginning. This process, called the "big bang nucleosynthesis", would have created helium and deuterium (plus trace amounts of elements like lithium and beryllium) out of an initial sea of energetic protons and neutrons.
In the early 1960s, spectroscopic studies of local stars showed that the abundance of helium was about 20-30% by mass, the rest being mostly hydrogen. Stars and hydrogen bombs are the only things we know of that make helium in the present universe. They both combine hydrogen nuclei (protons) into helium nuclei through nuclear fusion, releasing great amounts of energy. Astronomers calculate that the night sky should be much brighter if all the helium we now observe had come from stars burning (or bombs exploding). Some, if not most, of the helium must have existed before star formation.
Based on theories of the big bang nucleosynthesis, physicists in the mid-1960s calculated that roughly 1/4 of mass was converted into helium in the beginning, while the rest remained as hydrogen. This would be consistent with the earlier measurements of 20-30% helium abundance if most of the observed helium were present, from the big bang, even before stars began producing more. In the early 1970s, spectroscopic studies in other galaxies have confirmed that the majority of the observed helium did exist before any star formation. The figure shows the helium abundance of many galaxies having various oxygen abundances. Oxygen abundance indicates in general how much nucleosynthesis has taken place in stars because stars produce "heavy" elements (like oxygen, nitrogen, carbon, and helium) from hydrogen through nuclear fusion. If all observed helium was created in stars like all oxygen was, we would expect to find no helium in galaxies that have no oxygen because these galaxies must have formed before any stars created heavy elements. Yet, as the plot shows, the abundance of helium decreases very little as the abundance of oxygen approaches zero. The galaxies must have formed with an initial composition of about 24% helium. This observational result supports the theory that there must have been a big bang in the beginning which converted about 1/4 of mass into helium through nucleosynthesis.
Measurements of the He abundance in other galaxies are plotted against the oxygen abundance, an indicator of stellar processing.
Observation of deuterium gives an additional support of the big bang nucleosynthesis. Deuterium, unlike helium, is not produced in stars at all. At temperatures above about one million degrees K, it dissociates into proton and neutron. Astronomers in the early 1970s realized that no known process in the present universe could have produced deuterium. This is because any deuterium created in stars will immediately dissociate or convert to helium due to the high temperatures in interiors of stars. However, in 1973, studies of absorption spectra of nearby stars showed that interstellar medium (material between stars) contains a trace of deuterium. Since stars could not have produced the deuterium, it must have been created either very early in the formation of the galaxy or even before. Despite the high temperature at the beginning, the big bang nucleosynthesis could create deuterium because the expansion of the universe lowered the density and temperature so quickly that there was hardly time for the deuterium to decay.
Thus, the abundance of helium and existence of deuterium provide strong evidence that the universe began with a hot, violent explosion consistent with the big bang model.
The most conclusive evidence for the big bang arises from the observation of the cosmic background radiation. In 1948, Gamow predicted that the radiation from the big bang nucleosynthesis must still be filling the universe. He calculated how hot the universe must have been to yield the observed abundance of helium, and estimated that the temperature of this radiation would have lowered to about 5 degrees above absolute zero (5K) in the present universe. Most theorists at that time, including him, thought that such radiation would be too weak to detect.
However in 1964, two radio astronomers Arno Penzias and Robert Wilson were struggling to get rid of a constant background "noise" from their radio antenna signals. Their efforts included catching pigeons that nested inside the horn-shaped antenna, and cleaning what they called "white dielectric material" produced by the pigeons on the antenna surface. After a year, they still could not remove the background noise. They learned that this constant signal was precisely uniform in every direction, whether they pointed the antenna toward the Sun or the Milky Way or the more empty parts of the sky. This meant that the signal was coming from far beyond our Galaxy; otherwise it would not be so uniform in all directions. The high degree of isotropy tells us that the signal originated very far away, or equivalently, very early in time. The source must have been enormously powerful for us to be detecting it. Physicists inferred that this must have been from the immense fireball of radiation at the big bang as Gamow predicted, but how could they be sure that this was really what Penzias and Wilson were detecting? After all, they were observing only one small part of the spectrum of radiation.
If the radiation really were from the big bang, it would have a type of spectrum called the "blackbody" spectrum. Radiation has blackbody spectrum if it is emitted by anything that absorbs and re-emits light freely but doesn't reflect it (hence blackbody). According to the big bang model, the universe at the beginning must have been crowded with particles and light, and must have been very hot. In that environment, the particles were constantly bumping into light, absorbing and re-emitting it. Light from such an environment would have a blackbody spectrum, and the spectrum's characteristic shape will be preserved while the light travels through the expanding space. In a blackbody spectrum, there is a contribution to the intensity at every wavelength. The amount of contribution varies continuously across the wavelengths in a characteristic way that depends only on the temperature of the emitting body. Therefore astronomers could verify that a spectrum is that of a blackbody by measuring the intensity of the radiation at different wavelengths.
During the 1970s, various groups observed the background radiation at various microwave and infrared wavelengths. They all confirmed that the background radiation has a blackbody spectrum with a characteristic temperature of about 3K. Relatively recently in 1991, a satellite observatory called COBE (Cosmic Background Explorer) made a precise measurement of the background radiation from Earth's orbit and produced an absolutely beautiful result. The plot shows thirty-four data points along with the best-fit blackbody spectrum. The data fit the blackbody spectrum so perfectly that the theoretical blackbody curve hides the error-bars of the data points. This is considered the finest fit between theoretical and observed results in the history of astronomy.
Cosmic microwave background spectrum, with its intensity plotted against frequency (in waves per centimeter). The solid curve shows the expected intensity from a 2.73K blackbody spectrum, as predicted by the hot big bang theory. The COBE data were taken at 34 positions equally spaced along this curve. The data match the theoretical curve so exactly that the width of the blackbody curve hides the error uncertainties of the data.
COBE measured the characteristic temperature of the background radiation to be 2.726+/-0.010K. This temperature is significantly lower than the original temperature of the radiation because the expanding universe stretched the wavelength of radiation by many factors, making it much less energetic. It took practically the entire age of the universe for the radiation to reach us. Astronomers now know that the expansion of the universe during this time stretched out the wavelength of the radiation by more than 1000 fold. The afterglow of the big bang comes from when the universe was only about 500,000 years old. This makes the cosmic background radiation the oldest thing we have ever observed. We are almost viewing the event of the big bang.
The 20th century saw a giant leap in how humans perceive the cosmos. No longer did people assume that the universe was static in size. By looking at how distant galaxies recede from us, we learned instead that the universe is expanding in volume. Tracing the expanding universe backward in time, we imagined a dense, hot beginning of our universe in a finite past. In the middle of the century, we found out that the nuclear reactions in this hot early universe accurately account for the previously mysterious abundance of helium and deuterium. Moreover, we detected a faint afterglow of the big bang that occurred billions of years ago. That the universe began with a big bang is essentially conclusive and may stand as the most profound discovery humans have ever made.
The big bang, however, is merely a global description of the origin of the universe. Today, particle physicists have consistent theories about the history of the universe down to only a trillionth of a second after its birth or even earlier. They can test their theories experimentally with particle accelerators that can simulate events involving enormous energies similar to the condition at the beginning. To learn more about how exactly the universe began, physicists must develop a theory that works at even earlier times after the big bang. Such theory must combine both the general relativity (because of the extreme gravitational field at the beginning) and quantum mechanics (because of the extreme compactness of the universe at the beginning). The goal of physics today is to develop this quantum theory of gravity so that we may one day understand what exactly happened around the moment of the big bang to get the universe started. |