- What is the Big Bang?
- The Puzzling Problem of Galaxies
- Birth of the Earth
Suggested Reading: Vindication of the Big Bang: Breakthroughs and Barriers, by Barry Parker, Plenum Press, 1993; The Secret Melody, by Trinh Xuan Thuan, Oxford University Press, 1995
Suggested Viewing: Vol. 1 ("The Third Planet"), "The Miracle Planet", PBS.
We are the first generation of human beings to glimpse the full sweep of cosmic history. The portals of our Universe's early origins have been opened by a vast new arsenal of sophisticated telescopes and intergalactic probes, revealing an incredible wealth of detail on the structure and movement of the cosmic world. In this lecture, we will examine this new evidence and see how it fits into current scientific thinking about the origins of the Universe and our planet. By gaining an understanding of the cosmic forces that govern the Universe, we can better appreciate the place our own planet occupies in the cosmic web.
What is the Big Bang?
As we look up at the sky at night, we often wonder, where did it all come from? What's really out there, beyond our vision, beyond the reach of our telescopes? How did it all get here? As children (or grandchildren) of the space age, we have witnessed a view of our planet and stars never before imagined by any humans before us. Certainly, many historic scholars and artists gave it a good shot, but the pictures and information that we are now receiving reveal a Universe that continues to amaze and confound us.
Not much more than 300 years ago, many people still thought the sun revolved around the Earth. The proposition by Copernicus in 1543 that the earth revolved around the sun took over 100 years to be accepted, requiring the combined efforts of such famous astronomers as Brahe, Galileo (who was arrested for his beliefs), Kepler, and Newton. Newton put the final nail in the earth-centric coffin in 1666 when he espoused the principle of gravity and demonstrated (at least mathematically) how gravity could explain the orbits of the planets and their moons.
Since that time, considerable advances have taken place in our understanding of the Universe. Astronomers discovered that as they gaze at greater and greater distances, they are also seeing further and further back in time. This is because light travels at a finite speed and by the time we see the "light" of a distant galaxy, a great amount of time has passed. Thus, as we look further back, we are seeing galaxies as they appeared millions and billions of years ago.
The field of study that looks at how the universe came into being, why it looks as it does now, and what the future of the Universe holds is call cosmology. Cosmologists make astronomical observations of the Universe that peer back billions of years in an attempt to deduce theories that describe how the Universe works. To accomplish this, cosmologists rely on the mathematical tools of modern physics and the observational tools of modern astronomy. Thus, cosmology might be thought of as a hybrid field of astronomy and physics (just like oceanography is a hybrid field of many disciplines).
Since the late 1920s, astronomers have known that the universe is expanding. Gigantic collections of stars, known as galaxies, appear to be flying away from each other. This phenomenon is known as the Hubble expansion, named after the man who discovered it, Edwin Hubble (who started his career as a lawyer - and yes, this is the same man who now has a telescope in space with his name on it!). By calculating the rate of expansion and working backwards, astronomers estimated that the age of the Universe must be between 10 and 15 billion years old and no older than 20 billion years. However, new data from the Hubble Space Telescope (HST) indicate the age of the Universe is about 8 billion years. For our purposes, we will assume that the age of the Universe is 15 billion years. For more information on the controversy concerning the age of the Universe, see the recent Time magazine article (March 6, 1995, pp 76-84).
To better understand natural phenomena like the expansion of the Universe, scientists invent models, which may be simple and descriptive or complex and mathematical. The most common "model" used to explain the origin of the Universe is the Big Bang model, which has been described as the largest explosion ever imagined - "like a vast nuclear explosion." The Big Bang theory, originally proposed in 1927 by the Belgian astrophysicist George Lemaitre (although the name Big Bang was coined later), explains that all the matter and space that make up the Universe were concentrated into a small volume, then rapidly expanded (the Big Bang!) at the speed of light.
Other theories have been proposed, but seven decades of research and observations have yielded a considerable body of evidence that supports the Big Bang theory, and none contradicts it. A article in the August 29, 1991, issue of the journal Nature reviews the evidence supporting the Big Bang theory.
Although what happened during the first second can only be speculated, the popular story goes like this. In the beginning, all the energy of the Universe was squeezed into a ball the size of a pinhead. The Universe began with a tremendous explosion, creating matter out of pure energy as it expanded. Within the first second, the density of the Universe was equal to the Earth squeezed down to the size of a marble. The temperature was 10 billion degrees centigrade, hotter than the interior of the largest stars. After three minutes, the Universe cooled to 1 billion degrees and had a density less than water. At half a million years old, the Universe had cooled to a few thousand degrees, like the surface of our Sun. After 1 billion years, the force of gravity caused matter to clump together forming a billion galaxies, each composed of a billion stars or more, each of these, perhaps, surrounded by planets.
Three major lines of evidence support the Big Bang Theory:
Let's briefly examine this evidence and see for ourselves how much of the Universe can be explained by the Big Bang Theory.
1) The Universe is expanding
Evidence that the Universe is expanding comes from observations of stars. You are all familiar with the changing sound of a train as it comes towards you and moves away. This is known as the Doppler effect.
For example, as a whistling train comes towards you, the sound you hear appears to get higher. As it passes and moves away, the sound appears to get lower in pitch. Obviously, the sound from the train isn't changing (which you could perceive if you were on the train), but the compression of sound waves as it moves toward you or the expansion of sound waves as it moves away from you changes the pitch that you hear.
The same thing is true about stars. When early astronomers looked at stars in our galaxy, they discovered that some were "red-shifted" meaning they were moving away. Others were blue-shifted, meaning they were moving towards us. This discovery was made by Edwin Hubble, who looked at 41 galaxies using the telescope on Mount Wilson. Of the 41 galaxies he examined, 36 of them showed a shift towards the red; the remaining five were blue-shifted. Thus, in 1929, the idea was born that the majority of galaxies were receding from our own.
Without going into all the details, suffice it to say that Hubble discovered that the further away a galaxy was from earth, the more it was "red-shifted". In other words, the greater the distance the faster they were rushing away, as expected if the universe was expanding. Knowing how fast they were rushing away allows us to calculate the age of the Universe. This phenomenon is known as Hubble's Law.
2) The helium:hydrogen ratio is consistent with Big Bang
Another puzzle solved by Big Bang was related to the amount of helium in the Universe. There was just too much helium to be explained by the nuclear fusion reactions in existing stars which convert hydrogen into helium. These reactions could only account for a few percent of the helium.
However, using Big Bang as a model, scientists have been able to calculate how much helium should be present using information on the formation of helium gained from high-energy particle accelerators and speculating on the early composition of the Universe. At 1 billion K (the temperature of the early Universe), the density of protons and neutrons was just right, such that helium and other light elements could have formed in the amounts we observe in today's Universe. Therefore, the bulk of helium in our Universe must have come from the Big Bang.
3) The Cosmic Background Radiation is 3 Kelvin
To prove Big Bang, astronomers also had to show that the Universe had a beginning. For all the matter to be compressed as the Big Bang predicts, the temperature of the Universe in its early stages had to very hot (around a billion degrees Kelvin). If it was this hot, then some of that "heat" had to be left over for us to observe today, according to the theory.
We all know that light bulbs give off light and heat. Physicists have calculated that there should be some "heat" or infrared energy left over from Big Bang. Using the 15 billion year age, they calculated that the Universe should have cooled to 3 degrees Kelvin (or -270 centigrade). At this temperature, the energy should be in the form of microwaves (much like the same energy used to heat food).
The first "discovery" of this background radiation, or Cosmic Noise, came in 1964 when two Bell Telephone laboratory scientists, Arno Penzias and Robert Wilson, were testing an antenna to communicate with satellites. When they switched to microwave wavelengths, they found a hissing noise and thought there was something wrong. After taking the antenna apart and reassembling it several times, the hissing still remained. Finally, after talking to Princeton astronomers who were actually looking for the Cosmic Noise, they realized they had found it. Penzias and Wilson were awarded a Nobel prize in physics for stumbling across this discovery.
The discovery of Cosmic Noise, or Cosmic Microwave Background Radiation (CMBR), as it is now known, provided key evidence to support Big Bang. Much like forensic scientists who find clues as to the source of an explosion, astronomers now had pretty good data that an explosion occurred. This research continues today through the use of a satellite launched in 1990 known as COBE, the Cosmic Background Explorer. Early data from COBE showed that the intensity of background radiation matched the theory. Furthermore, COBE discovered that the Cosmic Noise wasn't smooth across the Universe. Cosmic noise had slight ripples in it. This was a big relief to many cosmologists.
One distressing piece of evidence that astronomers couldn't reconcile was the fact that the Universe is not uniform. In 1989, astronomers discovered what is known as the "Great Wall", a thin sheet of galaxies stretching 500 million light years across, 200 million light years in height, and 15 light years thick. What caused this wall to form was a source of great debate and actually caused many people to think that the Big Bang theory was wrong. Big Bang predicted that the Universe was smooth and homogenous, like one big cosmic milk shake. Within a few short years of this discovery, other astronomers "discovered" walls, and it is now believed that the Universe is made up of a series of walls. The data provided by COBE (which scientists had been seeking for 25 years) helps explain why the distribution of galaxies in the Universe is lumpy (i.e. not uniform). Cosmologists now have a plausible link between Cosmic Noise and the structure of the Universe, and research continues to explain how this lumpiness occurred within the context of the Big Bang.
Question: How do models help us understand natural phenomena? What "models" do you use in your everyday life to explain the world around you?
The Puzzling Problem of Galaxies
A quick glance at our Universe, either in the evening sky or through high-powered telescopes, reveals clumps of stars known as galaxies. Galaxies are discrete collections of stars (numbering 10 million in a dwarf galaxy and 10 trillion in a giant galaxy) and they appear to be the stuff the Universe is made of. Our own galaxy, the Milky Way Galaxy (which comes from the Greek word Galaktos, meaning "milk"), consists of several hundred billion stars, and is shaped like a very thin disk that measures 90,000 light years in diameter. Our Earth (and Sun) is about 30,000 light years from the Center, or about two-thirds of the way from the center to the edge. The Sun carries our Solar System with it at a speed of 230 kilometers per second, and it completes an orbit of the galaxy every 250 million years. Thus, we have circled the galaxy about 18 times since the Earth was formed, approximately 4.6 billion years ago.
Astronomers have found that galaxies don't like to live alone. Thus, they tend to occur in groups. Our own Milky Way and its neighbors are known as the "Local Group". It includes our nearest neighboring galaxy, Andromeda, as mentioned above, and about 15 dwarf galaxies, along with some "clouds" called the Large and Small Magellanic Clouds. In total, our Local Group extends over 10 million light years. Interestingly enough, Andromeda, which is 2.3 million light years away from the Milky Way, is "blue-shifted", meaning it is one of the few galaxies moving towards us. Given its distance, I don't think we have to worry about a collision anytime soon!
While it has been observed that galaxies tend to occur in groups, and while we know that groups of galaxies appear to move in relation to each other according to laws of gravity (fondly known as the Great Cosmic Ballet), we simply don't know how galaxies are formed. Cosmologists have a few ideas (like random quantum fluctuations in the early Universe), but for the most part, the question of galaxy formation remains unanswered. The discovery of fluctuations in the Cosmic Background Radiation has given impetus to the "fluctuation" model of galaxy formation, but nothing is concrete at this point.
Another problem is that the mass of the galaxies in the Universe is not sufficient (by a long shot) to explain the "motions" of the galaxies. This has led to the "missing mass" or "dark matter" problem. What it boils down to is this: a certain amount of matter is needed in the Universe to prevent it from flying apart forever. If there is less mass in the Universe than we think, then the Universe can be termed "open" and it will forever expand. If there is more mass, then the Universe will be "closed", and will collapse upon itself after a period of time, calculated to be 40 billion years. This is known as the "Big Crunch"! Most cosmologists (and humans, I suspect) prefer a "closed" Universe. Thus, for the time being, the search is on for dark matter (matter that is invisible to us), which comprises 90 percent of our Universe!
Several other "inconsistencies" will need to be explained before the Big Bang Theory is fully established. As outlined by Parker (1993), there is the galaxy and dark matter problems (which we briefly discussed), the horizon problem, the flatness problem, the entropy problem, the antimatter problem, the monopole problem, the photon problem, the rotation problem, and Dirac's coincidences, which shows that the number 10 to the minus 40 is important, but we don't know why. Check out the book. It is easy reading and most fascinating!
These problems and more will continue to fuel the controversies surrounding the formation and structure of our Universe. However, the rate at which these controversies will come and go is sure to accelerate as we probe further, deeper, and faster into the mysteries of our Universe.
Question: How has your perception of outer space and astronomy changed during your lifetime? What significant "astronomical" events can you recall?
The Birth of Planet Earth
Within the last couple years, at least seventeen planets have been discovered outside of our own solar system. While none of these planets appear to be of the type that support life, the discovery of these planets greatly accelerates our knowledge of how planets form and their evolution.
One popular theory for the formation of our solar system is that the Sun and planets evolved from a nebula or the remains of an exploded star. A whirling disc-like cloud of dust and gas was formed, becoming highly turbulent, with dust particles colliding and combining in what has been described as "a demolition derby of cosmic proportions". In the center of this cloud, where the gravitational pull was strongest, the compression of gases caused superheating, leading to thermonuclear reactions that converted hydrogen into helium, producing intense energy. This intense radiant energy became the Sun.
Other whirlpools were formed, colliding and coalescing into larger whirlpools. These whirlpools of dust packed together into larger and larger particles which formed planetesimals, similar in size and shape to the asteroids. As the larger planetesimals grew in size, their gravitational attraction drew more planetesimals towards them, forming the planets. It is believed that if the largest of these planets, Jupiter, had gotten bigger, that we might even have two Suns in our solar system. Binary stars comprise about half the solar systems in our galaxy.
Eventually, a single Sun and at least nine planets and an asteroid belt with planetoids were formed in our solar system. Astronomers speculate that our solar system must have a tenth planet to explain the orbit of Pluto, but it hasn't been discovered yet, to my knowledge. In addition, it is believed that the asteroid belt is a ring of planetesimals that failed to "get it together" to form a planet, or are the result of a collision between two early planets. In the final outcome, Earth stood as the third planet from the Sun, along with the other planets (in order) Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
The formation of the Earth as described above is believed to have occurred about 4.6 billion years ago. At this time, we should note that the rotation of the Earth was considerably faster. Sunrise to sunset took only 2.5 hours, as the Earth rotated every 5 hours.
The formation of our planet was only the beginning of a very significant chain of events for the Earth. Upon its initial formation, the Earth heated over time by three methods:
As planetesimals and meteorites impacted the Earth, the heat of impact managed to keep the Earth in a molten state. This was particularly true in the early stages of development when the Earth had no atmosphere. In addition, as the impacts continued, the Earth grew in size, causing compression of rocks in the interior as the Earth's gravity increased. Finally, the radioactive decay of elements within the rocks also added heat to the Earth. This radioactive decay continues to this day (albeit at a much slower rate) and it is surmised that if you put a gallon-size cube of granite in a perfectly insulated coffee pot and let it stand for 850,000 years, you could make a cup of coffee off the heat of the granite!
One side note on the impacting planetesimals concerns the formation of the moon. Evidence brought back from the moon astronauts suggests that a giant planetesimal, possibly the size of Mars, struck the Earth. The "splash" of the lighter elements of Earth's molten material into space was sufficient to cause the formation of the moon. One still wonders...
Scientists figure that the early Earth had a temperature of 1000 degrees C when first formed as a organized planet. But the rate of heating was faster than the rate of cooling, and the earth heated up. This was very important because after 1 billion years, the temperature became hot enough to melt iron at depths between 400-800 kilometers (250-500 miles).
Iron is abundant in the Earth and because it is heavy, it began to sink as it melted, displacing lighter elements. This "iron sink" caused a tumultuous reorganization of the whole structure of the earth. This process is known as planetary differentiation, and the homogeneous composition of the earth became a stratified one. It has been said that "planetary differentiation of the earth is perhaps the single most important event in the history of the Earth." It led to the formation of the crust and the continents. It also likely led to the escape of gases from the interior of the planet which led to the formation of the atmosphere and the oceans. This "outgassing" has been described as the Big Burp.
After all this overturning and reshuffling of the elements of the earth, things began to settle down and the Earth assumed the layered composition that we know today. In our next lecture, we will examine the formation of the Earth's oceans and look at the role of the seas in the evolution of life on our planet. Of course, we will always be reminded that the physical, chemical, geological, and biological processes on Earth did not evolve independently. This notion of the synergistic evolution of life and the planet will provide more fodder for our notion that the Earth is one giant system.
Question: How is the position of the Earth in our solar system ideally suited for Life? Why is the place Earth occupies sometimes called the Goldilocks position?
Within this grand and faster-than-light-speed survey of our vast Universe, it is appropriate to conclude on a philosophical and poetical note, never forgetting that creativity is the hallmark of great science. What the Big Bang theory has accomplished at the end of this century is truly remarkable. For the first time, we now have a plausible model of the Universe that unites the infinitesimally small with the infinitely large. From elementary particle physics to astronomy in space, the search is on to build a comprehensive pictures of cosmological processes within our Universe.
Still, we shouldn't be worried that everything is going to be figured out in our lifetime, or that everything will ever be figured out to completion. The quantum physics on which we perceive our world is a slippery rascal, and it's not likely to let us out of its grip anytime soon. We must remember that everything we perceive is just that, a perception, and our model of the Universe is just that, a model.
With that in mind, I'll leave you with a bit from the great physicist Erwin Schrodinger (after whom a whole set of well-known quantum physical equations are named), who waxed poetic in his book, My View of the World (Cambridge University Press, 1964). Schrodinger writes:
All of us living beings belong together in as much as we are all in reality sides or aspects of one single being, which may perhaps in western terminology be called God while in the Upanishads its name is Brahman...For we are all, in every particle of our being, precipitations of consciousness; as are, likewise, the animals and plants, metals cleaving to a magnet and waters tiding to the moon...we are to recognize in this whole Universe a reflection magnified of our own most inward nature; so that we are indeed its ears, its eyes, its thinking, and its speech -- or, on theological terms, God's ears, God's eyes, God's thinking, and God's Word; and, by the same token, participants here and now in an act of creation that is continuous in the whole infinitude of that space of our mind through which the planets fly, and our fellows of earth now among them.
Perhaps the next galaxy isn't so far, far away...
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