Illustration of the 15-billion history of the Universe by Pat Rawlings for the Jet Propulsion Laboratory of NASA
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. 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. He found that, no matter which direction he looked into space, distant galaxies appeared to be moving away from us. The farther away the galaxy is from our galaxy the faster it moves away from us. Hubble was observing the expansion of the Universe. Thus, in 1929, the idea was born that the majority of galaxies were receding from our own.
What does it mean that the Universe is expanding? Well, a simple way to think of it is to imagine baking a loaf of raisin bread. As the bread rises it also expands. All of the raisins move farther apart from one another. Every single raisin would see all of the others moving away from it. So to complete the analogy, all of the galaxies in the universe are like the raisins in the bread.
Knowing how fast those raisin are moving allows us to calculate the age of the Universe (replace the raisins with galaxies). This phenomenon is known as Hubble's Law and the rate at which the Universe is expanding is called Hubble's constant.
For the past 70 years astronomers have sought a precise measurement of the Hubble constant, ever since astronomer Hubble realized that galaxies were rushing away from each other at a rate proportional to their distance, i.e. the farther away, the faster the recession. For many years, right up until the launch of the Hubble telescope - the range of measured values for the expansion rate was from 50 to 100 kilometers per second per megaparsec (a megaparsec, or mpc, is 3.26 million light-years).
This summer a team of scientists measured the Hubble constant at 70 km/sec/mpc, with an uncertainty of 10 percent. This means that a galaxy appears to be moving 160 thousand miles per hour faster for every 3.3 million light-years away from Earth.
In other words, the greater the distance from Earth, the faster a galaxy is rushing away from us, which is to be expected if the universe is expanding.
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.
Highly Recommended Reading:
Please review the online tutorial provided by NASA. It summarizes what I've provided here and offers a more colorful and detailed account of the origins of the Universe and all it contains.
Online Tutorial for Evolution of the Universe, NASA-JPL
Big Bang>>Galaxies>>Giant Molecular Clouds>>Element Formation in Stars
The information below is optional reading that will help you better understand this material.
The text below is from this link:
Structure and Evolution of the Universe
What is space expanding into?
Amazingly, space is not actually expanding "into" anything. Put another way, a given region of space doesn't actually "push" the rest of the universe out of the way as it expands. How can this be? The Forum's pop-up tutorial will help you visualize what's going on.
Where did the Big Bang scenario come from?
If space (and everything with it) is expanding now, then the universe must have been much denser in the past. That is, all the matter and energy (such as light) that we observe in the universe would have been compressed into a much smaller space in the past. Einstein's theory of gravity enables us to run the "movie" of the universe backwards � i.e., to calculate the density that the universe must have had in the past. The result: any chunk of the universe we can observe � no matter how large � must have expanded from an infinitesimally small volume of space.
How do we know when the Big Bang took place?
By determining how fast the universe is expanding now, and then "running the movie of the universe" backwards in time, using Einstein's theory of gravity. The result is that space started expanding about 15 billion years ago, give or take a few billion years. This number is uncertain, in part because of uncertainties in our current measurements of how fast the universe is expanding, how much matter and energy there is, and even what kind of energy there is in the universe.
Does this mean that the entire universe began from a point?
No, it's a common myth that the entire universe began from a point. If the whole universe is infinitely large today (and we don't know yet), then it would have been infinitely large in the past, including during the Big Bang. But any finite chunk of the universe � such as the part of the universe we can observe today � is predicted to have started from an extremely small volume.
Part of the confusion is that scientists sometimes use the term "universe" when they're referring to just the part we can see ("the observable universe"). And sometimes they use the term universe to refer to everything, including the part of the universe beyond what we can see.
On the other hand, if the whole universe is finite, then according to Einstein's theory, it would have sprung from an infinitesimally small region. However, we know that Einstein's theory of gravity cannot be the whole story when considering very small regions: the strange quantum laws of nature must apply. Unfortunately, we don't yet have a theory of nature that combines Einstein's description of the large-scale world with the quantum description of the microscopic world. (To visualize how a universe could be finite, try the Forum's pop-up tutorial.)
Do we know where, in space, the Big Bang took place?
It's a common misconception that the Big Bang was an "explosion" that took place somewhere in space. But the Big Bang was an expansion of space itself. Every part of space participated in it. For example, the part of space occupied by the Earth, the Sun, and our Milky Way galaxy was once, during the Big Bang, incredibly hot and dense. The same holds true of every other part of the universe we can see.
Artists may find it more dramatic to draw a "fireball" expanding into space, but as far as we know, there would have been no such "ball."
How do we know there really was a Big Bang?
As mentioned above, we observe that galaxies are rushing apart in just the way predicted by the Big Bang scenario. But there are other important clues.
Astronomers have detected, throughout the universe, two chemical elements that could only have been created during the Big Bang: hydrogen and helium. Furthermore, these elements are observed in just the proportions (roughly 75% hydrogen, 25% helium) predicted to have been produced during the Big Bang. This prediction is based on our well-established understanding of nuclear reactions � independent of Einstein's theory of gravity.
Second, we can actually detect the light left over from the era of the Big Bang. The blinding light that was present in our region of space has long since traveled off to the far reaches of the universe. But light from distant parts of the universe is just now arriving here at Earth, billions of years after the Big Bang. This light is observed to have all the characteristics expected from the Big Bang scenario and from our understanding of heat and light.
But I've heard on the news there are problems with the Big Bang theory. Is it still just a "theory"?
The Big Bang is actually not a "theory" at all, but rather a scenario about the early moments of our universe, for which the evidence is overwhelming. But the Big Bang scenario cannot be the whole story, and its details are a subject of intense research.
Was the Big Bang the origin of the universe?
It is a common misconception that the Big Bang was the origin of the universe. In reality, the Big Bang scenario is completely silent about how the universe came into existence in the first place. In fact, the closer we look to time "zero," the less certain we are about what actually happened, because our current description of physical laws do not yet apply to such extremes of nature.
The Big Bang scenario simply assumes that space, time, and energy already existed. But it tells us nothing about where they came from � or why the universe was born hot and dense to begin with.
Are there theories that go beyond the Big Bang?
Yes, there are theories that build on the Big Bang scenario by adding insights from physics about the structure of space itself. Coming soon at the Forum: Learn more about these latest theories and their amazing predictions.
More Optional Reading:
The text below is from Nick Strobel, a professor who teaches astronomy online at Bakersfield College. Here's the link to his online notes:
Elementary Astronomy by Nick Strobel
What evidence is there for thinking the Big Bang theory is correct? The Big Bang theory may be nice but it has to pass the judgement of observation. Nature and experiments are the final judge of the correctness of scientific ideas. Though some details of the Big Bang still need to be perfected, the general scheme of a early hot universe with a definite beginning is accepted by most astronomers today. Even so, we have to be open to the possibility that future observations could show it to be wrong. The observations given below are sometimes said to be ``proof'' of the Big Bang theory. Actually, the observations are consistent with the Big Bang theory, but do not provide proof. Recall from the discussion in the chapter on the scientific method that scientific theories cannot be proven to be correct. As of now, the Big Bang theory is the only one that can explain all of these observations.
Here are some other useful and fascinating links on this topic:
Building a Ladder to the Stars:
The 90-Year Quest for the Size of the Universe
Structure and Evolution of the Universe Home Page
NASA's Origin's Program
Latest Hubble Space Observations
Tour the Cosmos: a multimedia presentation of Hubble findings
Hubble Space Telescope Public Pictures
Mapping the Universe: Scientific American
Is Space Finite: Scientific American
Chandra X-Ray Observatory Home Page
List of All Missions Dedicated to Understanding the Structure and Evolution
of the Universe
Astronomy Picture of the Day
Windows to the Universe - lots of good brief explanations
Space Science Telescope Institute
Latest Discoveries StScI Page
Keck Observatory in Hawaii
McDonald Observatory in Texas
Palomar Testbed Interferometer
2 Micron All Sky Survey, University of Massachusetts
Infrared Processing and Analysis Center, California Institute of Technology
Sub-Millimeter Wave Astronomy Satellite
New Windows on the Structure and Evolution of the Universe
No Escape: The Truth About Black Holes
Star Light Star Bright: What does star color tell us about stars?
Hubble Deep Field Academy
Imagine the Universe
Basics of Radio Astronomy: Excellent Introduction
NASA's Deep Space Network: Bringing Images from Space
Telescopes in Education
Space Interferometry: Good explanations
Earth and Sky Radio Series
National Space Agency of Japan (NASDA)
NASDA Comsic Information Center