General Astronomy/The Big Bang and Cosmic Expansion
The Big Bang is a misnomer, as it implies an explosion. In fact, the model describes an expansion of the Universe, not into some pre-existing space, but rather that it created space as it expanded. There is nothing outside the Universe, so there was nothing to expand into. It's just at one time every point in the Universe was incredibly close to every other place (less than the size of an atom), and that at the moment of the Big Bang, they all started getting farther apart at a high rate. This expansion continues today, and in fact appears to be speeding up, possibly due to another force called "dark energy." By current thinking, this expansion will continue forever, continually cooling.
The basic idea dates back to the 1920s and Georges Lemaitre who, basing his idea on Einstein's general theory of relativity. Soon afterwards Hubble's discovery that all distant galaxies are receding from us served as a confirmation of the basic idea of the Universe having a beginning in time and having since expanded.
By today's measurements -- using the current size and rate of expansion as we understand them -- working backwards in time leads to the conclusion that approximately 13-14 billion years ago, the Universe was compacted into the volume of less than that of an atom. By understanding how the forces of Nature work, we can trace this backwards into earlier times and higher temperatures. As we do so, the nature of the laws of Nature change, such that at 10^-43 of a second after the Big Bang (the "Planck time"), the laws as we know them break down. Hence, we cannot look farther back in time than that -- and in such a situation it is impossible to look to a time before the Big Bang. [Of course, we cannot rule out the possibility that those ideas may someday change.]
At and before Planck time, most astronomers and physicists think that all the forces of the Universe -- electromagnetism, the two nuclear forces and gravity -- were all merged into one force, but we do not know how that force worked. After the Planck time, gravity separated from the "Grand Unified Force," the latter of which consisted of the strong nuclear force and the electroweak force. As things cooled further, the electroweak force split into electromagnetism and the weak nuclear force, and the regime we have today was born.
During the first three minutes of the expansion, energy cooled and matter in the form of hydrogen and helium "froze" out. After about three minutes, the expansion had cooled the Universe so much that no more elements were created ("nucleogenesis"). In fact, no new elements were formed for hundreds of millions of years until the first stars formed. All the hydrogen in the Universe, including that in you, and most of the helium can be traced back to the first three minutes of the Universe.
After that the Universe expanded for hundreds of millions of years before the first stars and galaxies formed in a highly energetic state (by today's standards). Since then, the stars and galaxies have evolved to what they are today.
There are three major pieces of evidence in favor of the Big Bang:
First and foremost is the expansion itself. This was discovered in the 1920s through the red shifts of distant galaxies. If all the universe is expanding today -- which is certainly what the evidence shows -- then it is reasonably easy to work the process backward in time to deduce that at earlier epochs the Universe was smaller.
The second piece of evidence was predicted by George Gamow, who is considered by many to be the "father" of the Big Bang, although he was not the first to come up with the idea. In the 1940s he made the calculations that showed how elements were formed at the time of the Big Bang. Basically, he said that if the Big Bang idea was correct, then the Universe should consist of roughly 75 percent hydrogen and 25 percent helium (other elements being very small constituents). This could not be confirmed at the time, but with the advance of technology, has since been confirmed.
The third and very strong piece of evidence also was predicted by Gamow long before it could be confirmed. Basically, he said that the Big Bang would have been very hot, and as it expanded the Universe would cool off. As matter cools off it gives off certain types of radiation. It is like a hot fireplace poker cooling off and changing colors, or the embers of a fire dying and turning dark. Even though they may not give off visible light, they continue to radiate infrared radiation. Although Gamow had a wrong estimate on the age of the Universe, he said that by today it should have cooled to a certain level, and if we had the technology to detect it, we could read the radiation and effectively take the temperature of the Universe. The technology to do this became available in the 1960s, and the radiation predicted by Gamow -- properly adjusted for today's more refined value of the age of the Universe -- has been discovered. It is called the "Cosmic Background Radiation," or "Cosmic Microwave Background Radiation," and is often abbreviated as CBR or CMBR. The CBR is especially good evidence in favor of the Big Bang, as there is little if anything else that can adequately explain it.
Historically, the Big Bang has had problems. The three main ones are called:
- The Horizon Problem. You might call this the Mixing Problem. If we look to the edge of the observable universe in any direction, we find that everything seems the same, as if it was well mixed. But material 10 billion light years in one direction cannot mix with material that is 10 billion light years in another direction simply because the age of the Universe is too short. The material on one edge of the observable Universe is over the "horizon" for material at the other edge. They cannot interact. Yet everything is well mixed. How did it get that way?
- The Flatness Problem. The actual density of matter in the Universe is too close to the critical density (that density required to just stop the expansion) to be coincidental. To be as close as it is today, the Universe had to have been "flat" (that is, critical density and observed density exactly the same) to within something like 60 decimal places. This is amazing. How could it possibly be so close?
- The Smoothness Problem. In the original Big Bang scenario, everything is expanding away from everything else in all directions. As such, no two particles of matter would have every gotten close enough to stick together by gravity. Hence, there would be no galaxies, no stars, no planets. Yet these all exist.
About 25 years ago a physicist named Alan Guth came up with an idea now known as Cosmic Inflation. Think of water cooling off and changing into ice. It takes energy (heat) to hold water in a liquid state. When the water cools and changes to ice, it gives up this energy into the environment. This is called a "symmetry breaking." Basically, what Guth said was that in the early Universe, just a tiny fraction of a second after the Big Bang, something similar happened as it expanded and cooled. Only here the heat energy solidified into matter (via E=mc^2). The energy released in this symmetry breaking caused the expansion to suddenly speed up for a very short while, much greater than the Big Bang alone. This super expansion (inflation) helps solve the three problems that previously plagued the Big Bang model.
- In a short introduction we must leave out detail, but in essence Inflation solves the Horizon Problem because at the very early stages of the Big Bang, everywhere was close enough to every other "where" such that they were well mixed. The sudden inflation essentially locked that well-mixed state in and it remained as the Universe has expanded to today.
- Inflation solved the Flatness Problem because the inflation naturally ran out of energy just as the density reached the right value.
- It solved the Smoothness Problem because in the very early Universe, quantum fluctuations caused small irregularities throughout the baby Universe (we know such "irregularities" occur all the time in space today). You can think of these as small density concentrations or "knots" in the fabric of space. Inflation suddenly blew these up to enormous size, trillions of times larger than the originally were. As the Universe expanded, matter collected around these spatial "kinks" and eventually formed galaxies.
Although there have been other theories, most notably the Steady State theory, none have been able to adequately explain all the evidence in at least a semi-consistent and coherent model. Some of the other theories are just variants on the Big Bang model.
We have a reasonable mechanism to explain Cosmic Inflation and the changes in the Universe since the Big Bang itself, but the cause of the original "bang" is unknown. There have been suggestions, but none offer an ultimate solution. One idea is that the Universe arose completely out of nothing, just as we observe (although on vastly smaller scales) in the quantum physics we know today. [Particles come into existence out of nothing all the time, but they do not stay around long enough to be counted. They are called "virtual particles" and there is strong evidence that they exist even though we normally are unaware of them. By the proper application of energy, we can make some of these particles stay around longer so we know they exist.]
What caused the Big Bang? This question probably does not have a scientific answer. It is in a category of questions like, "Why does something exist instead of nothing?" That, nobody knows.