|
|
 |
Universe without inflation |
Universe with inflation |
The fact that the temperature of the universe is almost perfectly uniform presents a most important puzzle that has its origins in the concept of the event horizon of the universe. As we described above in Section 5 of this lesson, more and more galaxies come within the cosmic event horizon as the universe expands and decelerates. That result implies that the number of galaxies within the cosmic horizon of any part of the early universe was much smaller than the number of galaxies within the observable universe today. An observer sitting on a galaxy in the early universe would have no way of knowing the existence of a galaxy beyond his horizon. (To be sure, those galaxies existed, because we can see them today.)
We can calculate the distance to the horizon of any piece of the universe at the recombination epoch, when it was 300,000 years old. Likewise, we can calculate how big such a piece should appear to us today. The answer is: the angular diameter should be about 1 - 1.6o -- about 2 - 3 times the angular diameter of the Sun.
Here's the puzzle: since there is no way that any part of the universe can know about the existence of any other part of the universe beyond its horizon, why does the cosmic photosphere have nearly uniform temperature? Scientists like to think they can explain the universe by physical mechanisms that obey the laws of nature. But one of the most important laws of nature is that no signal can travel faster than light. That means that there is no possible physical mechanism that can cause parts of the universe beyond each other's horizons to have the same temperature. That means that there is no reason for the temperature of the cosmic photosphere to be uniform over angles greater than 1.6o.
One explanation might be that the Creator just made the universe that way. But scientists are never satisfied with such an explanation. Rather than regard a natural phenomenon as the whim of the Creator, we prefer to regard it as a consequence of the laws of nature. If you like, we prefer to regard the Creator as the giver of the laws, so that our job is to understand those laws and all their natural consequences. So, rather than just regard the smoothness of the microwave background as God-given, we would prefer to find a mechanism that would account for the smoothness of the microwave background.
Today, the most popular hypothesis to account for this smoothness is called the theory of cosmic inflation. The idea is rather simple: if all the atoms in the observable universe had the same temperature at the recombination epoch, when they could not see each other, then, perhaps, at some earlier time they indeed were within sight of each other. But if some atoms at that earlier time were inside the horizon but later moved outside it, then the expansion of the universe during that earlier time must be accelerating, because only during the accelerating expansion does the horizon shrink in size. So, scientists called this period of accelerated expansion "cosmic inflation".
|
|
|
Universe without inflation |
Universe with inflation |
In fact, the expansion of the universe in these graphs was factored out so that we can see all the horizons on one plot (the so-called "comoving" view). The actual size of horizon of the universe at the inflationary epoch was much smaller than it is today, but it encompassed more matter. Because the universe expanded a great deal since the end of inflation, a very small size at that time expanded to a region much bigger than our horizon today.
But what kind of force could make the universe accelerate? It would have to push things apart, unlike gravity, which normally attracts. The strongest repulsive force we know is the nuclear force, which becomes repulsive when neutrons and protons get too close and helps to prevent atomic nuclei (and neutron stars) from collapsing. There is only one way (without violating the laws of nature) that we can imagine the universe to be controlled by a repulsive force strong enough to counter gravity. We must think about the universe when it was so young and that it had density greater than that of an atomic nucleus. Then, repulsive forces such as the nuclear force might be strong enough to counter gravity.
In fact, the most popular theory of inflation suggests that the repulsive force did its main work even earlier than that, at a time of about 10-34 seconds A.B.E. At that time the energy density of the universe was so high that three of the four known forces of nature (electromagnetism, the weak nuclear force, and the strong nuclear force) should all merge into one primitive Unified Force, according to a theory of matter called the Grand Unified Theory (GUT - and that's why the Unified force is sometimes called the "GUT Force").
The GUT theory predicts that at these extraordinary densities and temperatures there exists a new kind of matter - called inflaton - which possesses an amazing property: its gravity is repulsive! And if the universe contains enough of this inflaton matter, its repulsive gravity will send the whole universe into an accelerated expansion - cosmic inflation (now you can guess why this new kind of matter is called inflaton).
Before enough of the inflaton matter formed, all atoms (or, more precisely, subatomic particles - atoms and even atomic nuclei could not have existed at the extraordinary densities and temperatures of the GUT era) in the present and future universe were in contact, so the universe could reach a more-or-less uniform temperature. After that time, the universe had accelerated to such a speed that the horizon of any piece of the universe shrank to a size far smaller than an atomic nucleus. Then, the universe began to decelerate and all these little horizons began to grow, so that more and more pieces of the universe came into contact with each other. But now there is a reason that these disconnected pieces of the universe would all find themselves having the same temperature when they come into contact: they all were in contact before the epoch of inflation.
Is this theory correct? We don't have any direct evidence to prove it, but there are quite a few indirect clues that inflation indeed took place: the geometry of the universe is flat, the universe is homogeneous and isotropic, and the variations in the CMB are exactly what inflation predicts. Elementary particle physicists believe that the inflaton matter really exists, but they do not yet have atom smasher machines that are energetic enough to test these theories. But it's fun to think about the theory of inflation, and even more fun to think that such a theory might someday be tested experimentally.
To summarize the theory of inflation: at a time earlier than 10-34 seconds A.B.E., the entire universe was extremely dense and hot. Then, inflaton matter was created in particle reactions, and it sent the whole universe into an incredibly fast expansion - the universe expanded by, perhaps, a factor of 101013 - before all the inflaton matter decayed into other particles with normal, attractive gravity (the expansion factor might be, of course, much smaller or much bigger - it depends on the details of inflation which we don't know). If you think your imagination is good - just try to imagine that unimaginable number: 101013. If our whole class decided to simply write this number down and we all got busy writing down zeros after the first one, it would take us... Well, I will leave it to you to guess how long it would take us just to write this number down, but it will be a veeeery looong time! (Click here if you want to check your guess.)
