Big Bang Mysteries

Despite its success, the SBBM cannot explain several cosmic mysteries. These mysteries are described below.

This is a real picture of the cosmic microwave background radiation, with circles and a red arrow added. For description of the background, please see the caption in the original image above. For a description of the black circles and red arrow, please see the text in this section. Credit: NASA / WMAP Science Team. Image Link.

The Horizon Problem

On the left, we again depict the CMBR. Notice, also, that we've placed two black circles on it. Suppose that, just after the Big Bang, an observer at the center of the bottom left circle sent a signal toward the top right. Because the speed of light is finite and the universe continuously expands, this light cannot travel infinitely far; but will it travel to the other side of the universe by the time the CMBR formed? In other words, if the black circle represents how far this signal could have traveled up to some time after the Big Bang, will this circle eventually encompass the entire universe (the entire sky) at CMBR formation? The answer depends on how quickly the universe expands, and we can compute this via the SBBM.

This computation shows that, regardless of whether the universe is made out of radiation or matter, in whatever combination, these black circles cannot stretch to envelope the entire CMBR sky. In fact, a famous computation shows that the CMBR sky can be arbitrarily divided into around 3000 of these circles. Why is this problematic?

The black circles tell us the parts of the CMBR that could have communicated from the Big Bang to the formation of the CMBR; namely, the center of one of these circles could only have communicated with other points inside that circle. Yet, all points in the CMBR have the same temperature--up to one part in 100,000. This is a bit like a room that has a powerful heater in one corner. If you walk into the room and the temperature is the same everywhere, you would conclude that the heater was turned off a while ago, so that the air molecules near the heater have had time to disperse their excess energy throughout the room. If those air molecules were heated, that means that they were moving relatively fast compared to other air molecules in the room. Over time, they will bump into adjacent air molecules, which will then bump into others; thus, this energy will be distributed evenly throughout the room if the heater is turned off. This will make the temperature of the air in all parts of the room almost the same.

Imagine, however, if you entered such a room immediately after the heater was turned off and noticed that the temperature was constant across the room. This would be strange, because there has not been enough time for the temperature to even out. How would you then explain why the temperature was even? This is the horizon problem. Since different parts of the CMBR could not have been in contact, even assuming signals had been sent from the Big Bang, there's no obvious explanation for the CMBR's homogeneity.

The Flatness Problem

By applying Einstein's general theory of relativity to the universe, we can derive equations which govern its expansion. These indicate that the overall geometry of the universe can be flat, saddle-shaped, or spherical. The image to the left depicts these geometries. Observation indicates that the universe is either perfectly flat, or very close to it.

Image Credit: NASA; you can find the image here.

The SBBM, however, provides no evident reason for its flatness. To make matters worse, one can show that, according to the SBBM, any deviation from flatness in the early universe would have become more substantial by now; in other words, if the universe is very close to flat today, it must have been really flat in the early universe. What caused this?

Both of the above-mentioned problems are solved via cosmic inflation, which is what we study. There are many other fascinating mysteries, such as dark matter and dark energy, for example. Just below, we briefly discuss some other cosmic mysteries.