Pedagoguery

The Big Bang can be a tricky subject, since so much of it is well outside of our daily experience. As a result, many misconceptions are formed about it. Here are six common ones.

Misconception #1: The big bang was like a bomb going off at a certain location in previously empty space. This is probably the most common misconceptions about the big bang. The vary name conjures up titanic explosions. The reality is very different. There was no explosion into space simply because there was no space to explode into. Space itself started to expand in the big bang. The “explosion” therefore took place at all points in space simultaneously. Galaxies are not receding from each other due to the impetus of some past explosion, but instead because the space between them is expanding, pushing them apart. That is why inhabitants of every galaxy would see more or less the same view of galaxies flying away, as if they were at the center of an explosion. In a sense, they were.

Misconception #2: Galaxies cannot recede faster than the speed of light. It seems like a straightforward hypothesis, but it is wrong. Consider what the universe would look like if this were true. Galaxies that are farther away are traveling faster, but at some point that velocity becomes an appreciable fraction of the speed of light. What happens? We would observe large numbers of galaxies with recession velocities very close to the speed of light. In fact, the closer to that velocity we came, the more galaxies we would observe, well out of proportion to the greater volume of space we would be observing. In reality, relativity does not forbid recession velocities greater than the speed of light. That is because relativity prevents an object from traveling that fast within spacetime. However, recession velocities are not a measure of the galaxies speed within spacetime. Instead, it is a measure of how stretched the light has become due to the expansion of space itself. Relativity has no limit on how fast space itself can expand. That's why the inflationary ideas work. So what happens when a galaxy has a recession velocity greater than the speed of light? The answer to that question is somewhat complicated, and it leads us to our next misconception.

Misconception #3: We can never see light from galaxies that are receding faster than the speed of light. If the expansion rate of the universe were constant, this would actually be true. However, since the expansion rate changes over time, it is possible for a photon that was emitted by a galaxy out side our observable universe (also known as the Hubble distance) to get close enough to us to allow the Hubble distance to grow to meet it. Once the photon is inside our Hubble sphere, it will eventually be able to reach us, if it is headed in our direction. However, if the expansion of space continues to accelerate, the Hubble sphere will not be able to grow fast enough to overcome the expansion of space, and this particular trick will not be achievable for much longer.

Misconception #4: The cosmic redshift is due to the motions of the galaxies themselves. We touched on this one briefly in the second misconception. This belief arises from the fact that we think of things moving within the bounds of spacetime. However, that is not how the universe's expansion works. True, galaxies do have some motion within spacetime, but compared to the expansion of space, it is negligible. What really happens is this. A photon is emitted from a distant galaxy. As you would normally expect, the farther away the galaxy is, the longer it takes for the photon to reach us. While it is traveling, the universe continues to expand. This expansion actually stretches the wavelength of light, reddening it. The longer the photon has traveled, the more time the expansion of space has had to stretch the wavelength of that light. Hence, the direct relationship between the distance a galaxy is from us and its cosmological redshift.

Misconception #5: The observable universe has a radius of 14 billion light years, since the universe is about 14 billion years old. This is another case in which simplistic assumptions turn out wrong. The radius of the observable universe is actually about 46 billion light years. But, you say, how can that be? The answer is that the universe continues to expand. When the photon of a galaxy that is currently 46 billion light years away left its galaxy, it was much closer to us. Part of the distance between the galaxy and us actually grew between the galaxy and the emitted photon during the time the photon traveled to us. So, the photon did not have to travel the entire 46 billion light years, but only about one third of that distance.

Misconception #6: The expansion of the universe causes objects within the universe to expand as well. Consider a galaxy cluster. As the universe expands, would the cluster get larger as well? The answer is no. The gravity that binds together objects like galaxies and clusters is stronger than the force causing the universe to expand. So, while initially galaxies in a cluster would be pulled a little further apart, eventually their gravity would pull them closer together until an equilibrium size is reached. By the same token, stars and planets would not grow in size because the dominant forces that hold them together – gravity, electromagnetism, and the nuclear forces – are stronger than expansion. If the universe continues to accelerate its expansion, however, that could change, and we could reach a time when molecules, and eventually atoms would be ripped apart by the expansion of the universe. We will not have to worry about that for many trillions of years, however.

Next issue: Is the Earth's magnetic field about to reverse its polarity?