In my last post in the Religion and Science series I discussed astronomical evidences for the antiquity of creation. As part of the discussion John asked some questions about the Big Bang theory, and I promised to write a post answering his questions and explaining the theory. So, in a thousand words I will do my best to convey the current model for the evolution of the universe since its creation.
Big Bang cosmology is inherently a general relativistic theory, so I begin with a brief explanation of how this theory relates to cosmology. In general relativity, mass and space have an intimate relationship. Space is not a static homogeneous entity as it is in Newtonian theory. Instead space is capable of being stretched and bent and warped, and in fact any object with any mass will warp space. In turn, space affects how masses move through it. When they aren’t undergoing acceleration due to external forces, masses always like to move on the shortest path through space (a geodesic), but if space is warped, then that path will not be a straight line. This is analogous to the great circles on earth. The shortest route for a plane to fly from one point on the surface to another is not a straight line but a curved geodesic. So in general relativity space affects mass and mass affects space. Though it has many bizarre and counter-intuitive implications, general relativity is actually experimentally one of the best-confirmed of all scientific theories.
When one applies general relativity to a really big chunk of space, like, say, the universe, then one can derive a set of equations that govern the behavior of the space and how it is affected by the distribution of mass. As it turns out, it is difficult to have a static configuration. The space always either wants to be contracting down or expanding out, depending on how dense of a mass distribution you have. Einstein found this idea philosophically repulsive, because it meant that you couldn’t have a static universe just sitting there, and consequently it implied that the universe had a beginning. Consequently, he added a constant, called the cosmological constant, into the equation that would perfectly balance the contraction and create a static universe. Twelve years later, in 1929, Hubble discovered through observations of galaxy spectra that the further away a galaxy was the faster it was receding from us. This is precisely the sort of relationship that one would expect if the universe (space itself) were expanding. The further away something is, the more space there is between you and it, and therefore if space is expanding objects further away will be moving away from you faster than nearby objects. This was the first physical evidence for an expanding universe. Einstein later referred to the cosmological constant as his greatest blunder.
If all of space is expanding then it doesn’t take a genius to realize that if you extrapolate back in time eventually everything will be really close together. Eventually you reach what is called a singularity, where the equations break down and the universe has an infinite density. This singularity is called the big bang, and when scientists refer to the age of the universe they are talking about the time since the big bang. The way cosmologists calculate the age of the universe is by carefully adding up the mass within the observed universe and using the equations governing the expansion to extrapolate back the amount of time since the Big Bang.
John also raised an interesting point when he asked, “Where in the universe are we?” This is an age-old question that historically has often taken a central role in cosmology. Ever since the Copernican revolution each successive step forward in scientific theory has taken earth to a less-central place in the universe. However, with the discoveries made by modern cosmology this progression has reached an end.
Let’s do a thought experiment in order to understand the map of the universe as seen in modern cosmology. At the beginning of the Big Bang space and time as we know it have just been created. Therefore light has had no time to travel anywhere. We can imagine ourselves at some point in this universe looking out into total darkness in all directions. As time goes on, light has a chance to travel to us from other places. However, to begin with all of the matter in the universe is so hot and so dense that all you can see is the stuff right in front of your face. No photons can travel very far without bumping into something else and getting redirected. Basically, it’s thick as pea soup. At about 400,000 years after the big bang things have cooled down and thinned out enough that photons can travel on their merry way without getting bumped around to much. This is the age at which the universe becomes transparent. Now with each passing second light is able to travel to us from further and further away. Imagine a sphere. We are at the center and the edge represents the distance that we can see – the distance that light could have travelled in that amount of time. As time goes on, the edge of that sphere is moving outward at the speed of light and more and more of the universe comes into view. Cosmologists call the edge of this sphere the cosmological horizon. At the horizon we can still actually see that dense pea soup, because the light from the pea soup is just getting to us from those distant locations. This pea soup light is called the cosmic microwave background radiation, and is another key evidence that supports the big bang theory. Now we can imagine today’s universe where the horizon has receeded to the extreme distance. With today’s space-based telescopes we are actually able to see all of the way out to the cosmological horizon. It is the same distance away from us in all directions. Thus, we are in the center of the observable universe simply because observations are limited by light travel time. So far as we know, the universe extends to infinity in all directions. However, we will never be able to see anything outside of our horizon. In fact we are at a special time in the universe’s history where it is starting to expand faster and faster. Because of this the horizon will eventually begin to shrink and the observable universe will get smaller and smaller. So at this point in the life of the universe we’re able to see about as much as any astronomer will ever be able to see.
There’s actually a nice movie that depicts the current map of the observed universe. The rainbow colored sphere at the end is the cosmic microwave background and we are at the center of the sphere. Also the strange pie-shaped structures aren’t real structures but are just the parts of the universe that we’ve observed so far. There are more details and a newer movie with more galaxies and no narration here.
-Matt, the elder brother