Space, Time, and the Beginning

The most common questions asked by students about the big bang are "what came before the big bang?" and "where did the explosion happen?" The basic answer is that these questions are without very well defined meaning!

Remember that in general relativity space and time are not a passive stage on which events take place, but are themselves shaped by energy and mass. The most consistent interpretation of the big bang is that space and time as we experience them were themselves created in the big bang. But if there was no time and space, the very words "before" and "where" lose their meaning! Indeed, if you analyze the preceding statements the word "created" itself is a problem. In normal usage, "creation" implies that something did not exist before a certain time, landing us in the linguistic predicament already noted--normal language seems to fail us in addressing these questions, which are (somewhat) better defined in the language of mathematics.

The Big Bang

All current evidence points to an early time when the Universe was much hotter and much denser than it is now.
Extrapolating Back in Time
If the properties of the Universe are extrapolated back in time, we come to a point 10-20 billion years ago (depending on the exact value of the Hubble constant) when the part of the Universe that we can presently see (the observable universe; see the discussion below) was concentrated at a point of infinite or nearly infinite density and temperature. This leads to the idea that the Universe sprang from a state of incredibly high temperature and density in a gigantic "explosion" some 10-20 billion years ago, and that the present expanding Universe is just the aftermath of that cataclysm. This "creation event" for the Universe is called the big bang. We place the word "explosion" in quotation marks because, as described in the top right box, the big bang was not an explosion in the usual sense. Since space and time were created in the big bang, the big bang cannot be thought of as an explosion happening in space and time. Nevertheless, we shall speak of it loosely as an explosion.

Early Radiation Domination
The big bang starts off with a state of extremely high density and pressure for the Universe. Under those conditions, the Universe is dominated by radiation. This means that the majority of the energy is in the form of photons and other massless or nearly massless particles (like neutrinos) that move at light speed or close to it. As the big bang evolves in time, the Universe expands, the temperature drops rapidly, the average velocity of particles decreases, and the growing volume of the Universe means that the vacuum or dark energy becomes increasingly important (see the discussion of cosmology in this chapter).

Later Matter Domination
Finally, one reaches a state where the energy of the Universe is primarily contained in dark energy and nonrelativistic matter (matter sufficiently massive that its average velocity is very much less than the speed of light). This is called a dark energy and matter dominated universe. The early Universe was radiation dominated, but the present Universe is dominated by matter and dark energy. Let us now give a brief description of the most important events in the big bang and how the Universe evolved from an early, hot, radiation-dominated phase to a later, cooler, matter and dark energy dominated one.

The Observable Universe
In the subsequent discussion, it is important to distinguish the "Universe" from the "observable Universe". The distinction comes because the Universe has existed for a finite time and because light has finite speed. The adjacent animation illustrates.

The part of the Universe that we can now see (the "observable Universe") is limited to a sphere around the Earth with radius equal to the maximum distance from which light could have reached us since the big bang. This radius defines our horizon. The Universe itself is presumably much larger (perhaps infinite), but we can't see beyond the horizon. Note, however, that the horizon grows with time, so the size of our observable Universe also grows with time. As the Universe evolves, our horizon encompasses ever larger portions of it.