It is a 3D timeline from the big bang to now. The diameter represents the size of the universe or more precisely the scale factor of the universe. It is hard to imagine but the universe did not start from a single point and explode into growing ball of gas. I know that’s what they imply when you listen to the TV pop science shows but the universe is everywhere, right? There is no center either. Its just that space happened all at once with all its bits in several dimensions and it started to expand in all those dimensions all at once.
The rate of expansion is illustrated in this picture, really fast at the beginning and then it slowed down. Now, as we have recently learned, it is starting to speed up again. So what does the diameter of the bell illustrate? In the beginning the bits of the universe were close together, now they are further apart. The diameter of the bell is proportional to how far each bit is from the next. This is called the scale factor. The outward expansion is just the scale factor growing at an accelerating rate. If nothing else changes things the bell will look like a trumpet pretty soon.
Something else cool about this picture is that it shows what we see in the WMAP and now the Planck images, the early microwave glow of the big bang. Using the satellites we capture photons from 13 billion years ago so they see what things looked like back then. The amazing thing about the image is that it is so uniform. In an explosion you’d expect to see billows, and clumps and gaps. But we don’t see much of that in the cosmic microwave background (CMB). This means the universe must have expanded very quickly indeed. Maybe faster than the speed of light (in fact almost certainly). That is why the first part of the bell gets big so quickly, the scale factor was growing so fast that each bit of the universe was moving away from every other at faster than the speed of light. The bits weren’t moving, it was space getting bigger, fluffier.
The interesting implication of this faster than light expansion, physicists call it inflation, is that some matter may be out there that we cannot see, past the edge of the observable universe. And that is the subject of the previous post, dark flow caused by ominous clumps of stuff outside our ability to see.
That is round about 390 words so there must be more in the picture but it is late and I will close with the other 610 left unwritten—for now.