The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [80]
A pencil mark drawn long ago on your child’s wall records how tall she was at the date specified. A series of pencil marks gives her height at a series of dates. Given enough marks, you can determine how quickly she was growing at various times in the past. A growth spurt at nine, a slower period until eleven, another rapid spurt at thirteen, and so on. When astronomers measure a Type Ia supernova’s redshift, they’re determining an analogous “pencil mark” for space. Much like your child’s height marks, a series of such redshift measurements of various Type Ia supernovae would enable them to calculate how quickly the universe was growing over various intervals in the past. With those data, in turn, the astronomers could determine the rate at which the expansion of space has been slowing. That was the plan of attack laid out by the research teams.
To execute it, they would have to complete one remaining step: dating the universe’s pencil marks. The teams needed to determine when the light from a given supernova was emitted. This is a straightforward task. Since the difference between a supernova’s apparent and intrinsic brightness reveals its distance, and since we know light’s speed, we should be able to immediately calculate how long ago the supernova’s light was emitted. The reasoning is right, but there is one essential subtlety, to do with the “post-facto” stretching of light’s trajectory mentioned above, that’s worth emphasizing.
When light travels in an expanding universe, it covers a given distance partly because of its intrinsic speed through space, but partly also because of the stretching of space itself. You can compare this with what happens on an airport’s moving walkway. Without increasing your intrinsic speed, you travel farther than you otherwise would because the moving walkway augments your motion. Similarly, without increasing its intrinsic speed, light from a distant supernova travels farther than it otherwise would because during its journey the stretching space augments its motion. To judge correctly when the light we now see was emitted, we must take account of both contributions to the distance it covers. The math gets a little involved (see the notes if you are curious), but it is by now thoroughly understood.7
Being careful about this point, as well as numerous other theoretical and observational details, both groups were able to work out the size of the universe’s scale factor at various identifiable times in the past. They were able, that is, to find a series of dated pencil marks delineating the universe’s size, and therefore to determine how the expansion rate has been changing over the history of the cosmos.
Cosmic Acceleration
After checking, and rechecking, and checking again, both teams released their conclusions. For the last 7 billion years, contrary to long-held expectations, the expansion of space has not been slowing down. It’s been speeding up.
A summary of this pioneering work, together with subsequent observations that cinched the case even more tightly, is given in Figure 6.2. The observations revealed that until about 7 billion years ago, the scale factor did indeed behave as expected: its growth gradually slowed down. Had this continued, the graph would have leveled off or even turned downward. But the data show that at about the 7-billion-year mark, something dramatic happened. The graph turned upward, which means that the growth rate of the scale factor began to increase. The universe kicked into high gear as the expansion of space started to accelerate.
Figure 6.2 The scale factor of the universe over time, showing that cosmic expansion slowed down until about 7 billion years ago, when it began to speed up.
Our cosmic destiny turns on the shape of this graph. With accelerated expansion, space will continue to spread indefinitely, dragging away distant galaxies ever farther and ever faster. A hundred billion years from