What Expansion of Space Means
It might seem obvious that expanding space is simply objects such as stars moving away from each other. This assumption, however, is not correct. The expansion of space is not like an explosion on Earth, for example, where pieces fly apart and the volume of debris expands while the underlying space stays constant. Space is measured as a metric tensor that changes over time (to understand what a tensor is, see the article Gauge Theory where it is explored). This means that distance is not a fixed measurement. It's not that stars are moving at all; space is expanding in between them. If a tape measure could be strung between two stars that are stationary with respect to each other, the actual markings on the tape measure would spread out, the scale would change in other words, showing the growing space between them. This is the subtle but often overlooked metric nature of space.
The following 5.5 minute video called "What is the Universe Expanding Into?" is by Deep Astronomy:
Two periods of accelerated expansion mark the history of the universe. The first acceleration, cosmic inflation, took place at an astonishing rate. Some models suggest that the universe doubled in size every 10-35 seconds. The second, current, acceleration is even more stupendous. The volume may now be doubling at a rate about 50 times higher than that of cosmic inflation.
No one knows why the expansion rate is increasing. This is one of the most fundamental puzzles in physics today, and it is where the dark energy story begins.
One thing is certain: The universe is far more mysterious to us today than it has ever been. Twenty years ago, when we gazed at the night sky with our best telescopes, we saw the vastness of distant stars, galaxies and glowing nebulae, and we thought that, with the exception of dark material such as planets, brown dwarfs, non-glowing gases and black holes, we could see everything out there. Dark matter and dark energy tell us that the visible universe is a mere fraction of what really exists.
Up next: Dark Energy Part 3.