Lesson Objectives:- Structure formation
- Accelerating expansion of the universe
Gravitationally bound systems refer to star systems, galaxies, and clusters of galaxies where their gravity is strong enough to hold them together while the universe as a whole is expanding.
As we have discussed previously, these galaxies probably formed from slight density enhancements that were present in the early universe. Gravity due to dark matter would have been the main force that pulled matter together in these regions of enhanced density until they became protogalactic clouds, which eventually became galaxies and galaxy clusters.
There are signs that clusters of galaxies may even be pulling on each other to form larger gravitationally bound systems known as superclusters.
Looking at three-dimensional maps of the distribution of galaxies in space have revealed the existence of large-scale structures much larger than clusters or superclusters -- over a billion light-years across. These structures which are arranged in chains and sheets of galaxies surround great empty regions called voids. Such large-scale structures most likely mirror the original distribution of dark matter in the early universe.
There are four different models for how the size of the universe is changing: the recollapsing universe, critical universe, coasting universe, and accelerating universe models.
By studying the recession speeds of galaxies at different distances, astronomers can determine whether the expansion of the universe has been speeding up or slowing down over time. Observations from such studies have matched up best with the accelerating universe model, indicating that the expansion is getting faster with time.
This has led scientists to believe that there is a mysterious form of energy called dark energy that is acting as a repulsive force to cause the universe to expand more quickly.
The overall flat geometry of the universe also acts as evidence for the existence of dark energy, since the amount of normal matter and dark matter in the universe would not be enough to maintain that flat geometry, according to Einstein's general theory of relativity. Many scientists now believe that 70% of the total mass-energy of the universe takes the form of dark energy.