Lesson 61: Structure Formation
In this lesson,
we will look at the role dark matter played in the formation of structure in
the universe. We will also learn about dark energy and its role in the
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.