How did the universe get to be filled with structures ranging up from galaxies to
superclusters with a delicate pattern in three-dimensional space?
We think the single force of gravity caused this to occur over ten
billion years.
The best insight on how this happens now comes from supercomputer simulations.
Pure theory gives us some ideas, but
the details are often fleshed out in a computer simulation.
The power of the computers allows us to put a billion or more particles
into the simulation and see how they would interact by gravity over cosmic time.
We're making a miniature universe in a computer, we're speeding up time,
and we're calculating the gravity between each particle and every other particle.
As you can see, this will be trillions of calculations every time the particles
move, and so this is very intensive of computation.
Really, supercomputers were the only way to do it until about ten years ago but
now desktop clusters are able to produce the same results.
These simulations,
in a virtual space inside the computer, need the key ingredient of dark matter
to produce anything like what we see in the real universe.
Simulators use various numerical tricks so
that the calculations can be done fairly quickly.
For example, since gravity is an inverse square law, it's not necessary to
calculate the gravity of every particle on every other particle in the simulation.
For a billion particles, that would be a billion squared calculations
every time there was a time step.
In practice, you can calculate the gravity from the nearest particles and
then use averaging for the more distant particles.
These various tricks and techniques have matured a lot in the last 10 or
15 years such that astronomers have quite detailed views of how structure form,
both within galaxies and on the very largest scales.
The general properties of structure formation in the universe
stem from the properties of dark matter.
Astronomers talk about dark matter as being cold dark matter,
where the word cold refers to the speed of those particles
before the first structures formed early in the universe.
Cold means that the particles were sub-relativistic.
With this property of dark matter, what happens is that the smallest structures
form first, and then grow by merging.
This is called hierarchical structure formation.
So we should expect to see the largest galaxies having
formed relatively recently.
The story can't be quite this simple, because there are some massive and
large galaxies whose ages seem quite substantial.
But we see structure formation happening this way.
We know the Milky Way's devouring dwarf galaxies, right now.
And we can see galaxy mergers taking place.
The structure formation process is non linear.
Because as gravity draws gas into a smaller region,
the collapse accelerates in a form of gravitational free fall.
And so once structure formation begins, it proceeds more and more rapidly.
In a computer simulation, using dark matter,
which is the key to how structures form since it dominates normal material,
the particles are placed in a three-dimensional volume.
The expansion of the universe, which is occurring,
is factored out because it'll be hard to watch the wave volume growing.
But it is part of the calculations.
In this simulation we watch the cube rotating so we can get a sense of
the three dimensional structures form, and we follow it from a high red shift 100
million years after the Big Bang, to the present day red shift zero.
Red shift is denoted by z in cosmology.
One of the most impressive simulations in the last decade is called the millennium
simulation, which included 10 billion particles,
the largest simulation ever attempted.
It simulates the entire visible universe and
zooming in through this simulation we see the phenomenal level of detail that can be
attained with modern super computers.
Remember, in this simulation we're only looking at dark matter,
the visible matter is like the froth on the tops of the crest of the waves
you might see on an ocean surface.
The millennium simulation spans 3 gigaparsecs, or over 10 billion light
years, a substantial fraction of the volume of the visible universe.
[MUSIC]
Chris ImpeyDistinguished Professor
