Formation of dark matter structures

In summary, dark matter can form structures from an initial near uniform distribution through the amplification of initial density fluctuations and the gain of kinetic energy from gravitational potential energy. Unlike ordinary matter, dark matter cannot lose energy through electromagnetic interactions, but can do so through gravitational radiation and momentum transfer between particles. On very large scales, structures do not collapse due to the use of linearized gravity, but instead settle into stable orbits. However, on smaller scales, gravity and dark energy play a more complex role in the evolution of structures.
  • #1
andrew s 1905
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TL;DR Summary
I can't understand how dark matter can form structures from the initial near uniform distribution.
So my question is what physics allows this to happen?
I can't understand how dark matter can form structures from the initial near uniform distribution as shown in simulation like those of the Eagle project http://icc.dur.ac.uk/Eagle/index.php. I understand how the initial density fluctuation are amplified and dark (and ordinary) matter start to collapse However, unlike ordinary matter which can lose energy via electromagnetic interactions, dark matter will gain kinetic energy from the gravitational potential energy of the initial distribution but this cannot be dissipated and so should in my naive view oscillate about the local center of mass.

This must be wrong but what physics am I missing in understanding this. I have tried various google searches but not found a solution. Any pointers would be most welcome.

Regards Andrew
 
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Dark matter can lose energy by gravitational interaction via the emission of gravitational radiation. Given that the gravitational interaction is much weaker than the electromagnetic, This is a much slower process. Also, gravitational interactions between DM particles can transfer momentum from one to another( much like a gravitational slingshot does), which can also aid in forming structures.
 
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Thanks, for the replies. I think I have worked it out. My assumption about the potential energy was wrong.
Regards Andrew
 
  • #5
One interesting thing about this problem is that on very large scales, structures don't collapse. This derives from using linearized gravity to estimate how structures evolve over time. This fact probably matches more with your original intuition: that conservation of energy would prevent collapse.

So how can there be these huge structures at all if they don't collapse? The answer is pretty simple: at some point, the rate of expansion was slow enough and the systems were dense enough that they settled into stable orbits instead of continuing to expand. Meanwhile, the overdense regions continue to get further apart and the rest of the universe gets less dense. So the density of these large regions is sort of "baked in" by how long ago they stopped expanding, with some settling into stable configurations younger than others.

That said, gravity isn't a linear theory, so this approximation breaks down as you look at smaller structures (such as galaxy clusters and individual galaxies). This leads to more complicated evolution. And dark energy also changes things so that the gravitational potentials of these large regions aren't actually constant.

Finally, note that my description is compatible with the one that Keith posted above. It's just described from a different perspective.
 
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1. What is dark matter and how does it form?

Dark matter is a type of matter that does not interact with light or other forms of electromagnetic radiation, making it invisible to telescopes. Its formation is still not fully understood, but it is thought to have originated from the early universe and has since clumped together to form structures such as galaxies and galaxy clusters.

2. How do scientists study the formation of dark matter structures?

Scientists use a variety of methods to study the formation of dark matter structures, including observations from telescopes, computer simulations, and particle accelerators. These methods allow them to study the effects of dark matter on the visible matter in the universe and make predictions about its behavior.

3. What is the role of dark matter in the formation of galaxies?

Dark matter is thought to play a crucial role in the formation of galaxies. Its gravitational pull helps to pull together gas and dust, which eventually form stars and galaxies. Without dark matter, galaxies as we know them would not be able to form and evolve.

4. How does dark matter affect the evolution of the universe?

Dark matter is believed to have a significant impact on the evolution of the universe. Its presence helps to shape the large-scale structure of the universe, influencing the distribution of galaxies and clusters. Dark matter also affects the expansion of the universe, as its gravitational pull slows down the rate of expansion.

5. Is there more than one type of dark matter?

Currently, there is only one type of dark matter that is widely accepted by scientists, known as Cold Dark Matter (CDM). However, there are some alternative theories that propose the existence of other types of dark matter, such as Warm Dark Matter (WDM) or Self-Interacting Dark Matter (SIDM). Further research is needed to fully understand the nature of dark matter and its possible variations.

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