Created by: Joseph Silk (Click to see a larger image: SOURCE)


I found a nice article that delved into one of the problems of the Cold Dark Matter Model and galaxy formation. Ultimately the article discusses the way in which feedback resolves all of the problems with galaxy formation in the early universe and what we see in our current universe.


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When using the CDM model, we’ve seen that feedback is definitely required when doing simulations for us to get an accurate rendition of the universe that we see today. it is apparent that Dark Matter of some description was required to help the first stars form. These first stars formed in mini dark matter halos of about 10^5 solar masses. Simulations have shown that the stars has to be formed in Cold Dark Matter halos and not Hot Dark Matter halos, as in many of the simulations that involved Hot Dark Matter (HDM) resulted in situations in which, nearly 14 billion years after the big bang, we still did not have any stars form. The basic reason for this is that Cold Dark Matter (CDM) consists of slow moving particles that clump together due to gravity. HDM, however, moves near or at the speed of light, so it can actually have the opposite effect and disrupt the formation of stars by creating gravitational turbulence within any star forming region.


Using the CDM simulations more or less perfectly demonstrates what we see in the universe at large scales. However, on the smaller scales there are some issues. He states: “All studies to date produce too many small galaxies, too many big galaxies in the nearby universe, too few massive galaxies at high redshift, and too many baryons within the galaxy halos. In addition there are structural problems: for example, massive galaxies with thin disks and/or without bulges are missing, and the concentration and cuspiness of cold dark matter is found to be excessive in barred galaxies and in dwarfs.”


Although I’m not going to go through the whole article, I thought I’d look specifically at the formation of the first galaxies in the mini dark matter halos. At this point in the universes’ early history, there wasn’t likely to be feedback from Supermassive Black Holes; however, at the time of the formation of the first stars and the first galaxies (200-500 million years after Big Bang), there were a lot of very powerful supernova going off that this time.


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What we can consider then is that not all of the dark matter halos were as small as 10^5 solar masses, we just have to think of this as being the minimum mass for them and expect that some of them would range between 10^5-10^10 solar masses. After the first flurry of supernova bursts, dwarf halo’s with masses between 10^5-10^8 are thought to be disrupted and halt star formation. There is a bit of scientific backing to the argument in that ultra faint objects have been found in the Milky Ways Galactic Halo. Score one for feedback.

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