MadSci Network: Astronomy

Re: Is dark energy responsible for the distribution of galaxies?

Date: Tue Jun 19 21:25:21 2001
Posted By: Juan Cabanela, Faculty, Physics, Astronomy, & Engineering Science, Saint Cloud State University
Area of science: Astronomy
ID: 990661759.As

One warning in this answer, I am not a cosmologist, I will be giving my best answer based on my current understanding of the field, but things have been changing rapidly in the last few years...I may not be completely up to date. Now my answer...

This is an interesting idea, but as you point out, we are not quite clear on what "dark energy" is. "Dark Energy" is a term that is best described by saying it is an "energy source" for driving the accelerating expansion of the universe against the self-gravity (which should be causing the expansion of the universe to slow down). I've heard various candidates for "dark energy" but in the end they usually are ascribed to some quantum vacuum energy.

Large-scale structures such as the walls and voids observed for the distribution of galaxies are typically explained by assuming that there were some primordial perturbations of the originally smooth mass density distribution of the universe (very early on, during inflation). These density perturbations will be unstable against self-collapse if they exceed a certain size (called a Jean's mass). The large-scale structure we see today is at about the level we would expect given the temperature fluctuations (which can be tied to mass density perturbations) seen in the Cosmic Microwave Background (the famous COBE results) if we allow for the presence of dark matter. There are details to be worked out about the nature of this dark matter (hot dark matter tends to create only large structures, cold dark matter concentrates power in small structures), but the formation of the large scale structure seems fairly well explained by such scenarios. In other words, there doesn't appear to be a need for any terribly fancy physics to explain how the structures given what we the perturbations we see present at the time the cosmic microwave background was created (roughly 300,000 years post-Big-Bang).

However, there was a similar model to yours in the past. It is suspected that very early stars could have been extremely massive due to their being composed only of hydrogen and helium (and very few more massive atoms). Ostriker and Cowie proposed in 1981 that these massive stars could have detonated and the shockwave would have swept up material into bubble-like structures. The major problem is that there is simply no known way for these super "supernova" to sweep up supercluster masses worth of material into the structures seen today. You appear to repair that by providing dark energy as a source, assuming it is "centered in voids". However, I think most candidate sources of "dark energy" are not centered on small structures but rather the "dark energy" is a uniformly distributed energy.

Now for the true speculation, I can imagine that the presence of a uniform "dark energy" would dramatically affect the formation of large-scale structure. It's presence would accelerate the expansion of the universe which should enhance the density perturbations initially present and allow for quicker formation of large-scale structure. In other words, I don't think any of the "dark energy" candidates would form large-scale structure, but they certainly could, by accelerating universal expansion, aid in its creation all over the universe.

[Moderator's Note: Despite Dr. Cabanela's disclaimer, he is quite correct! There are some models like Ostriker and Cowie's that postulate that explosions could seed structure, but these have fallen out of favor because the structures they predict do not correspond to observations. The basic problems with a picture in which the dark energy comes from specific locations is that there's no motivation for how that could happen and because all the other observations mentioned above (such as COBE) are very nicely explained by the traditional, uniformly distributed dark energy. But your idea could still be a nice way to think about what's going on!]

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