Why is There So Much Empty Space in Space?

09/13/2017 03:04 pm ET
DigitalVision/Getty Images
DigitalVision/Getty Images

After the Big Bang, galaxies should have been equally distributed through space evenly, so what caused cosmic voids? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by Viktor T. Toth, IT pro, part-time physicist, on Quora:

After the Big Bang, there were no galaxies at all. Just very homogeneous matter, with only minor density fluctuations (imperfections).

So in some places, matter was ever so slightly more dense, in other places, less dense. Where matter was more dense, it had more gravity, causing it to clump a little more. Conversely, where matter was less dense, there was less gravity, so less clumping.

Over time, these clumps grew. They grow on characteristic scales, determined not just by gravity, but other material properties of the gas. There is an entire body of theory devoted to this in cosmology, starting with the Newtonian theory of small fluctuations.

The result is that the clumping manifests itself on characteristic scales. Galaxies come in certain typical sizes. So do clusters of galaxies. The statistical distribution of this can be analyzed, plotted, and compared against theoretical predictions. This is the so-called “matter power spectrum.” Here is an example, plotting data from various measurements against the theoretical prediction:

Without going into confusing detail, the horizontal axis in this plot represents various characteristic scales, whereas the vertical axis shows how likely or how unlikely it is for galaxies to appear at corresponding distances from each other. The solid red line represents the prediction of the standard cosmological theory, whereas the crosses (data points with horizontal and vertical error bars) are actual measurements.

So no, cosmic voids are not full of dark matter or dark energy. They are, in fact, genuine voids, i.e., under-dense regions of our universe, exactly as the theory predicts.

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