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17.12 Evolution of Structure

The early universe must have contained the "seeds" for today's large-scale structure. After all, if the universe were perfectly smooth and homogeneous, we would not exist! The contrast between the early universe and the present day is striking. The cosmic background radiation dates from just 380,000 years after the big bang, yet it is almost perfectly smooth. By contrast, matter today is clumped into stars and galaxies, with virtually nothing in between. Astronomers expected that the cosmic background radiation would show tiny departures from isotropy, representing the first irregularities out of which galaxies and stars and planets and people would eventually form. So it was with great excitement, and more than a little relief, that scientists associated with the COBE satellite announced the discovery in 1992 of microwave background fluctuations. These tiny variations amount to only 70 millionths of a degree between different parts of the sky. In general, the microwave background is almost perfectly smooth, but with a highly magnified temperature scale, departures from isotropy can be seen. If the inflationary scenario for an exponential growth of the infant universe is correct, the seed for galaxies are quantum fluctuations that were amplified by inflation!


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The Cosmic Microwave Background, mapped from WMAP. Click here for original source URL


These tiny radiation ripples were the starting points for galaxy formation. How long did it take for galaxies to form? The microwave background dates from a red shift of z = 1000, when the universe was a thousandth of its current size. Galaxies have been found at red shifts as high as z = 7. The time between z = 1000 and z = 7 depends on the cosmological model, but is typically 500 million years. This figure is the amount of time available for the transition from shimmers in the primeval fireball to the stark beauty of spiral and elliptical galaxies.

Galaxy formation is one of the most hotly debated topics in modern cosmology. Why is a galaxy the basic mass unit of the universe? Why is space not filled with planets, or grains of dust, or objects much larger than galaxies? Luckily, physics give us some guidance. The temperature fluctuations of the microwave background are signs of matter beginning to clump. The slightly hotter and cooler regions on the sky represent radiation that is slightly blue shifted and slightly red shifted. Regions of slightly higher and lower density were interspersed. Denser regions had stronger gravity and so attracted nearby material. As the collapse began, it was resisted by higher pressure within the compressed region — the same tussle between gravity and pressure that governs the life history of a star. An object can form only if gravity wins the battle. At a time 380,000 years after the big bang, the size of the region where gravity and pressure were just balanced contained about 100,000 times the mass of the Sun. Lumps smaller than this could not form. There were no lumps larger than about a trillion times the mass of the Sun, because a gas cloud this size could not cool enough to collapse, and radiation kept it puffed up. This mass range — 105 to 1012 solar masses — is just the mass range of present-day galaxies!

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Map of voids and superclusters within 500 million light years from Milky May. Click here for original source URL.

Clusters and super clusters of galaxies range all the way up to 1017 solar masses. Which formed first, galaxies or these larger structures? Not surprisingly, the answer depends on the nature of the dark matter. We do not know what type of particle the dark matter is made of. However, if dark matter particles move quickly, close to the speed of light, then they would have "washed out" anything smaller than a super cluster when structures started forming. Galaxies would have formed later, by fragmentation within the super cluster. This scenario is called top-down structure formation and it is based on hot dark matter (HDM). On the other hand, if dark matter particles move slowly, then galaxies would have formed first, with larger and larger structures forming later. This scenario is called bottom-up structure formation, and it is based on cold dark matter (CDM).

Which scenario is correct, a universe filled with hot or cold dark matter? Astronomers believe that clusters are considerably younger than the galaxies they contain. As a cluster evolves it becomes more and more symmetrical; the irregular shapes of some clusters suggest that they are very young. On the largest scales, super clusters may only just now be forming. This is good indirect evidence that dark matter is cold.

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Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. Click here for original source URL.

No set of equations can describe the rich structure of the universe. The most powerful way to test the models is to create a "universe in a computer." Just as it is possible to simulate a galaxy by calculating the gravity of many stars, it is also possible to simulate the universe by calculating the gravity of many galaxies! In the simulations, a galaxy is treated as a point, and the gravity among many galaxies is calculated according to Newton's law of gravity. The volume described by the calculation is steadily increased to represent the expanding universe. The enormous power of super computers allows hundreds of millions of calculations to be carried out each second. Beautiful structures emerge within the computer model. These simulations give us an important clue about dark matter: models with cold dark matter particles give a better match to the clustering of the observed universe than models with hot dark matter particles. The simulations also reproduce the rich texture of clusters, voids, and filaments that we see in the real universe. The simulations have built into them the cold dark matter that causes structure to form and the dark energy that is making the universal expansion accelerate. The consensus model is called lambda-CDM or ΛCDM, where the Λ refers to Einstein's original cosmological constant.