It seems like every few weeks that we hear news of a new study of the early universe showing that black holes formed earlier than expected and that structure in the pattern of galaxies extends to larger distances than expected.
The ruling paradigm says that galaxies formed when hydrogen gas and dark matter slowly clumped together under its gravitational pull. Stars were formed which continued to collapse togther to form galaxies. The early stars which were large would die quickly and form black holes which would coallesque to form supermassive black holes at the centres of galaxies.
The process was seeded by density perturbations in the gas that existed at the time of last light scattering. The effects of these perturbations are seen in the cosmic microwave background and are very familiar to cosmologists. They are believed to be due to fluctuations during the inflationary epoch and they have the right scale invariant spectrum to fit that hypothesis. You can model the formation of galaxies and large scale galactic structure in cold dark matter models using computer simulations. With the right parameters set for the mass of dark matter particles you can get good agreement with observations.
But the agreement is not good enough. It predicts that the black holes form after the stars, yet we see quasars appearing in the early universe containing huge black holes that must have formed much earlier. The latest example is a quasar with a mass of 2 billion suns observed at just 770 million years after the big bang by ESO’s Very Large Telescope. We have seen proto-galaxies and gamma ray bursts from even earlier.
We also observe structure in the distribution of galaxies that extends out to very large scales. This is not predicted by the cold dark matter theory of structure formation. An example is the Great Sloan Wall, a vast planar structure covering 5% of the size of the observable universe. Up to these scales the distribution of galaxies forms clusters and filaments as well as voids separated by these walls. How could these have formed so soon and so big?
One possible answer is that they did not form through gravitational collapse at all, but instead by a process of caustic focusing of dark matter by gravitational waves. Let me explain.
We know very little about how the inflationary epoch ended. The vacuum state would have changed as the inflationary scalar field dropped into a broken phase. There may have been a phase transition but it may have been a soft second order transition or even a smooth crossover. We don’t even know when it happened. It may have been the elctro-weak transition or something earlier. With new physics from the LHC we may be able to work out how it happened.
It is likely that the transition did not happen simultaneously at all points in space. Fluctuations would mean that inflation continued a little longer in some places than others. This would leave a remnant gravitational wave background in the universe which in time would have cooled and weakened as the universe expanded more slowly. It would be hard to detect directly today because of its very low frequency and weak amplitude, but in the early universe during baryogenesis it would have been stronger.
The effect on baryonic matter would however have been washed out by electromagnetic forces acting more strongly than anything these waves could do. Dark matter on the other hand is uncharged and only interacts weakly. The gravitational waves, if strong enough could have influenced the distribution of dark matter. This is more true if dark matter particles are heavy so that they move more slowly at a given temperature. So what would happen?
In fact heavy particles would follow geodesics through the gravitational waves which would focus them onto caustic surfaces. The process is very similar to the focusing of light through the waves on the surface of the sea of a swimming pool creating familiar patterns of light on the bottom. The caustic lines are replaced with surfaces stretching across the universe just like the ones seen in the Sloan Survey. Where the walls meet even denser concentraions of matter would form.
Caustic light patterns formed by water waves quickly shift to disappear and reform elsewhere, but when enough dark matter is concentrated into one place it will itself gravitate and form dark stars or black holes which lock in the pattern. This could have happened very early in the universe, possibly even before the cosmic radiation background last scattered off hot baryonic matter.
The ordinary stars would form around these structures either by gravitational attraction or due to the pressure of radiation from dark stars and gas falling into the black holes. Either way the structure in the distribution of galaxies would be largely determined by the caustic patterns provided by the gravitational waves so it can extend much further depending on the spectrum of the waves at large wavelengths. The large black holes that form quasars would originate from concentrations of dark matter at the densest points where the caustic planes meet.
Even as more discoveries appear to contradict the ΛCDM theory cosmologists stick to the old paradigm because it almost works. Λ, the cosmological constant is there and so is Cold Dark Matter, but that does not mean that they explain the formation of super-massive black holes and large scale structure in the universe. Cosmologists need to wake up to this fact and start exploring alternatives such as the caustic theory outlined here. As usual I will quietly wait while they ignore it and eventually reinvent the idea for themselves. Good luck guys. 🙂