Alternative Cosmologies

There is mounting, if not overwhelming, evidence that the cold dark matter (CDM) model provides the most accurate description of our Universe. Observations point towards a Universe comprised of 28% dark matter and 68% dark energy, with luminous baryonic matter (i.e. galaxies, stars, gas, and dust) at a mere 4%.

There are several, independent observational pillars supporting this picture. These include:

This so-called concordance model is based on hierarchical structure formation, whereby small objects form first and subsequently merge to form progressively larger objects.

While generally successful, the CDM model does face several problems. On scales larger than galaxies, the predictions obtained from N-body simulations of CDM models are in very good agreement with observations. But when we turn and look inside galaxies (i.e. to investigate the internal properties of them), discrepancies between theory and observations become apparent. For example, it is predicted that one to two orders of magnitude more satellite/dwarf galaxies should be orbiting within galactic halos than are actually observed. Moreover, N-body simulations also predict far more mass inside the very central parts of galaxies than their rotation curves would allow for. This situation is quite surprising, as dark matter models were designed to match observational and theoretical rotation curves of galaxies (but only in the outer regions of the galaxy).

The so-called ‘CDM crisis’ has led to a vast number of publications trying to solve the problems which seem to be associated with an excess of small scale structures. One possibility is to introduce warm dark matter. Alternatives include making dark matter self-interactive, fiddling with Newtonian dynamics or simply decreasing its ‘power’ on certain scales. All these methods have advantages and shortcomings, meaning that while they solve at least some of the problems, they also introduce new ones.

It is important to note that these problems are mainly associated with computer simulations that only deal with dark matter. However, astrophysics is based on observing photons emitted due to atomic processes. Only when these processes, and the physics governing them, are included in the theoretical modeling (i.e. the N-body simulations) will we have a fair chance of understanding how the Universe formed and evolved. Not only do we need to model dark matter and dark energy, but baryonic processes also have to be included in any cosmological simulation if there is to be a reasonable hope that it will capture the essence of galaxy formation. Since the problems outlined above operate on scales where baryonic physics might dominate, they may disappear once the modeling improves.

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