Abstract
In the standard cosmological model, the Universe consists mainly of two invisible substances: vacuum energy with constant mass-density (where is a ‘cosmological constant’ originally proposed by Einstein and is Newton’s gravitational constant) and cold dark matter (CDM) with mass density that is currently . This ‘CDM’ model has the virtue of simplicity, enabling straightforward calculation of the formation and evolution of cosmic structure against the backdrop of cosmic expansion. Here, we review apparent discrepancies with observations on small galactic scales, which CDM must attribute to complexity in the baryon physics of galaxy formation. Yet, galaxies exhibit structural scaling relations that evoke simplicity, presenting a clear challenge for formation models. In particular, tracers of gravitational potentials dominated by dark matter show a correlation between orbital size, , and velocity, , that can be expressed most simply as a characteristic acceleration, kmspc cm s, perhaps motivating efforts to find a link between localised and global manifestations of the Universe’s dark components.
Highlights
Gravity regulates the Universe’s expansion and the growth of its structure
Dark matter reduces to a fluid made of ‘cold’, ‘collisionless’ particles that form with negligible velocity dispersion and avoid non-gravitational interactions, letting cold dark matter (CDM) particles clump together gravitationally on small—i.e., subgalactic—scales
The right-hand panel is generated from real observational data and shows the spatial distribution of luminous stars observed in a recent survey of the Andromeda galaxy [22], which is similar to the Milky Way in terms of size, luminosity and mass and—unlike the Milky Way—offers Earthlings an external view
Summary
Gravity regulates the Universe’s expansion and the growth of its structure. Observations of both phenomena reveal accelerations that cannot be attributed to classical gravitational fields sourced by known particles. The current cosmological paradigm is built on the hypothesis that both substances take extremely simple forms, such that their influence on cosmic evolution can be calculated given the values of a few quantities that specify initial conditions [7,8,9] Dark matter reduces to a fluid made of ‘cold’, ‘collisionless’ particles that form with negligible velocity dispersion and avoid non-gravitational interactions, letting CDM particles clump together gravitationally on small—i.e., subgalactic—scales. It is this ability that distinguishes CDM1 astrophysically from alternatives like ‘warm’, ‘hot’, or ‘self-interacting’ dark matter. We review efforts to find such a scale and discuss possible implications for the ΛCDM model
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