Dark matter, dark energy, and alternate models: A review
Introduction
One of the most unexpected revelations about our understanding of the universe is that the universe is not dominated by the ordinary baryonic matter, but instead, by a form of non-luminous matter, called the dark matter (DM), and is about five times more abundant than baryonic matter (Ade et al., 2014). While DM was initially controversial, it is now a widely accepted part of standard cosmology due to observations of the anisotropies in the cosmic microwave background, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements from galaxy clusters.
Another unresolved problem in cosmology is that the detailed measurements of the mass density of the universe revealed a value that was 30% that of the critical density. Since the universe is very nearly spatially flat, as is indicated by measurements of the cosmic microwave background, about 70% of the energy density of the universe was left unaccounted for. This mystery now appears to be connected to the observation of the non-linear accelerated expansion of the universe deduced from independent measurements of Type Ia supernovae (Riess et al., 1998, Perlmutter et al., 1999, Peebles and Ratra, 2003, Sivaram, 2009).
Generally one would expect the rate of expansion to slow down, as once the universe started expanding, the combined gravity of all its constituents should pull it back, i.e. decelerate it (like a stone thrown upwards). So the deceleration parameter was expected to be a positive value. A negative would imply an accelerating universe, with repulsive gravity and negative pressure. And the measurements of Type Ia supernovae have revealed just that. This accelerated expansion is attributed to the so-called dark energy (DE).
There are several experiments to detect postulated DM particles running for many years that have yielded no positive results so far. Only lower and lower limits for their masses are set with these experiments so far. The motto seems to be ‘absence of evidence is not evidence of absence’. But if future experiments still do not give any clue about the existence of DM, one may have to consider looking forward for alternate theories (Sivaram, 1994a, Sivaram, 1999).
The best example of this is that of the orbit and position of Vulcan, which was theoretically inferred from the observation of Mercury orbit (Hsu and Fine, 2005). The deviation of its orbit, as predicted by Newtonian gravity, was attributed to the missing planet (DM). But the resolution of this discrepancy came through the modification of Newtonian gravity by Einstein and not by DM. This is unlike in the case of Uranus were the prediction and discovery were successful using DM (Neptune) theory (Kollerstrom, 2001).
Section snippets
Observational evidence for dark matter
The evidence for the existence of such non-radiating matter goes back to more than eighty years ago, when Zwicky (1937) was trying to estimate the masses of large clusters of galaxies. Surprisingly it was found that the dynamical mass of the cluster, deduced from the motion of the galaxies (i.e. their dispersion of velocities), in a large cluster of galaxies were at least a hundred times their luminous mass. This led Zwicky to conclude that most of the matter in such clusters is not made up of
Dark energy
Various observations of the dynamics of the universe have implied the dominance of DE. This has led to the introduction of a repulsive gravity source to make the deceleration parameter negative (Jones and Lambourne, 2004). The dimensionless quantity, deceleration parameter q measures the cosmic acceleration of the universe’s expansion:where ‘a’ is the scale factor of the universe.
All postulated forms of matter yield a deceleration parameter (positive q), except in the case of DE.
Dark matter and dark energy
As of today, we don't know if dark matter and dark energy are manifestations of the same dark “thing”. For now, we think of them as separate entities. But the difference between the two is in the pressure exerted by them. The dark energy and cosmic repulsion is associated with negative pressure, given by:Quantum vacuum energy exerts a negative pressure, contributing a cosmological constant term to gravity. But both ordinary matter (atoms, molecules, and photons) and dark matter exert
Alternate models to dark matter and dark energy
As mentioned earlier, if future experiments still do not give any clue about the existence of DM, one may have to consider looking forward for alternate theories to DM. These alternatives range from modification of Newtonian dynamics and modification of Newtonian gravity to modifying the Einstein-Hilbert action. These models are still not complete and even in the modified scenarios; some amount of DM is still required to account for certain observations.
Summary and outlook
In this review, we have given an overview of the current understanding of Dark Matter and Dark Energy, along with the possible alternative theories. The only observational evidence we have so far is that we require some amount of DM to account for certain observations, but we do not yet understand the nature of these particles. The proposed candidates range from WIMPS and Axions to exotic particles. This review covers the entire spectrum of these DM candidates highlighting the different
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