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Dark Matter: The Search for the Invisible Universe
Abstract
The quest to unravel the mysteries of dark matter, an elusive and invisible form of matter that pervades our Universe, has captivated the minds of physicists and cosmologists for decades. Despite its profound impact on the formation and evolution of galaxies and cosmic structures, the true nature of dark matter remains an enigma. This paper delves into the compelling evidence for the existence of dark matter, explores the various theoretical candidates and detection methods, and examines the ongoing search for this enigmatic substance. We discuss the observational data from galaxies, galaxy clusters, and cosmological observations that have led to the inference of dark matter's presence. Furthermore, we investigate the diverse range of proposed dark matter candidates, from weakly interacting massive particles (WIMPs) to more exotic possibilities like axions and primordial black holes. We also examine the various experimental approaches employed in the quest to detect and characterize dark matter, including direct detection experiments, indirect detection methods, and collider searches. Additionally, we explore the potential implications of dark matter for our understanding of particle physics and cosmology, and the challenges that lie ahead in unraveling this profound mystery. Ultimately, this paper aims to provide a comprehensive overview of the search for dark matter, highlighting the significant progress made thus far and the exciting prospects that lie ahead in this vital field of research.
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We present a weak-lensing mass reconstruction of the interacting cluster 1E 0657-558, in which we detect both the main cluster and a subcluster. The subcluster is identified as a smaller cluster that has just undergone initial infall and pass-through of the primary cluster and has been previously identified in both optical surveys and X-ray studies. The X-ray gas has been separated from the galaxies by ram pressure-stripping during the pass-through. The detected mass peak is located between the X-ray peak and galaxy concentration, although the position is consistent with the galaxy centroid within the errors of the mass reconstruction. We find that the mass peak for the main cluster is in good spatial agreement with the cluster galaxies and is offset from the X-ray halo at 3.4 sigma significance, and we determine that the mass-to-light ratios of the two components are consistent with those of relaxed clusters. The observed offsets of the lensing mass peaks from the peaks of the dominant visible mass component (the X-ray gas) directly demonstrate the presence, and dominance, of dark matter in this cluster. This proof of dark matter existence holds true even under the assumption of modified Newtonian dynamics (MOND); based on the observed gravitational shear-optical light ratios and the mass peak-X-ray gas offsets, the dark matter component in a MOND regime would have a total mass that is at least equal to the baryonic mass of the system. Based on observations made with ESO telescopes at the Paranal Observatories under program IDs 60.A-9203 and 64.O-0332.
We compare new maps of the hot gas, dark matter, and galaxies for 1E 0657-56, a cluster with a rare high-velocity merger occurring nearly in the plane of the sky. The X-ray observations reveal a bullet-like gas subcluster just exiting the collision site. A prominent bow shock gives an estimate of the subcluster velocity, 4500 km s-1, which lies mostly in the plane of the sky. The optical image shows that the gas lags behind the subcluster galaxies. The weak-lensing mass map reveals a dark matter clump lying ahead of the collisional gas bullet but coincident with the effectively collisionless galaxies. From these observations, one can directly estimate the cross section of the dark matter self-interaction. That the dark matter is not fluid-like is seen directly in the X-ray-lensing mass overlay; more quantitative limits can be derived from three simple independent arguments. The most sensitive constraint, sigma/m
WMAP precision data enable accurate testing of cosmological models. We find that the emerging standard model of cosmology, a flat Λ-dominated universe seeded by a nearly scale-invariant adiabatic Gaussian fluctuations, fits the WMAP data. For the WMAP data only, the best-fit parameters are h = 0.72 ± 0.05, Ωbh2 = 0.024 ± 0.001, Ωmh2 = 0.14 ± 0.02, τ = 0.166, ns = 0.99 ± 0.04, and σ8 = 0.9 ± 0.1. With parameters fixed only by WMAP data, we can fit finer scale cosmic microwave background (CMB) measurements and measurements of large-scale structure (galaxy surveys and the Lyα forest). This simple model is also consistent with a host of other astronomical measurements: its inferred age of the universe is consistent with stellar ages, the baryon/photon ratio is consistent with measurements of the [D/H] ratio, and the inferred Hubble constant is consistent with local observations of the expansion rate. We then fit the model parameters to a combination of WMAP data with other finer scale CMB experiments (ACBAR and CBI), 2dFGRS measurements, and Lyα forest data to find the model's best-fit cosmological parameters: h = 0.71, Ωbh2 = 0.0224 ± 0.0009, Ωmh2 = 0.135, τ = 0.17 ± 0.06, ns(0.05 Mpc-1) = 0.93 ± 0.03, and σ8 = 0.84 ± 0.04. WMAP's best determination of τ = 0.17 ± 0.04 arises directly from the temperature-polarization (TE) data and not from this model fit, but they are consistent. These parameters imply that the age of the universe is 13.7 ± 0.2 Gyr. With the Lyα forest data, the model favors but does not require a slowly varying spectral index. The significance of this running index is sensitive to the uncertainties in the Lyα forest.
We review progress in understanding dark matter by astrophysics, and particularly via the effect of gravitational lensing. Evidence from many different directions now all imply that five sixths of the material content of the universe is in this mysterious form, separate from and beyond the ordinary "baryonic" particles in the standard model of particle physics. Dark matter appears not to interact via the electromagnetic force, and therefore neither emits nor reflects light. However, it definitely does interact via gravity, and has played the most important role in shaping the Universe on large scales. The most successful technique with which to investigate it has so far been the effect of gravitational lensing. The curvature of space-time near any gravitating mass (including dark matter) deflects passing rays of light - observably shifting, distorting and magnifying the images of background galaxies. Measurements of such effects currently provide constraints on the mean density of dark matter, and its density relative to baryonic matter; the size and mass of individual dark matter particles; and its cross section under various fundamental forces. Comment: 48 page Rep Prog Phys review. Matches published version.
Although dark matter is a central element of modern cosmology, the history of how it became accepted as part of the dominant paradigm is often ignored or condensed into a brief anecdotical account focused around the work of a few pioneering scientists. The aim of this review is to provide the reader with a broader historical perspective on the observational discoveries and the theoretical arguments that led the scientific community to adopt dark matter as an essential part of the standard cosmological model.
For 21 Sc galaxies whose properties encompass a wide range of radii,
masses, and luminosities, we have obtained major axis spectra
extending to the faint outer regions, and have deduced rotation
curves. The galaxies are of high inclination, so uncertainties in the
angle of inclination to the line of sight and in the position angle of
the major axis are minimized. Their radii range from 4 to 122 kpc (H =
50km s-1 Mpc-1); in general, the rotation curves
extend to 83% or R25i.b. When plotted on a
linear scale with no scaling, the rotation curves for the smallest
galaxies fall upon the initial parts of the rotation curves for the
larger galaxies. All curves show a fairly rapid velocity rise to V
∼ 125 km s-1 at R ∼ 5 kpc, and a slower rise
thereafter. Most rotation curves are rising slowly even at the
farthest measured point. Neither high nor low luminosity Sc galaxies
have falling rotation curves. Sc galaxies of all luminosities must
have significant mass located beyond the optical image. A linear
relation between log Vmax and log R follows from the shape
of the common rotation curve for all Sc's, and the tendency of smaller
galaxies, at any R, to have lower velocities than the large galaxies
at that R. The significantly shallower slope discovered for this
relation by Tully and Fisher is attributed to their use of galaxies of
various Hubble types and the known correlation of Vmax with
Hubble type.
The galaxies with very large central velocity gradients tend to be
large, of high luminosity, with massive, dense nuclei. Often their
nuclear spectra show a strong stellar continuum in the red, with
emission lines of [N II] stronger than Hα. These galaxies also
tend to be 13 cm radio continuum sources.
Because of the form of the rotation curves, small galaxies undergo
many short-period, very differential, rotations. Large galaxies
undergo (in their outer parts) few, only slightly differential,
rotations. This suggests a relation between morphology, rotational
properties, and the van den Bergh luminosity classification, which is
discussed. UGC 2885, the largest Sc in the sample, has undergone fewer
than 10 rotations in its outer parts since the origin of the universe
but has a regular two-armed spiral pattern and no significant velocity
asymmetries. This observation puts constraints on models of galaxy
formation and evolution.
We introduce a new adaptive and fully Bayesian grid-based method to model strong gravitational lenses with extended images. The primary goal of this method is to quantify the level of luminous and dark mass substructure in massive galaxies, through their effect on highly magnified arcs and Einstein rings. The method is adaptive on the source plane, where a Delaunay tessellation is defined according to the lens mapping of a regular grid on to the source plane. The Bayesian penalty function allows us to recover the best non-linear potential-model parameters and/or a grid-based potential correction and to objectively quantify the level of regularization for both the source and potential. In addition, we implement a Nested-Sampling technique to quantify the errors on all non-linear mass model parameters – marginalized over all source and regularization parameters – and allow an objective ranking of different potential models in terms of the marginalized evidence. In particular, we are interested in comparing very smooth lens mass models with ones that contain mass substructures. The algorithm has been tested on a range of simulated data sets, created from a model of a realistic lens system. One of the lens systems is characterized by a smooth potential with a power-law density profile, 12 include a Navarro, Frenk and White (NFW) dark matter substructure of different masses and at different positions and one contains two NFW dark substructures with the same mass but with different positions. Reconstruction of the source and lens potential for all of these systems shows the method is able, in a realistic scenario, to identify perturbations with masses ≳107 M⊙ when located on the Einstein ring. For positions both inside and outside of the ring, masses of at least 109 M⊙ are required (i.e. roughly the Einstein ring of the perturber needs to overlap with that of the main lens). Our method provides a fully novel and objective test of mass substructure in massive galaxies.
The identity of dark matter is a question of central importance in both
astrophysics and particle physics. In the past, the leading particle candidates
were cold and collisionless, and typically predicted missing energy signals at
particle colliders. However, recent progress has greatly expanded the list of
well-motivated candidates and the possible signatures of dark matter. This
review begins with a brief summary of the standard model of particle physics
and its outstanding problems. We then discuss several dark matter candidates
motivated by these problems, including WIMPs, superWIMPs, light gravitinos,
hidden dark matter, sterile neutrinos, and axions. For each of these, we
critically examine the particle physics motivations and present their expected
production mechanisms, basic properties, and implications for direct and
indirect detection, particle colliders, and astrophysical observations.
Upcoming experiments will discover or exclude many of these candidates, and
progress may open up an era of unprecedented synergy between studies of the
largest and smallest observable length scales.
Some time ago, R. W. Mandi paid me a visit and asked me to publish the results of a little calculation, which I had made at his request. This note complies with his wish.
Cosmic microwave background (CMB) temperature anisotropies have and will
continue to revolutionize our understanding of cosmology. The recent discovery
of the previously predicted acoustic peaks in the power spectrum has
established a working cosmological model: a critical density universe
consisting of mainly dark matter and dark energy, which formed its structure
through gravitational instability from quantum fluctuations during an
inflationary epoch. Future observations should test this model and measure its
key cosmological parameters with unprecedented precision. The phenomenology and
cosmological implications of the acoustic peaks are developed in detail. Beyond
the peaks, the yet to be detected secondary anisotropies and polarization
present opportunities to study the physics of inflation and the dark energy.
The analysis techniques devised to extract cosmological information from
voluminous CMB data sets are outlined, given their increasing importance in
experimental cosmology as a whole.
Rotation curves of spiral galaxies are the major tool for determining the distribution of mass in spiral galaxies. They provide fundamental information for understanding the dynamics, evolution and formation of spiral galaxies. We describe various methods to derive rotation curves, and review the results obtained. We discuss the basic characteristics of observed rotation curves in relation to various galaxy properties, such as Hubble type, structure, activity, and environment. Comment: 44 pages, 6 gif figures; High-quality ps figs avail. at http://www.ioa.s.u-tokyo.ac.jp/~sofue/pp.htm; Submitted to Ann. Rev. Astron. Astrophys. Vol. 39, 2001
Planck 2018 results. VI. Cosmological parameters
Planck Collaboration. (2020). Planck 2018 results. VI. Cosmological parameters. Astronomy & Astrophysics,
Cosmological Structure Formation
- J R Primack
Primack, J. R. (2015). Cosmological Structure Formation. Publications of the Astronomical Society of the Pacific, 127(956), 963.