Showing papers on "Dark fluid published in 2001"

The Power Spectrum Dependence of Dark Matter Halo Concentrations

Abstract: High-resolution N-body simulations are used to examine the power spectrum dependence of the concentration of galaxy-sized dark matter halos. It is found that dark halo concentrations depend on the amplitude of mass fluctuations as well as on the ratio of power between small and virial mass scales. This finding is consistent with the original results of Navarro, Frenk, and White (NFW) and allows their model to be extended to include power spectra substantially different from cold dark matter (CDM). In particular, the single-parameter model presented here fits the concentration dependence on halo mass for truncated power spectra, such as those expected in the warm dark matter scenario, and predicts a stronger redshift dependence for the concentration of CDM halos than proposed by NFW. The latter conclusion confirms recent suggestions by Bullock and coworkers, although this new modeling differs from theirs in detail. These findings imply that observational limits on the concentration, such as those provided by estimates of the dark matter content within individual galaxies, may be used to constrain the amplitude of mass fluctuations on galactic and subgalactic scales. The constraints on ΛCDM models posed by the dark mass within the solar circle in the Milky Way and by the zero point of the Tully-Fisher relation are revisited, with the result that neither data set is clearly incompatible with the "concordance" (Ω0 = 0.3, Λ0 = 0.7, σ8 = 0.9) ΛCDM cosmogony. This conclusion differs from that reached recently by Navarro and Steinmetz, a disagreement that can be traced to inconsistencies in the normalization of the ΛCDM power spectrum used in that work.

Probing dark energy: methods and strategies

Abstract: The presence of dark energy in the Universe is inferred directly from the accelerated expansion of the Universe, and, indirectly, from measurements of cosmic microwave background (CMB) anisotropy. Dark energy contributes about two-thirds of the critical density, is smoothly distributed, has large negative pressure, and is very mysterious. For now, all of its discernible cosmological consequences follow from its effect on the expansion rate of the Universe. Absent a compelling theoretical model (or even a class of models), we describe the dark energy by its equation of state ${w=p}_{X}/{\ensuremath{\rho}}_{X}$ which is allowed to vary with time. We describe and compare different approaches for determining $w(t),$ including a magnitude-redshift (Hubble) diagram, number counts of galaxies and clusters, and CMB anisotropy. We focus particular attention on the use of a sample of several thousand type Ia supernova with redshifts $z\ensuremath{\lesssim}1.7,$ as might be gathered by the proposed SNAP satellite. Among other things, we derive optimal strategies for constraining cosmological parameters using type Ia supernovae. The redshift range $z\ensuremath{\simeq}0.2\ensuremath{-}2$ has the most leverage for probing ${w}_{X};$ supernovae and number counts appear to have the most potential to probe dark energy. Because the expansion rate depends upon both $w(t)$ and ${\ensuremath{\Omega}}_{M},$ an independent measurement of the matter density is critical for obtaining the most information about dark energy from cosmological observations.

Dark Halos: The Flattening of the Density Cusp by Dynamical Friction

Abstract: N-body simulations and analytical calculations of the gravitational collapse in an expanding universe predict that halos should form with a diverging inner density profile, the cusp. There are some observational indications that the dark matter distribution in galaxies might be characterized by a finite core. This core catastrophe has prompted a search for alternatives to the cold dark matter (CDM) cosmogony. It is shown here that the discrepancy between theory and observations can be very naturally resolved within the standard CDM model, provided that gas is not initially smoothly distributed in the dark matter halo but rather is concentrated in clumps of mass ?0.01% of the total mass of the system. Dynamical friction acting on these lumps moving in the background of the dark matter particles dissipates the clumps' orbital energy and deposits it in the dark matter. Using Monte Carlo simulations, it is shown that the dynamical friction provides a strong enough drag and that with realistic baryonic mass fractions, the available orbital energy of the clumps is sufficient to heat the halo and turn the primordial cusp into a finite, nondiverging core?overcoming the competing effect of adiabatic contraction due to the gravitational influence of the shrinking baryonic component. Depending on the initial conditions, the total density distribution may become either more or less centrally concentrated. Possible consequences of the proposed mechanism for other problems in the CDM model and for the formation and early evolution of the baryonic component of galaxies are also briefly discussed.

Cosmic antifriction and accelerated expansion

Abstract: We explain an accelerated expansion of the present Universe, suggested from observations of supernovae of type la at high redshift, by introducing an antifrictional force that is self-consistently exerted on the particles of the cosmic substratum. Cosmic antifriction. which is intimately related to "particle production," is shown to give rise to an effective negative pressure of the cosmic medium. While other explanations for an accelerated expansion (cosmological constant, quintessence) introduce a component of dark energy in addition to "standard" cold dark matter (CDM) we resort to a phenomenological one-component model of CDM with internal self-interactions. We demonstrate how the dynamics of the cold dark matter model with a cosmological constant may be recovered as a special case of cosmic antifriction. We discuss the connection with two-component models and obtain an attractor behavior for the ratio of the energy densities of both components which provides a possible phenomenological solution to the coincidence problem.

Stationary dark energy: The Present universe as a global attractor

Abstract: We propose a cosmological model that makes a significant step toward solving the coincidence problem of the near similarity at the present of the dark energy and dark matter components. Our cosmology has the following properties: (a) among flat and homogeneous spaces, the present universe is a global attractor; all the possible initial conditions lead to the observed proportion of dark energy and dark matter; once reached, it remains fixed forever; (b) the expansion is accelerated at the present; (c) the model is consistent with the large-scale structure and microwave background data; (d) the dark energy and the dark matter densities scale similarly after equivalence and are close to within one order of magnitude.

Dwarf galaxy rotation curves and the core problem of dark matter haloes

Abstract: ABSTRA C T The standard cold dark matter (CDM) model has recently been challenged by the claim that dwarf galaxies have dark matter haloes with constant-density cores, whereas CDM predicts haloes with steeply cusped density distributions. Consequently, numerous alternative dark matter candidates have recently been proposed. In this paper we scrutinize the observational evidence for the incongruity between dwarf galaxies and the CDM model. To this end, we analyse the rotation curves of 20 late-type dwarf galaxies studied by Swaters. Taking the effects of beam smearing and adiabatic contraction into account, we fit mass models to these rotation curves with dark matter haloes with different cusp slopes, ranging from constantdensity cores to r 22 cusps. Even though the effects of beam smearing are small for these data, the uncertainties in the stellar mass-to-light ratio and the limited spatial sampling of the halo’s density distribution hamper a unique mass decomposition. Consequently, the rotation curves in our sample cannot be used to discriminate between dark haloes with constant-density cores and r 21 cusps. We show that the dwarf galaxies analysed here are consistent with CDM haloes in a LCDM cosmology, and that there is thus no need to abandon the idea that dark matter is cold and collisionless. However, the data are also consistent with any alternative dark matter model that produces dark matter haloes with central cusps less steep than r 21.5 . In fact, we argue that based on existing H I rotation curves alone, at best weak limits can be obtained on cosmological parameters and/or the nature of the dark matter. In order to make progress, rotation curves with higher spatial resolution and independent measurements of the mass-to-light ratio of the disc are required.

Structure of Dark Matter Halos from Hierarchical Clustering

Abstract: We investigate the structure of the dark matter halo formed in the cold dark matter scenario using N-body simulations. We simulated 12 halos with masses of 6.6 × 1011-8.0 × 1014 M☉. In all runs, the halos have density cusps proportional to r-1.5 developed at their centers, which is consistent with the results of recent high-resolution calculations. The density structure evolves in a self-similar way and is universal in the sense that it is independent of the halo mass and initial random realization of density fluctuation. The density profile is in good agreement with the profile proposed by Moore et al., which has a central slope proportional to r-1.5 and an outer slope proportional to r-3. The halo grows through repeated accretion of diffuse smaller halos. We argue that the cusp is understood as a convergence slope for the accretion of tidally disrupted matter.

Halo Cores and Phase-Space Densities: Observational Constraints on Dark Matter Physics and Structure Formation

Abstract: We explore observed trends in the cores of a wide range of relaxed dark matter-dominated halos (about 7 orders of magnitude in mass) to constrain hypothetical dark matter candidates and scenarios of structure formation. First, we argue that neither generic warm dark matter (collisionless or collisional) nor self-interacting dark matter can be responsible for the observed cores on all scales. Both scenarios predict smaller cores for higher mass systems, in conflict with observations; some cores must instead have a dynamical origin. Second, we show that the mean core phase-space densities Q of dwarf spheroidal galaxies, rotating dwarf and low surface brightness galaxies, and clusters of galaxies decrease with increasing velocity dispersion like Q ∝ σ-3 ∝ M-1, as predicted by a simple scaling argument based on quietly merging equilibrium systems over a range of about 8 orders of magnitude in Q. We discuss the processes that set the overall normalization of the observed phase density hierarchy. We note the resemblance between the observed phase-space scaling behavior and density profiles of dark matter halos and stellar components in elliptical galaxies and conjecture that dark matter halos may suffer from the same systematic departures from homology as seen in elliptical galaxies, thus explaining the shallower density profiles observed in low-mass halos. Finally, we use the maximum observed phase-space density in dwarf spheroidal galaxies to fix a minimum mass for relativistically decoupled warm dark matter candidates of roughly 700 eV for thermal fermions and 300 eV for degenerate fermions.

Spectral gamma-ray signatures of cosmological dark matter annihilations.

Abstract: We propose a new signature for weakly interacting massive particle (WIMP) dark matter, a spectral feature in the diffuse extragalactic gamma-ray radiation. This feature, a sudden drop of the gamma-ray intensity at an energy corresponding to the WIMP mass, comes from the asymmetric distortion of the line due to WIMP annihilation into two gamma rays caused by the cosmological redshift. Unlike other proposed searches for a line signal, this method is not very sensitive to the exact dark matter density distribution in halos and subhalos.

Testing the cosmic coincidence problem and the nature of dark energy

Abstract: Dark energy models which alter the relative scaling behavior of dark energy and matter could provide a natural solution to the cosmic coincidence problem-why the densities of dark energy and dark matter are comparable today. A generalized class of dark energy models is introduced which allows noncanonical scaling of the ratio of dark matter and dark energy with the Robertson-Walker scale factor a(t). We show that determining whether there is a coincidence problem, and the extent of cosmic coincidence, can be addressed by several forthcoming experiments.

Peculiar velocities of galaxies and clusters

Abstract: We present a simple model for the shape of the distribution function of galaxy peculiar velocities. We show how both non-linear and linear theory terms combine to produce a distribution which has an approximately Gaussian core with exponential wings. The model is easily extended to study how the statistic depends on the type of particle used to trace the velocity field (dark matter particles, dark matter haloes, galaxies), and on the density of the environment in which the test particles are located. Comparisons with simulations suggest that our model is accurate. We also show that the evolution of the peculiar velocities depends on the local, rather than the global, density. Since clusters populate denser regions on average, using cluster velocities with the linear theory scaling may lead to an overestimate of the global value of Ω0. Conversely, using linear theory with the global value of Ω0 to scale cluster velocities from the initial to the present time results in an underestimate of their true velocities. In general, however, the directions of motions of haloes are rather well described by linear theory. Our results help to simplify models of redshift-space distortions considerably.

Q-ball candidates for self-interacting dark matter.

Abstract: We show that nontopological solitons, known as Q-balls, are promising candidates for self-interacting dark matter. They can satisfy the cross-section requirements for a broad range of masses. Unlike previously considered examples, Q-balls can stick together after collision, reducing the effective self-interaction rate to a negligible value after a few collisions per particle. This feature modifies predictions for halo formation. We also discuss the possibility that Q-balls have large interaction cross sections with ordinary matter.

Limits on Collisional Dark Matter from Elliptical Galaxies in Clusters

Abstract: The dynamical evolution of galaxies in clusters would be modified if dark matter were self-interacting. Heat conduction from the hot cluster halo would lead to evaporation of the relatively cooler galactic halos. The stellar distribution would adiabatically expand as it readjusted to the loss of dark matter, reducing the velocity dispersion and increasing the half-light radius. If the dark matter content within that radius was fdm = 25%-50% of the total, as indicated by current observations, the elliptical galaxies in clusters would be offset from the fundamental plane relation beyond observational scatter. The requirement that their halos survive for a Hubble time appears to exclude just that range of the dark matter cross section, 0.3 σp/mp 104 cm2 g-1, thought to be optimal for reducing central halo cusps, unless fdm 1.8. In either case, the problem of central density cusps remains.

Sources and Detection of Dark Matter and Dark Energy in the Universe

Abstract: List of Talks from DM 2012.- List of Attendees at DM 2012.- Brief Overview.- Theoretical Precision: Cosmology and Dark Matter.- Radiative natural supersymmetry with mixed axion/higgsino cold dark matter.- Finite Temperature density profile in SFD.- An argument for axion dark matter.- Supersymmetric Dark Matter at XENON100 and the LHC: No-Scale F-SU(5) Stringy Correlations.- Approaches on self-gravitating Bose-Einstein condensates.- Search for Turbulent Gas through Interstellar Scintillation.- Search for Dark Matter - LHC, CMB, Fermi LAT- The missing energy events at the LHC and implications for dark matter search.- Where is SUSY?- Bounds on dark matter from CMB observations- Searches for Galactic Dark Matter Substructure with the Fermi LAT- Current Search Results and New Detectors- DAMA.- Xenon100 Detector.- Xenon 1 Ton.- Super CDMS.- Antideuterons.- Light WIMPs.- Directional Detection.- Cogent.- Low-mass WIMPs.

Self-Interacting Dark Matter

Abstract: Spergel and Steinhardt have recently proposed the concept of dark matter with strong self-interactions as a means to address numerous discrepancies between observations of dark matter halos on subgalactic scales and the predictions of the standard collisionless dark matter picture. We review the motivations for this scenario and discuss some recent, successful numerical tests. We also discuss the possibility that the dark matter interacts strongly with ordinary baryonic matter, as well as with itself. We present a new analysis of the experimental constraints and re-evaluate the allowed range of cross-section and mass.

Dark matter in draco and the local group: implications for direct detection experiments

Abstract: We use a cosmological simulation of the Local Group to make quantitative and speculative predictions for direct detection experiments. Cold dark matter (CDM) halos form via a complex series of mergers, accretion events and violent relaxation which precludes the formation of significant caustic features predicted by axially symmetric collapse. The halo density profiles are combined with observational constraints on the galactic mass distribution to constrain the local density of cold dark matter to lie in the range 0.18 <~ rho_CDM(R_solar)/GeV cm^-3 <~ 0.30. In velocity space, coherent streams of dark matter from tidally disrupted halos fill the halo and provide a tracer of the merging hierarchy. The particle velocities within triaxial CDM halos cannot be approximated by a simple Maxwellian distribution and is radially biased at the solar position. The detailed phase space structure within the solar system will depend on the early merger history of the progenitor halos and the importance of major mergers over accretion dominated growth. We follow the formation of a ``Draco'' sized dSph halo of mass 10^8M_solar with several million particles and high force accuracy. Its internal structure and substructure resembles that of galactic or cluster mass halos: the density profile has a singular central cusp and it contains thousands of sub-halos orbiting within its virial radius demonstrating a self-similar nature to collisionless dark matter sub-clustering. The singular cores of substructure halos always survive complete tidal disruption although mass loss is continuous and rapid. Extrapolating wildly to earth mass halos with velocity dispersion of 1 m s^-1 (roughly equal to the free streaming scale for neutralinos) we find that most of the dark matter may remain attached to bound subhalos. (Abridged)

Problems for Modified Newtonian Dynamics in Clusters and the Lyα Forest

Abstract: The observed dynamics of gas and stars on galactic and larger scales cannot be accounted for by self-gravity, indicating that there are large quantities of unseen matter or that gravity is non-Newtonian in these regimes. Milgrom's modified Newtonian dynamics (MOND) postulates that Newton's laws are modified at very low acceleration, and can account for the rotation curves of galaxies and some other astrophysical observations, without dark matter. Here we apply MOND to two independent physical systems: Lyα absorbers and galaxy clusters. While physically distinct, both are simple hydrodynamical systems with characteristic accelerations in the MOND regime. We find that, because MOND violates the strong equivalence principle, the properties of Lyα absorbers depend strongly on the (unknown) background acceleration field in which they are embedded. If this field is small compared to their internal accelerations, then the absorbers are more dense and about 10 times smaller than in Newtonian gravity with dark matter, in conflict with sizes inferred from quasar pair studies. If, however, the background field is rather large, then the absorbers take on properties similar to those predicted in the cold dark matter picture. In clusters MOND appears to explain the observed (baryonic) mass-temperature relation. However, given observed gas density and enclosed mass profiles and the assumption of hydrostatic equilibrium, MOND predicts radial temperature profiles that disagree badly with observations. We show this explicitly for the Virgo, Abell 2199, and Coma Clusters, but the results are general and seem very difficult to avoid. If this discrepancy is to be resolved by positing additional (presumably baryonic) dark matter, then this dark matter must have ~1-3 times the cluster gas mass within 1 Mpc and about 10 times the gas mass within 200 kpc. This result strongly disfavors MOND as an alternative to dark matter.

The constant-density region of the dark haloes of spiral galaxies

Abstract: We determine a crucial feature of the dark halo density distribution from the fact that the luminous matter dominates the gravitational potential at about one disc scalelength Rd, but at the optical edge the dark matter has already become the main component of the galaxy density. From the kinematics of 137 spirals we find that the dark matter halo density profiles are self-similar at least out to Ropt and show core radii much larger than the corresponding disc scalelengths. The luminous regions of spirals consist of stellar discs embedded in dark haloes with roughly constant density. This invariant dark matter profile is very difficult to reconcile with the fundamental properties of the density distribution of cold dark matter haloes. With respect to previous work, the present evidence is obtained by means of a robust method and for a large and complete sample of normal spirals.

Decaying Cold Dark Matter Model and Small-Scale Power

Abstract: The canonical cosmological constant-dominated cold dark matter model (ΛCDM) may possess too much power on small scales at z = 0, manifested as central overconcentration of dark matter and overabundance of dwarf galaxies. We suggest an alternative model, ΛDCDM, where one-half of the cold dark matter particles decay into relativistic particles by z = 0. The model successfully lowers the concentration of dark matter in dwarf galaxies as well as in large galaxies like our own at low redshift while simultaneously retaining the virtues of the ΛCDM model. The model solves the problem of overproduction of small dwarf galaxies in the ΛCDM, not by removing them but by identifying them with failed, "dark" galaxies, where star formation is quenched as a result of dark matter evaporation and consequent halo expansion. A dramatic difference between the ΛDCDM model and other proposed variants of the ΛCDM model is that the small-scale power at high redshift (z > 2) in the ΛDCDM model is enhanced compared to the ΛCDM model. A COBE- and cluster-normalized ΛDCDM model can be constructed with the following parameters: H0 = 60 km s-1 Mpc-1, λ0 = 0.60, Ω0, CDM = 0.234, Ω0, b = 0.044, n = 1.0, and σ8 = 1.06. A clean test of this model can be made by measuring the evolution of the gas fraction in clusters. The prediction is that the gas fraction should decrease with redshift and is smaller by 31% at z = 1 than at z = 0. X-ray and Sunyaev-Zeldovich effect observations should provide such a test.

Giant cluster arcs as a constraint on the scattering cross-section of dark matter

Abstract: We carry out ray tracing through five high resolution simulat ions of a galaxy cluster to study how its ability to produce giant gravitationally lens ed arcs is influenced by the collision cross-section of its dark matter. In three cases typical dar k matter particles in the cluster core undergo between 1 and 100 collisions per Hubble time; two more explore the long (“collisionless”) and short (“fluid”) mean free path limits. We stud y the size and shape distributions of arcs and compute the cross-section for producing “extreme” arcs of various sizes. Even a few collisions per particle modify the core structure enou gh to destroy the cluster’s ability to produce long, thin arcs. For larger collision frequencie s the cluster must be scaled up to unrealistically large masses before it regains the ability to produce giant arcs. None of our models with self-interacting dark matter (except the “fluid ” limit) is able to produce radial arcs; even the case with the smallest scattering cross-sect ion must be scaled to the upper limit of observed cluster masses before it produces radial a rcs. Apparently the elastic collision cross-section of dark matter in clusters must be very small, below 0.1 cm 2 g −1 , to be compatible with the observed ability of clusters to produce both radial arcs and giant arcs.

Measuring Time Dependence of Dark Energy Density from Type Ia Supernova Data

Abstract: Observations of high-redshift supernovae imply an accelerating universe that can only be explained by an unusual energy component such as vacuum energy or quintessence. To assess the ability of current and future supernova data to constrain the properties of the dark energy, we allow its density to have arbitrary time dependence, ρX(z). This leads to an equation of state for the dark energy, wX(z) = pX(z)/ρX(z), which is a free function of redshift z. We find that current data of type Ia supernovae (SNe Ia) are consistent with a cosmological constant, with large uncertainties at z 0.5. We show that ρX(z)/ρX(z = 0) can be measured reasonably well to about z = 1.5 using SN Ia data from realistic future SN Ia pencil beam surveys, provided that the weak energy condition (energy density of matter is nonnegative for any observer) is imposed. While it is only possible to differentiate between different models (say, quintessence and k-essence) at z 1.5 using realistic data, the correct trend in the time dependence of the dark energy density can be clearly detected out to z = 2, even in the presence of plausible systematic effects. This would allow us to determine whether the dark energy is a cosmological constant or some exotic form of energy with a time-dependent density.

Structure Formation and the Time Dependence of Quintessence

Abstract: We discuss the influence of dark energy on structure formation, especially the effects on \sigma_8. Our interest is particularly focused on quintessence models with time-dependent equation of state and non-negligible quintessence component in the early universe. We obtain an analytic expression for \sigma_8 valid for a large class of dark energy models. We conclude that structure formation is a good indicator for the history of dark energy and use our results to set constraints on quintessence models.

Bimetric gravity and ``dark matter''

Abstract: We present a bimetric theory of gravity containing a length scale of galactic size. For distances less than this scale the theory satisfies the standard tests of general relativity. For distances greater than this scale the theory yields an effective gravitational constant much larger than the locally observed value of Newton's constant. The transition from one regime to the other through the galactic scale can explain the observed rotation curves of galaxies and hence the effects normally attributed to the presence of dark matter. Phenomena on an extragalactic scale such as galactic clusters and the expansion of the universe are controlled by the enhanced gravitational coupling. This provides an explanation of the missing matter normally invoked to account for the observed value of Hubble's constant in relation to observed matter.

Astrophysical limits on massive dark matter

Abstract: Annihilations of weakly interacting dark matter particles provide an important signature for the possibility of indirect detection of dark matter in galaxy haloes. These self-annihilations can be greatly enhanced in the vicinity of a massive black hole. We show that the massive black hole present at the centre of our galaxy accretes dark matter particles, creating a region of very high particle density. Consequently the annihilation rate is considerably increased, with a large number of e+e− pairs being produced either directly or by successive decays of mesons. We evaluate the synchrotron emission (and self-absorption) associated with the propagation of these particles through the galactic magnetic field, and are able to constrain the allowed values of masses and cross sections of dark matter particles.

Dark Energy and the BOOMERANG Data

Abstract: The recent high-quality BOOMERANG data allow the testing of many competing cosmological models. Here I present a seven-parameter likelihood analysis of dark energy models with exponential potential and explicit coupling to dark matter. The BOOMERANG data constrain the dimensionless coupling $\ensuremath{\beta}$ to be smaller than 0.1, an order of magnitude better than previous limits. In terms of the constant $\ensuremath{\xi}$ of nonminimally coupled theories, this amounts to $\ensuremath{\xi}l0.01.$ On the other hand, BOOMERANG does not have enough sensitivity to put constraints on the potential slope.

Dark Energy and the Epoch of Galaxy Formation

Abstract: The influence of a dark component on the first epoch of galaxy formation is analyzed by using the ages of the three oldest high-redshift galaxies known in the literature. Our results, based on a spatially flat accelerated universe driven by a "quintessence" component (px = ωρx), show that, if the inferred ages of these objects are correct, the first formation era is pushed back to extremely high redshifts. For the present best-fit quintessence model (Ωx = 0.7, ω < -0.6), we find a lower bound of zf ≥ 7.7, whereas in the extreme case of the Λ + cold dark matter model (ω = -1) the limit is slightly smaller (zf ≥ 5.8). The case for open cold dark matter models has also been discussed. For Ωm 0.3, the formation redshift is restricted by zf ≥ 18. As a general result, if Ωm ≥ 0.37, these galaxies are not formed in FRW cosmologies with no dark energy since for all these cases zf → ∞.

Neutralino gamma-ray signals from accreting halo dark matter

Abstract: There is mounting evidence that a self-consistent model for particle cold dark matter has to take into consideration spatial inhomogeneities on sub-galactic scales seen, for instance, in high-resolution N-body simulations of structure formation. Also in more idealized, analytic models, there appear density enhancements in certain regions of the halo. We use the results from a recent N-body simulation of the Milky Way halo and investigate the gamma-ray flux which would be produced when a specific dark matter candidate, the neutralino, annihilates in regions of enhanced density. The clumpiness found on all scales in the simulation results in very strong gamma-ray signals which seem to already rule out some regions of the supersymmetric parameter space, and would be further probed by upcoming experiments, such as the GLAST gamma-ray satellite. As an orthogonal model of structure formation, we also consider Sikivie's simple infall model of dark matter which predicts that there should exist continuous regions of enhanced density, caustic rings, in the dark matter halo of the Milky Way. We find, however, that the gamma-ray signal from caustic rings is generally too small to be detectable.