2,739 research outputs found
End of a Dark Age?
We argue that dark matter and dark energy phenomena associated with galactic
rotation curves, X-ray cluster mass profiles, and type Ia supernova data can be
accounted for via small corrections to idealized general relativistic spacetime
geometries due to disordered locality. Accordingly, we fit THINGS rotation
curve data rivaling modified Newtonian dynamics, ROSAT/ASCA X-ray cluster mass
profile data rivaling metric-skew-tensor gravity, and SCP Union2.1 SN Ia data
rivaling CDM without non-baryonic dark matter or a cosmological
constant. In the case of dark matter, we geometrically modify proper mass
interior to the Schwarzschild solution. In the case of dark energy, we modify
proper distance in Einstein-deSitter cosmology. Therefore, the phenomena of
dark matter and dark energy may be chimeras created by an errant belief that
spacetime is a differentiable manifold rather than a disordered graph.Comment: This version was accepted for publication in the International
Journal of Modern Physics D; revised version of an essay that won Honorable
Mention in the Gravity Research Foundation 2016 Awards for Essays on
Gravitation. 10 pages, 3 figures. arXiv admin note: text overlap with
arXiv:1509.0928
The Missing Mass Problem as a Manifestation of GR Contextuality
In Newtonian gravity, mass is an intrinsic property of matter while in
general relativity (GR), mass is a contextual property of matter, i.e., matter
can simultaneously possess two different values of mass when it is responsible
for two different spatiotemporal geometries. Herein, we explore the possibility
that the astrophysical missing mass attributed to non-baryonic dark matter (DM)
actually obtains because we have been assuming the Newtonian view of mass
rather than the GR view. Since an exact GR solution for realistic astrophysical
situations is not feasible, we explore GR-motivated ansatzes relating proper
mass and dynamic mass for one and the same baryonic matter, as justified by GR
contextuality. We consider four GR alternatives and find that the GR ansatz
motivated by metric perturbation theory works well in fitting galactic rotation
curves (THINGS data), the mass profiles of X-ray clusters (ROSAT and ASCA data)
and the angular power spectrum of the cosmic microwave background (CMB, Planck
2015 data) without DM. We compare our galactic rotation curve fits to modified
Newtonian dynamics (MOND), Burkett halo DM and Navarro-Frenk-White (NFW) halo
DM. We compare our X-ray cluster mass profile fits to metric skew-tensor
gravity (MSTG) and core-modified NFW DM. We compare our CMB angular power
spectrum fit to scalar-tensor-vector gravity (STVG) and CDM. Overall,
we find our fits to be comparable to those of MOND, MSTG, STVG, CDM,
Burkett, and NFW. We present and discuss correlations and trends for the best
fit values of our fitting parameters. For the most part, the correlations are
consistent with well-established results at all scales, which is perhaps
surprising given the simple functional form of the GR ansatz.Comment: 18 pages text. Twice revised per referee/reviewer comments. Fit of
CMB angular power spectrum and dark matter halo fits adde
Massive prompt cusps: A new signature of warm dark matter
Every dark matter halo and subhalo is expected to have a prompt central density cusp, which is a relic of its condensation out of the
smooth mass distribution of the early universe. The sizes of these prompt cusps
are linked to the scales of the peaks in the initial density field from which
they formed. In warm dark matter (WDM) models, the smoothing scale set by free
streaming of the dark matter can result in prompt cusps with masses of order
M. We show that WDM models with particle masses ranging from 2
to 6 keV predict prompt cusps that could detectably alter the observed
kinematics of Local Group dwarf galaxies. Thus, prompt cusps present a viable
new probe of WDM. A prompt cusp's properties are highly sensitive to when it
formed, so prospects can be improved with a better understanding of when the
haloes of the Local Group dwarfs originally formed. Tidal stripping can also
affect prompt cusps, so constraints on satellite galaxy orbits can further
tighten WDM inferences.Comment: 5 pages, 6 figures; accepted by MNRAS Letters. Includes more detail
on the sampling of prompt cusp
Simulations of Gravitational Heating Due to Early Matter Domination
In cosmologies with an early matter-dominated era (EMDE) prior to Big Bang
nucleosynthesis, the boosted growth of small-scale matter perturbations during
the EMDE leads to microhalo formation long before halos would otherwise begin
to form. For a range of models, halos can even form during the EMDE itself.
These halos would dissipate at the end of the EMDE, releasing their
gravitationally heated dark matter and thereby imprinting a free-streaming
cut-off on the matter power spectrum. We conduct the first cosmological
-body simulations of the formation and evaporation of halos during and after
an EMDE. We show that in these scenarios, the free-streaming cut-off after the
EMDE can be predicted accurately from the linear matter power spectrum.
Although the free streaming can erase much of the EMDE-driven boost to density
perturbations, we use our findings to show that the (re-)formation of halos
after the EMDE nevertheless proceeds before redshift . Early-forming
microhalos are a key observational signature of an EMDE, and our prescription
for the impact of gravitational heating will allow studies of the observational
status and prospects of EMDE scenarios to cover a much wider range of
parameters.Comment: 33 pages, 16 figures. Comments welcom
Prompt cusps and the dark matter annihilation signal
Dark matter is the dominant form of matter in today's universe. Its
gravitational effects drive the formation of galaxies and all larger structure,
yet its nature is unknown. As gravitational collapse creates the first cosmic
objects, a dark matter cusp forms immediately at every initial density maximum.
Such prompt cusps have a density profile extending up to
a limiting density dependent on the nature of the dark matter. Numerical
simulations and theoretical arguments suggest that the bulk of these cusps
survive until the present day. Here we show that if dark matter is a thermally
produced weakly interacting massive particle, many thousands of prompt cusps
with individual masses similar to that of the earth should be present in every
solar mass of dark matter. This radically alters predictions for the amount and
spatial distribution of dark matter annihilation radiation, substantially
tightening observational constraints on the relevant cross sections. In
particular, the cross section required to explain the observed -ray
excess near the Galactic Centre predicts prompt cusp emission from the Milky
Way's outer halo and from extragalactic dark matter at levels in tension with
the observed diffuse -ray background.Comment: 19 pages, 10 figures; submitte
Inner cusps of the first dark matter haloes: Formation and survival in a cosmological context
We use very high resolution cosmological zoom simulations to follow the early
evolution of twelve first-generation haloes formed from gaussian initial
conditions with scale-free power spectra truncated on small scales by a
gaussian. Initial collapse occurs with a diverse range of sheet- or
filament-like caustic morphologies, but in almost all cases it gives rise to a
numerically converged density cusp with and total mass
comparable to that of the corresponding peak in the initial linear density
field. The constant can be estimated to within about 10 per cent from the
properties of this peak. This outcome agrees with earlier work on the first
haloes in cold and warm dark matter universes. Within central cusps, the
velocity dispersion is close to isotropic, and equidensity surfaces tend to
align with those of the main body of the halo at larger radii. As haloes grow,
their cusps are often (but not always) overlaid with additional material at
intermediate radii to produce profiles more similar to the Einasto or NFW forms
typical of more massive haloes. Nevertheless, to the extent that we can resolve
them, cusps survive at the smallest radii. Major mergers can disturb them, but
the effect is quite weak in the cases that we study. The cusps extend down to
the resolution limits of our simulations, which are typically a factor of
several larger than the cores that would be produced by phase-space
conservation if the initial power spectrum cutoff arises from free streaming.Comment: 23 pages, 28 figures; to be submitted to MNRA
How an era of kination impacts substructure and the dark matter annihilation rate
An era of kination occurs when the Universe's energy density is dominated by
a fast-rolling scalar field. Dark matter that is thermally produced during an
era of kination requires larger-than-canonical annihilation cross sections to
generate the observed dark matter relic abundance. Furthermore, dark matter
density perturbations that enter the horizon during an era of kination grow
linearly with the scale factor prior to radiation domination. We show how the
resulting enhancement to the small-scale matter power spectrum increases the
microhalo abundance and boosts the dark matter annihilation rate. We then use
gamma-ray observations to constrain thermal dark matter production during
kination. The annihilation boost factor depends on the minimum halo mass, which
is determined by the small-scale cutoff in the matter power spectrum.
Therefore, observational limits on the dark matter annihilation rate imply a
minimum cutoff scale for a given dark matter particle mass and kination
scenario. For dark matter that was once in thermal equilibrium with the
Standard Model, this constraint establishes a maximum allowed kinetic
decoupling temperature for the dark matter. This bound on the decoupling
temperature implies that the growth of perturbations during kination cannot
appreciably boost the dark matter annihilation rate if dark matter was once in
thermal equilibrium with the Standard Model.Comment: 23 pages, 18 figures. References added; matches accepted versio
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