123 research outputs found
Milky Way and Andromeda past-encounters in different gravity models: the impact on the estimated Local Group mass
The Two-body problem of and the Milky Way (MW) galaxies with a
Cosmological Constant background is studied, with emphasis on the possibility
that they experienced Past Encounters. By implementing the Timing Argument
(TA), it is shown that if and the MW have had more than one encounter
then the deduced mass of the Local Group (LG) would be larger. Past encounters
are possible only for non-zero transverse velocity, and their viability is
subject to observations of the imprints of such near collisions. Using a recent
- based measurement of the transverse velocity we show that the presence
of the Cosmological Constant requires the mass for the LG to be higher:
with no Cosmological Constant or
with a Cosmological Constant
background. If the LG has had one past encounter, the LG mass is
with a Cosmological Constant
background. Modified Newtonian Dynamics (MOND) is studied as the accelerations
of the Local Group are fully in the deep-MOND regime. MOND yields the order of
magnitude for the expected baryonic mass only if at least one encounter
occurred. While we only consider the LG as two point masses, our calculations
provide a benchmark for future work with simulations to test Dynamical Friction
and other effects. This model can be also used to test screening mechanisms and
alternative theories of gravity.Comment: 16 pages. A revised versio
Decaying coupled Fermions to curvature and the tension
A formulation of cosmology driven by fermions is studied. Assumption
of condensation for the spinor field simplifies the homogeneous solution of the
Dirac equations and connects the spinor field with the scale parameter of the
universe. With coupling between the Einstein term and spinor field, the
possibility for a late time interaction emerges. In that way, the early
universe agrees with CDM model, but for the late universe the new
integrating term dominates. From late time expansion data we obtain the
from the SH0ES experiment. The data include the Pantheon Type Ia supernova,
Quasars, Gamma Ray Bursts (for the Hubble diagram), cosmic chronometers and
Byron Acoustic Oscillations. The decaying coupling reveals the capabilities of
the scenario and makes it a good candidate for the description of nature. The
tension can be reduced even further by including the local measurement of the
Hubble constant.Comment: 5 pages, 2 figure
Unified Dark Energy and Dark Matter from Dynamical Spacetime Cosmology
A model of unified dark matter and dark energy based on a Dynamical Spacetime
Theory (DST) is studied. By introducing a Dynamical Spacetime vector field
, a conservation of an energy momentum tensor
emerges. The action allows for two different potentials, while one represents a
dark energy. For constant potentials, the cosmological solution yields a non
singular bouncing solutions that rapidly approaches the CDM model. The
Dynamical Time corresponds to the cosmic time as well. The theory fits with the
late time expansion data of the Universe. With higher dimensions a mechanism
for inflation and compactification appears, with exponential growth for some
dimensions and exponential contraction of the others. By demanding that the
Dynamical Spacetime vector field be a gradient of a scalar the DST becomes a
theory with diffusive interacting, which asymptotically returns to the
CDM model as a stable point. These formulations lead to scenarios
which address our understanding about the origin of the Universe.Comment: PhD Thesi
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