Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userToward a Possible Solution to the Cosmic Coincidence Problem
157
0
0.0
(
0
)
Authors
Kim Griest
Ask ChatGPT about the research
No Arabic abstract
It is a mystery why the density of matter and the density of vacuum energy are nearly equal today when they scale so differently during the expansion of the Universe. We suggest a paradigm that might allow for a non-anthropic solution to this cosmic coincidence problem. The fact that the half life of Uranium 238 is very near to the age of the solar system is not considered a coincidence since there are many nuclides with half lives ranging over a huge range of time scales implying that there is likely to be some nuclide with a half life near to any given time scale. Likewise it may be that the vacuum field energy causing the universal acceleration today is just one of a large ensemble of scalar field energies, which have dominated the Universe in the past and then faded away. Given that in standard cosmology and particle physics there are already several scalar fields that probably contribute to universal vacuum energy (the Higgs field, the inflaton, and whatever quintessence/dark energy field causes the current acceleration), the idea of a large ensemble of fields does not seem far fetched. Predictions of the idea include: 1) The current vacuum energy driving the acceleration is not constant and will eventually fade away, 2) The ratio w of scalar field pressure to density is currently changing and is not precisely negative one, 3) There were likely periods of vacuum dominance and acceleration in the past, 4) the current phase of acceleration will end but there may be additional periods of acceleration in the future, 5) the ultimate fate of the Universe cannot be decided until the nature of these fields is known, but the eventual sum of densities from all scalar fields could be zero, as was usually assumed before the discovery of the current universal acceleration.
rate research
Read More
It is argued that cosmological models that feature a flow of energy from dark energy to dark matter may solve the coincidence problem of late acceleration (i.e., why the energy densities of both components are of the same order precisely today?). However, much refined and abundant observational data of the redshift evolution of the Hubble factor are needed to ascertain whether they can do the job.
Faced by recent evidence for a flat universe dominated by dark energy, cosmologists grapple with deep cosmic enigmas such as the cosmological constant problem, extreme fine-tuning and the cosmic coincidence problem. The extent to which we observe the dimming of distant supernovae suggests that the cosmic acceleration is as least as severe as in cosmological constant models. Extrapolating this to our cosmic future implies terrifying visions of either a cold and empty universe or an explosive demise in a ``Big Rip. We construct a class of dynamical scalar field models of dark energy and dark matter. Within this class we can explain why supernovae imply a cosmic equation of state $wlesssim-1$, address fine tuning issues, protect the universe from premature acceleration and predict a constant fraction of dark energy to dark matter in the future (thus solving the coincidence problem), satisfy the dominant energy condition, and ensure that gravitationally bound objects remain so forever (avoid a Big Rip). This is achieved with a string theory inspired Lagrangian containing standard kinetic terms, exponential potentials and couplings, and parameters of order unity.
Hot stars with hot Jupiters have a wide range of obliquities, while cool stars with hot Jupiters tend to have low obliquities. An enticing explanation for this pattern is tidal realignment of the cool host stars, although this explanation assumes that obliquity damping occurs faster than orbital decay, an assumption that needs further exploration. Here we revisit this tidal realignment problem, building on previous work identifying a low-frequency component of the time-variable tidal potential that affects the obliquity but not the orbital separation. We adopt a recent empirically-based model for the stellar tidal quality factor and its sharp increase with forcing frequency. This leads to enhanced dissipation at low frequencies, and efficient obliquity damping. We model the tidal evolution of 46 observed hot Jupiters orbiting cool stars. A key parameter is the stellar age, which we determine in a homogeneous manner for the sample, taking advantage of Gaia DR2 data. We explore a variety of tidal histories and futures for each system, finding in most cases that the stellar obliquity is successfully damped before the planet is destroyed. A testable prediction of our model is that hot-Jupiter hosts with orbital periods shorter than 2--3 days should have obliquities much smaller than $1^circ$. With the possible exception of WASP-19b, the predicted future lifetimes of the planets range from $10^8$,yr to more than $10^{10}$,yr. Thus, our model implies that these hot Jupiters are probably not in immediate danger of being devoured by their host stars while they are on the main sequence.
The possibility that the so-called lithium problem, i.e. the disagreement between the theoretical abundance predicted for primordial $^7$Li assuming standard nucleosynthesis and the value inferred from astrophysical measurements, can be solved through a non-thermal BBN mechanism has been investigated by several authors. In particular, it has been shown that the decay of a MeV-mass particle, like, e.g., a sterile neutrino, decaying after BBN not only solves the lithium problem, but also satisfies cosmological and laboratory bounds, making such a scenario worth to be investigated in further detail. In this paper, we constrain the parameters of the model with the combination of current data, including Planck 2015 measurements of temperature and polarization anisotropies of the CMB, FIRAS limits on spectral distortions, astrophysical measurements of primordial abundances and laboratory constraints. We find that a sterile neutrino with mass $M_S=4.35_{-0.17}^{+0.13},MeV$ (at $95%$ c.l.), a decay time $tau_S=1.8_{-1.3}^{+2.5}cdot 10^5,s$ (at $95%$ c.l.) and an initial density $bar{n}_S/bar{n}_{cmb}=1.7_{-0.6}^{+3.5}cdot 10^{-4}$ (at $95%$ c.l.) in units of the number density of CMB photons, perfectly accounts for the difference between predicted and observed $^7$Li primordial abundance. This model also predicts an increase of the effective number of relativistic degrees of freedom at the time of CMB decoupling $Delta N_{eff}^{cmb}equiv N_{eff}^{cmb}-3.046=0.34_{-0.14}^{+0.16}$ at $95%$ c.l.. The required abundance of sterile neutrinos is incompatible with the standard thermal history of the Universe, but could be realized in a low reheating temperature scenario. We provide forecasts for future experiments finding that the combination of measurements from the COrE+ and PIXIE missions will allow to significantly reduce the permitted region for the sterile lifetime and density.
Despite the ultraviolet problems with canonical quantum gravity, as an effective field theory its infrared phenomena should enjoy fully quantum mechanical unitary time evolution. Currently this is not possible, the impediment being what is known as the problem of time. Here, we provide a solution by promoting the cosmological constant $Lambda$ to a Lagrange multiplier constraining the metric volume element to be manifestly a total derivative. Because $Lambda$ appears linearly in the Hamiltonian constraint, it unitarily generates time evolution, yielding a functional Schroedinger equation for gravity. Two pleasant side effects of this construction are that vacuum energy is dissociated from the cosmological constant problem, much like in unimodular gravity, and the natural foliation provided by the time variable defines a sensible solution to the measure problem of eternal inflation.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
