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Register a new userMulti-field dark energy: cosmic acceleration on a steep potential
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We argue that dark energy with multiple fields is theoretically well-motivated and predicts distinct observational signatures, in particular when cosmic acceleration takes place along a trajectory that is highly non-geodesic in field space. Such models provide novel physics compared to $Lambda$CDM and quintessence by allowing cosmic acceleration on steep potentials. From the theoretical point of view, these theories can easily satisfy the conjectured swampland constraints and may in certain cases be technically natural, potential problems which are endemic to standard single-field dark energy. Observationally, we argue that while such multi-field models are likely to be largely indistinguishable from the concordance cosmology at the background level, dark energy perturbations can cluster, leading to an enhanced growth of large-scale structure that may be testable as early as the next generation of cosmological surveys.
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A web of interlocking observations has established that the expansion of the Universe is speeding up and not slowing, revealing the presence of some form of repulsive gravity. Within the context of general relativity the cause of cosmic acceleration is a highly elastic (psim -rho), very smooth form of energy called ``dark energy accounting for about 75% of the Universe. The ``simplest explanation for dark energy is the zero-point energy density associated with the quantum vacuum; however, all estimates for its value are many orders-of-magnitude too large. Other ideas for dark energy include a very light scalar field or a tangled network of topological defects. An alternate explanation invokes gravitational physics beyond general relativity. Observations and experiments underway and more precise cosmological measurements and laboratory experiments planned for the next decade will test whether or not dark energy is the quantum energy of the vacuum or something more exotic, and whether or not general relativity can self consistently explain cosmic acceleration. Dark energy is the most conspicuous example of physics beyond the standard model and perhaps the most profound mystery in all of science.
We study the effect of an explicit interaction between two scalar fields components describing dark matter in the context of a recent proposal framework for interaction. We find that, even assuming a very small coupling, it is sufficient to explain the observational effects of a cosmological constant, and also overcome the problems of the $Lambda$CDM model without assuming an exotic dark energy.
In the forthcoming decades, the redshift drift observations in optical and radio bands will provide accurate measurements on $H(z)$ covering the redshift ranges of $2<z<5$ and $0<z<1$. In addition, gravitational wave (GW) standard siren observations could make measurements on the dipole anisotropy of luminosity distance, which will also provide the $H(z)$ measurements in the redshift range of $0<z<3$. In this work, we propose a multi-messenger and multi-wavelength observational strategy to measure $H(z)$ based on the three next-generation projects, E-ELT, SKA, and DECIGO, and we wish to see whether the future $H(z)$ measurements could provide tight constraints on dark-energy parameters. It is found that E-ELT, SKA1, and DECIGO are highly complementary in constraining dark energy models using the $H(z)$ data. We find that E-ELT, SKA1, and DECIGO can tightly constrain $Omega_m$, $w$ (or $w_0$), and $H_0$, respectively, and thus the combination of them could effectively break the cosmological parameter degeneracies. The joint E-ELT+SKA1+DECIGO data give $sigma(w)approx 0.02$ in the $w$CDM model and $sigma(w_0)approx 0.03$ in the CPL model, which are better than the results of {it Planck} 2018 TT,TE,EE+lowE+lensing+SNe+BAO. But even the joint data cannot well constrain $w_a$ in the CPL model.
We investigate the creation of cold dark matter (CCDM) cosmology as an alternative to explain the cosmic acceleration. Particular attention is given to the evolution of density perturbations and constraints coming from recent observations. By assuming negligible effective sound speed we compare CCDM predictions with redshift-space-distortion based f(z) sigma_8(z) measurements. We identify a subtle issue associated with which contribution in the density contrast should be used in this test and then show that the CCDM results are the same as those obtained with LambdaCDM. These results are then contrasted with the ones obtained at the background level. For the background tests we have used type Ia supernovae data (Union 2.1 compilation) in combination with baryonic acoustic oscillations and cosmic microwave background observations and also measurements of the Hubble parameter at different redshifts. As a consequence of the studies we have performed at both the background and perturbation levels, we explicitly show that CCDM is observationally degenerate with respect to LambdaCDM (dark degeneracy). The need to overcome the lack of a fundamental microscopic basis for the CCDM is the major challenge for this kind of model.
We study a phenomenological dark energy model which is rooted in the Veneziano ghost of QCD. In this dark energy model, the energy density of dark energy is proportional to Hubble parameter and the proportional coefficient is of the order $Lambda^3_{QCD}$, where $Lambda_{QCD}$ is the mass scale of QCD. The universe has a de Sitter phase at late time and begins to accelerate at redshift around $z_{acc}sim0.6$. We also fit this model and give the constraints on model parameters, with current observational data including SnIa, BAO, CMB, BBN and Hubble parameter data. We find that the squared sound speed of the dark energy is negative, which may cause an instability. We also study the cosmological evolution of the dark energy with interaction with cold dark matter.
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