Lam Hui, Austin Joyce, Michael J. Landry, Xinyu Li

In a galactic halo like the Milky Way, bosonic dark matter particles lighter than about $100$ eV have a de Broglie wavelength larger than the average inter-particle separation and are therefore well described as a set of classical waves. This applies to, for instance, the QCD axion as well as to lighter axion-like particles such as fuzzy dark matter. We show that the interference of waves inside a halo inevitably leads to vortices, locations where chance destructive interference takes the density to zero. The phase of the wavefunction has non-trivial winding around these points. This can be interpreted as a non-zero velocity circulation, so that vortices are sites where the fluid velocity has a non-vanishing curl. Using analytic arguments and numerical simulations, we study the properties of vortices and show they have a number of universal features: (1) In three spatial dimensions, the generic defects take the form of vortex rings. (2) On average there is about one vortex ring per de Broglie volume and (3) generically only single winding ($\pm 1$) vortices are found in a realistic halo. (4) The density near a vortex scales as $r^2$ while the velocity goes as $1/r$, where $r$ is the distance to vortex. (5) A vortex segment moves at a velocity inversely proportional to its curvature scale so that smaller vortex rings move faster, allowing momentary motion exceeding escape velocity. We discuss observational/experimental signatures from vortices and, more broadly, wave interference. In the ultra-light regime, gravitational lensing by interference substructures leads to flux anomalies of $5-10 \%$ in strongly lensed systems. For QCD axions, vortices lead to a diminished signal in some detection experiments but not in others. We advocate the measurement of correlation functions by axion detection experiments as a way to probe and capitalize on the expected interference substructures.

Betty X. Hu, Abraham Loeb

Gravitational waves (GWs) are produced by colliding particles through the gravitational analogue of electromagnetic bremsstrahlung. We calculate the contribution of free-free emission in the radiation-dominated Universe to the stochastic GW background. We find that the energy density of the resulting GW radiation is heavily dependent on the number of elementary particles, $N_{\mathrm{tot}}$, and the maximum initial temperature, $T_{\mathrm{max}}$. We rule out $N_{\mathrm{tot}}\gtrsim N_{\mathrm{SM}}$ for $T_{\mathrm{max}}\sim T_{\mathrm{Planck}}\approx10^{19}$ GeV and $N_{\mathrm{tot}}\gtrsim10^{13}\times N_{\mathrm{SM}}$ for $T_{\mathrm{max}}\sim10^{16}$ GeV, where $N_{\mathrm{SM}}$ is the number of particles in the Standard Model. In the case of inflation, existing cosmological data constrain $T_{\mathrm{max}}\lesssim10^{16}$ GeV. However, alternative models to inflation such as bouncing cosmologies allow for $T_{\mathrm{max}}$ near $T_{\mathrm{Planck}}$. At the energy scales we are considering, the extra number of particles arise naturally in models of extra dimensions.

Yuchen Du, Shammi Tahura, Diana Vaman, Kent Yagi

We study the effect of compact extra dimensions on the gravitational wave luminosity and waveform. We consider a toy model, with a compactified fifth dimension, and matter confined on a brane. We work in the context of five dimensional ($5d$) general relativity, though we do make connections with the corresponding Kaluza-Klein effective $4d$ theory. We show that the luminosity of gravitational waves emitted in $5d$ gravity by a binary with the same characteristics (same masses and separation distance) as a $4d$ binary is 20.8\% less relative to the $4d$ case, to leading post-Newtonian order. The phase of the gravitational waveform differs by 26\% relative to the $4d$ case, to leading post-Newtonian order. Such a correction arises mainly due to the coupling between matter and dilaton field in the effective $4d$ picture and agrees with previous calculations when we set black holes' scalar charges to be those computed from the Kaluza-Klein reduction. The above correction is inconsistent with the recent gravitational-wave observations and it thus effectively rules out the possibility of such a simple compactified higher dimensions scenario. We also comment on how our results change if there are several compactified extra dimensions, and show that the discrepancy with $4d$ general relativity only increases.

Lerh Feng Low, Shaun Hotchkiss, Richard Easther

Landscape cosmology posits the existence of a convoluted, multidimensional, scalar potential -- the "landscape" -- with vast numbers of metastable minima. Random matrices and random functions in many dimensions provide toy models of the landscape, allowing the exploration of conceptual issues associated with these scenarios. We compute the relative number and slopes of minima as a function of the vacuum energy $\Lambda$ in an $N$-dimensional Gaussian random potential, quantifying the associated probability density, $p(\Lambda)$. After normalisations $p(\Lambda)$ depends only on the dimensionality $N$ and a single free parameter $\gamma$, which is related to the power spectrum of the random function. For a Gaussian landscape with a Gaussian power spectrum, the fraction of positive minima shrinks super-exponentially with $N$; at $N=100$, $p(\Lambda>0) \approx 10^{-780}$. Likewise, typical eigenvalues of the Hessian matrices reveal that the flattest approaches to typical minima grow flatter with $N$, while the ratio of the slopes of the two flattest directions grows with $N$. We discuss the implications of these results for both swampland and conventional anthropic constraints on landscape cosmologies. In particular, for parameter values when positive minima are extremely rare, the flattest approaches to minima where $\Lambda \approx 0$ are much flatter than for typical minima, increasingly the viability of quintessence solutions.

Metin Gurses, Tahsin Cagri Sisman, Bayram Tekin

No! We show that the field equations of Einstein-Gauss-Bonnet theory defined in generic $D>4$ dimensions split into two parts one of which always remains higher dimensional, and hence the theory does not have a non-trivial limit to $D=4$. Therefore, the recently introduced four-dimensional, novel, Einstein-Gauss-Bonnet theory does not admit an intrinsically four-dimensional definition as such it does not exist in four dimensions. The solutions (the spacetime, the metric) always remain $D>4$ dimensional. As there is no canonical choice of 4 spacetime dimensions out of $D$ dimensions for generic metrics, the theory is not well defined in four dimensions.