Engineering topics
Clark, Steven J.
Publications and source records attributed to Clark, Steven J..
H 0 and S 8 tensions necessitate early and late time changes to Λ CDM
An only early or only late time alteration to Λ CDM has been inadequate at resolving both the H 0 and S 8 tensions simultaneously; however, a combination of early and late time alterations to Λ CDM can provide a solution to both tensions. As an illustration, we examine a combined early dark energy, decaying dark matter model. While early dark energy has the ability to resolve the H 0 tension, it leads to a discrepancy in S 8 measurements. We show that the addition of decaying dark matter helps resolve the S 8 discrepancy that would otherwise be enhanced in an early dark energy model, while the latter is able to relieve the H 0 disagreement to within the 95th percentile interval. Finally, our results show a preference for the combined model over Λ CDM with ΔAIC = –6.72, hinting that both early and late Universe modifications may be necessary to address the cosmological tensions.
Correlating gravitational wave and gamma-ray signals from primordial black holes
Asteroid-mass primordial black holes (PBHs) can explain the observed dark matter abundance while being consistent with the current indirect detection constraints. These PBHs can produce gamma-ray signals from Hawking radiation that are within the sensitivity of future measurements by the AMEGO and e-ASTROGAM experiments. PBHs which give rise to such observable gamma-ray signals have a cosmic origin from large primordial curvature fluctuations. There must then be a companion, stochastic gravitational wave (GW) background produced by the same curvature fluctuations. We demonstrate that the resulting GW signals will be well within the sensitivity of future detectors such as LISA, DECIGO, BBO, and the Einstein Telescope. The multimessenger signal from the observed gamma-rays and GWs will allow a precise measurement of the primordial curvature perturbation that produces the PBH. Indeed, so we argue that the resulting correlation between the two types of observations can provide a smoking-gun signal of PBHs.
Nonlocal effects from boosted dark matter in indirect detection
Here, indirect dark matter (DM) detection typically involves the observation of standard model (SM) particles emerging from DM annihilation/decay inside regions of high dark matter concentration. We consider an annihilation scenario in which this reaction has to be initiated by one of the DMs involved being boosted while the other is an ambient nonrelativistic particle. This “trigger” DM must be created, for example, in a previous annihilation or decay of a heavier component of DM. Remarkably, boosted DM annihilating into gamma rays at a specific point in a galaxy could actually have traveled from its source at another point in the same galaxy or even from another galaxy. Such a “nonlocal” behavior leads to a nontrivial dependence of the resulting photon signal on the galactic halo parameters, such as DM density and core size, encoded in the so-called “astrophysical” J-factor. These nonlocal J-factors are strikingly different than the usual scenario. A distinctive aspect of this model is that the signal from dwarf galaxies relative to the Milky Way tends to be suppressed from the typical value to various degrees depending on their characteristics. This feature can thus potentially alleviate the mild tension between the DM annihilation explanation of the observed excess of approximately GeV photons from the Milky Way’s Galactic Center vs the apparent nonobservation of the corresponding signal from dwarf galaxies.