Find my publications on INSPIRE, ADS, and arXiv.
How did the first stars and black holes form?
How do we make headway in observing this "cosmic dawn" and the preceding dark ages?
How can we use these and other cosmological observables to probe the nature of dark matter?
These are the kinds of questions that I work to answer.
I am broadly interested in cosmology and dark matter phenomenology,
and my research has spanned topics such as constraining dark matter microphysics,
primordial black holes, 21 cm cosmology, CMB spectral distortions,
stochastic gravitational wave backgrounds, and the formation of supermassive black holes.
For more details, see a list of my projects below!
The change to the critical virial temperature required for halos to form stars over
the parameter space for dark matter that decays to electrons and positrons.
Enhanced density fluctuations with amplitudes that are not large enough to form primordial black holes can still lead to collapsed dark matter halos at very early times. For halos forming prior to 1+z≈200, the Cosmic Microwave Background (CMB) is energetic enough to suppress the formation of molecular hydrogen, hence preventing cooling and fragmentation, as a consequence of which baryons falling into the potential well of the halo may undergo "direct collapse" into a black hole. These "not-quite-primordial black holes" (NQPBHs) could provide the seeds for supermassive black holes, as well as explain the abundance of high-redshift black holes recently inferred from observations by the James Webb Space Telescope.
Relevant publications: W. Qin, S. Kumar, P. Natarajan, and N. Weiner. Not-quite-primordial black holes. arXiv:2506.13858 [astro-ph.CO]
The change to the critical virial temperature required for halos to form stars over
the parameter space for dark matter that decays to electrons and positrons.
Dark matter interactions with Standard Model particles, such as decay or annihilation, can inject a significant amount of energy into the early universe, producing observable changes in the global temperature and ionization histories, as well as the background spectrum of radiation. These changes can alter observables such as the cosmic microwave background, the temperature of the intergalactic medium (IGM), and the timing of features in 21cm cosmology. Using the DarkHistory code package, we can quantify these effects and set constraints on dark matter masses/interactions in a model-independent way.
Relevant publications: C. Xu, W. Qin, and T. R. Slatyer. CMB limits on decaying dark matter beyond the ionization threshold. Phys.Rev.D, 110 (2024) 12, 123529W. Qin, J. B. Muñoz, H. Liu, and T. R. Slatyer. Birth of the first stars amidst decaying and annihilating dark matter. arXiv:2308.12992 [astro-ph.CO]
H. Liu, W. Qin, G. W. Ridgway, and T. R. Slatyer. Exotic energy injection in the early universe I: a novel treatment for low-energy electrons and photons. arXiv:2303.07366 [astro-ph.CO]
H. Liu, W. Qin, G. W. Ridgway, and T. R. Slatyer. Exotic energy injection in the early universe II: CMB spectral distortions and constraints on light dark matter. arXiv:2303.07370 [astro-ph.CO]
H. Liu, W. Qin, G. W. Ridgway, and T. R. Slatyer. Lyman-$\alpha$ Constraints on Cosmic Heating from Dark Matter Annihilation and Decay. Phys. Rev. D, 104(4):043514, 2021.
Now that experiments such as HERA are on the cusp of detecting the 21cm power spectrum, the ability to make theoretical predictions about the signal from reionization will be important for fully profiting from this new data. Previously, it was widely believed that the 21cm signal is nonperturbative and too difficult to treat analytically—however, recent studies have shown that an effective field theory description of 21cm radiation is valid at wavenumbers probed by experiments and ionization fractions less than ~0.8. We have further developed this effective field theory, e.g. by including the effect of redshift space distortions, since observations of the 21cm signal actually occur in redshift space. This is an important step for many future directions, such as incorporating spin temperature fluctuations, constraining new physics, and reconstructing modes lost to foregrounds.
Relevant publications: W. Qin, K.-F. Chen, K. Schutz, and A. Liu. Effective bias expansion for circumventing 21 cm foregrounds. arXiv:2508.13268 [astro-ph.CO].W. Qin, K. Schutz, O. Rosenstein, S. O'Neil, and M. Vogelsberger. Supersizing hydrodynamical simulations of reionization using perturbative techniques. arXiv:2504.02929 [astro-ph.CO].
W. Qin, K. Schutz, A. Smith, E. Garaldi, R. Kannan, T. R. Slatyer, and M. Vogelsberger. An Effective Bias Expansion for 21 cm Cosmology in Redshift Space. Phys. Rev. D, 106(12):123506, 2022.
Primordial black holes (PBHs) are of great interest today as dark matter candidates and potential seeds for supermassive black holes. It is generally agreed that black holes smaller than 10^17 g are ruled out because they evaporate too quickly to make up the dark matter abundance today, while black holes larger than 10^21 g are ruled out by microlensing and other constraints. In the window in between these constraints, it may still be possible for PBHs to make up all of the dark matter abundance. Previous studies have found that certain models of single-field inflation are able to generate spikes in the curvature power spectrum that can result in PBHs without spoiling agreement with CMB measurements, but generally require a high degree of fine-tuning. We are seeking to understand whether it is possible to generate this behavior more generically in multifield models.
Relevant publications:W. Qin, S. Geller, S.Balaji, E. McDonough, and D. I. Kaiser. Planck Constraints and Gravitational Wave Forecasts for Primordial Black Hole Dark Matter Seeded by Multifield Inflation. arXiv:2303.02168 [astro-ph.CO]
S. Geller, W. Qin, E. McDonough, and D. I. Kaiser. Primordial Black Holes from Multifield Inflation with Nonminimal Couplings. Phys. Rev. D, 106(6):063535, 2022.
When gravitational waves interact with the light from stars, they can cause the time of the light's arrival to change, which is what pulsar timing arrays seek to measure, as well as deflect the light's trajectory, which manifests in astrometry as a fluctuation in the apparent positions of stars. These deviations will be correlated in certain patterns, depending on the polarizations of the incident waves. Knowing what these correlations look like can then allow us to disentangle signals from theories of modified gravity from the standard signal predicted by general relativity. Drawing an analogy to how one derives the statistics of cosmic microwave background polarization, we showed how one can construct the auto-correlations and cross-correlations of these observables. We also demonstrated that the power spectrum of observables induced by each polarization can be calculated with relative ease using total-angular-momentum waves, which provide an alternative to the plane wave basis that simplifies calculations on the sphere of the sky.
Relevant publications:W. Qin, K. K. Boddy, and M. Kamionkowski. Subluminal stochastic gravitational waves in pulsar-timing arrays and astrometry. Phys. Rev. D, 103(2):024045, 2021.
W. Qin, K. K. Boddy, M. Kamionkowski, and L. Dai. Pulsar-timing arrays, astrometry, and gravitational waves. Phys. Rev. D, D99(6):063002, 2019.