Title: Excess of substructure due to primordial black holes
Authors: P.E. Colazo, N. Padilla, and F. Stasyszyn
First Author’s Institution: Facultad de Matemática, Astronomía, Física y Computación, UNC, Argentina
Status: Submitted to Astronomy & Astrophysics [open access]
Another Dark Matter Candidate
Dark matter: you may know it as that elusive stuff that we know exists but still can’t find. Physicists and astronomers have proposed many potential dark matter candidates (theories about what dark matter might be), including WIMPs, axions, and even modified gravity. One of the stranger theories is that the gravitational anomalies we ascribe to dark matter could be due to primordial black holes (PBHs). Unlike “normal” black holes, which emerge from the death of a star, PBHs are hypothetical objects that could have formed directly from the gravitational collapse of extremely dense regions almost immediately following the Big Bang. Without the typical constraints of stellar evolution, PBHs could be less massive than normal black holes, and remain as yet undetected.
So, if practically invisible PBHs do exist, is there a way we could detect them indirectly? Today’s authors set out to answer this question using simulations that include PBHs to see how they influence the scale of structures in the universe, and if their influence could be detectable.
Tuning the Cosmological Dials
To simulate a universe that could create PBHs, we need to slightly alter the way our standard cosmological model (ΛCDM) treats inflation. Inflation was the rapid growth of the universe in the fraction of a second after the Big Bang which resulted in the initial density fluctuations that seeded all the large-scale structure we see today, like galaxies, clusters, and the cosmic web. The scale of these fluctuations is characterized by the primordial power spectrum, which is usually inferred from observations of the Cosmic Microwave Background. In order for PBHs to form, the primordial power spectrum needs to be adjusted to include more small-scale fluctuations. The authors of today’s work ran three simulations, each with a different primordial power spectrum input to its initial conditions, to examine various PBH model scenarios:
CDM: The trusty ΛCDM, “cold dark matter” model, generally accepted by all, with no PBHs.
NB: The “blue-tilted” model, which has nothing to do with color, but instead a primordial power spectrum that allows smaller-scale fluctuations than the CDM model. Smaller-scale fluctuations could result in PBHs, but don’t guarantee them.
FCT: The PBH model, which includes the same smaller-scale fluctuations as the NB model, as well as the influence of PBHs on the gravitational potential, called the Poisson effect.
The resulting primordial power spectra for all three models are shown in Figure 1.
The simulations were run using SWIFT, at multiple resolutions per model to ensure the results were not biased by computational effects. Each began at a redshift of z = 1200, the time of the formation of the Cosmic Microwave Background, and ran until present day.
Primordial Black Holes Increase Substructure
To see how PBHs affect the scale of structures in the universe, the authors examined the masses of dark matter halos and subhalos in each simulation. Both halos and subhalos were identified by overdense regions with a sufficient number of dark matter particles (depending on the resolution), however subhalos were those regions that were also encapsulated within a larger dark matter halo. Figure 2 shows the excess of halos and subhalos in the NB and FCT models compared to the CDM model, as a function of mass. Both the NB and FCT models had increased substructure (peaks in the low mass regime), which was especially pronounced for subhalos. The subhalo excess for the NB model peaked at 3 times that of the CDM model, while it was 6 times higher for subhalos in the FCT model.
But Can We Detect It?
Even though the presence of PBHs resulted in more substructure, actually detecting this difference is complicated. First, if PBHs exist, the excess in substructure they produce could be anywhere between the NB and FCT models. The NB model assumed only small-scale fluctuations that could result in PBHs, while the FCT model assumed dark matter consists of 100% PBHs. The actual percentage could lie somewhere in the middle. Second, the shaded gray region of Figure 1 represents dark matter halos that are not massive enough to host luminous galaxies. Most of the substructure excess is in this region, so unless the percentage of dark matter made up of PBHs is nearly as high as the FCT model assumes, it would be extremely difficult to detect directly through galaxy emission.
Finally, the authors tested the feasibility of substructure detection in stacked observations. (Stacking is a common trick used in astronomy when dealing with weak signals.) They created projected images of their most massive simulated halos, which were galaxy cluster-sized, because they contained the most subhalos, or substructure excess. Next, they stacked the images together as shown in the bottom row of Figure 3. The top row of Figure 3 shows the same images, but the subhalos have been removed to highlight the amount of substructure produced by each model. Both the NB and FCT models show more substructure, or clumpiness, than the CDM model. It is especially pronounced in the FCT model, so again, if PBHs make up a percentage of dark matter near that of the FCT model, this substructure could be detected as brightness fluctuations near the center of galaxy clusters. Alternatively, the substructure excess produced by just the NB model is enough to produce perturbations in strong gravitational lensing.
If dark matter is made up partially or completely of PBHs, the resulting substructure may leave detectable signatures in upcoming surveys from Roman, Euclid, and JWST. As is true for all dark matter candidates, the theory explored in works like today’s paper tells us what to look for as we continue to search.
Astrobite edited by William Smith
Featured image credit: NASA’s Goddard Space Flight Center
