Ultracompact Minihalos Research Articles

Minihalos as probes of the inflationary spectrum:accurate boost factor calculation and new CMB constraints

Although the spectrum of primordial fluctuations has been accurately measured on scales above ∼ 0.1 Mpc, only upper limits exist on smaller scales. In this study, we investigate generic monochromatic enhancements to the ΛCDM spectrum that trigger the collapse of ultracompact minihalos (UCMHs) well before standard structure formation. We refine previous treatments by considering a mixed population of halos with different density profiles, that should realistically arise as a consequence of late-time accretion and mergers. Assuming that dark matter (DM) can self-annihilate, we find, as expected, that UCMHs can greatly enhance the annihilation rate around recombination, significantly imprinting the cosmic microwave background (CMB) anisotropies. However, we provide additional insight on the theoretical uncertainties that currently impact that boost and which may affect late-time probes such as the 21 cm line or γ-ray signals. We derive constraints on the primordial power spectrum on small scales using the ExoCLASS/HYREC codes and the Planck legacy data. We account for the velocity dependence of the DM annihilation cross-section (s- or p-wave), annihilation channel, the DM particle mass and the inclusion of late-time halo mergers. Our s-wave constraints are competitive with previous literature, excluding primordial amplitudes \U0001d49c*≳ 10-6.5 at wavenumbers k ∼ 104 - 107 Mpc-1. For the first time, we highlight that even p-wave processes have constraining power on the primordial spectrum for cross-sections still allowed by currently the strongest astrophysical constraints. Finally, we provide an up-to-date compilation of the most stringent limits on the primordial power spectrum across a wide range of scales.

Cosmological contribution from population III stars in ultracompact minihalos

In this work, we investigate the effect of Population~III~(Pop.~III) stars in ultracompact minihalos~(UCMHs) on the cosmic ionization history using the Planck observation data. Although high-redshift astrophysics is not understood yet, UCMHs could host the Pop.~III stars like the halos formed in the standard structure formation scenario. Such Pop.~III stars would emit ionizing photons during their main sequence and facilitate cosmic reionization in high redshifts. To study their effects on the global ionization, we model the cosmic reionization evolution based on the ``tanh"-type reionization model which is expressed by $z_{\mathrm{reio}}$ with additional two parameters characterizing the initial mass of UCMHs and the number density of UCMHs. We implement the Monte Carlo Markov Chain analysis with the latest Planck observation data for our reionization model. As the result, we found that if the UCMH initial mass is larger than $10^{8.4}\mathrm{M}_{\odot}$, the number density of UCMHs is strictly limited. Then we obtained the constraint on the amplitude of the primordial power spectrum through the constraint on the UCMH number density like $\mathcal{A}_{\zeta}\lesssim 10^{-8}$ in the scales, $k\lesssim 50\mathrm{Mpc}^{-1}$, when we assume that the standard ``tanh"-type reionization occurs by $z=3$, so that we set $z_{\mathrm{reio}}>3$.

PBH assisted search for QCD axion dark matter

The entropy production prior to BBN era is one of ways to prevent QCD axion with the decay constant Fa ∈ [1012 GeV, 1016 GeV] from overclosing the universe when the misalignment angle is θ i = \U0001d4aa(1). As such, it is necessarily accompanied by an early matter-dominated era (EMD) provided the entropy production is achieved via the decay of a heavy particle. In this work, we consider the possibility of formation of primordial black holes during the EMD era with the assumption of the enhanced primordial scalar perturbation on small scales (k > 104 Mpc-1). In such a scenario, it is expected that PBHs with axion halo accretion develop to ultracompact minihalos (UCMHs). We study how UCMHs so obtained could be of great use in the experimental search for QCD axion dark matter with Fa ∈ [1012 GeV, 1016 GeV].

Constraints on primordial black holes with CMB spectral distortions

In the mixed dark matter scenarios consisting of primordial black holes (PBHs) and weakly interacting massive particles (WIMPs), WIMPs can be accreted onto PBHs to form ultracompact minihalos (UCMHs) with a density spike in the early universe. Compared with the classical dark matter halo, UCMHs are formed earlier and have a higher density of center. Since the annihilation rate is proportional to the squared number density of WIMPs, it is expected that WIMPs annihilation within UCMHs is enhanced and has influences on the early universe. Between the time of recombination and matter-radiation equality, the energy released from WIMPs annihilation within UCMHs is injected into the Universe resulting in cosmic microwave background $y$-type distortion. We investigate these effects and derive the upper limits on the abundance of PBHs taking advantage of the observational results of a far infrared absolute spectrophotometer (FIRAS). We find that for the WIMPs mass range $1\ensuremath{\le}{m}_{\ensuremath{\chi}}\ensuremath{\le}1000\text{ }\text{ }\mathrm{GeV}$, the upper limits on the abundance of PBHs are $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\ensuremath{\le}{\mathrm{\ensuremath{\Omega}}}_{\mathrm{PBH}}\ensuremath{\le}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$.

Radio bounds on the mixed dark matter scenarios of primordial black holes and WIMPs

We study the synchrotron radio emission in the mixed dark matter scenarios consisting of the primordial black holes (PBHs) and the self-annihilating WIMPs (weakly interacting massive particles). The WIMPs can form the ultracompact minihalos around PBHs and the annihilation enhancement from these dense halos can lead to the efficient synchrotron radiation at the radio frequency in the presence of galactic magnetic fields. The upper bound of PBH fraction with respect to the total dark matter abundance is of order 10-8 ∼ 10-5 depending on the electroweak scale WIMP mass (mχ = 10 ∼ 1000 GeV) and the WIMP annihilation channel (e.g. a hadronic χχ ⟶ bb̅ or a leptonic χχ ⟶ e + e - channel). The PBH contribution to the total dark matter abundance is hence negligible when the other component of dark matter is composed of the conventional electroweak scale WIMPs.

Constraints on small-scale primordial density fluctuation from cosmic microwave background through dark matter annihilation

The cosmic microwave background (CMB) observation by the Planck satellite precisely determines primordial curvature fluctuations on larger scales than \U0001d4aa(1) Mpc, while the small-scale curvature fluctuation is still less constrained.The constraint on small-scale fluctuations is highly improved if we assume the standard thermal relic dark matter scenario.When small-scale fluctuations are large enough, dense regions collapse to form small halos even in a redshift z ≳ 103, which is called “ultracompact minihalos”.These minihalos enhance the annihilation of the dark matter and it is constrained by observations such as extragalactic gamma rays and the CMB. We revisit the effect of minihalos formed by the small-scale density fluctuations and calculate the ionization history modified by the dark matter annihilation assuming the thermally produced WIMP accounting for all dark matter with e + e −-channel annihilation.We perform the Markov Chain Monte Carlo method to constrain the size of small-scale curvature fluctuations by the CMB power spectrum.It is found that the constraint from the CMB power spectrum is comparable to that from the extragalactic gamma rays.Assuming a pair-annihilation cross section 〈σ v〉 = 3 × 10-26cm3/s and a mass of dark matter 1 TeV,we constrain the curvature power spectrum \U0001d4abℛ(k) < \U0001d4aa(10-6) over 1 Mpc-1 < k < 106 Mpc-1.We confirm that our constraint mainly comes from the energy injection in early time (z ≳ 100) and hence it is independent of the uncertainty of minihalo properties in the late time.

Constraints on ultracompact minihalos from the extragalactic gamma-ray background observation

Ultracompact minihalo (UCMH) is a special type of dark matter halo with a very steep density profile that may form in the early Universe seeded by an overdense region or a primordial black hole. Constraints on its abundance give valuable information on the power spectrum of primordial perturbation. In this work, we update the constraints on the UCMH abundance in the Universe using the extragalactic gamma-ray background observation. Comparing to previous works, we adopt the updated Fermi-LAT extragalactic gamma-ray background measurement and derive constraints based on a full consideration of the astrophysical contributions. With these improvements, we place constraints on UCMH abundance 1--2 orders of magnitude better than previous results. With the background components considered, we can also attempt to search for possible additional components beyond the known astrophysical contributions.

Shining primordial black holes

We study the well-motivated mixed dark matter (DM) scenario composed of a dominant thermal WIMP, highlighting the case of $SU(2{)}_{L}$ triplet fermion ``winos,'' with a small fraction of primordial black holes (PBHs). After the wino kinetic decoupling, the DM particles are captured by PBHs leading to the presence of PBHs with dark minihalos in the Milky Way today. The strongest constraints for the wino DM come from the production of narrow line gamma rays from wino annihilation in the Galactic Center. We analyse in detail the viability of the mixed wino DM scenario, and determine the constraints on the fraction of DM in PBHs assuming a cored halo profile in the Milky Way. We show that already with the sensitivity of current indirect searches, there is a significant probability for detecting a gamma ray signal characteristic for the wino annihilation in a single nearby dressed PBH when ${M}_{\mathrm{PBH}}\ensuremath{\sim}{M}_{\ensuremath{\bigodot}}$, which we refer to as a ``shining black hole.'' Similar results should apply also in more general setups with ultracompact minihalos or other DM models, since the accretion of DM around large overdensities and DM annihilation are both quite generic processes.

The abundance of primordial black holes from the global 21cm signal and extragalactic gamma-ray background

Primordial black holes (PBHs) formed in the early Universe can accret dark matter particles due to gravity and form ultracompact minihalos (UCMHs). The theoretical researches and simulations have shown that the density profile of dark matter in UCMHs is in the form of $\rho (r) \sim r^{-2.25}$. Compared with the popular dark matter halo model, e.g. NFW model, dark matter annihilation rate is larger in UCMHs. Considering dark matter annihilation, there is a maximum core density $\rho_{\rm max}$ in UCMHs which has been treated independent on redshift. While in this work, we point out that $\rho_{\rm max}$ depends on redshift and dark matter annihilation rate in UCMHs also changes with time. We re-investigate the $\gamma$-ray flux from UCMHs due to dark matter annihilation and focus on their contributions to the extragalactic gamma-ray background (EGB). Utilizing the EGB data of Fermi, the constraints on the abundance of PBHs are derived, $\Omega_{\rm PBH,EGB} \lesssim 2\times 10^{-8}$. Motivated by the recent observations of global 21cm signals by the EDGES experiment, we investigate the influences of dark matter annihilation in UCMHs on global 21cm signals and derive the constraints on the abundance of PBHs, $\Omega_{\rm PBH,21cm} \lesssim 2\times 10^{-10}$. The derived constraints are valid for mass range of $10^{-6} M_{\odot} \lesssim M_{\rm PBH} \lesssim 10^{3} M_{\odot}$.

Compact dark matter objects via N dark sectors

We propose a novel class of compact dark matter objects in theories where the dark matter consists of multiple sectors. We call these objects $N$-MACHOs. In such theories neither the existence of dark matter species nor their extremely weak coupling to the observable sector represent additional hypotheses but instead are imposed by the solution to the Hierarchy Problem and unitarity. The crucial point is that particles from the same sector have non-trivial interactions but interact only gravitationally otherwise. As a consequence, the pressure that counteracts the gravitational collapse is reduced while the gravitational force remains the same. This results in collapsed structures much lighter and smaller as compared to the ordinary single-sector case. We apply this phenomenon to a dark matter theory that consists of $N$ dilute copies of the Standard Model. The solutions do not rely on an exotic stabilization mechanism, but rather use the same well-understood properties as known stellar structures. This framework also gives rise to new microscopic superheavy structures, for example with mass $10^8\,$g and size $10^{-13}\,$cm. By confronting the resulting objects with observational constraints, we find that, due to a huge suppression factor entering the mass spectrum, these objects evade the strongest constrained region of the parameter space. Finally, we discuss possible formation scenarios of $N$-MACHOs. We argue that, due to the efficient dissipation of energy on small scales, high-density regions such as ultra-compact mini-halos could serve as formation sites of $N$-MACHOs.

WIMPs and stellar-mass primordial black holes are incompatible

We recently showed that postulated ultracompact minihalos with a steep density profile do not form in realistic simulations with enhanced initial perturbations. In this paper we assume that a small fraction of the dark matter consists of primordial black holes (PBHs) and simulate the formation of structures around them. We find that in this scenario halos with steep density profiles do form, consistent with theoretical predictions. If the rest of the dark matter consists of weakly interacting massive particles (WIMPs), we also show that WIMPs in the dense innermost part of these halos would annihilate and produce a detectable gamma-ray signal. The non-detection of this signal implies that PBHs make up at most one billionth of the dark matter, provided that their mass is greater than one millionth of the mass of the Sun. Similarly, a detection of PBHs would imply that the remaining dark matter could not be WIMPs.

Ultracompact minihalos associated with stellar-mass primordial black holes

The possibility that primordial black hole binary mergers of stellar mass can explain the signals detected by the gravitational-wave interferometers has attracted much attention. In this scenario, primordial black holes can comprise only part of the entire dark matter, say, of order 0.1 %. This implies that most of the dark matter is accounted for by a different component, such as Weakly Interacting Massive Particles. We point out that in this situation, very compact dark matter minihalos, composed of the dominant component of the dark matter, are likely to be formed abundantly in the early Universe, with their formation redshift and abundance depending on primordial non-Gaussianity. They may be detected in future experiments via pulsar observations.

Through a mini halo, darkly

In this Letter we explore the effects of the scattering of photons incident on a dark matter halo through their interaction with either electrons or photons produced by dark matter annihilation. Particularly, we examine the effects of this scattering upon the observed spectrum of a distant AGN or of the Cosmic Microwave Background. Our results indicate that ultra-compact mini halos and other dense dark matter clumps can induce an observable Comptonisation of AGN spectra as well as a Sunyaev-Zel'dovich effect (SZE) with an optical depth similar to that attained by thermal electrons in the Coma cluster. The rate of encounters between a distant AGN and these dense mini-halos is also estimated using micro-lensing limits existing on the population of dark compact bodies.

Molecular Ionization Rates and Ultracompact Dark Matter Minihalos.

Molecular ionization in the central molecular zone of our Galaxy is enhanced over the typical galactic value by an order of magnitude or more. This cannot be easily explained for dense Galactic center molecular complexes in the absence of embedded sources of low energy cosmic rays. We provide such a source in the form of ultracompact minihalos of self-annihilating dark matter for a variety of annihilation channels that depend on the particle mass and model. Such sources are motivated for plausible inflationary power spectrum parameters, and while possibly subdominant in terms of the total dark matter mass within the Galactic bulge, might also account for, or at least not be in tension with, the Fermi Galactic center γ-ray excess.

Are ultracompact minihalos really ultracompact?

Ultracompact minihalos (UCMHs) have emerged as a valuable probe of the primordial power spectrum of density fluctuations at small scales. UCMHs are expected to form at early times in regions with ${\delta\rho/\rho \gtrsim 10^{-3}}$, and they are theorized to possess an extremely compact ${\rho\propto r^{-9/4}}$ radial density profile, which enhances their observable signatures. Nonobservation of UCMHs can thus constrain the primordial power spectrum. Using $N$-body simulations to study the collapse of extreme density peaks at ${z \simeq 1000}$, we show that UCMHs forming under realistic conditions do not develop the ${\rho\propto r^{-9/4}}$ profile and instead develop either ${\rho\propto r^{-3/2}}$ or ${\rho\propto r^{-1}}$ inner density profiles depending on the shape of the power spectrum. We also demonstrate via idealized simulations that self-similarity---the absence of a scale length---is necessary to produce a halo with the ${\rho\propto r^{-9/4}}$ profile, and we argue that this implies such halos cannot form from a Gaussian primordial density field. Prior constraints derived from UCMH nonobservation must be reworked in light of this discovery. Although the shallower density profile reduces UCMH visibility, our findings reduce their signal by as little as $\mathcal O(10^{-2})$ while allowing later-forming halos to be considered, which suggests that new constraints could be significantly stronger.

Uncertainties in primordial black-hole constraints on the primordial power spectrum

The existence (and abundance) of primordial black holes (PBHs) is governed by the power spectrum of primordial perturbations generated during inflation. So far no PBHs have been observed, and instead, increasingly stringent bounds on their existence at different scales have been obtained. Up until recently, this has been exploited in attempts to constrain parts of the inflationary power spectrum that are unconstrained by cosmological observations. We first point out that the simple translation of the PBH non-observation bounds into constraints on the primordial power spectrum is inaccurate as it fails to include realistic aspects of PBH formation and evolution. We then demonstrate, by studying two examples of uncertainties from the effects of critical and non-spherical collapse, that even though they may seem small, they have important implications for the usefulness of the constraints. In particular, we point out that the uncertainty induced by non-spherical collapse may be much larger than the difference between particular bounds from PBH non-observations and the general maximum cap stemming from the condition $\Omega \leq 1$ on the dark-matter density in the form of PBHs. We therefore make the cautious suggestion of applying only the overall maximum dark-matter constraint to models of early Universe, as this requirement seems to currently provide a more reliable constraint, which better reflects our current lack of detailed knowledge of PBH formation. These, and other effects, such as merging, clustering and accretion, may also loosen constraints from non-observations of other primordial compact objects such as ultra-compact minihalos of dark matter.

Are ultracompact minihalos really ultracompact?

Are ultracompact minihalos really ultracompact?

3D simulations with boosted primordial power spectra and ultracompact minihalos

We perform three-dimensional simulations of structure formation in the early Universe, when boosting the primordial power spectrum on approximately kpc scales. We demonstrate that our simulations are capable of producing power-law profiles close to the steep $\rho\propto r^{-9/4}$ halo profiles that are commonly assumed to be a good approximation to ultracompact minihalos (UCMHs). However, we show that for more realistic initial conditions in which halos are neither perfectly symmetric nor isolated, the steep power-law profile is disrupted and we find that the Navarro-Frenk-White profile is a better fit to most halos. In the presence of background fluctuations even extreme, nearly spherical initial conditions do not remain exceptional. Nonetheless, boosting the amplitude of initial fluctuations causes all structures to form earlier and thus at larger densities. With sufficiently large amplitude of fluctuations we find that values for the concentration of typical halos in our simulations can become very large. However, despite the signal coming from dark matter annihilation inside the cores of these halos being enhanced, it is still orders-of-magnitude smaller compared to the usually assumed UCMH profile. The upper bound on the primordial power spectrum from the non-observation of UCMHs should therefore be re-evaluated.

Signatures of compact halos of sterile-neutrino dark matter

We investigate compact halos of sterile-neutrino dark matter and examine observable signatures with respect to neutrino and photon emission. Primarily, we consider two cases: primordial black-hole halos and ultra-compact mini-halos. In both cases, we find that there exists a broad range of possible parameter choices such that detection in the near future with X-ray and gamma-ray telescopes might be well possible. In fact, for energies above $10\,{\rm TeV}$, the neutrino telescope IceCube would be a splendid detection machine for such macroscopic dark-matter candidates.

New bound on low reheating temperature for dark matter in models with early matter domination

We investigate a new bound on the low reheating temperature in a scenario where the Universe experiences early matter-domination before reheating after which the standard big bang cosmology begins. In many models of dark matter (DM), the small scale fluctuations of DM grow during the early matter-domination era and seed the formation of the ultracompact minihalos (UCMHs). Using the constraints on the number of UCMHs from gamma-ray observations, we find a lower bound on the reheating temperature between ${\cal O}(10)~{\rm MeV} - {\cal O}(100)~{\rm MeV}$ for WIMP dark matter depending on the nature of DM. A similar bound could be obtained for non-WIMP dark matter by observing UCMHs gravitationally such as pulsar timing, microlensing and so on in some future observations.