The Event Horizon Telescope has recently provided the first image of the dark shadow around the supermassive black hole M87*. The observation of a highly circular shadow provides strong limits on ...deviations of M87*'s quadrupole moment from the Kerr value. We show that the absence of such a deviation can be used to constrain the physics of extra dimensions of spacetime. Focusing on the Randall-Sundrum AdS5 brane-world scenario, we show that the observation of M87*'s dark shadow sets the limit ℓ≲170 AU, where ℓ is the AdS5 curvature radius. This limit is among the first quantitative constraints on exotic physics obtained from the extraordinary first ever image of the dark shadow of a black hole.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Understanding the nature of the Dark Matter (DM) is one of the current challenges in modern astrophysics and cosmology. Knowing the properties of the DM particle would shed light on physics beyond ...the Standard Model and even provide us with details of the early Universe. In fact, the detection of such a relic would bring us information from the pre-Big Bang Nucleosynthesis (BBN) period, an epoch from which we have no direct data, and could even hint at inflation physics. In this work, we assume that the expansion rate of the Universe after inflation is governed by the kinetic energy of a scalar field ϕ , in the so-called “kination” model. Adding to previous work on the subject, we assume that the ϕ field decays into both radiation and DM particles, which we take to be Weakly Interacting Massive Particles (WIMPs). The present abundance of WIMPs is then fixed during the kination period through either a thermal “freeze-out” or “freeze-in” mechanism, or through a non-thermal process governed by the decay of ϕ . We explore the parameter space of this theory with the requirement that the present WIMP abundance provides the correct relic budget. Requiring that BBN occurs during the standard cosmological scenario sets a limit on the temperature at which the kination period ends. Using this limit and assuming the WIMP has a mass m χ = 100 GeV, we obtain that the thermally averaged WIMP annihilation cross section has to satisfy the constraints 4 × 10 − 16 GeV − 2 ≲ ⟨ σ v ⟩ ≲ 2 × 10 − 5 GeV − 2 in order for having at least one of the production mechanism to yield the observed amount of DM. This result shows how the properties of the WIMP particle, if ever measured, can yield information on the pre-BBN content of the Universe.
Full text
Available for:
IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
In the simplest picture, the masses of string axions populating the axiverse depend on two parameters: the supersymmetry-breaking scale M susy and the action S of the string instantons responsible ...for breaking the axion shift symmetry. In this work, we explore whether cosmological data can be used to probe these two parameters. Adopting string-inspired flat priors on log 10 M susy and S and imposing that M susy be sub-Planckian, we find S = 198 ± 28 . These bounds suggest that cosmological data complemented with string-inspired priors select a quite narrow axion mass range within the axiverse, log 10 ( m a / eV ) = − 21. 5 + 1.3 − 2.3 . We find that M susy remains unconstrained due to a fundamental parameter degeneracy with S . We explore the significant impact of other choices of priors on the results, and we comment on similar findings in recent previous literature.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
The Event Horizon Telescope (EHT) collaboration has recently released the first image of a black hole (BH), opening a new window onto tests of general relativity in the strong field regime. In this ...paper, we derive constraints on the nature of M87* (the supermassive object at the center of the galaxy M87), exploiting the fact that its shadow appears to be highly circular, and using measurements of its angular size. We first consider the simple case where M87* is assumed to be a Kerr BH. We find that the inferred circularity of M87* excludes Kerr BHs with observation angle θobs≳45° for dimensionless rotational parameter 0.95≲a*≤1 whereas the observation angle is unbounded for a*≲0.9. We then consider the possibility that M87* might be a superspinar, i.e., an object described by the Kerr solution and spinning so fast that it violates the Kerr bound by having |a*|>1. We find that, within certain regions of parameter space, the inferred circularity and size of the shadow of M87* do not exclude the possibility that this object might be a superspinar.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration has recently reported strong evidence for a stochastic process affecting the 12.5 yr dataset of pulsar timing ...residuals. We show that the signal can be interpreted in terms of a stochastic gravitational wave background emitted from a network of axionic strings in the early Universe. The spontaneous breaking of the Peccei-Quinn symmetry originate the axionic string network and the QCD axion, the dark matter particle in the model. We explore a nonstandard cosmological model driven by an exotic scalar field ϕ which evolves under the influence of a self-interacting potential; the axion field starts to oscillate during the modified cosmology, and provides the dark matter observed. For an equation of state wϕ<1/3, the QCD axion mass is smaller than expected in the standard cosmology and the GW spectrum from axionic strings is larger. We assess the parameter space of the model which is consistent with the NANOGrav-12.5 yr detection, which can be explained within 95% limit by a QCD axion field evolving in a dust-like scenario, as well as within 68% limit in a cosmology with wϕ<0.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Recently, Tsupko et al have put forward the very interesting proposal to use the shadows of high-redshift supermassive black holes (SMBHs) as standard rulers. This would in principle allow us to ...probe the expansion history within a redshift range which would otherwise be challenging to access. In this note, we critically examine this proposal, and identify a number of important issues which had been previously overlooked. These include difficulties in obtaining reliable SMBH mass estimates and reaching the required angular resolution, and an insufficient knowledge of the accretion dynamics of high-redshift SMBHs. While these issues currently appear to prevent high-redshift SMBH shadows from being used as robust standard rulers, we hope that our flagging them early will help in making this probe theoretically mature by the time it will be experimentally feasible.
The search for extra dimensions is a challenging endeavor to probe physics beyond the Standard Model. The joint detection of gravitational waves (GW) and electromagnetic (EM) signals from the merging ...of a binary system of compact objects like neutron stars can help constrain the geometry of extra dimensions beyond our 3+1 spacetime ones. A theoretically well-motivated possibility is that our observable Universe is a 3+1-dimensional hypersurface, or brane, embedded in a higher 4+1-dimensional anti–de Sitter (AdS5) spacetime, in which gravity is the only force which propagates through the infinite bulk space, while other forces are confined to the brane. In these types of brane-world models, GW and EM signals between two points on the brane would, in general, travel different paths. This would result in a time lag between the detection of GW and EM signals emitted simultaneously from the same source. We consider the recent near-simultaneous detection of the GW event GW170817 from the LIGO/Virgo collaboration, and its EM counterpart, the short gamma-ray burst GRB170817A detected by the Fermi Gamma-ray Burst Monitor and the International Gamma-Ray Astrophysics Laboratory Anti-Coincidence Shield spectrometer. Assuming the standard Λ-cold dark matter scenario and performing a likelihood analysis which takes into account astrophysical uncertainties associated to the measured time lag, we set an upper limit of ℓ≲0.535 Mpc at 68% confidence level on the AdS5 radius of curvature ℓ. Although the bound is not competitive with current Solar System constraints, it is the first time that data from a multimessenger GW-EM measurement is used to constrain extra-dimensional models. Thus, our work provides a proof of principle for the possibility of using multimessenger astronomy for probing the geometry of our space-time.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Dilute and dense axion stars Visinelli, Luca; Baum, Sebastian; Redondo, Javier ...
Physics letters. B,
02/2018, Volume:
777, Issue:
C
Journal Article
Peer reviewed
Open access
Axion stars are hypothetical objects formed of axions, obtained as localized and coherently oscillating solutions to their classical equation of motion. Depending on the value of the field amplitude ...at the core |θ0|≡|θ(r=0)|, the equilibrium of the system arises from the balance of the kinetic pressure and either self-gravity or axion self-interactions. Starting from a general relativistic framework, we obtain the set of equations describing the configuration of the axion star, which we solve as a function of |θ0|. For small |θ0|≲1, we reproduce results previously obtained in the literature, and we provide arguments for the stability of such configurations in terms of first principles. We compare qualitative analytical results with a numerical calculation. For large amplitudes |θ0|≳1, the axion field probes the full non-harmonic QCD chiral potential and the axion star enters the dense branch. Our numerical solutions show that in this latter regime the axions are relativistic, and that one should not use a single frequency approximation, as previously applied in the literature. We employ a multi-harmonic expansion to solve the relativistic equation for the axion field in the star, and demonstrate that higher modes cannot be neglected in the dense regime. We interpret the solutions in the dense regime as pseudo-breathers, and show that the life-time of such configurations is much smaller than any cosmological time scale.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Constraints on stupendously large black holes Carr, Bernard; Kühnel, Florian; Visinelli, Luca
Monthly Notices of the Royal Astronomical Society,
02/2021, Volume:
501, Issue:
2
Journal Article
Peer reviewed
Open access
ABSTRACT
We consider the observational constraints on stupendously large black holes (SLABs) in the mass range $M \gtrsim 10^{11}\, \mathrm{ M_{\odot}}$. These have attracted little attention ...hitherto, and we are aware of no published constraints on a SLAB population in the range (1012–$10^{18})\, \mathrm{ M_{\odot}}$. However, there is already evidence for black holes of up to nearly $10^{11}\, \mathrm{ M_{\odot}}$ in galactic nuclei, so it is conceivable that SLABs exist and they may even have been seeded by primordial black holes. We focus on limits associated with (i) dynamical and lensing effects, (ii) the generation of background radiation through the accretion of gas during the pre-galactic epoch, and (iii) the gamma-ray emission from the annihilation of the halo of weakly interacting massive particles expected to form around each SLAB if these provide the dark matter. Finally, we comment on the constraints on the mass of ultralight bosons from future measurements of the mass and spin of SLABs.
ABSTRACT
We consider constraints on primordial black holes (PBHs) in the mass range $(10^{-18}\!-\!10^{15})\, \mathrm{M}_{\odot }$ if the dark matter (DM) comprises weakly interacting massive ...particles (WIMPs) that form haloes around them and generate γ-rays by annihilations. We first study the formation of the haloes and find that their density profile prior to WIMP annihilations evolves to a characteristic power-law form. Because of the wide range of PBH masses considered, our analysis forges an interesting link between previous approaches to this problem. We then consider the effect of the WIMP annihilations on the halo profile and the associated generation of γ-rays. The observed extragalactic γ-ray background implies that the PBH DM fraction is $f^{}_{\rm PBH} \lesssim 2 \times 10^{-9}\, (m_{\chi } / {\rm TeV})^{1.1}$ in the mass range $2 \times 10^{-12}\, \mathrm{M}_{\odot }\, (m_{\chi } / {\rm TeV})^{-3.2} \lesssim M \lesssim 5 \times 10^{12}\, \mathrm{M}_{\odot }\, (m_{\chi } / {\rm TeV})^{1.1}$, where mχ and M are the WIMP and PBH masses, respectively. This limit is independent of M and therefore applies for any PBH mass function. For $M \lesssim 2\times 10^{-12}\, \mathrm{M}_{\odot }\, (m_{\chi }/ {\rm TeV})^{-3.2}$, the constraint on $f^{}_{\rm PBH}$ is a decreasing function of M and PBHs could still make a significant DM contribution at very low masses. We also consider constraints on WIMPs if the DM is mostly PBHs. If the merging black holes recently discovered by LIGO/Virgo are of primordial origin, this would rule out the standard WIMP DM scenario. More generally, the WIMP DM fraction cannot exceed 10−4 for $M \gt 10^{-9}\, \mathrm{M}_{\odot }$ and $m_{\chi } \gt 10\,$ GeV. There is a region of parameter space, with $M \lesssim 10^{-11}\, \mathrm{M}_{\odot }$ and $m_{\chi } \lesssim 100\,$ GeV, in which WIMPs and PBHs can both provide some but not all of the DM, so that one requires a third DM candidate.
PDF
