Narrow Results

Searches for dark matter (DM) constituents are presently mainly focused on axions and weakly interacting massive particle (WIMPs) despite the fact that far higher mass constituents are viable. We discuss and dispute whether axions exist and those arguments for WIMPs which arise from weak scale supersymmetry. We focus on the highest possible masses and argue that, since if they constitute all DM, they cannot be baryonic, they must uniquely be primordial black holes. Observational constraints require them to be of intermediate masses mostly between ten and a hundred thousand solar masses. Known search strategies for such PIMBHs include wide binaries, cosmic microwave background (CMB) distortion and, most promisingly, extended microlensing experiments.

We discuss the hypothesis that the constituents of dark matter in the galactic halo are primordial intermediate-mass black holes (PIMBHs). The status of axions and weakly interacting massive particles (WIMPs) is discussed, as are the methods for detecting PIMBHs with emphasis on microlensing. The role of the angular momentum ${𝒥}$ of the PIMBHs in their escaping previous detection is considered.

We investigate the wave effect in the gravitational lensing by a black hole with very tiny mass less than 10^-19 M_sun (solar mass), which is called attolensing, motivated by a recent report that the lensing signature might be a possible probe of a modified gravity theory in the braneworld scenario. We focus on the finite source size effect and the effect of the relative motion of the source to the lens, which are influential to the wave effect in the attolensing. Astrophysical condition that the lensed interference signature can be a probe of the modified gravity theory is demonstrated. The interference signature in the microlensing system is also discussed.

It is discussed how the ideas of entropy and the second law of thermodynamics, conceived long ago during the nineteenth century, underly why cosmological dark matter exists and originated in the first three years of the universe in the form of primordial black holes, a very large number of which have many solar masses including up to the supermassive black holes at the centres of galaxies. Certain upper bounds on dark astrophysical objects with many solar masses based on analysis of the CMB spectrum and published in the literature are criticised. For completeness we discuss WIMPs and axions which are leading particle theory candidates for the constituents of dark matter. The PIMBHs (Primordial Intermediate Mass Black Holes) with many solar masses should be readily detectable in microlensing experiments which search the Magallenic Clouds and measure light curves with durations of from one year up to several years.

Among particle theory candidates for the dark matter constituents. Axions and WIMPs are the most popular. In this paper, we discuss these then focus on our preferred astrophysical candidate, the Primordial Intermediate Mass Black Holes in the acronym DM=PIMBHs. The earliest experimental confirmation may come from microlensing of the Magellanic Clouds at the LSST 8 m telescope in the mid-2020s, or possibly a few years earlier in 2021 from work being pursued, using DECam data from the smaller Blanco 4 m telescope, at LLNL.

Intermediate Mass Black Holes (IMBHs) are a class of black holes with masses in the range 10²÷10⁵M_⊙, which cannot directly derive from stellar evolution. Looking for these objects and estimating their abundance is important not only for a deeper understanding of their origin but also for unveiling the nature and distribution of the dark matter in the galactic halo. Since February 2018 to January 2020, the Large and Small Magellanic Cloud have been intensively monitored by the DECAM instrument, installed on the 4m V. Blanco Telescope (CTIO, Chile) with the main objective to discover microlensing events possibly due to IMBHs.

Here we outline the developed data analysis pipeline. We have tested it versus known variable sources finding many not previously known variables objects. A few sources show a light curve similar to that expected for a microlensing event, but further analysis is required to confirm the microlensing nature of these events.

For these sources, and in particular for the uncatalogued variable stars, we try to determine if they are periodic or not via a periodogram analysis.

The EROS-2 collaboration has monitored during 6.7 years millions of stars in the Magellanic Clouds, in order to search for microlensing phenomena due to hypothetical compact dark objects in the Milky Way Halo ('machos'), constituting a component of Galactic dark matter. I will present the limit obtained by the EROS experiment on the macho content of the galactic halo, using only the brightest, well resolved stars (~ 7 million). The number of microlensing effects found is by far not numerous enough to account for the galactic dark matter. Our robust limit (f < 0.07 at 95% CL for 0.4 M_⊙) is now clearly in conflict with the MACHO experiment positive result (0.05 < f < 0.42 for 0.4 M_⊙). More generally, machos in the mass range 10^-7M_⊙ < M < 5M_⊙ are ruled out as the primary occupants of the Milky Way Halo. Furthermore, a new constraint for higher machos masses (up to 150 solar masses) has been performed using the complete star sample. A preliminary limit is presented here.

We discuss the hypothesis that the constituents of dark matter in the galactic halo are primordial intermediate-mass black holes (PIMBHs). The status of axions and weakly interacting massive particles (WIMPs) is discussed, as are the methods for detecting PIMBHs with emphasis on microlensing. The role of the angular momentum $\mathcal{J}$ of the PIMBHs in their escaping previous detection is considered.

We present the results of the EROS-2 search for microlensing of stars in the Magellanic clouds and in the Milky-Way plane. More than hundred million of stars were monitored over a period of about 7 years. Hundreds of microlensing candidates have been found in the galactic plane, but only one was found towards the subsample of bright –well measured– Magellanic stars. This result implies that massive compact halo objects (machos) in the mass range 10⁻⁷M⊙ < M < 5M⊙ are ruled out as a major component of the Milky Way Halo. The optical depth estimates and the event duration distributions comply with the simple standard Milky-Way model.

We discuss pixel lensing observations towards M31 carried out at the Loiano telescope. We have established a fully automatic pipeline for the detection and the characterization of microlensing events. We have evaluated the efficiency of the pipeline. We have estimated the expected signal by means of a Monte Carlo simulation. As a result we select 2 microlensing candidates. This is compatible with the expected M31 self-lensing signal. The small statistics of events at disposal does not still allow us, however, to draw definite conclusions on the content of compact halo objects.