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Register a new userLensing of gravitational waves as a probe of compact dark matter
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We study gravitational lensing of gravitational waves from compact object binaries as a probe of compact dark matter (DM) objects such as primordial black holes. Assuming a point mass lens, we perform parameter estimation of lensed gravitational wave signals from compact object binaries to determine the detectability of the lens with ground based laser interferometers. Then, considering binary populations that LIGO-Virgo has been probing, we derive a constraint on the abundance of compact DM from non-observation of lensed events. We find that the LIGO-Virgo observations imply that compact objects heavier than $M_l = 50M_odot$ can not constitute all DM and less than $15%$ of DM can be in compact objects heavier than $M_l = 200M_odot$. We also show that the DM fraction in compact objects can be probed by LIGO in its final sensitivity for $M_l > 20M_odot$ reaching $0.7%$ of the DM abundance at $M_l > 100M_odot$, and by ET for $M_l > 0.4M_odot$ reaching DM fraction as low as $3times 10^{-5}$ at $M_l > 20M_odot$.
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Compact dark matter has been efficiently constrained in the M <~ 10 M_sun mass range by null searches for microlensing of stars in nearby galaxies. Here we propose to probe the mass range M >~ 10 M_sun by seeking echoes in gamma-ray-burst light curves induced by strong lensing. We show that strong gravitational lensing of gamma ray bursts (GRBs) by massive compact halo objects (MACHOs) generates superimposed GRB images with a characteristic time delay of >~ 1 ms for M >~ 10 M_sun. Using dedicated simulations to capture the relevant phenomenology of the GRB prompt emission, we calculate the signal-to-noise ratio required to detect GRB lensing events as a function of the flux ratio and time delay between the lensed images. We then analyze existing data from the Fermi/GBM and Swift/BAT instruments to assess their constraining power on the compact dark matter fraction f_DM. We find that this data is noise limited, and therefore localization-based masking of background photons is a key ingredient. Future observatories with better sensitivity will be able to probe down to the f_ DM >~ 1% level across the 10 M_sun <~ M <~ 1000 M_sun mass range.
Whereas considerable effort has been afforded in understanding the properties of galaxies, a full physical picture, connecting their baryonic and dark-matter content, super-massive black holes, and (metric) theories of gravity, is still ill-defined. Strong gravitational lensing furnishes a powerful method to probe gravity in the central regions of galaxies. It can (1) provide a unique detection-channel of dark-matter substructure beyond the local galaxy group, (2) constrain dark-matter physics, complementary to direct-detection experiments, as well as metric theories of gravity, (3) probe central super-massive black holes, and (4) provide crucial insight into galaxy formation processes from the dark matter point of view, independently of the nature and state of dark matter. To seriously address the above questions, a considerable increase in the number of strong gravitational-lens systems is required. In the timeframe 2010-2020, a staged approach with radio (e.g. EVLA, e-MERLIN, LOFAR, SKA phase-I) and optical (e.g. LSST and JDEM) instruments can provide 10^(2-4) new lenses, and up to 10^(4-6) new lens systems from SKA/LSST/JDEM all-sky surveys around ~2020. Follow-up imaging of (radio) lenses is necessary with moderate ground/space-based optical-IR telescopes and with 30-50m telescopes for spectroscopy (e.g. TMT, GMT, ELT). To answer these fundamental questions through strong gravitational lensing, a strong investment in large radio and optical-IR facilities is therefore critical in the coming decade. In particular, only large-scale radio lens surveys (e.g. with SKA) provide the large numbers of high-resolution and high-fidelity images of lenses needed for SMBH and flux-ratio anomaly studies.
We show that hidden hot dark matter, hidden-sector dark matter with interactions that decouple when it is relativistic, is a viable dark matter candidate provided it has never been in thermal equilibrium with the particles of the standard model. This hidden hot dark matter may reheat to a lower temperature and number density than the visible Universe and thus account, simply with its thermal abundance, for all the dark matter in the Universe while evading the typical constraints on hot dark matter arising from structure formation. We find masses ranging from ~3 keV to ~10 TeV. While never in equilibrium with the standard model, this class of models may have unique observational signatures in the matter power spectrum or via extra-weak interactions with standard model particles.
Although gravitational waves only interact weakly with matter, their propagation is affected by a gravitational potential. If a gravitational wave source is eclipsed by a star, measuring these perturbations provides a way to directly measure the distribution of mass throughout the stellar interior. We compute the expected Shapiro time delay, amplification and deflection during an eclipse, and show how this can be used to infer the mass distribution of the eclipsing body. We identify continuous gravitational waves from neutron stars as the best candidates to detect this effect. When the Sun eclipses a far-away source, depending on the depth of the eclipse the time-delay can change by up to $sim 0.034$ ms, the gravitational-wave strain amplitude can increase by $sim 4$%, and the apparent position of the source in the sky can vary by $4$. Accreting neutron stars with Roche-lobe filling companion stars have a high probability of exhibiting eclipses, producing similar time delays but undetectable changes in amplitude and sky location. Even for the most rapidly rotating neutron stars, this time delay only corresponds to a few percent of the phase of the gravitational wave, making it an extremely challenging measurement. However, if sources of continuous gravitational waves exist just below the limit of detection of current observatories, next-generation instruments will be able to observe them with enough precision to measure the signal of an eclipsing star. Detecting this effect would provide a new direct probe to the interior of stars, complementing asteroseismology and the detection of solar neutrinos.
Microlensing started with the seminal paper by Paczynski in 1986, first with observations towards the Large Magellanic Cloud and the galactic bulge. Since then many other targets have been observed and new applications have been found. In particular, it turned out to be a powerful method to detect planets in our galaxy and even in the nearby M31. Here, we will present some results obtained so far by microlensing without being, however, exhaustive.
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