text: 307,78,1105569383085158401,2427184074,Christopher Berry,"New on the arXiv today: an #Astro2020 white paper on the awesomeness of extreme-mass-ratio inspirals, a unique #GravitationalWave source for @LISACommunity <LINK> <LINK> Extreme-mass-ratio inspirals are when one smaller stellar-mass black hole orbits a supermassive one. The resulting orbits are extremely complicated (as illustrated in this rather lovely figure). It is this structure which encodes lots of information into the #GravitationalWaves <LINK> We're not sure how many inspirals we'll see—measuring the number would teach us lots about galactic cores. We estimated ~1–2000 per year, so they're a bankable source across a 4 year @LISACommunity mission! I describe more in my blog <LINK> #Astro2020 Since the orbits are so complicated, we'll be able to measure the properties of the source *really* well. Black hole spins are hard to measure with @LIGO, but here we'll get them to 1 part in 10,000–1,000,000! The spins encode lots of information about how black holes grow Because we'll get such ridiculously detailed measurements of the properties, we can really test the structure of the massive black hole. If there's a missing piece to @AlbertEinstein's theory of general relativity, here is an excellent place to look! #Astro2020 Putting together a few extreme-mass-ratio inspirals, we can start to reconstruct the mass distribution of massive black holes. @LISACommunity is sensitive to black holes with masses 10^4–10^7 times the mass of our Sun. We don't know what the distribution is like at the lower end! If we can cross-correlate the location of the source we work out from the #GravitaitonalWave signal with galaxy catalogues, we can also measure the Hubble constant, perhaps to 1% after 20 detections! This is a great check of other methods #Astro2020 Extreme-mass-ratio inspirals are a wonder opportunity for #GravitationalWave astronomy. There is only one currently scheduled mission that can detect them: @LISACommunity <LINK> 💞💓❤️ #Astro2020",https://arxiv.org/abs/1903.03686,"The inspiral of a stellar-mass compact object into a massive ($\sim 10^{4}$-$10^{7} M_{\odot}$) black hole produces an intricate gravitational-wave signal. Due to the extreme-mass ratios involved, these systems complete $\sim 10^{4}$-$10^{5}$ orbits, most of them in the strong-field region of the massive black hole, emitting in the frequency range $\sim10^{-4}-1~$Hz. This makes them prime sources for the space-based observatory LISA (Laser Interferometer Space Antenna). LISA observations will enable high-precision measurements of the physical characteristics of these extreme-mass-ratio inspirals (EMRIs): redshifted masses, massive black hole spin and orbital eccentricity can be determined with fractional errors $\sim 10^{-4}$-$10^{-6}$, the luminosity distance with better than $\sim 10\%$ precision, and the sky localization to within a few square degrees. EMRIs will provide valuable information about stellar dynamics in galactic nuclei, as well as precise data about massive black hole populations, including the distribution of masses and spins. They will enable percent-level measurements of the multipolar structure of massive black holes, precisely testing the strong-gravity properties of their spacetimes. EMRIs may also provide cosmographical data regarding the expansion of the Universe if inferred source locations can be correlated with galaxy catalogs. ","The unique potential of extreme mass-ratio inspirals for