Maximum Redshift of Gravitational Wave Merger Events

Abstract

Future generation of gravitational wave detectors will have the sensitivity to detect gravitational wave events at redshifts far beyond any detectable electromagnetic sources. We show that if the observed event rate is greater than one event per year at redshifts z ≥ 40, then the probability distribution of primordial density fluctuations must be significantly non-Gaussian or the events originate from primordial black holes. The nature of the excess events can be determined from the redshift distribution of the merger rate.PACS numbers: 95.35.+d, 04.30.Db, 04.30.?w The discovery of gravitational waves from merging pairs of massive black holes [1][2][3][4] has opened a new window to the astrophysics of black holes, their formation, and cosmic evolution. Black holes of stellar masses have been observed with LIGO [1-4] and supermassive black holes in galaxies are expected to be detected by LISA over the next couple of decades [5][6][7]. The sensitivity of the next generation of ground-based gravitational wave detectors is expected to improve by at least an order of magnitude [8], thus allowing the detection of merging black holes events out to the highest redshifts, potentially exceeding the reach of electromagnetic observations which respond to amplitude squared and not amplitude.The expected rates of black hole mergers has been calculated based on the number and properties of the few events discovered to-date (see, e.g. [9][10][11]). The rate depends on a multitude of factors: black holes must be formed and they must find a way to get close enough so that gravitational waves can take-over as the dominant energy loss mechanism. The redshift distribution encodes information about the origin of black hole pairs. If black holes originate from massive stellar progenitors then the redshift distribution should relate to the formation, accretion, and cooling of gas in galaxies. If on the other hand the black holes are primordial [12][13][14][15][16], then the redshirt distribution will extend to earlier cosmic times due to primordial binaries [17].A key difference between these two scenarios is that in the case of a baryonic origin, black holes must form out of cold gas, which accreted into a dark matter gravitational potential well, and then cooled to form black hole progenitors. This path follows the abundance of appropriate potential wells.In this letter we calculate the maximum redshift of ex- * Electronic address: koushiappas@brown.edu † Electronic address: loeb@cfa.harvard.edu pected black hole merger events that have baryonic origin in the standard cosmological model. That is, the black holes are formed in galaxies as opposed to primordial black holes, or black holes that are formed in nonstandard cosmological scenarios, e.g., cosmologies with a significant non-Gaussianity in the primordial density fluctuations of the dark matter.The significance of this calculation is two-fold: first, it defines a maximum redshift over which baryonic structures can form, and second any detection above the derived bound will si...

Disentangling the Potential Dark Matter Origin of LIGO’s Black Holes

“…It has since been pointed out that the monochromatic scenario has two problems. First, constraints from dwarf galaxy dynamics and radio emissions imply that not all of dark matter can be explained by ∼ 30M black holes (Brandt 2016;Koushiappas & Loeb 2017;Gaggero et al 2017). Second, the expected merger rate predicted by this model would be above the inferred merger rate provided by LIGO (Sasaki et al 2016;Eroshenko 2016;Abbott et al 2017).…”

“…Second, it will have a functional form and predicted rate consistent with LIGO observations to date. Third, it will not violate the tightest constraints on primordial black hole binaries in the mass range already observed by LIGO (Koushiappas & Loeb 2017;Brandt 2016) or the microlensing constraints set by , but it will violate those set by the EROS2 collaboration (Tisserand et al 2007). Finally, our proposed mass function will account for 100% of the dark matter.…”

“…In the LIGO region, microlensing (Alcock et al 1996Allsman et al 2001;Lasserre 2000;Tisserand et al 2007;Wyrzykowski et al 2011) and star cluster dynamics in the Eridanus II (Brandt 2016) and Segue I dwarf galaxies (Koushiappas & Loeb 2017) place the tightest constraints on PBH dark matter (Fig. 1).…”

“…The long and short dashed lines show the MA-CHO (Allsman et al 2001) and EROS2 (Tisserand et al 2007) collaboration constraints on compact objects for the standard halo. The blue line shows a 99.9% confidence limit derived from mass segregation in Segue I (Koushiappas & Loeb 2017) while the green lines are derived from the survival of a star cluster in Eridanus II (Brandt 2016;Carr et al 2016) for velocity dispersions and dark-matter densities at the galactic center of σ = 5, 10 km s −1 respectively and ρ = .1M pc −3 . The shaded regions are excluded by these constraints.…”

Constraining primordial black holes with the EDGES 21-cm absorption signal

“…The yellow, purple and light blue regions are excluded by the microlensing results from EROS [58] and MACHO (M) [59], and by lack of lensing signatures in type Ia supernovae (SNe) [60], respectively. The dark blue, orange, red and green regions are ruled out by Planck data [42], survival of stars in Segue I (Seg I) [61] and Eridanus II (Eri II) [62], and the distribution of wide binaries (WB) [63], respectively. The gray region (LIGO) is excluded by the nonobservation of the stochastic GW background [56].…”