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The Call of Dark Sirens in Precision Cosmology

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illustration of two merging black holes in the night sky above an L-shaped detector on earth.

Gravitational waves have revealed a wealth of information about distant black holes and neutron stars — but they can also provide large-scale insights into how our universe works. A new study explores how gravitational-wave detections may soon resolve the long-lived tension in measurements of our universe’s expansion.

An Expanding Problem

plot showing the different measurements made of H0 through the years. Local and global measurements are clearly not in agreement.

Some past measurements of H0 (click to enlarge). Black data points are local-universe distance-ladder measurements, which cluster around 73 km/s/Mpc; red data points are early-universe CMB measurements, which cluster around 67 km/s/Mpc. [Renerpho]

We know the universe is expanding, but we’re still not sure how quickly. The empirically derived value H0 — referred to as the Hubble constant or the Hubble-Lemaître constant — parametrizes the universe’s expansion rate. This controversial parameter, which describes how quickly galaxies are receding from us as a function of their distance from us, is traditionally measured in one of two ways:

  1. In the local universe, by determining the distances to and recession speeds of visible astronomical objects. This method relies on the distance ladder: the distances measured to far-off objects are built upon measured distances to nearer objects.
  2. On global scales, estimated by modeling measurements of the cosmic microwave background (CMB), relic radiation from the Big Bang.

The trouble? The values we come up with for H0 from these two methods are not consistent with one another! To resolve this tension, we need another way of measuring H0 that’s independent of these approaches. Enter: dark sirens.

The Call of Hidden Collisions

Dark sirens are the collisions of two black holes that, though they produce no light, can provide us with valuable distance information. When black holes merge, their gravitational-wave signal encodes a distance luminosity. By combining this piece of information with the physical distance to the merging black holes — identifiable if we can precisely pinpoint the host galaxy of the collision — we can get an independent measure of H0.

map showing the localization of GW170817 on the sky

This map shows the localization of the gravitational-wave, gamma-ray, and optical signals of the neutron-star merger detected on 17 August 2017. Upgrades to LIGO and Virgo should improve gravitational-wave localization, making it possible to identify the host galaxy of a merger even without electromagnetic counterparts. [Abbott et al. 2017]

The catch? Right now, our gravitational-wave technology isn’t quite good enough to identify a precise value of H0. This problem is rooted in two issues: the large uncertainty on the measured distance luminosity in the gravitational-wave signal, and the difficulty in accurately identifying the host galaxy of a black hole merger, which produces no electromagnetic counterpart.

But there’s hope! Future advancements in technology at gravitational-wave detectors may soon bring these precise measurements into reach, according to the calculations of a team of scientists led by Ssohrab Borhanian (Pennsylvania State University).

A Precise Future

Borhanian and collaborators explore a series of models for future detections of gravitational-wave events using current and upcoming detectors with increasingly advanced technology.

plot of delta(H0)/H0 for four different networks.

Estimated distribution of fractional errors in the measurement of H0 after 2 years of observing time for different networks of detectors. The left cluster (HLV+) represents 2G+ technology on a network consisting of only LIGO-Livingston, LIGO-Hanford, and Virgo detectors; the right cluster (ECC) represents the 3G technology of a future detector network. The different colors represent the authors’ different population models. [Adapted from Borhanian et al. 2020]

Upgrades to the Laser Interferometer Gravitational-wave Observatory (LIGO) and the European detector Virgo are currently underway. The authors show that this advancement to next-generation (known as 2G+) technology could allow these detectors to uniquely identify the host galaxies of binary black hole mergers, without the need for an electromagnetic counterpart to the merger.

Borhanian and collaborators estimate that with 2G+ technology, we’ll be able to measure H0 to a precision of 2% within 5 years — which is sufficient to resolve the Hubble tension. And if you want something to dream about, consider this: third-generation detector technology (which will include the proposed Einstein Telescope and Cosmic Explorer) will be able to measure H0 to within less than a percent. The call of dark sirens is leading us to a beautifully precise future!

Citation

“Dark Sirens to Resolve the Hubble–Lemaître Tension,” Ssohrab Borhanian et al 2020 ApJL 905 L28. doi:10.3847/2041-8213/abcaf5