D15 Observational Cosmology: Gravitational Waves as Standard Sirens

Topic

The key component of this model is the network of laser interferometers—featuring arms several kilometers long—designed to detect spacetime distortions caused by the merging of black holes and neutron stars; the effect is so minute that the change in each arm's length is far smaller than the diameter of a single proton.

The underlying principle is simple: a laser beam is split and travels along two mutually perpendicular arms, and the difference in optical path length between the two arms—induced by a passing gravitational wave—manifests as a measurable shift in the resulting interference pattern.

For this network, neutron star mergers serve as particularly valuable "standard sirens": the amplitude of the gravitational wave itself directly yields the luminosity distance—bypassing the traditional distance ladder entirely—while any associated electromagnetic counterpart provides the redshift. Together, these two pieces of data allow for the measurement of the universe's expansion rate via a method completely independent of previous approaches.

The most remarkable feature of these instruments is their ability to suppress virtually all other sources of noise until the signal from spacetime itself becomes detectable; this is achieved through multi-stage suspensions that isolate the mirrors from ground vibrations by many orders of magnitude, an ultra-high vacuum within the beam tubes, and laser power amplified manifold within resonant optical cavities.

The first neutron star merger detected in this manner yielded the luminosity distance directly from the wave's amplitude, while a separate spectrograph measured the redshift of the event's visible glow, thereby producing the first measurement of the universe's expansion rate using this entirely new method. Thus, from the network of laser interferometers with which this document begins to the initial measurement with which it concludes—and taking in the principle of interference and standard sirens along the way—observational cosmology enables a single gravitational wave to measure the expansion rate of the universe on its own, without relying on any rung of the traditional distance ladder.

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