Cosmological Gravitational Radiation

Interesting new paper coming out in Nature this week, presenting early scientific results from LIGO on the amplitude of stochastic gravitational radiation background of cosmological origin.

LIGO went into science operation a few years ago, achieving phase I design sensitivity at the end of that set of operation, the S5 science run from Nov 5th 2005 till Sep 30th 2007. During that time a year of coincident operation at the Hanford and Livingston interferometers was achieved, and the results of this data set are coming out.

LIGO Hanford (click to embiggen)

LIGO shut down for interim upgrades for a year, and is now ramping up to a new set of science operations, hoping to achieve twice the sensitivity of the first run, before shutting down again to upgrade to the Advanced LIGO phase, which ought to achieve an order of magnitude improvement in sensitivity, and hence approximately a factor of thousand in search volume.

S5 sensitivity curve (click to embiggen)

Gravitational radiation is expected to come from a number of sources, including close stellar binaries and compact binaries, and from the inspiral and merger of neutron stars and black holes.
One class of sources is "cosmological sources", which are predicted to occur due to fluctuations in energy density during inflation, due to any phase transitions in the early universe, or from topological defects in the structure of space-time at early times, such as from cosmic strings.

Constraint curves from S5 and other measurements (click to embiggen)

The paper, by a cast of hundreds, provides a strong upper limit on the frequency independent spectrum of gravitational radiation, centered around 100 Hz, to be Ω_GW < 6.9 x 10^-6, which is an improvement on all previous observaional upper bounds.

ie the energy density in gravitational radiation, if it is independent of frequency, is less than a few parts per million of the critical energy density of the universe, extrapolating from the measurement in a frequency window around 100 Hz. They also provide estimates for tilted (no frequency independent spectra) for some plausible ranges of tilt, and they are similarly in the few parts per million.

The observation also constrains some cosmological string models, providing some additional constraints on the allowed string tension, and it shows the prospect for testing non-standard and pre-Big Bang models with Advanced LIGO and future advances gravitational radiation observatories.

This is a nice result, albeit a null result, which is important both in demonstrating the quantitative ability to do science with observations constraining the gravitational radiation flux, and in showing what future detections might measure.

Associated Press Release