Synopsis: Homing in on Primordial Gravitational Waves

An analysis of data spanning 29 orders of magnitude in gravitational-wave frequency provides insights into the physics of the early cosmos.

Gravitational waves—the distortions in spacetime first spotted by the LIGO Scientific Collaboration in September 2015—are fundamentally distinct from electromagnetic waves. But like their electromagnetic counterparts, they can come in a broad range of frequencies. Different experiments to detect them target specific frequency bands, each of which links to particular wave sources, such as the two colliding black holes that generated the signal detected by LIGO. In a study that combines data across an unprecedented swath of gravitational-wave frequencies, Paul Lasky from Monash University, Australia, and colleagues now put stringent bounds on a source that should produce a signal in all experiments: quantum fluctuations in the early cosmos.

Researchers believe that a fraction of a second after the big bang, quantum fluctuations of the gravitational field were magnified by a rapid expansion of space called inflation, generating a primordial background of gravitational waves. Lasky and co-workers pulled together data from several experiments that should carry direct or indirect imprints of this background. Spanning 29 orders of magnitude in gravitational-wave frequency, these data included observations of the cosmic microwave background radiation obtained by the Planck satellite and the BICEP2 experiment, of the arrival times of pulses from rotating neutron stars, and of mirror displacements in ground-based interferometers such as LIGO. The combined data allowed the authors to put stringent bounds on the slope of the background’s energy spectrum, which quantifies how the energy density of the waves varies with frequency. From these bounds, they were able to rule out exotic models of inflation that produce large slopes.

This research is published in Physical Review X.

–Ana Lopes


More Features »


More Announcements »

Subject Areas


Previous Synopsis

Materials Science

Topological Origami

Read More »

Next Synopsis

Condensed Matter Physics

Electron–Phonon Affair Comes to Light

Read More »

Related Articles

Synopsis: Self-Interacting Dark Matter Scores Again

Synopsis: Self-Interacting Dark Matter Scores Again

Dark matter that interacts with itself provides a better description of the speeds of stars in galaxies than dark matter that doesn’t self-interact. Read More »

Synopsis: Searching for Neutron Star Gravitational Waves

Synopsis: Searching for Neutron Star Gravitational Waves

The first run of LIGO and Virgo’s gravitational-wave search shows no evidence of spinning asymmetric neutron stars, but recent upgrades could make the detection possible. Read More »

Viewpoint: Neutron-Star Implosions as Heavy-Element Sources

Viewpoint: Neutron-Star Implosions as Heavy-Element Sources

A dramatic scenario in which a compact black hole eats a spinning neutron star from inside might explain a nearby galaxy’s unexpectedly high abundance of heavy elements. Read More »

More Articles