Hunting for MHz Gravitational Waves with the Fermilab Holometer

Abstract

A new ground-based detector, the Fermilab Holometer, has extended the accessible gravitational wave frequency range from kHz up to MHz. At these higher frequencies, exotic sources that were produced shortly after the Big Bang could radiate. The existence of nearby remnants is tested using the Holometer, which consists of separate yet identical 39-meter Michelson interferometers operated at Fermi National Accelerator Laboratory. Utilizing a 130-hour dataset collected between July 15, 2015 and August 15, 2015, constraints are made on both the stochastic gravitational wave background and primordial black hole binaries.

The first result is a 3-sigma upper limit of the stochastic gravitational wave background at MHz frequencies. This is the only direct gravitational wave measurement at these frequencies. The Holometer 3-sigma upper limit on the energy density, Omega_GW is 5.6e12 at 1 MHz and goes up to 8.4e15 at 13 MHz. This result is much higher than existing indirect measurements. However, this does place constraints on early universe models that predict large bursts of gravitational radiation in a narrow MHz band.

The second result is a measurement of in-spiraling primordial black hole binaries from 1 to 1.92 MHz. We report that there are no detectable primordial black hole binaries in the mass range 0.7e21 - 3.5e21 g between the earth and the moon. Utilizing the same dataset, an alternative analysis path can constrain primordial black hole binaries in the mass range from 0.6e25-2.5e25 g which would increase the distance out to Jupiter. Additionally, the sensitivity of the Holometer with a new data acquisition system can constrain merging black hole binary pairs up to ~1e30 g within the Milky Way halo. This instrument opens up a new opportunity to improve measurements on one of the least constrained mass ranges for primordial black holes.