Far out cosmology: looking for gravitational wave background (Introduction)

by David Turell , Friday, November 11, 2022, 23:16 (741 days ago) @ David Turell

The GWB should be there:

https://www.sciencemagazinedigital.org/sciencemagazine/library/item/11_november_2022/40...

"Gravitational waves are ripples in the fabric of spacetime that are caused by events such as the merging of black holes. In principle, many types of events occur that could create gravitational waves with frequencies ranging from as high as a few kilohertz to as low as a few nanohertz. Sources of gravitational waves in the nanohertz frequency range include cosmic strings, quantum fluctuations from the early Universe, and, notably, supermassive black hole binaries (SMBHBs). Some gravitational wave sources are so numerous that they are all expected to contribute to a gravitational wave background (GWB). This GWB has been the target of pulsar timing arrays (PTAs) for decades.

"Some gravitational wave sources are so numerous that they are all expected to contribute to a gravitational wave background (GWB). This GWB has been the target of pulsar timing arrays (PTAs) for decades.

"PTAs use the correlations between dozens of pulsar pairs to observe the GWB. Recently, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) (1), the European Pulsar Timing Array (EPTA) (2), the Parkes Pulsar Timing Array (PPTA) (3), and the International Pulsar Timing Array (IPTA) (4) have all detected a low-frequency noise in their pulsar data, which may be the first hint of the GWB

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"The noise in each pulsar should be independent, whereas the GWB signal should be a common signal in each pulsar—hence, the more pulsar pairs that can be observed, the lower the noise and the larger the signal. The smoking gun of the GWB is the Hellings and Downs curve (5), for which we expect the recently detected red noise to eventually conform to a specific functional form (see the figure).

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"Although the focus is on SMBHBs because of their expected presence in the PTA frequency range, other sources are possible. A network of cosmic strings, the existence of which has never been directly demonstrated, is another potential source of a GWB. A third source, a GWB of primordial origin, would provide evidence of an ekpyrotic Universe, where the Big Bang is eventually followed by a Big Crunch. It is not known for sure how long it will take to distinguish between different sources, but Pol et al. (8) showed that at the time of an initial detection of spatial correlations in pulsar pairs with a signal-to-noise ratio of three, current PTAs should have the capability to distinguish a SMBHB from at least some such exotic sources.

"Once the GWB is detected, the next task is to make maps of it, akin to the cosmic microwave background. For instance, individual nearby SMBHB systems and potentially large-scale structures could contribute to or trace the anisotropy in the GWB (12). Indeed, GWB anisotropy may enable us to constrain the cosmic population of SMBHBs. Moreover, it will be interesting to see where the anisotropic (excess) power on the sky originates and whether this can be associated with SMBHB activity. However, obtaining upperlimit maps of GWB anisotropy may be challenging because the distribution of pulsars in the sky is itself anisotropic, thwarting the use of the usual spherical harmonics (13).

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"The detection of the GWB may be imminent, and as such, a new low-frequency era of GW astronomy is at hand. Assuming that the GWB is astrophysical, its detection will likely cast aside any remaining doubt that SMBHs do eventually merge. Moreover, it will yield insights into the expected number density of SMBHBs as a function of redshift, the volume enclosing the GWB, and the minimum mass of a SMBHB that contributes to the background (10). All these values are fundamental properties of SMBHBs on which there are extremely limited observational constraints (which also come from PTAs). At present, PTA datasets span about 15 years, and with 5 more years of data, it should be possible to measure a low-frequency turnover in the GWB strain spectrum due to the presence of, for example, gas and stars surrounding the cosmic population of SMBHBs (8). Underlying all of this exciting astrophysics will be IPTA datasets formed by combining data from all the major PTAs, substantially increasing detection prospects for all nanohertz gravitational wave sources."

Comment: the GWB background map will be a tremendous advance. It may settle the Hubble tension problem of theoretically two values that don't agree. It may settle the flatness issue of the shape of space time. To understand the technicalities at least look at the illustrations.