Uncovering the secrets of ultra-low frequency gravitational waves

Date:
October 18, 2021
Source:
University of Birmingham
Summary:
New methods of detecting ultra-low frequency gravitational waves
can be combined with other, less sensitive measurements to deliver
fresh insights into the early development of our universe, according
to researchers.



FULL STORY ==========================================================================
New methods of detecting ultra-low frequency gravitational waves can be combined with other, less sensitive measurements to deliver fresh insights
into the early development of our universe, according to researchers at
the University of Birmingham.


========================================================================== Gravitational waves -- ripples in the fabric of Einstein's spacetime
-- that cross the universe at the speed of light have all sorts of
wavelengths, or frequencies. Scientists have not yet managed to detect gravitational waves at extremely low 'nanohertz' frequencies, but new approaches currently being explored are expected to confirm the first
low frequency signals quite soon.

The main method uses radio telescopes to detect gravitational waves using pulsars -- exotic, dead stars, that send out pulses of radio waves with extraordinary regularity. Researchers at the NANOGrav collaboration,
for example, use pulsars to time to exquisite precision the rotation
periods of a network, or array, of millisecond pulsars -- astronomers'
best approximation of a network of perfect clocks -- spread throughout
our galaxy. These can be used to measure the fractional changes caused
by gravitational waves as they spread through the universe.

The question of what is producing these signals, however, has yet to be determined. Scientists in the University of Birmingham's Institute for Gravitational Wave Astronomy, argue that it will be extremely difficult
to settle on an answer using only data from pulsar timing arrays (PTAs).

Instead, in a letter published today (18 October 2021) in Nature
Astronomy,they suggest that combining this new data with observations
made by other projects such as the European Space Agency's Gaia mission,
will help the different signals still lingering from the earliest periods
of our universe to be disentangled and interpreted.

The main theory for ultra-low frequency gravitational waves is that they
are caused by a population of the supermassive black holes at the centre
of merging galaxies. As galaxies merge, their central black holes pair
up, forming binaries and generating gravitational waves. In this case,
a detection of gravitational waves by PTA would offer exciting new ways
to study the astrophysics of the assembly and growth of galaxies.

But there are other possibilities too. Nanohertz gravitational waves could
tell the story of our infant universe, well before galaxies and black
holes form. In fact, it has been suggested that extremely low frequency gravitational wave signals could instead be generated shortly after the
big bang by other processes; for example if the Universe underwent what physicists refer to as a phase transition at the correct temperature.

Lead author, Dr Christopher Moore, said: "The first tentative hints of
a gravitational wave signal using pulsar timing arrays might recently
have been seen by NANOGrav and we expect the next few years to be
a golden age for this type of science. The variety of explanations
for these signals is exciting, but also a maze. We need a way to
tell the different possible sources apart from each other. Currently,
this is extremely difficult to do with pulsar timing array data alone." Co-author Professor Alberto Vecchio said: "Pulsar timing arrays may offer unprecedented insights into ancient cosmological processes. Developing
the sophisticated methods to interpret these insights will mean we can
truly begin to understand how our universe was formed and took shape." ========================================================================== Story Source: Materials provided by University_of_Birmingham. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Moore, C.J., Vecchio, A. Ultra-low-frequency gravitational
waves from
cosmological and astrophysical processes. Nat Astron, 2021 DOI:
10.1038/ s41550-021-01489-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211018112529.htm

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