Astrophysicists propose a new way of measuring cosmic expansion: Lensed gravitational waves

Date:
July 1, 2023
Source:
University of California - Santa Barbara
Summary:
The universe is expanding; we've had evidence of that for about a
century. But just how quickly celestial objects are receding from
each other is still up for debate.


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FULL STORY ==========================================================================
The universe is expanding; we've had evidence of that for about a
century. But just how quickly celestial objects are receding from each
other is still up for debate.

It's no small feat to measure the rate at which objects move away from
each other across vast distances. Since the discovery of cosmic expansion,
its rate has been measured and re-measured with increasing precision,
with some of the latest values ranging from 67.4 up to 76.5 kilometers
per second per megaparsec, which relates the recession velocity (in
kilometers per second) to the distance (in megaparsecs).

The discrepancy between different measurements of cosmic expansion
is called the "Hubble tension." Some have called it a crisis in
cosmology. But for UC Santa Barbara theoretical astrophysicist Tejaswi Venumadhav Nerella and colleagues at the Tata Institute of Fundamental
Research in Bangalore, India, and the Inter-University Center for
Astronomy and Astrophysics in Pune, India, it is an exciting time.

Since the first detection of gravitational waves in 2015, detectors have
been significantly improved and are poised to yield a rich haul of signals
in the coming years. Nerella and his colleagues have come up with a method
to use these signals to measure the universe's expansion, and perhaps
help to settle the debate once and for all. "A major scientific goal of
future detectors is to deliver a comprehensive catalog of gravitational
wave events, and this will be a completely novel use of the remarkable dataset," said Nerella, co-author of a paper published in Physical
Review Letters.

Measurements of the cosmic expansion rate boil down to velocity and
distance.

Astronomers use two kinds of methods to measure distances: the first
start with objects with a known length ("standard rulers") and look at
how big they appear in the sky. These "objects" are features in cosmic background radiation, or in the distribution of galaxies in the universe.

A second class of methods starts with objects of known luminosity
("standard candles") and measures their distances from Earth using
their apparent brightness. These distances are connected to those of
farther bright objects and so on, which builds up a chain of measurement schemes that is often called the "cosmic distance ladder." Incidentally, gravitational waves themselves can also help measure cosmic expansion,
since the energy released by the collision of neutron stars or black
holes can be used to estimate the distance to these objects.

The method that Nerella and his co-authors propose belongs to the second
class but uses gravitational lensing. This is a phenomenon that occurs
when massive objects warp spacetime, and bend waves of all kinds that
travel near the objects. In rare cases, lensing can produce multiple
copies of the same gravitational wave signal that reach Earth at different times -- the delays between the signals for a population of multiple
imaged events can be used to calculate the universe's expansion rate,
according to the researchers.

"We understand very well just how sensitive gravitational wave detectors
are, and there are no astrophysical sources of confusion, so we can
properly account for what gets into our catalog of events," Nerella
said. "The new method has sources of error that are complementary
to those of existing methods, which makes it a good discriminator."
The sources of these signals would be binary black holes: systems of
two black holes that orbit each other and ultimately merge, releasing
massive amounts of energy in the form of gravitational waves. We haven't
yet detected strongly lensed examples of these signals, but the upcoming generation of ground-based detectors is expected to have the necessary
level of sensitivity.

"We expect the first observation of lensed gravitational waves in the
next few years," said study co-author Parameswaran Ajith. Additionally,
these future detectors should be able to see farther into space and
detect weaker signals.

The authors expect these advanced detectors to start their search
for merging black holes in the next decade. They anticipate recording
signals from a few million black hole pairs, a small fraction (about
10,000) of which will appear multiple times in the same detector due
to gravitational lensing. The distribution of the delays between these
repeat appearances encodes the Hubble expansion rate.

According to lead author Souvik Jana, unlike other methods of
measurement, this method does not rely on knowing the exact locations
of, or the distances to, these binary black holes. The only requirement
is to accurately identify a sufficiently large number of these lensed
signals. The researchers add that observations of lensed gravitational
waves can even provide clues on other cosmological questions, such as
the nature of the invisible dark matter that makes up much of the energy content of the universe.

* RELATED_TOPICS
o Space_&_Time
# Black_Holes # Astrophysics # Cosmology # Astronomy #
Cosmic_Rays # Asteroids,_Comets_and_Meteors # Big_Bang
# Galaxies
* RELATED_TERMS
o Physical_cosmology o Ultimate_fate_of_the_universe o
Astronomy o Shape_of_the_Universe o Radio_telescope o Big_Bang
o Cosmic_microwave_background_radiation o Extraterrestrial_life

========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Barbara. Original written by Sonia
Fernandez. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Souvik Jana, Shasvath J. Kapadia, Tejaswi Venumadhav, Parameswaran
Ajith.

Cosmography Using Strongly Lensed Gravitational Waves from Binary
Black Holes. Physical Review Letters, 2023; 130 (26) DOI: 10.1103/
PhysRevLett.130.261401 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230701135624.htm

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