- 4063 - GENERAL
RELATIVITY - new ways to
test? The theory of general
relativity (GR), proposed by Einstein over a century ago, remains one of the
most well-known scientific postulates of all time. This theory, which explains
how spacetime curvature is altered in the presence of massive objects, remains
the cornerstone of our most widely-accepted cosmological models.
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4063 - GENERAL
RELATIVITY - new ways to test?
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- Scientists have
mounted several observation campaigns to test GR using SagittariusA (Sgr A*),
the supermassive black hole at the center of the Milky Way. In 2020 the Event Horizon Telescope (EHT)
announced they had taken the first images of Sag A*, which came just two years
after the release of the first-ever images of an SMBH (M87).
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- In 2014, the
European members of the EHT launched another initiative known as “BlackHoleCam”
to gain a better understanding of SMBHs using a combination of radio imaging,
pulsar observations, astrometry, and GR.
The BHC initiative described how they tested GR by observing pulsars
orbiting Sgr A*.
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- Astronomers have
observed binary neutron star systems for over forty years. In these systems,
where one or both stars are active radio pulsars, precision tests of
gravitation have been possible. Similarly, a pulsar in a close orbit around Sgr
A* would be the ideal laboratory for testing predictions made by GR and
properties that cannot otherwise be measured.
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- This includes the “no-hair
theorem”, which states that the matter that formed a black hole is
inaccessible, and the cosmic censorship conjecture (CCC), which theorizes about
the structure of singularities in GR.
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- Several searchers
have been made for pulsars located within about 240 light-years (73 parsecs) of
the galactic center (GC). In 2013, the pulsar population in this area was
brought to a total of six with the detection of PSR J1745–2900 (a
radio-emitting magnetar) in multiple wavelengths.
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- One technique is
to search for pulsars at "higher than normal" frequencies, more than
ten gigahertz (GHz), and at longer integration lengths. This reduces the
effects of interstellar dispersion and scattering, which are highest for
objects within GC. -
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- Unfortunately,
this approach comes with a tradeoff, as these searches are limited by the steep
emissions spectrum of pulsars, leading to a higher signal-to-noise ratio. This
can make surveys for binary pulsars at GC very challenging, restricting
searches to isolated pulsars with flatter spectrums.
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- It will come down to using the same methodology, very
long baseline interferometry (VLBI). This consists of multiple radio telescopes
working together and combining data to create higher-resolution images. So far,
most pulsar searches have relied on the most sensitive element of the EHT: the
"fully phased" ALMA.
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- Both EHT VLBI
imaging and pulsar observations can utilize the same raw data product from each
array element, EHT VLBI and pulsar observations. In the future they can envisage using a
phased array of the largest components of the EHT to further increase
sensitivity or to mitigate site specific interference contamination.
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- As always,
advances in astronomy create new opportunities for study that go beyond the
original mission. Originally designed to image the event horizons of
supermassive black holes (SMBHs) at the centers of galaxies, the EHT has opened
doors for next-generation interferometry research.
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- In the coming
years, the unparalleled sensitivity these arrays offer could test the laws of
physics under the most extreme conditions, providing new insight into the laws
governing the universe.
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June 21, 2023 GENERAL
RELATIVITY - new ways to test? 4063
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Friday, June 23, 2023
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