Friday, June 23, 2023

4063 - GENERAL RELATIVITY - new ways to test?

 

-    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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