Friday, June 23, 2023

4064 - GALAXIES - are blackholes at the centers?

 

-    4064  -   GALAXIES  -  are blackholes at the centers?     The theory of general relativity (GR) 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.


---------------------   4064   -     GALAXIES  -  are blackholes at the centers?

-   General Relativity (GR)  has been verified under the most extreme conditions. Scientists have mounted several observation campaigns to test GR using Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way.

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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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-  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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-    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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-    The same technology used to snap the first image of Sgr A* will be used to spot binary pulsars orbiting it. It will also come down to the same methodology: very long baseline interferometry (VLBI). This consists of multiple radio telescopes working together and combining data to create higher-resolution images.

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-     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. 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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-   For example, the expansion of the universe could be a mirage.  This rethinking  also suggests solutions for the puzzles of dark energy and dark matter, which scientists believe account for around 95% of the universe's total energy and matter but remain shrouded in mystery.

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-    Scientists know the universe is expanding because of redshift, the stretching of light's wavelength towards the redder end of the spectrum as the object emitting it moves away from us.  Distant galaxies have a higher redshift than those nearer to us, suggesting those galaxies are moving ever further from Earth.

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-   Scientists have found evidence that the universe's expansion isn't fixed, but is actually accelerating faster and faster. This accelerating expansion is captured by a term known as the “cosmological constant”, or “lambda”.

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-    The cosmological constant has been a headache for cosmologists because predictions of its value made by particle physics differ from actual observations by 120 orders of magnitude. The cosmological constant has therefore been described as "the worst prediction in the history of physics."

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-   Cosmologists often try to resolve the discrepancy between the different values of lambda by proposing new particles or physical forces.  In a new mathematical interpretation, the universe isn't expanding but is flat and static, as Einstein once believed. The effects we observe that point to expansion are instead explained by the evolution of the masses of particles, such as protons and electrons, over time.

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-   In this picture, these particles arise from a field that permeates space-time. The cosmological constant is set by the field's mass and because this field fluctuates, the masses of the particles it gives birth to also fluctuate. The cosmological constant still varies with time, but in this model that variation is due to changing particle mass over time, not the expansion of the universe.

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-   In the model, these field fluctuations result in larger redshifts for distant galaxy clusters than traditional cosmological models predict. And so, the cosmological constant remains true to the model's predictions.

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-    This new framework also tackles some of cosmology's other pressing problems, including the nature of dark matter. This invisible material outnumbers ordinary matter particles by a ratio of 5 to 1, but remains mysterious because it doesn't interact with light.

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-    Axions being hypothetical particles that are one of the suggested candidates for dark matter.

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-    These fluctuations could also do away with dark energy, the hypothetical force stretching the fabric of space and thus driving galaxies apart faster and faster. In this model, the effect of dark energy would be explained by particle masses taking a different evolutionary path at later times in the universe.

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-    Everything in the universe has gravity. Yet this most common of all fundamental forces is also the one that presents the biggest challenges to physicists. “General relativity” has passed many years of observational tests, from Eddington’s measurement of the deflection of starlight by the Sun in 1919 to the recent detection of gravitational waves.

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-    However, gaps in our understanding start to appear when we try to apply it to extremely small distances, where the laws of quantum mechanics operate, or when we try to describe the entire universe as a whole.

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-    We have now tested Einstein’s theory on the largest of scales. We hope to resolve some of the biggest mysteries in cosmology, and the results hint that the theory of general relativity may need to be tweaked on this scale.

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-    Quantum theory predicts that empty space, the vacuum, is packed with energy. We do not notice its presence because our devices can only measure changes in energy rather than its total amount.

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-   However, according to Einstein, the vacuum energy has a repulsive gravity, it pushes the empty space apart. In 1998, it was discovered that the expansion of the universe is in fact accelerating .

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-   However, the amount of vacuum energy, or “dark energy” as it has been called, necessary to explain the acceleration is many orders of magnitude smaller than what quantum theory predicts.

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-    The big question, dubbed “the old cosmological constant problem”, is whether the vacuum energy actually gravitates, exerting a gravitational force and changing the expansion of the universe.

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-    If yes, then why is its gravity so much weaker than predicted? If the vacuum does not gravitate at all, what is causing the cosmic acceleration?  We don’t know what dark energy is, but we need to assume it exists in order to explain the universe’s expansion.

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-    Similarly, we also need to assume there is a type of invisible matter presence, dubbed dark matter, to explain how galaxies and clusters evolved to be the way we observe them today.

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-    These assumptions are baked into scientists’ standard cosmological theory, called the “lambda cold dark matter” (LCDM) model, suggesting there is 70% dark energy, 25% dark matter and 5% ordinary matter in the cosmos. And this model has been remarkably successful in fitting all the data collected by cosmologists over the past 20 years.

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-   But the fact that most of the universe is made up of dark forces and substances, taking odd values that don’t make sense, has prompted many physicists to wonder if Einstein’s theory of gravity needs modification to describe the entire universe.

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-   A few years ago when it became apparent that different ways of measuring the rate of cosmic expansion, dubbed the Hubble constant, give different answers,  This is a problem known as the “Hubble tension”.

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-    The disagreement, or tension, is between two values of the Hubble constant. One is the number predicted by the LCDM cosmological model, which has been developed to match the light left over from the Big Bang (the cosmic microwave background radiation). The other is the expansion rate measured by observing exploding stars known as supernovas in distant galaxies.

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-    Many theoretical ideas have been proposed for ways of modifying LCDM to explain the Hubble tension. Among them are alternative gravity theories.   We can design tests to check if the universe obeys the rules of Einstein’s theory.

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-    General relativity describes gravity as the curving or warping of space and time, bending the pathways along which light and matter travel.  It predicts that the trajectories of light rays and matter should be bent by gravity in the same way.

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-    Using a statistical method known as the “Bayesian inference”, astronmers reconstructed the gravity of the universe through cosmic history in a computer model based on these three parameters. They could estimate the parameters using the cosmic microwave background data from the Planck satellite, supernova catalogues as well as observations of the shapes and distribution of distant galaxies by the SDSS and DES telescopes.

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-    They then compared our reconstruction to the prediction of the LCDM model (essentially Einstein’s model).

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-    The full solution would probably require a new ingredient in the cosmological model, present before the time when protons and electrons first combined to form hydrogen just after the Big Bang, such as a special form of dark matter, an early type of dark energy or primordial magnetic fields.

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-    This means that astronomers will be able to use these statistical methods to continue tweaking general relativity, exploring the limits of modifications, to pave the way to resolving some of the open challenges in cosmology

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June 22,  2023        GALAXIES  -  are blackholes at the centers?       4064

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