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