- 4128 - GRAVITY - how
gravity works? There is evidence for
“modified gravity” at low acceleration from Gaia observations of wide binary
stars. A new study reports conclusive
evidence for the breakdown of standard gravity in the low acceleration limit from
a verifiable analysis of the orbital motions of long-period, widely separated,
binary stars.
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4128 - GRAVITY
- how gravity works?
-
- This study of 26,500 wide binaries within
650 light years (LY) observed by European Space Agency's Gaia space telescope
was published in August 2023. The study focused on calculating gravitaional
accelerations experienced by binary stars as a function of their separation or,
equivalently the orbital period.
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- Gravity could be most directly and
efficiently tested by calculating accelerations because gravitational field
itself is an acceleration. Galactic
disks and wide binaries share some similarity in their orbits, though wide
binaries follow highly elongated orbits while hydrogen gas particles in a
galactic disk follow nearly circular orbits.
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- The study finds that when two stars orbit
around with each other with accelerations lower than about one nanometer per
second squared start to deviate from the prediction by Newton's universal law
of gravitation and Einstein's general relativity.
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- For accelerations lower than about 0.1
nanometer per second squared, the observed acceleration is about 30 to 40%
higher than the Newton-Einstein prediction. The significance is very high
meeting the conventional criteria of 5 sigma for a scientific discovery.
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- In a sample of 20,000 wide binaries within
a distance limit of 650 LY two independent acceleration bins respectively show
deviations of over 5 sigma significance in the same direction.
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- Because the observed accelerations are
stronger than about 10 nanometer per second squared agree well with the
Newton-Einstein prediction from the same analysis, the observed boost of
accelerations at lower accelerations is a mystery.
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- What is intriguing is that this breakdown
of the Newton-Einstein theory at accelerations weaker than about one nanometer
per second squared was suggested 40 years ago by theoretical physicist, called modified Newtonian dynamics (MOND) or
Milgromian dynamics.
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- Moreover, the boost factor of about 1.4 is
correctly predicted by a MOND-type Lagrangian theory of gravity called AQUAL.
What is remarkable is that the correct boost factor requires the external field
effect from the Milky Way galaxy that is a unique prediction of MOND-type
modified gravity. What the wide binary
data show are not only the breakdown of Newtonian dynamics but also the
manifestation of the external field effect of modified gravity.
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- Unlike galactic rotation curves in which
the observed boosted accelerations can, in principle, be attributed to dark
matter in the Newton-Einstein standard gravity, wide binary dynamics cannot be
affected by it even if it existed. The standard gravity simply breaks down in
the weak acceleration limit in accordance with the MOND framework.
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- Now anomalies in wide binaries require a
new theory extending general relativity to the low acceleration MOND limit.
Despite all the successes of Newton's gravity, general relativity is needed for
relativistic gravitational phenomena such as black holes and gravitational
waves.
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- Despite all the successes of general
relativity, a new theory is needed for MOND phenomena in the weak acceleration
limit. The weak-acceleration catastrophe of gravity may have some similarity to
the ultraviolet catastrophe of classical electrodynamics that led to quantum
physics.
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- Wide binary anomalies are a disaster to the
standard gravity and cosmology that rely on dark matter and dark energy
concepts. Because gravity follows MOND, a large amount of dark matter in
galaxies (and even in the universe) are no longer needed.
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- A new revolution in physics seems now under
way. If this anomaly is confirmed as a
breakdown of Newtonian dynamics, and especially if it indeed agrees with the
most straightforward predictions of MOND, it will have enormous implications
for astrophysics, cosmology, and for fundamental physics.
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- The unprecedented accuracy of the Gaia
satellite, the large and meticulously selected sample used in this detailed
analysis, make this result sufficiently robust to qualify as a discovery.
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- With this test on wide binaries as well as
our tests on open star clusters nearby the sun, the data now compellingly imply
that gravitation is Milgromian rather than Newtonian. The implications for all
of astrophysics are immense.
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- Although dark matter is a central part of
the standard cosmological model, it's not without its issues. There continue to
be nagging mysteries about the stuff, not the least of which is the fact that
scientists have found no direct particle evidence of it.
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- Despite numerous searches, we have yet to
detect dark matter particles. Some astronomers favor “modified Newtonian dynamics” (MoND) or
modified gravity model. And this new study of galactic rotation seems to
support them.
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- The idea of MoND was inspired by this
galactic rotation. Most of the visible matter in a galaxy is clustered in the
middle, so you'd expect that stars closer to the center would have faster orbital
speeds than stars farther away, similar to the planets of our solar system.
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- What we observe is that stars in a galaxy
all rotate at about the same speed. The rotation curve is essentially flat
rather than dropping off. The dark matter solution is that galaxies are
surrounded by a halo of invisible matter, but in 1983 Mordehai Milgrom argued
that our gravitational model must be wrong.
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- At interstellar distances, the
gravitational attraction between stars is essentially Newtonian. So rather than
modifying general relativity, Milgrom proposed modifying Newton's universal law
of gravity. He argued that rather than the force of attraction as a pure
inverse square relation, gravity has a small remnant pull regardless of
distance. This remnant is only about 10 trillionths of a G, but it's enough to
explain galactic rotation curves.
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- Just adding a small term to Newton's gravity
means that you also have to modify Einstein's equations, as well. So MoND has
been generalized in various ways, such as AQUAL, which stands for "a
quadradic Lagrangian." Both AQUAL and the standard LCDM model can explain
observed galactic rotation curves, but there are some subtle differences.
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- One difference between AQUAL and LCDM is in
the rotation speeds of inner orbit stars vs. outer orbit stars. For LCDM, both
should be governed by the distribution of matter, so the curve should be
smooth. AQUAL predicts a tiny kink in the curve due to the dynamics of the
theory. It's too small to measure in a single galaxy, but statistically, there
should be a small shift between the inner and outer velocity distributions.
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- Astronomers looked at high-resolution
velocity curves of 152 galaxies as observed in the Spitzer Photometry and
Accurate Rotation Curves (SPARC) database. They found a shift in agreement with
AQUAL. The data seems to support modified gravity over standard dark matter
cosmology.
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- The result doesn't conclusively overturn
“dark matter”. The AQUAL model has its own issues, such as its disagreement with
observed gravitational lensing by galaxies. An astronomers work is never done!
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August 25, 2023 GRAVITY
- how gravity works? 4128
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