- 4114 - MODIFIED GRAVITY - does MOND replace Dark Matter? There is a smoking-gun evidence for modified gravity at low acceleration from Gaia observations of wide binary stars. Evidence for the breakdown of standard gravity equation in the low acceleration limit from a verifiable analysis of the orbital motions of long-period, widely separated, binary stars, usually referred to as wide binaries.
----- 4114 - MODIFIED GRAVITY - does MOND replace Dark Matter?
- Up to 26,500 wide
binaries within 650 light years were observed by European Space Agency's “Gaia
space telescope”. This study focused on
calculating gravitational accelerations experienced by binary stars as a
function of their separation or, equivalently the orbital period.
-
- 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.
-
- The studying 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.
-
- 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.
-
- 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.
-
- Because the
observed accelerations 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.
-
- 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 in a new
theoretical framework called modified Newtonian dynamics (MOND) or Milgromian
dynamics in current usage.
-
- 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.
-
- Unlike galactic
rotation curves in which the observed boosted accelerations can 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 this MOND framework.
-
- Anomalies in
Mercury's orbits observed in the nineteenth century eventually led to Einstein's
general relativity. 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. Likewise,
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. Who knows?
-
- 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.
-
- 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 at large.
-
- The unprecedented
accuracy of the “Gaia satellite” make his results sufficiently robust to
qualify as a discovery. 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.
-
- As these results
show although dark matter is a central part of the standard cosmological model,
it's not without its issues. The idea of
MoND was inspired by 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.
-
- 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 argue that our
gravitational model must be wrong.
-
- At interstellar
distances, the gravitational attraction between stars is essentially Newtonian.
So rather than modifying general relativity they proposed modifying Newton's
universal law of gravity. 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.
-
- 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.
-
- 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.
-
- High-resolution
velocity curves of 152 galaxies as observed in the “Spitzer Photometry and
Accurate Rotation Curves” (SPARC) database found a shift in agreement with AQUAL.
The data seems to support modified gravity over standard dark matter cosmology.
-
- The result is
exciting, but it doesn't conclusively overturn dark matter. The AQUAL model has
its own issues, such as its disagreement with observed gravitational lensing by
galaxies.
-
- Astronomers have
more work to do. Stay in school!
-
August 9, 2023 MODIFIED GRAVITY - replace Dark Matter? 4114
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