- 4401 - EMERGENT GRAVITY - instead of dark matter? - In 2009, theoretical physicists proposed a radical reformulation of gravity. In this theory, gravity is not a fundamental force but rather a manifestation of deeper hidden processes. But in the 15 years since then, there hasn't been much experimental support for the idea. So where do we go next?
------------------------- 4401 - EMERGENT GRAVITY - instead of dark matter?
- “Emergence” is common throughout physics.
The property of temperature, for example, isn't an intrinsic property of gases.
Instead, it's the emergent result of countless microscopic collisions. We have
the tools to match those microscopic collisions to temperature; indeed, there
is an entire branch of physics, known as statistical mechanics, that makes
these temperatures connections known.
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- In other areas, the connections between
microscopic behaviors and emergent properties aren't so clear. For example,
while we understand the simple mechanisms behind superconductivity, we do not
know how microscopic interactions lead to the emergence of high-temperature
superconductors.
-
- “Verlinde's theory” is based on what
Stephen Hawking and Jacob Bekenstein observed in the 1970s: Many properties of
black holes can be expressed in terms of the laws of thermo-dynamics. However,
the laws of thermodynamics are themselves emergent from microscopic processes.
TheVerlinde says there was more than a mere coincidence and indicated that what
we perceive as gravity may be emerging from some deeper physical process.
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- “Statistical mechanics” can be used for
describing these “emergent” properties.
These techniques applied to gravity arrive at an alternate formulation
of gravity. And because gravity is also tied to our concepts of motion,
inertia, space and time, this means our entire universe is also emergent from
those same deeper processes.
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- In 2016, Verlinde expanded his theory by
discovering that a universe containing dark energy naturally leads to a new
emergent property of space, thus allowing it to push inward on itself in
regions of low density.
-
- This discovery led to a flurry of
excitement, as it provided an alternative explanation for “dark matter” .
Currently, astronomers believe that dark matter is a mysterious, invisible
substance that makes up the bulk of all the mass of every galaxy . While that
hypothesis has been able to explain a vast wealth of observations, from the
rotation rates of stars within galaxies to the evolution of the largest
structures in the cosmos, we have yet to identify the mysterious particle.
-
- Dark matter is a form of matter that cannot
be detected by telescopes as it emits no radiation. It is thought that cold
dark matter first formed after the Big Bang. This matter then collapsed under
its own weight to form vast halos which sucked in normal matter to form visible
matter, such as galaxies.
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- In Verlinde's theory of emergent gravity, as
soon as you enter low-density regions, basically, anything outside the solar
system, gravity behaves differently than we would expect from Einstein's theory
of general relativity . At large scales, there is a natural inward pull to
space itself, which forces matter to clump up more tightly than it otherwise
would.
-
- This idea was exciting because it allowed
astronomers to find a way to test this new theory. Observers could take this
new theory of gravity and put it in models of galaxy structure and evolution to
find differences between it and models of dark matter.
-
- Over the years, however, the experimental
results have been mixed. Some early tests favored emergent gravity over dark
matter when it came to the rotation rates of stars. But more recent
observations haven't found an advantage. And dark matter can also explain much
more than galaxy rotation rates; tests within galaxy clusters have found
emergent gravity coming up short.
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- This isn't the end of “emergent gravity”.
The idea is still new and requires a lot of assumptions in its calculations to
make it work. Without a fully realized theory, it's hard to tell if the
predictions it makes for the behavior of galaxies and clusters accurately
represent what emergent gravity would tell us. And astronomers are still trying
to develop more stringent tests, like using data from the cosmic microwave
background .
-
- Emergent gravity remains an intriguing idea.
If it's correct, we would have to radically reshape our understanding of the
natural world and see gravity and motion, and even more fundamental concepts,
like time and space, through a lens of emergence from deeper, more complicated
interactions. But for right now, it remains simply an intriguing idea. Only
time and extensive observational testing will tell us if we're on the right
track.
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March 23, 2023 EMERGENT
GRAVITY - instead of dark matter? 4401
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