- 4227 -
EINSTEIN'S THEORY -
must be wrong? Einstein's
“general relativity” fails to explain the universe. As new and powerful telescopes gather fresh
data about the universe, they reveal the limits of older theories like
Einstein's relativity
-
--------------------- 4227 - EINSTEIN'S THEORY - must be wrong?
- Einstein's theory
of gravity, “general relativity” has been very successful for more than a
century. However, it has theoretical shortcomings. The theory predicts its own
failure at spacetime singularities inside black holes and the Big Bang itself.
-
- Unlike physical
theories describing the other three fundamental forces in physics, the
electromagnetic and the strong and weak nuclear interactions, the general
theory of relativity has only been tested in weak gravity.
-
- According to
Einstein, our universe originated in a Big Bang. Other singularities hide
inside black holes. Space and time cease
to have meaning there, while quantities such as energy density and pressure
become infinite.
-
- These signal that
Einstein's theory is failing there and must be replaced with a more fundamental
one. These spacetime singularities
should be resolved by quantum mechanics, which apply at very small scales.
-
- Quantum physics
relies on two simple ideas: point particles make no sense; and the Heisenberg
uncertainty principle, which states that one can never know the value of
certain pairs of quantities with absolute precision, for example, the position
and velocity of a particle. This is because particles should not be thought of
as points but as waves; at small scales they behave as waves of matter.
-
All attempts to blend general relativity and quantum physics
necessarily introduce deviations from Einstein's theory. Therefore, Einstein's gravity cannot be the
ultimate theory of gravity.
-
- It was not long
after the introduction of general relativity by Einstein in 1915 that Arthur
Eddington, best known for verifying this theory in the 1919 solar eclipse,
started searching for alternatives just to see how things could be different.
-
- Einstein's theory
has survived all tests to date, accurately predicting various results from the
precession of Mercury's orbit to the existence of gravitational waves. So,
where are these deviations from general relativity hiding?
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- A century of
research has given us the standard model of cosmology known as the Λ-Cold Dark
Matter (ΛCDM) model. Here, Λ stands for either Einstein’s famous cosmological
constant or a mysterious dark energy with similar properties.
-
- Dark energy was
introduced to explain the acceleration of the cosmic expansion. Despite fitting
cosmological data extremely well until recently, the ΛCDM model is
spectacularly incomplete and unsatisfactory from the theoretical point of view.
-
- In the past five
years, it has also faced severe observational tensions. The “Hubble constant”,
which determines the age and the distance scale in the universe, can be
measured in the early universe using the “cosmic microwave background” and in
the late universe using “supernovae as standard candles”.
-
- These two
measurements give incompatible results. Even more important, the nature of the
main ingredients of the ΛCDM model, dark energy, dark matter and the field
driving early universe inflation (a very brief period of extremely fast
expansion originating the seeds for galaxies and galaxy clusters), remains a mystery.
-
- The most compelling
motivation for modified gravity is the acceleration of the universe discovered
in 1998 with Type Ia supernovae, whose luminosity is dimmed by this
acceleration. The ΛCDM model based on general relativity postulates an
extremely exotic dark energy with negative pressure permeating the universe.
-
- The problem is that
this dark energy has no physical justification. Its nature is completely
unknown, although a numbere of models has been proposed. The proposed
alternative to dark energy is a cosmological constant Λ which, according to
quantum-mechanical calculations, should be huge.
-
- However, Λ must
instead be incredibly fine-tuned to a tiny value to fit the cosmological
observations. If dark energy exists, our ignorance of its nature is deeply
troubling.
-
- Type Ia
supernovae were discovered in 1998, and revealed more about the rate of the
universe's acceleration. Could it be
that troubles arise, instead, from wrongly trying to fit the cosmological
observations into general relativity. Are we observing the first deviations
from general relativity while the mysterious dark energy simply does not exist?
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- How can we tell?
Deviations from Einstein gravity are constrained by solar system experiments,
the recent observations of gravitational waves and the near-horizon images of
black holes. There is now a large
literature on theories of gravity alternative to general relativity, going back
to Eddington's 1923 early investigations.
-
- “Scalar-tensor
gravity is conceptually very simple since it only introduces one additional
ingredient (a scalar field corresponding to the simplest, spinless, particle)
to Einstein's geometric description of gravity.
-
- A striking
phenomenon is the "chameleon effect," consisting of the fact that
these theories can disguise themselves as general relativity in high-density
environments (such as in stars or in the solar system) while deviating strongly
from it in the low-density environment of cosmology.
-
- As a result, the
extra gravitational field is effectively absent in the first type of systems,
disguising itself as a chameleon does, and is felt only at the largest
cosmological scales.
-
- Nowadays the
spectrum of alternatives to Einstein gravity has widened dramatically. Even
adding a single massive scalar excitation (namely, a spin-zero particle) to
Einstein gravity, and keeping the resulting equations "simple" to
avoid some known fatal instabilities, has resulted in the much wider class of
Horndeski theories.
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- Theorists have
spent the last decade extracting physical consequences from these theories. The
recent detections of gravitational waves have provided a way to constrain the
physical class of modifications of Einstein gravity allowed.
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-
November 18, 2023 EINSTEIN'S THEORY
- must be wrong? 4227
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