- 4226 - GENERAL RELATIVITY - does not explain? How 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.
--------------------- 4226 - GENERAL RELATIVITY - does not explain?
- How 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.
-
- Hubble captured
images of the universe's many galaxies, with an Einstein ring. When the light
from distant galaxies warps around an extremely large mass, like a galaxy
cluster, it creates this elegant ring
-
- Einstein's theory
of gravity, general relativity, has been very successful for more than a
century. However, it has theoretical shortcomings. This is not surprising: 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.
-
- Deviations of
gravity from general relativity are by no means excluded nor tested everywhere
in the universe. And, according to theoretical physicists, deviation must
happen.
-
- 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.
-
- 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.
-
- This is enough to
understand that a theory that embraces both general relativity and quantum
physics should be free of such pathologies. However, 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. Indeed, 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?
-
- 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 ad hoc by astronomers 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.
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.
-
- From the
observational point of view, 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.
-
- Problem is, this
dark energy has no physical justification. Its nature is completely unknown,
although a number of models has been proposed. The proposed alternative to dark
energy is a “cosmological constant” “Λ”
which, according to quantum-mechanical back-of-the-envelope 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. 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? That we are observing the first deviations from general
relativity while the mysterious dark energy simply does not exist?
-
- 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. A very popular class of alternatives is the so-called
'scalar-tensor gravity'. It 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.
-
- The consequences
of this program is the striking phenomenon, 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”.
-
- 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.
-
- However, much work
still needs to be done, with the hope that future advances in multi-messenger
astronomy lead to discovering modifications of general relativity where gravity
is extremely strong.
-
-
November 17, 2023 GENERAL RELATIVITY
- does not explain? 4222
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