Over a century has seen great achievement with
general relativity, Einstein's theory of gravity. It has theoretical flaws, though. This is
not shocking as the theory predicts that it will fail at spacetime
singularities within black holes, as well as at the
Big Bang.
The general theory of relativity has only been tested in weak gravity, in
contrast to physical theories that describe the other three fundamental
forces in physics: the electromagnetic and the strong and weak nuclear
interactions.
By no means are deviations of gravity from general relativity checked or
rejected anywhere in the cosmos. Theoretical physicists also assert that
deviation is inevitable.
Inconsistencies and quantum physics
Our universe began in a Big Bang, according to a theory first put out by
Georges Lemaître and largely embraced by the astronomical community. Within
black holes, there are more singularities: There, pressure and energy
density become unlimited, and space and time lose all significance. These
indicate that there are problems with Einstein's theory and that a more
basic one has to be put in its place.
Naively, quantum physics, which operates on extremely tiny scales, ought
should be able to resolve spacetime singularities.
Two fundamental principles underpin quantum physics: first,
point particles
are illogical; second, the Heisenberg uncertainty principle asserts that it
is impossible to know with perfect confidence the value of some pairs of
values, such as a particle's location and velocity. This is due to the fact
that particles behave like waves of matter at microscopic sizes and should
not be seen as points.
This is sufficient to realize that such pathologies should not exist in a
theory that incorporates both general relativity and quantum physics. All
attempts to combine quantum physics with general relativity, however,
inevitably include departures from Einstein's theory.
As a result, Einstein's theory of gravity cannot be the final one. In fact,
Arthur Eddington—best known for confirming general relativity in the
1919 solar eclipse—began looking for alternatives soon after Einstein introduced the theory
in 1915, simply to see whether things might be any different.
Up until now, all of the tests conducted on Einstein's theory have been
successful in forecasting a wide range of outcomes, from the precession of
Mercury's orbit to the presence of gravitational waves. Thus, where are these general
relativity aberrations hidden?
The cosmos is relevant
The Λ-Cold Dark Matter
(ΛCDM) model
is the mainstream model of cosmology that has been developed over a century
of research. In this case, κ represents either the well-known cosmological
constant of Einstein or an enigmatic dark energy with comparable
characteristics.
Astronomers invented dark energy on the spur of the moment to explain why
the
expansion
of the universe was accelerating. Up until recently, the ΓCDM model fit
cosmological evidence quite well, yet it is remarkably inadequate and
unsatisfying from a theoretical standpoint.
It has also seen extreme
observational tensions
during the last five years. The cosmic microwave background can be used to
estimate the Hubble constant in the early universe, and supernovae can be
used as standard candles in the late universe to calculate the age and
spatial scale of the universe.
The findings of these two measurements are
inconsistent. More importantly, the nature of dark energy, dark matter, and the field
responsible for early universe
inflation—a brief period of extraordinarily rapid expansion that gave rise to the
first galaxies and galaxy clusters—remains a mystery.
The acceleration of the cosmos seen in 1998 with
Type Ia supernovae, whose brilliance is diminished by this acceleration, is the most
persuasive observational argument for modified gravity. The general
relativity-based ΓCDM model proposes that the cosmos is filled with a highly
exotic dark energy that exerts negative pressure.
The issue is that there is no physical basis for this dark energy. Its
nature remains entirely unknown despite a multitude of
models being put forth. The cosmological constant Ϋ, which is the suggested
substitute for dark energy, should be enormous based on dubious
quantum-mechanical
back-of-the-envelope calculations.
To suit the cosmic findings, however, κ has to be extremely fine-tuned to a
minuscule number. If dark energy is real, it is really concerning that we
know so little about it.
Einstein's theory alternatives
Is it possible that problems stem instead from an incorrect attempt to
incorporate the cosmic findings into general relativity, much like
attempting to put an oversize pair of pants on someone? That the enigmatic
dark energy just does not exist, and that we are seeing the first departures
from general relativity?
The University of Naples scholars who initially put out this theory have
seen enormous growth in support, despite the ongoing fervor of the dark
energy group.
How do we find out?
Experiments on the solar system, current
gravitational wave
observations, and
near-horizon black hole imaging
restrict deviations from Einstein gravity.
A vast body of
work
has been written about alternative theories of gravity to general relativity
since Eddington's pioneering studies in 1923. A highly favored category of
substitutes is known as scalar-tensor gravity. Since it adds just one scalar
field—which corresponds to the most basic spinless particle—to Einstein's
geometric account of gravity, it is theoretically extremely
straightforward.
However, this program has far-reaching repercussions. One remarkable
occurrence is the "chameleon effect," which is the ability of these theories to pass for general relativity in
high-density settings (like stars or the solar system) while drastically
departing from it in the low-density setting of cosmology.
Because of this, the additional (gravitational) field only manifests at the
biggest (cosmological) scales in the first class of systems, when it
basically vanishes like a chameleon.
The circumstances of today
The range of alternatives to Einstein gravity has significantly expanded in
the modern era. A significantly larger class of theories known as
Horndeski theories
and its later extensions have been produced even by adding a single massive
scalar excitation (a spin-zero particle) to Einstein gravity and keeping the
resultant equations "simple" to avoid some known catastrophic
instabilities.
For the past ten years, theorists have worked to determine the practical
implications of these theories.
Gravitational waves have just been detected, and this has made it possible to
limit the physical class
of Einstein gravity changes that are possible.
There is still more to be done, nevertheless, in the hopes that
developments in
multi-messenger astronomy
in the future will reveal alterations to general relativity when gravity is
extraordinarily strong.
Provided by
The Conversation
