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.
Deviations and quantum mechanics
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.
Naively, 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. 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.
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 plethora 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 (but questionable) 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.
Alternatives to Einstein's theory
Could it be that troubles arise, instead, from wrongly trying to fit the cosmological observations into general relativity, like fitting a person into a pair of trousers that are too small? That we are observing the first deviations from general relativity while the mysterious dark energy simply does not exist?
This idea, first proposed by researchers at the University of Naples, has gained tremendous popularity while the contending dark energy camp remains vigorous.
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, however, are far from trivial. 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.
The current situation
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, and subsequent generalizations.
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.
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PhD in Astrophysics, supervisor George F.R. Ellis, worked in relativity, cosmology, and alternative theories of gravity for 30 years, been at Bishop's University for 16 years, currently full professor in the Physics & Astronomy Department. Author of 210 refereed journal articles and 7 books, funded by NSERC and volunteered extensively for NSERC, the Canadian Association of Physicists, and occasionally for other organizations worldwide.
I have always felt that general relativity and the theory of gravity are wrong. Unfortunately, I can't not prove why.Reply
Georges Lemaître was the person who proposed the Big Bang, not Einstein.Reply
What a difference two months do, the origibal Conversation article was written as the first Webb observations - as hoped for - would entice and confound the astronomical community. Since then the early question have had - as expected - mainstream explanations and Webb has joined the ever increasing supportive tests of general relativity and the rejection of "alternative theories".Reply
Re the mischaracterization of the space expansion that is at the core of "big bang" cosmologies the original idea of "a primeval atom" has been replaced by observation of inflation. The observed low energy slow roll inflation is quite possibly eternal - so no need for an initial condition - and seem to be unavoidably multiverse spawning - so no need for anthropic "finetuning" of vacuum energy.
Zbigniew Osiak published a paper in 2019 saying that following the relativity principle faithfully leads to a mass energy equivalency of E_0 = m c^2/2, and a relativistic energy equation E = g^2 m c^2/2, where g here is the Lorentz factor. The extra Lorentz factor would imply that the early universe of Osiak relativity is vastly more energetic than under Einstein relativity. So it seems reasonable it would lead to earlier formation of stars and galaxies than predicted by standard cosmology.Reply
I had some reasons that I needed to know if Osiak is right or not so I rederived all of his equations, and then I kept going. I was aware (from J.D. Jackson's textbook) that it's claimed that E_0 = m c^2 is the unique mass-energy equivalency that leads to conservation of energy in relativistic particle mechanics. I was able to confirm pretty easily with a matlab routine that E0_ = m c^2 /2 doesn't conserve energy. (Osiak states in his paper that his alternative conserves momentum and energy, but the equation he is talking about is about four-momentum, which in Osiak relativity is no longer the energy-momentum four-vector, so it is conserving four-momentum but not energy.) From Einstein's comment on the energy formulas, "It's best to keep things simple," I think he probably arrived at the same equations as Osiak, but rejected them in order to obtain energy conservation. But the energy-nonconservation of Osiak would only be observable in relativistic conditions. Also, four-momentum conservation still exists in Osiak relativity, as needed to explain, e.g., particle creation thresholds.
I started working on this is 2019, soon after Osiak's paper was published, so before Webb was launched. There are a lot of other interesting things about the Osiak form of relativity and it seems compatible with existing physics (particularly the Dirac relativistic electron theory), and since dark energy was inferred, people have been proposing energy non-conservation as an explanation. Also, so far as I have figured out from some limited literature review, cosmic inflation still does not have a widely accepted explanation, although it is considered well supported by observation. Guth says in his book he thought it was a Higgs field driving inflation but that fell apart and it seems that nothing has clearly filled the need. Anyhow in addition to dark energy it seems that relativistic energy nonconservation is not an unreasonable cause of inflation. An additional Lorentz factor in the early, highly relativistic, universe is a vastly greater energy density than per Einstein relativity. I get that up to half is nonconserved, and it straightforwardly becomes gravitational potential energy, seems to me. In any case, to modify special relativity is necessarily to modify general relativity as well.
I have a draft version of my Osiak review paper posted on Researchgate. Right now googling "Osiak relativity" turns it up as the first thing, then Osiak's paper.
Could Einstein Be Wrong? Theory of Gravity May Have Shortcomings in Explaining the UniverseKendra StacyReply
Nov 15, 2023
While the popular theory of gravity, or general relativity, of Einstein boasts of success that has lasted for over a century, it does have its own theoretical shortcoming when it comes to explaining the Universe.
The theory of gravity has only been tested in gravity that is weak. This is unlike other physical theories that describe the three other fundamental physics forces, namely, the strong, weak, and electromagnetic nuclear interactions.
General relativity's gravity deviations are not tested nor excluded anywhere in the Universe. Theoretical physicists think that deviation is necessary.
held that the Universe started with the Big Bang. There are other singularities that can be found within black holes. Within these massive cosmic mysteries, time and space become meaningless, while pressure and energy density end up becoming infinite. These show that the theory of Einstein fails there and that a more fundamental theory should replace it.
Quantum mechanics should be able to resolve singularities in spacetime. Quantum physics typically depends on two ideas, namely, the Heinsenberg uncertainty principle that holds that no one can know a certain quantity pair's value with absolute accuracy and that point particles do not have sense.
This is sufficient enough to understand that such pathologies should not be present in a theory that embraces quantum physics and general relativity. However, Einstein's theory ends up with deviations when attempts are made to mix quantum physics and general relativity.
This means that the general theory of relativity proposed by Einstein cannot be the utmost theory of gravity. Interestingly, Arthur Eddington began looking for alternatives shortly after Einstein's theory was introduced. Eddington is known for verifying the theory during the solar eclipse in 1919.
The theory of Einstein has lived through all the tests. The question now is where the general relativity deviations could be.
See: Breakthrough Physics Discovery: Gravity Can Actually Create Light, Study Says
With a century's worth of research, scientists have been gifted with the Λ-Cold Dark Matter (ΛCDM) model. The symbol Λ refers to the cosmological constant of Einstein or a similar dark energy, which was introduced to explain cosmic expansion and acceleration. Though the model was found to fit cosmological data, it was found to be unsatisfactory and incomplete from a theoretical stance.
The model has also seen various observational tensions throughout the past five years. The constant of the Hubble, which notes the scale of distance and age across the Universe, can be gauged in the early Universe via cosmic microwave background. It can also be assessed in the late Universe via supernovae.
Both measurements offer results that are not compatible. The ΛCDM components also largely remain mysterious.
From an observational perspective, the most convincing reason for modified gravity would be the Universe's acceleration. The ΛCDM model postulates dark energy that is remarkably exotic and that has negative pressure that permeates through the Universe. However, the issue lies in the lack of physical justification of dark energy. The proposed dark energy alternative is the Λ cosmological constant, which calculations show should be huge. However, the constant Λ should be fine tuned into a small value for it to align with cosmic observations.
The idea of troubles surfacing from wrongly fitting cosmological data into the theory is an idea that has been gaining great popularity. This comes as the camp for dark energy stays vigorous.
This can be told by how the deviations of Einstein's theory are constrained by experiments in the solar systems.
Literature pertaining to alternative gravity theories are widely present and have shown dramatic growth. In the last decades, theorists have been trying to see the physical consequences that result from other theories.
Recent gravitational wave detections have offered an approach to manage the physical class modifications that were allowed by Einstein gravity. However, there is a need to conduct further work and research.
Einstein's Theory Of General Relativity has been under assault for the better part of a century. No one can really prove it wrong but it's commonly assumed to be wrong.
Sergei Kopeikin, associate professor of physics and astronomy at University of Missouri-Columbia, has spent the last five years defending Einstein's prediction that gravity moves at the speed of light - they want to prove Einstein right and they say they can measure the speed of propagation of tiny ripples of space-time known as gravitational waves.
He says his paper, "Gravimagnetism, causality, and aberration of gravity in the gravitational light-ray deflection experiments" published along with Edward Fomalont from the National Radio Astronomical Observatory, arrives at a consensus in the continuing debate that has divided the scientific community.
The experiment conducted by Kopeikin and Edward Fomalont five years ago found that the gravity force of Jupiter and light travel at the same speed, which validates Einstein's suggestion that gravity and electromagnetic field properties, are governed by the same principle of special relativity with a single fundamental speed.
In observing the gravitational deflection of light caused by motion of Jupiter in space, Kopeikin concluded that mass currents cause non-stationary gravimagnetic fields to form in accordance with Einstein's point of view.
Einstein believed that in order to measure any property of gravity, one has to use test particles. “By observing the motion of the particles under influence of the gravity force, one can then extract properties of the gravitational field,” Kopeikin said. “Particles without mass – such as photons – are particularly useful because they always propagate with constant speed of light irrespectively of the reference frame used for observations.”
The property of gravity tested in the experiment with Jupiter also is called causality. Causality denotes the relationship between one event (cause) and another event (effect), which is the consequence (result) of the first. In the case of the speed of gravity experiment, the cause is the event of the gravitational perturbation of photon by Jupiter, and the effect is the event of detection of this gravitational perturbation by an observer. The two events are separated by a certain interval of time which can be measured as Jupiter moves, and compared with an independently-measured interval of time taken by photon to propagate from Jupiter to the observer. The experiment found that two intervals of time for gravity and light coincide up to 20 percent. Therefore, the gravitational field cannot act faster than light propagates.”
Other physicists argue that the Fomalont-Kopeikin experiment measured nothing else but the speed of light. “This point of view stems from the belief that the time-dependent perturbation of the gravitational field of a uniformly moving Jupiter is too small to detect,” Kopeikin said.
“However, our research article clearly demonstrates that this belief is based on insufficient mathematical exploration of the rich nature of the Einstein field equations and a misunderstanding of the physical laws of interaction of light and gravity in curved space-time.”
- University of Missouri-Columbia
Scientists used a half-century of lunar laser ranging data to confirm with 100 times greater precision that all properties of mass are equivalent. This finding significantly bolsters Einstein’s equivalence principle, a cornerstone of relativity theory.
One of the most basic assumptions of fundamental physics is that the different properties of mass – weight, inertia, and gravitation – always remain the same in relation to each other. Without this equivalence, Einstein’s theory of relativity would be contradicted and our current physics textbooks would have to be rewritten. Although all measurements to date confirm the equivalence principle, quantum theory postulates that there should be a violation. This inconsistency between Einstein’s gravitational theory and modern quantum theory is the reason why ever more precise tests of the equivalence principle are particularly important.
A team from the Center of Applied Space Technology and Microgravity (ZARM) at University of Bremen, in collaboration with the Institute of Geodesy (IfE) at Leibniz University Hannover, has now succeeded in proving with 100 times greater accuracy
How close the measured value conforms to the correct value.
Inertial mass resists acceleration. For example, it causes you to be pushed backward into your seat when the car starts. Passive gravitational mass reacts on gravity and results in our weight on Earth. Active gravitational mass refers to the force of gravitation exerted by an object, or more precisely, the size of its gravitational field. The equivalence of these properties is fundamental to general relativity. Therefore, both the equivalence of inertial and passive gravitational mass and the equivalence of passive and active gravitational mass are being tested with increasing precision.
https://scitechdaily.com/images/Vishwa-Vijay-Singh-400x400.jpgFirst Author of the Publication, Vishwa Vijay Singh. Credit: Singh
What was the study about?
If we assume that passive and active gravitational mass are not equal – that their ratio depends on the material – then objects made of different materials with a different center of mass would accelerate themselves. Since the Moon consists of an aluminum shell and an iron core, with centers of mass offset against each other, the Moon should accelerate. This hypothetical change in speed could be measured with high precision, via “Lunar Laser Ranging.” This involves pointing lasers from Earth at reflectors on the Moon placed there by the Apollo missions and the Soviet Luna program. Since then, round trip travel times of laser beams are recorded. The research team analyzed “Lunar Laser Ranging” data collected over a period of 50 years, from 1970 to 2022, and investigated such mass difference effects. Since no effect was found, this means that the passive and active gravitational masses are equal to approximately 14 decimal places. This estimate is a hundred times more accurate than the best previous study, dating back to 1986.
LUH’s Institute of Geodesy – one of only four centers worldwide analyzing laser distance measurements to the Moon – has unique expertise in assessing the data, particularly for testing general relativity. In the current study, the institute analyzed the Lunar Laser Ranging measurements, including error analysis and interpretation of the results.
Vishwa Vijay Singh, Jürgen Müller and Liliane Biskupek from the Institute of Geodesy at Leibniz University Hannover, as well as Eva Hackmann and Claus Lämmerzahl from the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen published their findings in the journal Physical Review Letters, where the paper was highlighted in the category “editors’ suggestion.”
Reference: “Equivalence of Active and Passive Gravitational Mass Tested with Lunar Laser Ranging” by Vishwa Vijay Singh, Jürgen Müller, Liliane Biskupek, Eva Hackmann and Claus Lämmerzahl, 13 July 2023, Physical Review Letters.
Albert Einstein published his full theory of general relativity in 1915, followed by a flurry of research papers by Einstein and others exploring the predictions of the theory. In general relativity (GR), concentrations of mass and energy curve the structure of spacetime, affecting the motion of anything passing near — including light. The theory explained the anomalous orbit of Mercury, but the first major triumph came in 1919 when Arthur Eddington and his colleagues measured the influence of the Sun’s gravity on light from stars during a total solar eclipse.
Physicists made many exotic predictions using general relativity. The bending of light around the Sun is small, but researchers realized the effect would be much larger for galaxies, to the point where gravity would form images of more distant objects — the phenomenon now called gravitational lensing. GR also predicted the existence of black holes: objects with gravity so intense that nothing getting too close can escape again, not even light.
General relativity showed that gravitation has a speed, which is the same as the speed of light. Catastrophic events like collisions between black holes or neutron stars produce gravitational waves. Researchers finally detected these waves in 2015 using the Laser Interferometer Gravitational Observatory (LIGO), a sensitive laboratory that took decades to develop.
For many aspects of astronomy — the motion of planets around stars, the structure of galaxies, etc. — researchers don’t need to use general relativity. However, in places where gravity is strong, and to describe the structure of the universe itself, GR is necessary. For that reason, researchers continue to use GR and probe its limits.
Black holes are extremely common in the universe. Stellar-mass black holes, the remnants of massive stars that exploded, are sometimes the source of powerful X-ray emissions when they are in binary systems with stars. In addition, nearly every galaxy harbors a supermassive black hole at its center, some of which produce powerful jets of matter visible from across the universe. GR is essential to understanding how these objects become so bright, as well as studying how black holes form and grow. The Event Horizon Telescope (EHT) is a world-spanning array of observatories that captured the first image of a supermassive black hole, providing a new arena for testing GR’s predictions.
Gravitational waves are a new branch of astronomy, providing a complementary way to study astrophysical systems to the standard light-based observations. Researchers use GR to provide “templates” of many possible gravitational wave signals, which is how they identify the source and its properties. Gravitational wave astronomy combines with light-based astronomy to characterize some of the most extreme events in the cosmos: collisions of black holes and neutron stars.
Astronomers use gravitational lensing to locate some of the earliest galaxies in the universe, which are too faint to be seen without the magnification provided by gravity. In addition, the distortion created by lensing allows researchers to study dark matter, and map the structure of the universe on the largest scales.
Not long after Einstein published GR, researchers realized the theory predicts that the universe changes in time. Observations in the 1920s found that prediction was true: the universe is expanding, with galaxies moving away from each other. Using GR, cosmologists found the cosmos had a beginning, and was once hotter and denser than it is today. GR provides the mathematical framework for describing the structure and evolution of the universe from its beginnings 13.8 billion years ago, and into the future.See: https://pweb.cfa.harvard.edu/research/science-field/einsteins-theory-gravitation
General relativity explains how the universe can obey physical laws that apply to any form of motion. It’s at the heart of identifying and investigating key questions about space and time, existence and reality. Its implications are not limited to esoteric concerns on cosmic scales — it has its down-to-Earth impacts as well. Without general relativity, for instance, GPS devices would be worthless. Satellite signals designed to keep your car on the right road would be off by miles if not corrected for the effects predicted by Einstein’s math.
Special relativity showed that the laws of nature don’t depend on how you are moving, as long as it’s uniform motion — constant speed in a straight line. But in real life, objects and people move in all sorts of nonuniform ways. Even some “simple” motions, like the rotation of a sphere or orbit of a planet, are nonuniform, as they constantly change direction and are therefore accelerating. Einstein wanted to extend relativity to all forms of accelerated motions. But he didn’t know how.
Today, Einstein’s cosmological constant has been revived. Rather than preventing the universe from collapse, the vacuum energy it describes can explain why the universe now expands at an accelerating pace. General relativity, cosmological constant and all, today forms the core science for analyzing the history of the universe and forecasting its future.
After Einstein died in 1955, general relativity came to life. About that time Wheeler, at Princeton University, began a program to explore its implications and train students to pursue them. By the early 1960s, new astronomical phenomena demanded explanations that Newtonian physics could not provide, and general relativity was poised for its renaissance. In the decades that followed, general relativity proved crucial for describing all sorts of celestial phenomena. At the same time, physicists devised ever more precise tests of its predictions, and Einstein passed them all. As Will has noted, “It is remarkable that this theory, born 100 years ago out of almost pure thought, has managed to survive every test.”