From Eddington’s 1919 measurement of the bending of starlight by the Sun to the current finding of gravitational waves, general relativity has stood the test of time in the observation laboratory.
Although successful in modelling the gravity of stars and planets, Albert Einstein’s theory of general relativity appears to only apply properly on some scales.
Gravity is present and felt by all things in the cosmos. Despite being the most ubiquitous of the fundamental forces, understanding it has proven to be one of the greatest hurdles for the physical sciences.
Although General Relativity by Albert Einstein has proven extremely successful in modelling stellar and planetary gravity, it appears only sometimes applicable.
From Eddington’s 1919 measurement of the bending of starlight by the Sun to the current finding of gravitational waves, general relativity has stood the test of time in the observation laboratory.
Applying it to extremely small distances, where quantum physics laws operate, or attempting to describe the entire cosmos reveals that we still have much to learn.
With our latest research, published in Nature Astronomy, Einstein’s hypothesis has been put to the ultimate test.
Our method could one day aid in solving some of cosmology’s greatest mysteries, and the results suggest that general relativity might need a change on this scale.
An Incorrect Model?
According to quantum theory, the vacuum contains a great deal of energy. However, because our instruments can only detect fluctuations in energy, we cannot detect its presence.
According to Einstein, however, space exerts repulsive gravity due to vacuum energy.
In 1998, scientists discovered that the universe’s expansion was accelerating (a finding awarded the 2011 Nobel prize in physics).
The acceleration requires many orders of magnitude less vacuum or dark energy than quantum theory predicts.
Thus, the essential issue, known as “the classic cosmological constant problem,” is whether or not the vacuum energy gravitates in the sense of exerting a gravitational attraction and thereby altering the universe’s expansion.
If true, why does it have significantly less gravity than anticipated? What drives the universe’s expansion if the vacuum has no gravitational pull? To account for the accelerating expansion of the cosmos, we must postulate the existence of a mysterious force called “dark energy.”
To account for the observed structure of galaxies and clusters, we must also postulate the existence of an invisible form of substance, nicknamed dark matter.
The current gold standard in cosmology, the lambda cold dark matter (LCDM) model, is based on these presuppositions. This model proposes that 70% of the universe comprises dark energy, 25% dark matter, and 5% conventional matter.
Incredibly, cosmologists have used this model to explain their evidence over the past 20 years.
Since much of the world is made up of dark forces and substances with unusual and unexplainable values, many physicists have wondered if Einstein’s theory of gravity must be updated to describe the entire cosmos.
In recent years, however, a new wrinkle known as the Hubble tension has emerged: it has become clear that different methods of calculating the rate of cosmic expansion, nicknamed the Hubble constant, yield different results.
In this case, the Hubble constant is the source of the discord.
The LCDM cosmological modelb, constructed to fit the light from the Big Bang, predicts a value of one (the cosmic microwave background radiation).
The other can be inferred from observations of supernovae in distant galaxies, which occur at a pace corresponding to the universe’s expansion rate.
There have been a lot of theoretical proposals for adjusting LCDM to account for the Hubble tension. Alternative theories of gravity are one example.
Looking For The Bottom of Things
Furthermore, in discussing Einstein’s general theory of relativity, we can set up experiments to determine whether the universe follows Einstein’s theory. According to general relativity, gravity warps the paths that light and matter take through space and time.
More importantly, it implies that gravity should affect light beam paths like ordinary stuff.
Our group of cosmologists tested the fundamental principles of general relativity. We also looked into the possibility that tweaking Einstein’s theory could solve some of cosmology’s unanswered questions, such as the Hubble tension.
We set out to do the first comprehensive study of general relativity, looking at all three of its major characteristics to determine whether it holds on huge scales.
These included the universe’s accelerating expansion, the warping of light by gravity, and the acceleration of matter due to gravity.
Using Bayesian inference, a statistical technique, we reconstructed the universe’s gravity across cosmic history in a computational model based on these three variables.
The Planck satellite’s measurements of the cosmic microwave background, supernova catalogues, and the Sloan Digital Sky Survey’s and the Dark Energy Survey’s studies of the shapes and distributions of distant galaxies might all be used to estimate the parameters.
The LCDM model (basically Einstein’s model) was then used to compare our reconstruction to its prediction.
Despite the modest statistical significance, we did find some intriguing signs of a probable discrepancy with Einstein’s prediction.
However, this still leaves open the potential that gravity behaves differently on extremely large scales, suggesting that general relativity may need some fine-tuning.
Our research also shows that more than adjusting gravity alone is needed to resolve the Hubble tension.
A new component in the cosmological model, such as a novel type of dark matter, an early form of dark energy, or primordial magnetic fields, would likely be necessary for a complete solution to be reached, as it would have to have existed before the time when protons and electrons first combined to form hydrogen shortly after the Big Bang.
Alternatively, there may be a systematic flaw in the data that has yet to be uncovered.
However, our research has shown that general relativity may be tested using observational evidence, even at cosmological distances.
We have yet to figure out the Hubble problem, but we’ll have more information in a few years, thanks to new probes.
This implies we can use these statistical tools to keep tinkering with general relativity, testing its boundaries as a framework for examining potential alterations that could help us solve some of the unresolved problems in cosmology.
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