General relativity may need tweaking on the grand scale of the Universe

Source: (https://bit.ly/3unUM5I)
Einstein's theory of general relativity, formulated in 1916, treats
gravity as a deformation of spacetime as a result of different
particles and fields. Together with the Standard Model describing
electromagnetic, weak and strong forces, it constitutes the basis
of our modern understanding of particle physics.
Despite the remarkable success of this theory in explaining
gravitational effects on the scales of planets, stars, and galaxies,
general relativity still has its limitations. It has been known for
around a century that it doesn't explain quantum effects, which
in the case of gravity are expected to become important at the
scale approximately 20 orders of magnitude smaller than the
size of an atom. This means that to describe the physics of gravity
at such extremely small distances, one cannot use general relativity.
However, recent astronomical observations indicate that on the
largest of scales - that of the entire Universe - general relativity
may also lose its validity.
To determine whether general relativity or any known modification
provides a correct theory of gravity on this grand scale, an
international team of physicists led by Professor Levon Pogosian
of Simon Fraser University analyzed a large body of astronomical
data and concluded that all known theories of gravity have some
form of tension with current observations.
Einstein's theory of general relativity has successfully passed many
experimental tests. The first dates back to the beginning of the
20th century, when it was found that general relativity could explain
the peculiarities of Mercury's trajectory, and correctly predicted
the angle of light deflection by the Sun.
Since then, the validity of this theory has been confirmed in
a series of experiments and observations, culminating in the
detection of gravitational waves in 2015 and the first image
of a black hole in 2019 - the existence of which was an exciting
prediction of general relativity.
However, in the 1933, it was noticed that in order for the observed
velocities of stars in distant galaxies to not contradict those
predicted by general relativity, the galaxies must contain a large
amount of an unknown substance dubbed dark matter.
Moreover, the 1998 discovery of the accelerated expansion of the
Universe forced scientists to introduce another ingredient into the
theory called dark energy, which is sometimes interpreted as the
energy of the vacuum, and according to Einstein's theory, causes
the expansion.
For general relativity to be consistent with this astronomical data,
these two entities - dark matter and dark energy - must account for
about 95% of all energy in the Universe. However, neither have
manifested themselves in any laboratory experiments, and their
origin and properties do not have a reliable theoretical explanation,
which has led some scientists to think that they may not exist
at all.
Even more difficulties arose after the discovery of what astronomers
called the Hubble tension, in which the values of the expansion rate
of the Universe measured in different experiments contradict one
another. For example, in observations made of supernova explosions
and the properties of the cosmic microwave background - radiation
left over from the Big Bang.
"The discrepancy does not automatically mean that general relativity
is incorrect or that something else is wrong with our cosmological
model," said Pogosian in an e-mail. "We have to keep an open mind
and allow for something yet unaccounted for in the data analysis.
However, this possibility has been (and continues to be) under tight
scrutiny and, so far, the tension only got stronger with further
analysis."
Nevertheless, many cosmologists wonder if the contradictions between
theory and observation should be resolved by a modification of the
theory of gravity. Possible improvements of general relativity have
been studied by researchers for decades, but the most popular of them
is a set of theories proposed in 1974 by Gregory Horndeski of the
University of Waterloo, Canada, in which the details of how matter
curves spacetime may differ from Einstein's theory.
According to general relativity and Horndeski's generalizations, the
influence of gravity on the evolution of the Universe can be divided
into three parts: gravity's effect on matter, radiation, and the
expansion of the Universe.
To determine if there is a theory of gravity whose description
of these effects agree with observations, Pogosian's team combined
astronomical data collected by a number of ground-based and
space-based observatories and compared the data with predictions
made using these different theories.
They found an unsurprising mismatch between Einstein's theory and
observations, which is expected due to the previously mentioned
Hubble tension. However, what is more interesting is that none
of the Horndeski's theories gave results consistent with
observations.
This means that if the observational data are accurate, a more
radical modification of general relativity is needed than that
proposed by Horndeski. It may include some new particles or new
interactions that contribute significantly to the evolution of the
Universe.
"The observational evidence suggests that the missing ingredient
is likely to be in our understanding of the early Universe, namely,
the epoch just before recombination - the time when the cosmic
microwave background was produced," said Pogosian. "Popular
ideas include early dark energy, primordial magnetic fields, new
types of neutrinos, interacting dark matter, and some others."
A possible problem with the accuracy of experimental data will
hopefully be solved with the next generation of telescopes. If future
observations confirm the correctness of the data used by the
scientists, this will mean that our current understanding of gravity
is incomplete and needs to be improved.
"The next generation of the cosmic microwave background
experiments, such as the Simons Observatory and CMB-S4, will
have much better sensitivity and resolution and could rule out or
confirm some of the contending theoretical proposals for solving
the Hubble tension," concluded Pogosian. "The ongoing and future
galaxy survey, such as DESI, Euclid, Vera Rubin and SKA, when
combined with the cosmic microwave background data, will offer
very stringent tests of general relativity."

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