Everything in the universe has gravity and feels it too. Yet the most common of all fundamental forces is also the one that presents physicists the greatest challenges.

Albert Einstein’s general theory of relativity It has been remarkably successful in describing the gravity of stars and planets, but it does not seem to apply perfectly at all scales.

general relativity It has passed years of observational testing. Eddington’s measurement Deflection of starlight by the sun in 1919 final detection of gravitational waves.

However, when we try to apply it to extremely small distances, gaps in our understanding begin to appear. the laws of quantum mechanics workor when we try to describe the entire universe.

Our new work published *Nature Astronomy*has now tested Einstein’s theory on the largest scales.

We believe our approach may one day help solve some of the biggest mysteries in cosmology, and the results imply that the general theory of relativity may need to be adjusted at this scale.

## Wrong model?

Quantum theory predicts that empty space, the vacuum, is full of energy. We don’t notice its existence because our devices can only measure changes in energy rather than its total amount.

However, according to Einstein, vacuum energy has a repulsive gravity – it separates empty space. Interestingly, in 1998 it was discovered that the expansion of the Universe is actually accelerating (this finding 2011 Nobel Prize in Physics).

However, the amount of vacuum energy or dark energy As said earlier, orders of magnitude smaller than what quantum theory predicts are required to explain acceleration.

Thus, the big question, called the “old cosmological constant problem,” is whether the energy of space is really gravitational—whether it exerts a gravitational force and alters the expansion of the universe.

If yes, why is gravity much weaker than anticipated? If the vacuum has no gravity at all, what is causing the cosmic acceleration?

We don’t know what dark energy is, but we have to assume it exists to explain the expansion of the Universe.

Similarly, we have to assume that there is some kind of invisible matter. dark matterto explain how galaxies and clusters evolved in the way we observe them today.

These assumptions are baked into the scientists’ standard cosmological theory, called the lambda cold dark matter (LCDM) model – showing that there is 70 percent dark energy, 25 percent dark matter and 5 percent ordinary matter in the cosmos. And this model has been quite successful in fitting all the data collected by cosmologists over the past 20 years.

But the fact that so much of the Universe is made up of dark forces and matter that takes on meaningless weird values has led many physicists to wonder if Einstein’s theory of gravity needed changes to describe the entire universe.

A new twist arose a few years ago when different ways of measuring the rate of cosmic expansion emerged. Hubble constantgive different answers – an issue known as Hubble voltage.

The disagreement, or tension, is between two values of the Hubble constant.

One is the number predicted by the LCDM cosmological model developed to match. Light from the Big Bang ( cosmic microwave background radiation).

The other is the expansion rate, measured by observing exploding stars known as supernovas in distant galaxies.

Many theoretical ideas have been proposed for ways to modify the LCDM to explain the Hubble voltage. Among these are alternative theories of gravity.

## dig for answers

We can design tests to check whether the universe obeys the rules of Einstein’s theory.

General relativity describes gravity as the bending or bending of space and time by bending the paths light and matter travel. More importantly, he predicts that the orbits of light rays and matter should be bent the same way by gravity.

Together with a team of cosmologists, we put the fundamental laws of general relativity to the test. We also explored whether changing Einstein’s theory would help solve some of the open problems of cosmology, such as the Hubble voltage.

To find out whether general relativity is true at large scales, we set out to examine three aspects of it simultaneously for the first time. These were the expansion of the Universe, the effects of gravity on light, and the effects of gravity on matter.

Using a statistical method known as Bayesian inference, we reconstructed the gravity of the Universe through cosmic history in a computer model based on these three parameters.

We can estimate parameters using cosmic microwave background data from the Planck satellite, supernova catalogs, as well as observations of the shapes and distributions of distant galaxies. SDSS and December telescopes.

We then compared our reconstruction with the prediction of the LCDM model (mainly Einstein’s).

We found interesting hints of a possible discordance with Einstein’s prediction, although it had fairly low statistical significance.

This still means that gravity is likely to work differently at large scales, and the general theory of relativity may need to be tweaked.

Our study also found that it is very difficult to solve the Hubble tension problem simply by changing the theory of gravity.

The full solution would likely require a new component in the cosmological model that was present just before the time when protons and electrons first combined to form hydrogen. Big Bangsuch as a special form of dark matter, an early form of dark energy, or primitive magnetic fields.

Or perhaps there is a yet unknown systematic error in the data.

However, our study has shown that it is possible to test the validity of general relativity over cosmological distances using observational data. While we haven’t solved the Hubble problem yet, in a few years we will have much more data from the new probes.

This means that we can use these statistical methods to continue fine-tuning general relativity, exploring the limits of modification, paving the way for solving some obvious challenges in cosmology.

*Kazuya Koyamacosmology professor, University of Portsmouth and Levon PogosyanProfessor of Physics, Simon Fraser University*

**This article has been republished from: Speech Under Creative Commons license. To read original article.**