Everything in the universe has and feels gravity. However, the most general of all the fundamental forces also poses the greatest challenges for physicists.

Albert Einstein’s general theory of relativity has been quite successful in describing the gravitational forces of stars and planets, but does not apply perfectly on all scales.

general relativity has undergone many years of observation tests Eddington measurement Variation of starlight by the Sun in 1919 the recent detection of gravitational wavesðŸ‡§ðŸ‡·

But when we try to apply this to very small distances, the gaps in our understanding begin to show the laws of quantum mechanics workor when we try to describe the entire universe.

Our new study, was published in *Natural Astronomy*now tested Einstein’s theory on the largest scale.

We believe that our approach may one day help solve some of the greatest mysteries of cosmology, and the results suggest that general relativity may need to be modified on this scale.

## A flawed model?

Quantum theory predicts that empty space, the vacuum, is full of energy. We do not feel its presence because our devices can only measure changes in energy, not the total amount of energy.

However, according to Einstein, vacuum energy has a repulsive force of gravity â€“ it pushes against empty space. Interestingly, in 1998 it was discovered that the expansion of the Universe is actually accelerating (this finding 2011 Nobel Prize in PhysicsðŸ‡§ðŸ‡·

However, the amount of vacuum energy or dark energy as said, it is necessary to explain that the acceleration is many times smaller than predicted by quantum theory.

So the big question, called the “old cosmological constant problem,” is whether the vacuum energy is actually attractiveâ€”exerting gravity and altering the expansion of the universe.

If so, why is its gravity weaker than predicted? If vacuum does not attract at all, what is the cause of cosmic acceleration?

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

Similarly, we must also assume the existence of some kind of invisible substance dubbed dark matterto explain how galaxies and clusters evolved into the way we observe them today.

These assumptions translate into scientists’ standard cosmological theory, called the lambda cold dark matter (LCDM) modelâ€”which suggests that the universe is 70 percent dark energy, 25 percent dark matter, and 5 percent ordinary matter. And this model has been quite successful in fitting all the data collected by cosmologists over the past 20 years.

But the fact that most of the Universe is made up of dark forces and matter, taking on strange values â€‹â€‹that do not make sense, has led many physicists to wonder if Einstein’s theory of gravity needs modification to describe the entire universe.

A new twist appeared a few years ago, when it became clear that there are different methods of measuring the rate of cosmic expansion, the so-called “Cut”. Hubble constantgive different answers â€“ known as a problem Hubble tensionðŸ‡§ðŸ‡·

The disagreement or tension is between the two values â€‹â€‹of the Hubble constant.

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

Another is the rate of expansion, measured by observing exploding stars known as supernovae in distant galaxies.

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

## Digging for answers

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

General relativity describes gravity as the warping or warping of space and time, the bending of the paths taken by light and matter. Importantly, it predicts that the trajectories of light and matter should be bent in the same way by gravity.

Together with a team of cosmologists, we tested the fundamental laws of general relativity. We also explored whether modifying Einstein’s theory could help solve some of the open problems of cosmology, such as the Hubble tension.

To find out whether general relativity holds true on a large scale, we set out to investigate three aspects of it simultaneously for the first time. These were the expansion of the Universe, the effect of gravity on light, and the effect 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 the parameters using cosmic microwave background data from the Planck satellite, supernova catalogs, and observations of the shapes and distributions of distant galaxies. SDSS and DES telescopes.

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

We found interesting hints of possible inconsistency with Einstein’s prediction, albeit with fairly low statistical significance.

This means that gravity is likely to work differently on large scales, and general relativity may need to be modified.

Our research also showed that it is very difficult to solve the Hubble stress problem just by changing the theory of gravity.

A complete solution would likely require a new ingredient that existed before the time when protons and electrons first combined to form hydrogen in the cosmological model. big bangfor example, a special form of dark matter, an early form of dark energy, or primordial magnetic fields.

Or, perhaps there is an as yet unknown systematic error in the data.

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

This means that we will be able to use these statistical methods to continue to change general relativity, to explore the limits of change, and to open the way to solving some open problems in cosmology.

*Kazuya Koyamaprofessor of cosmology, University of Portsmouth and Levon Pogosyanphysics professor, Simon Fraser University*

**This article is being republished Conversation Under Creative Commons license. read it original articleðŸ‡§ðŸ‡·**