Everything in the Universe has gravity, and you also feel it. However, this fundamental force, the most common of all, is also the one that presents the greatest challenges to physicists.

Albert Einstein’s Theory of General Relativity has been remarkably successful in describing the gravity of stars and planets, but it does not seem to apply perfectly to all scales.

General relativity has passed many years of observation tests, from Eddington measure from the deflection of starlight by the Sun in 1919 to the recent detection of gravitational waves.

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

Our new studio published in *nature astronomy*now he has proven Einstein’s theory on the largest of scales.

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

## Defective model?

Quantum theory predicts that empty space, the void, is teeming with energy. We don’t notice its presence because our devices can only measure changes in energy rather than its total amount.

However, according to Einstein, the energy of the vacuum has a repulsive gravity: it pushes empty space apart. Interestingly, in 1998 it was discovered that the expansion of the Universe is in fact accelerating (an award-winning finding). 2011 Nobel Prize in Physics).

However, the amount of vacuum energy, or dark energy as it has been called, necessary to explain that the acceleration is many orders of magnitude less than what quantum theory predicts.

So the big question, called “the old cosmological constant problem,” is whether vacuum energy actually gravitates, exerting a gravitational force and changing the expansion of the universe.

If so, why is its gravity so much weaker than anticipated? If the vacuum doesn’t gravitate at all, what is causing the cosmic acceleration?

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

Similarly, we must also assume that there is a kind of presence of invisible matter, called dark matterto explain how galaxies and clusters evolved to be the way we observe them today.

These assumptions feed into scientists’ standard cosmological theory, called the lambda model of cold dark matter (LCDM), which suggests that there is 70 percent dark energy, 25 percent dark matter, and 5 percent dark matter. ordinary in the cosmos. And this model has been remarkably successful in fitting all the data collected by cosmologists over the last 20 years.

But the fact that most of the Universe is made up of dark forces and substances, which take on weird values that don’t 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 different ways of measuring the rate of cosmic expansion, called hubble constantgive different answers – a problem known as Hubble stress.

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

One is the number predicted by the LCDM cosmological model, which has been developed to match the light left over from the Big Bang (the cosmic microwave background radiation).

The other 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 LCDMs to explain the Hubble strain. Among them are alternative theories of gravity.

## Digging for answers

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

General relativity describes gravity as the curvature or warping of space and time, bending the pathways along which light and matter travel. Importantly, it predicts that the paths of light rays and matter should be bent by gravity in the same way.

Together with a team of cosmologists, we put the basic laws of general relativity to the test. We also explore whether modifying Einstein’s theory could help solve some of the open problems in cosmology, such as the Hubble strain.

To find out whether general relativity is correct on a large scale, we set out, for the first time, to simultaneously investigate three aspects of it. 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 reconstruct the gravity of the Universe throughout cosmic history in a computer model based on these three parameters.

We could estimate the parameters using the cosmic microwave background data from the Planck satellite, catalogs of supernovae, as well as observations of the shapes and distribution of distant galaxies by the SDSS Y DES telescopes

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

We find interesting hints of a possible mismatch with Einstein’s prediction, albeit with fairly low statistical significance.

This means that, however, there is a possibility that gravity works differently on a large scale, and that the general theory of relativity needs to be modified.

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

The complete solution would probably require a new ingredient in the cosmological model, present before the time when protons and electrons first combined to form hydrogen just after the big Bangas a special form of dark matter, a primitive type of dark energy, or primordial magnetic fields.

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

That said, 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, we’ll have a lot more data from new probes in a few years.

This means that we will be able to use these statistical methods to further tune general relativity, exploring the limits of modifications, to pave the way for solving some of the open challenges in cosmology.

*kazuya koyamaprofessor of cosmology, University of Portsmouth Y levon pogosianPhysics teacher, Simon Fraser University*

**This article is republished from The conversation under a Creative Commons license. Read the Original article.**