Everything in the universe has gravity—and feels it, too. However, it is also this most common fundamental force that presents the greatest challenges to physicists.

Albert Einstein’s general theory of relativity It was remarkably successful in describing the gravitational pull of stars and planets, but it doesn’t seem to be quite true on all scales.

general relativity Passed many years of observation tests, from Eddington measurement From the diffraction of starlight from the sun in 1919 to The latest detection of gravitational waves.

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

Our new study, Posted in *natural astronomy*He 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 suggest that general relativity may need to be modified on this scale.

## defective model?

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

However, according to Einstein, the energy of vacuum has a repulsive attraction – it pushes empty space apart. Interestingly, in 1998, it was discovered that the expansion of the universe is in fact accelerating (a discovery that was granted with 2011 Nobel Prize in Physics).

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

Hence the big question, dubbed the “Old Cosmological Constant Problem”, is whether vacuum energy is really attracted – giving rise to the force of gravity and altering the expansion of the universe.

If yes, why is its attraction so much weaker than expected? If a vacuum is not attracted at all, what causes the cosmic acceleration?

We don’t know what dark energy is, but we need to postulate its existence in order to explain the expansion of the universe.

Likewise, we also need to suppose that there is some kind of existence of invisible matter dubbed dark matterTo explain how galaxies and clusters evolved to be the way we observe them today.

These assumptions have been incorporated into scientists’ standard cosmological theory, called the Cold Dark Matter Lambda Model (LCDM) – which suggests that there is 70 percent of dark energy, 25 percent of dark matter, and 5 percent of ordinary matter in the universe. This model has been remarkably successful in fitting all the data that cosmologists have collected over the past 20 years.

But the fact that most of the universe consists of forces and dark matter, which take on strange, meaningless values, has led many physicists to wonder whether Einstein’s theory of gravity needs to be modified to describe the entire universe.

A new development emerged a few years ago when it became apparent that different ways of measuring the rate of cosmic expansion, called Hubble constantgive different answers – a problem known as Hubble tension.

Disagreement or tension between two values of the Hubble constant.

The first is the number predicted by the LCDM cosmological model, which was developed to match The light left by the Big Bang (The cosmic microwave background radiation).

The other is the rate of expansion, which is measured by observing supernovae in distant galaxies.

Several theoretical ideas for LCDM modulation methods have been proposed to explain the Hubble tension. Among them are alternative theories of gravity.

## Searching 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 deflection of space and time, which bends the paths along which light and matter travel. Importantly, it predicts that the paths of light and matter rays should be bent by gravity in the same way.

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 open problems in cosmology, such as the Hubble tension.

To find out if general relativity is true on large scales, we set out, for the first time, to investigate three aspects of it simultaneously. 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 shapes and distribution of distant galaxies by SDSS And the DE telescopes.

Then we compared our reconstruction with the prediction to the LCDM model (essentially Einstein’s model).

We found interesting hints about a possible mismatch with Einstein’s predictions, albeit with rather low statistical significance.

This means that there is still a possibility that gravity will work differently on large scales, and that general relativity may need to be modified.

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

Perhaps a complete solution would require a new component of the cosmological model, which existed before the time when protons and electrons first combined to form hydrogen after the great explosionsuch as a special form of dark matter, an early type of dark energy, or primordial magnetic fields.

Or maybe there is an unknown systematic error in the data.

However, our study demonstrated that it is possible to test the validity of general relativity at cosmic distances using observational data. While we haven’t solved the Hubble problem yet, we’ll have a lot of data from the new probes in a few years.

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

*Kazuya Koyamaprofessor of cosmology, University of Portsmouth And the Levon Bogosianphysics professor, Simon Fraser University*

**This article has been republished from Conversation Under a Creative Commons License. Read the original article.**

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