# Scientists tested Einstein’s relativity on a cosmic scale and found something strange: ScienceAlert

Scientists tested Einstein’s relativity on a cosmic scale and found something strange: ScienceAlert

Everything in the universe has gravity – and feels it. However, this most common of all fundamental forces is also the one that presents the greatest challenge to physicists.

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

General relativity has passed several years of observational tests, since Eddington measurement from the deflection of starlight from the Sun in 1919 to 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 workor when trying to describe the entire universe.

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

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

## Faulty model?

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

However, according to Einstein, vacuum energy has repulsive gravity – it pulls empty space apart. It is interesting that in 1998 it was discovered that the expansion of the universe is actually accelerating (a discovery awarded with Nobel Prize in Physics 2011).

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

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

If so, then why is its gravity so much weaker than predicted? If the vacuum does not gravitate at all, what causes 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 should also assume that there is a kind of presence of invisible matter, called Black matterto explain how galaxies and clusters evolved into the way we see them today.

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

But the fact that much of the Universe is made up of dark forces and matter, taking on strange 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 apparent that different ways of measuring the rate of cosmic expansion, called Hubble constantgive different answers – a problem known as Hubble tension.

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

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

The second 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 account for the Hubble tension. 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 a curvature or bending of space and time, a bending of the paths 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 tested the basic laws of general relativity. We also investigated whether modifying Einstein’s theory could help solve some open problems in cosmology, such as the Hubble tension.

To find out whether general relativity is correct 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 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 throughout cosmic history in a computer model based on these three parameters.

We could estimate the parameters using cosmic microwave background data from the Planck satellite, supernova catalogs, and observations of the shape and distribution of distant galaxies by SDSS and OF THE telescopes.

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

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

This means that there is still a possibility that gravity works differently on large scales and that the theory of general relativity may need to be adjusted.

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

A 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 Big bangsuch as a special form of dark matter, an early type of dark energy, or primordial magnetic fields.

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

However, our study has shown that it is possible to test the validity of general relativity at cosmological distances using observational data. Although we haven’t solved the Hubble problem yet, we will 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 continue to fine-tune general relativity, exploring the limits of modification, to pave the way towards solving some of the open challenges in cosmology.

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

**This article was republished by Conversation under Creative Commons license. Read it original article.**

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