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

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

Albert Einstein’s General Theory of Relativity has been remarkably successful in describing the gravitational pull of stars and planets, but it doesn’t seem to apply perfectly at all scales.

General Theory of Relativity has withstood many years of observational tests, from Eddington’s measurement from the deflection of the starlight by the sun in 1919 to the recent gravitational wave detection.

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

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

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

## Defective 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 rather than the total amount.

However, according to Einstein, the vacuum energy has a repulsive gravitational pull — it pushes empty space apart. Interestingly, it was discovered in 1998 that the expansion of the Universe is in fact accelerating (a finding crowned with the 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 smaller than what quantum theory predicts.

Hence the big question, also called “the old cosmological constant problem”, is whether the vacuum energy actually attracts – exerting a gravitational pull and altering the expansion of the universe.

If so, why is gravity so much weaker than predicted? If the vacuum doesn’t pull at all, what causes 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.

Likewise, we must also assume that there is some kind of invisible matter called dark matterto explain how galaxies and clusters evolved into the way we perceive them today.

These assumptions are ingrained in scientists’ standard cosmological theory called the lambda cold dark matter (LCDM) model — suggesting that there is 70 percent dark energy, 25 percent dark matter and 5 percent ordinary matter in the cosmos. And this model has been remarkably 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 substances, which take on strange values that make no sense, has led many physicists to wonder whether Einstein’s theory of gravity should be adapted to describe the entire universe.

A few years ago, a new twist appeared when it became clear that several ways of measuring the rate of cosmic expansion, the so-called Hubble constantgive different answers – a problem known as the Hubble voltage.

The disagreement, or tension, lies 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 that remains from the big bang (the 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 put forward for ways to adapt LCDM to explain the Hubble voltage. Among them are alternative theories of gravity.

## Digging for answers

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

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

Together with a team of cosmologists, we test the basic laws of general relativity. We also explored whether adapting Einstein’s theory could help solve some of the outstanding problems of cosmology, such as the Hubble voltage.

To find out whether general relativity is true on a large scale, 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 the Bayesian inference, we reconstructed the gravity of the Universe through cosmic history in a computer model based on these three parameters.

We were able to estimate the parameters using the cosmic microwave background data from the Planck satellite, supernova catalogs and observations of the shapes and distribution of distant galaxies through the SDSS and OF THE telescopes.

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

We found interesting evidence of a possible mismatch with Einstein’s prediction, albeit with a rather low statistical significance.

This means that there is still a possibility that gravity works differently on a large scale and that general relativity may need to be modified.

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

The complete solution would likely require a new ingredient in the cosmological model, present before the time when protons and electrons first came together to form hydrogen just after the big bangsuch as a special form of dark matter, an early type of dark energy or primordial magnetic fields.

Or maybe there is an as 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, in a few years we will have a lot more data from new probes.

This means that we can use these statistical methods to continue modifying general relativity, exploring the limits of modifications, paving the way for solving some of the outstanding challenges in cosmology.

*Kazuya Koyamaprofessor of cosmology, University of Portsmouth and Levon Pogosianprofessor of physics, Simon Fraser University*

**This article was republished from The conversation under a Creative Commons license. Read the original article.**

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