We don’t know what dark energy is, but we have to assume it exists to explain the expansion of the universe. Similarly, we must also assume that there is a type of invisible matter present, called dark matter, to explain how galaxies and clusters evolved into how we observe them today.

These assumptions are embedded in scientists’ standard cosmological theory, called the cold dark matter lambda model (LCDM) – suggesting that there is 70% dark energy, 25% dark matter and 5% 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, taking on strange values that don’t make sense, has many physicists wondering if Einstein’s theory of gravity had need to be modified to describe the entire universe.

A new twist came a few years ago when it became apparent that different ways of measuring the rate of cosmic expansion, called the Hubble constant, gave different answers – a problem known as the Hubble strain.

Detuning, or tension, is between two values of the Hubble constant. One is the number predicted by the LCDM cosmological model, which was developed to match the light left behind by the Big Bang (cosmic microwave background radiation). The other is the rate of expansion measured by observing exploding stars called supernovae in distant galaxies.

Cosmic microwave background. NASA
Many theoretical ideas have been proposed to modify the LCDM to explain the Hubble voltage. Among them are alternative gravitational theories.

Look for answers
We can design tests to check whether the universe obeys the rules of Einstein’s theory. General relativity describes gravity as the bending or warping of space and time, bending the pathways along which light and matter travel. Importantly, he predicts that the paths of light rays and matter should be bent by gravity in the same way.

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 if general relativity is correct on a large scale, we undertook, for the first time, to study 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 into a computer model based on these three parameters. We were able to estimate the parameters using cosmic microwave background data from the Planck satellite, supernova catalogs as well as observations of the shapes and distribution of distant galaxies by the SDSS and DES telescopes. We then compared our reconstruction to the prediction of the LCDM model (essentially the Einstein model).

We found interesting hints of a possible discrepancy with Einstein’s prediction, albeit with rather low statistical significance. This means that there is nevertheless a possibility that gravity works differently on large scales and that the theory of general relativity must be modified.

Our study also revealed that it is very difficult to solve the Hubble tension problem by modifying only the theory of gravity. The complete solution would likely require a new ingredient in the cosmological model, present before the time protons and electrons combined to form hydrogen just after the Big Bang, such as a special form of dark matter, an early type of dark energy or primordial magnetic fields. Or, perhaps, there is a yet unknown systematic error in the data.

That said, 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 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 refine general relativity, to explore the limits of modifications, to open the way to solving some of the open challenges in cosmology.

Kazuya Koyama, professor of cosmology, Portsmouth University and Levon Pogosian, professor of physics, Simon Fraser University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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