107 years later, we're still testing Einstein's theory of gravity -- the results keep getting weirder

107 years later, we’re still testing Einstein’s theory of gravity — the results keep getting weirder

everything in The universe has gravity – and feels it too. But this most common of all fundamental forces is also the one that presents physicists with the greatest challenges. Albert Einstein’s general theory of relativity was remarkably successful in describing the gravitational forces of stars and planets, but it does not appear to apply perfectly at all scales.

General relativity has passed many years of observational tests, from Eddington’s measurement of the Sun’s deflection of starlight in 1919 to the recent discovery of gravitational waves. However, gaps in our understanding arise when we try to apply it to extremely small distances where the laws of quantum mechanics are at work, or when we try to describe the entire 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 suggest that general relativity may need to be adjusted to that extent.

A quantum problem

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

However, according to Einstein, vacuum energy has repulsive gravity—it pushes empty space apart. Interestingly, in 1998 it was discovered that the expansion of the universe was accelerating (a finding that won the Nobel Prize in Physics in 2011). However, the amount of vacuum energy, or dark energy as it is called, needed to explain the acceleration is many orders of magnitude smaller than what quantum theory predicts.

Therefore, the big question, dubbed “the ancient problem of the cosmological constant,” is whether the vacuum energy is gravitational – exerting a gravitational force and altering the expansion of the universe.

If so, why is its gravity so much weaker than predicted? If the vacuum doesn’t gravitate at all, then what’s causing 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. Similarly, to explain how galaxies and clusters evolved in the way we observe them today, we must also assume that there is a type of invisible matter called dark matter.

Cosmic Microwave Background.NASA

These assumptions are baked into scientists’ standard cosmological theory, called the Lambda Cold Dark Matter (LCDM) model, which proposes 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 of 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 that take on strange values ​​that don’t make sense has caused many physicists to wonder if Einstein’s theory of gravitation needs to be modified to describe the entire universe .

A new twist appeared a few years ago when different methods of measuring the rate of cosmic expansion, called the Hubble constant, were found to give different answers — a problem known as the Hubble voltage.

The disagreement or tension exists 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 over from the Big Bang (the cosmic microwave background radiation). The other is the expansion rate, which is measured by observing exploding stars, known as supernovae, in distant galaxies.

Many theoretical ideas have been proposed to modify LCDM to explain the Hubble voltage. Among them are alternative theories of gravity.

Search for answers

Einstein’s general theory of relativity still fascinates physicists.Gado/Stock Photos/Getty Images

We can develop 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 that warps the paths along which light and matter travel. Importantly, it predicts that the trajectories 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 investigated whether a modification of Einstein’s theory could help solve some of the open problems in cosmology, such as the Hubble voltage.

To determine whether general relativity is correct on a large scale, we set out, for the first time, to examine 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 universe’s gravity through cosmic history in a computer model based on these three parameters. We were able to estimate the parameters from the cosmic microwave background data from the Planck satellite, supernova catalogues, and 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 Einstein’s model).

We found interesting evidence for a possible disagreement with Einstein’s prediction, albeit with rather low statistical significance. This means that it is still possible that gravity works differently on large scales and that general relativity needs to be adjusted.

Our study also found that it is very difficult to solve the Hubble stress problem just by changing the theory of gravity. The full solution would likely require a new component in the cosmological model that existed before the time when protons and electrons first combined to form hydrogen shortly after the Big Bang, such as B. a special form of dark matter, an early form of dark energy, or pristine magnetic fields. Or maybe there is an as yet unknown systematic error in the data.

Our study has shown 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 more data from new probes in a few years.

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

This article was originally published on The conversation by Kazuya Koyama and Levon Pogosian at the University of Portsmouth and Simon Fraser University. Read the original article here.

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