Everything in the universe has gravity – and feels it too. Yet this most common fundamental force is also the one that poses the greatest challenges to physicists. Albert Einstein’s theory of general relativity was remarkably successful in describing the gravity of stars and planets, but it doesn’t seem to apply perfectly to all scales.
General relativity has passed many years of observational testing, from Eddington’s measurement of starlight deflection by the Sun in 1919 to the 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 operate, or when we try to describe the entire universe.
Our new study, published in Nature Astronomy, has now tested Einstein’s theory on the largest scale. We believe our approach could one day help solve some of cosmology’s greatest mysteries, and the results suggest that the theory of general relativity may need to be modified on this scale.
Quantum theory predicts that empty space, the vacuum, is filled with energy. We do not notice its presence because our devices can only measure changes in energy rather than its total amount.
However, according to Einstein, vacuum energy has repulsive gravity – it pulls empty space apart. Interestingly, in 1998 it was discovered that the expansion of the universe was actually accelerating (a discovery that won the 2011 Nobel Prize in Physics). However, the amount of vacuum energy, or dark energy as it has been called, needed to explain the acceleration is several orders of magnitude less than predicted by quantum theory.
Hence the big question, dubbed the “old cosmological constant problem”, is whether vacuum energy actually gravitates – exerting a gravitational force and altering the expansion of the universe.
If so, then why is its gravity so much lower than expected? If the vacuum does not gravitate 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. 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.
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 Hubble’s 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, 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.
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