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Gravity is experienced and felt by all things in the cosmos. Despite being the most ubiquitous of the fundamental forces, understanding it has proven to be the most significant barrier for physicists. General relativity, developed by Albert Einstein, has been extraordinarily effective in modeling star and planetary gravity. However, in a new study, it does not seem to apply precisely on all scales.
General relativity has been tested by observations for many years, from Eddington's 1919 measurement of how the sun bends starlight to the recent discovery of gravitational waves. But when we attempt to apply it to very short distances, where the rules of quantum physics operate, or when we try to characterize the whole universe, we start to see gaps in our knowledge.
Einstein's theory was recently put to the greatest challenge in new research published in Nature Astronomy. They think this method has the potential to one day explain some of cosmology's deepest riddles, and their findings suggest that general relativity may need to be tweaked on this scale.
So, what exactly is it about that model that makes it flawed?
According to quantum theory, the vacuum contains an abundance of energy. Due to the limitations of our measuring equipment, we are oblivious to its existence.
But Einstein claims that vacuum energy has a repulsive gravity, causing empty space to be pushed apart. In 1998, astronomers made the unexpected discovery that the universe's expansion is speeding up (a finding awarded the 2011 Nobel Prize in physics). In contrast to what one would expect from quantum theory, the quantity of vacuum energy, or dark energy, required to explain the acceleration is several orders of magnitude lower.
So, the main issue, which is called "the old cosmological constant problem," is whether or not vacuum energy gravitates, which means that it creates a gravitational force and changes the way the universe is expanding.
If that's the case, then it begs the question of why the planet's gravity is so much lower than anticipated. What is driving the expansion of the universe if the vacuum has no gravitational pull?
To account for the expansion of the cosmos, we must theorize the existence of something called "dark energy," the nature of which remains a mystery. To account for the observed properties of galaxies and clusters today, we must additionally theorize the existence of an unseen kind of stuff called dark matter.
The mainstream cosmological theory, the Lambda-cold dark matter (LCDM) model, which proposes that there is 70% dark energy, 25% dark matter, and 5% ordinary matter in the universe, is predicated on these assumptions. Incredibly, cosmologists have been able to fit all of their data from the previous 20 years into this model.
There have been a lot of theoretical suggestions on how to tweak LCDM to account for the Hubble tension. Alternative theories of gravity are one such example.
In order to determine whether or not the cosmos follows the predictions of Einstein's theory, we may develop tests to do so. According to general relativity, gravity warps the paths that light and matter take across space and time. What's more, it predicts that gravity should have the same effect on the paths of light beams and matter.
After testing, the research team found interesting hints of a possible mismatch 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 may need to be tweaked.
Their study also found that it is very difficult to solve the Hubble tension problem by only changing the theory of gravity. The full 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 the Big Bang, such as a special form of dark matter, an early type of dark energy, or primordial magnetic fields. Or, perhaps, there’s a yet unknown systematic error in the data.
Still, their study shows that it is possible to use observational data to test whether general relativity is true over cosmological distances. While they haven’t yet solved the Hubble problem, they will have a lot more data from new probes in a few years.
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