In 1915, Albert Einstein published the Theory of General Relativity, revolutionising our perception of the universe.
In the theory, space and time are a fabric, permeating even the furthest reaches of the universe.
As matter and energy press down on this fabric, they actually change its geometry.
This, Einstein said, is what causes gravity. As massive objects such as the sun press down on the fabric of spacetime, other objects, such as Earth, react, orbiting in that curved geometry.
One hundred years later, scientists are using this theory to make precise models of cosmology, allowing them to explore the evolution of the universe.
Two research teams on both sides of the Atlantic “have shown that precise modelling of the universe and its contents will change the detailed understanding of the evolution of the universe and the growth of structure in it,” a Case Western University press release said.
Smoothing out the lumps
Up until now, scientists had to study cosmology in an approximate way with pencil and paper.
“The equations of General Relativity are very complicated, involving many different variables,” Glenn Starkman, professor of physics and astronomy at Case Western Reserve University, told Business Insider. “You have to follow lots of things that are interrelated in very complicated ways.”
To do this, they had to take the universe, which is lumpy and full of objects denser than the space around them, and smooth it out.
“On top of that smooth universe, we could put very small differences place to place to describe how objects behaved,” Starkman said. “But we couldn’t fully incorporate it into a description of how the universe has evolved.”
But over the course of the last 100 years, modern computers have developed, becoming faster and faster.
And over the last 20 or 30 years, especially as scientists hunted for gravitational waves emitted by merging black holes, they developed new techniques for studying the complicated equations of General Relativity on computers.
“We didn’t have to make the universe smooth to follow its evolution,” Starkman said. “We could put an inhomogenous, lumpy universe into the computer and write a program to tell the computer how to use Einstein’s field equations to follow the universe from wherever we started into the future.”
You can think of it like a balloon, Starkman said. As you push on it, it responds, stretching and changing shape. When you put a piece of matter or energy somewhere in space and time, it creates a dimple in the fabric. Lots of pieces of matter, spread out over space, create lots of dimples.
What scientists want to know is how all of those dimples behave together and change space and time. Starkman called this “the dance of the dimples.”
“It’s such a complicated dance to really follow accurately,” Starkman said. “You need to follow it on a computer to be able to tell the difference between what those dimples do exactly and what they would do if you smoothed them out into one large dimple.”
These new models will allow scientists to closely follow this dance and check how accurate a picture of the universe their approximations have given them up until now.
“Some people expect dramatic changes,” Starkman said. “They expect the acceleration of the universe we found 20 years ago to be entirely due to general relativistic effects. Others say we now have to use a new form of energy called dark energy to account for this acceleration. Now we finally have these new tools to explore this.”
The symphony of the universe
So far, the scientists have created simulations of the universe, which you can think of as many, many consecutive slices of what the universe might have looked like at various times, Starkman said.
The dimples are kind of like musical notes in the universe. Up until this point, the Case Western University press release explained, what scientists have been doing is kind of like averaging the music made by a symphony.
The audience would hear a single average note, keeping the overall beat, growing generally louder and softer.
These new models allow them to hear the “individual notes and rhythms of each of the orchestra’s instruments.”
But, because of the limitations of computers, these models still aren’t perfect.
The next step is to make the models better by being able to make the regions they’re studying bigger. They want to add to these notes to fill out the symphony.
“So far we’ve only been able to put a few hundred notes in there,” Starkman said. “We want to put thousands or millions of notes in there to get more and more structure in the universe.”
The scientists also want to do an analogue of experiments, pretending that they are observers sitting in the universe, receiving light from every direction. Some of the light travels a short way from the nearest galaxy, and some of it travels nearly all the way across the universe taking billions of years. They want to know what this would look like compared to a smooth universe, with no dimples, to see to what extent the models are different.
“What’s exciting is that we do appear to be seeing a difference. It is likely there will be measurable differences between how the universe behaves if it’s perfectly smooth versus when it’s lumpy,” Starkman said. “It’s amazing that it’s taken 100 years, but we are really finally getting to the point where the full power of General Relativity can be brought to bear on the whole universe.”
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