Image NASA, ESA, CSA, and STScI
Scientists want to measure the distortion of time in ancient faraway galaxies in order to solve fundamental mysteries about our reality, including the potential existence of a “fifth force” beyond established physics, reports a new study.
By peering at galaxies located billions of light years from Earth, researchers hope to spot signs of an effect called gravitational redshift, wherein light becomes redder after escaping the gravitational pull of massive objects. This redshifted light contains clues about how the gravitational fields of massive objects warp time, which could constrain a host of unanswered questions about our universe.
For instance, time distortion measurements might reveal whether the laws of gravity change at extreme scales in ways that are not predicted by Albert Einstein’s theory of general relativity, or by Leonhard Euler’s equations describing celestial motions. This concept of “modified gravity” could assess whether a mysterious force called dark energy is accelerating the expansion of our universe, and shed light on dark matter, an unidentified substance that accounts for the vast majority of mass in the cosmos. It could also probe the existence of an undiscovered fifth force that only acts on dark matter particles, as opposed to the particles that make up planets, stars, and people.
Camille Bonvin, an associate professor of cosmology at the University of Geneva, has been developing a mathematical method to measure time distortion for years. Now, Bonvin and Levon Pogosian, a professor of physics at Simon Fraser University, have laid out a plan for capturing “a direct measurement of the distortion of time” and finding “a possible smoking gun for modified gravity,” according to a study published on Thursday in Nature Astronomy.
“The reason we want to test Einstein’s equations is because there is one big mystery in our universe that we currently don’t know how to explain,” said Bonvin in an email to Motherboard. “We have observed that roughly 8 billion years after the beginning of the universe, the expansion of the universe starts to accelerate, and we don’t know the reason for this. Such an accelerated expansion is not possible if gravity is described by general relativity (Einstein’s theory of gravity) and if there is just matter and radiation in our universe.”
“In addition to this, the reason why we want to test Euler’s equation is because we have never tested its validity for dark matter,” she continued. “Euler’s equation describes the motion of matter in the universe. It has been tested and validated very precisely for ordinary (visible) matter. However, we believe that besides ordinary matter, there is a large amount (85%) of dark invisible matter in the universe. We don’t know what this dark matter is, we have never detected it, and therefore we don’t know if it obeys Euler’s equation. It may very well be that dark matter is affected by extra forces (that we call fifth force) and that because of that it does not obey Euler’s equation. For this reason, it is very interesting to test the validity of this equation for dark matter.”
In other words, the researchers hope to discover if there is an entirely new fifth “dark force” that is not described by Euler’s equations, or whether gravity is modified in ways that fall outside of Einstein’s predictions. This study is part of a wider effort to both understand the nature of dark matter and to understand why the universe is expanding at an accelerated pace, a phenomenon known as cosmic acceleration.
“There are currently two ways of explaining this strange behavior,” explained Bonvin, referring to cosmic acceleration. “The first one is to add a new form of energy in the universe, called dark energy, that has some very strange properties and leads the universe into an accelerated expansion.”
“The second way of explaining the acceleration of the universe is to modify the theory of gravity, in such a way that gravity itself is responsible for the accelerated expansion,” she added. “These so-called ‘modified theories of gravity’ behave like general relativity at small distances (on Earth, in the solar system), where general relativity has been tested very precisely. But they differ from general relativity at very large distances: one needs a different behavior of gravity to put the universe into acceleration. To determine if the accelerated expansion is due to dark energy or to modified gravity, we need to test general relativity at very large distances.”
Time distortion is the missing piece of this puzzle, the team said, because it might distinguish between Here on Earth, we experience the flow of time as a predictable progression of identical units, such as seconds or hours, but Albert Einstein’s theory of general relativity revealed that time is not such a constant all over the universe.
“The distortion of time describes the fact that time is not absolute,” Bonvin said. “The rate at which time passes depends on the gravitational field. For example, time passes more slowly close to a massive object than in vacuum. We call this change in the passing of time: time distortion.”
“What is interesting is that time distortion exists in all modern theories of gravity,” she continued. “However, the amplitude of the time distortion (how much the presence of a massive object slows down time) varies from theory to theory. In particular, in general relativity, the distortion of time and the distortion of space (the gravitational well created by a massive object) are predicted to be the same. In other theories of gravity this is generically not the case. So if we can measure the distortion of time and the distortion of space and compare them, we can test if general relativity is valid or not.”
To detect time distortion, scientists require sophisticated astronomical surveys that can accurately measure gravitational redshift in galaxies that existed when the universe was just a few billion years old (it is currently about 13.8 billion years old). The Dark Energy Spectroscopic Instrument (DESI), a huge galactic survey in Arizona, has already started collecting observations of this caliber. European Space Agency’s Euclid space telescope, which is due to launch next week, will add to the unique dataset, as well as the Square Kilometer Array, which will be the biggest radio telescope on Earth when it is completed later in the 2020s.
“The measurement of the distortion of time, at the very large distances we are interested in, has never been done,” Bonvin said. “Current surveys are not precise enough to perform this measurement. The coming surveys, like DESI, Euclid and the SKA, will however change the game. It will be possible to measure the distortion of time with the data delivered by these surveys.”
“This is very interesting, because for the first time, we will be able to compare the distortion of time with that of space, to test if general relativity is valid, and we will also be able to compare the distortion of time with the velocity of galaxy, to see if Euler’s equation is valid,” she noted. “So with one new measurement, we will be able to test two fundamental theories.”
Bonvin is already a member of the Euclid and SKA collaborations, and also plans to collaborate with the DESI team. This wealth of new data will encompass regions of warped time that might reveal exotic physics beyond the domain of Einstein’s and Euler’s theories. If new measurements of time distortion do not add up to the sum of the distortion of space and time, the results will challenge Einstein’s theory of general relativity, whereas if the measurements don’t match the velocity of these distant galaxies, it means that Euler’s equations might fall short.
“Until now we had only measurements of the sum of the distortion of time and space, so we could not perform this test,” Bonvin said. “What scientists were doing instead, is to measure the velocity of galaxies, to assume that dark matter obeys Euler equation, in order to relate these velocities to the distortion of time, and then to compare this with the distortion of space plus time. The problem is that if dark matter does not obey Euler’s equation, this method simply does not work. The test is invalid. With our new method, we won’t need to do this extra assumption: we will be able to directly measure the distortion of time, and compare it with the distortion of space plus time.”
To that end, this new technique for measuring time distortion could play a role in solving the longstanding riddles of cosmic acceleration, dark matter, and the presence of forces beyond the well-corroborated standard model of cosmology.
“What we really want to do now is to apply our method to data,” Bonvin concluded. “With past and current data we cannot measure the distortion of time. It is smaller than the uncertainty in the measurement. With the coming generation of surveys this will change and we should be able to measure. So it is very exciting to prepare these analyses and measure this effect for the very first time.”
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