For more than a century, astronomers have known that the universe has been expanding since the Big Bang. During the first eight billion years, the rate of expansion was relatively constant because it was held back by the force of gravity.
However, thanks to missions like the Hubble Space Telescope, astronomers have since realized that about 5 billion years ago, the rate of expansion was accelerating.
And while some insist that the force of gravity may have changed over time, this is a controversial hypothesis because it means that Einstein’s general theory of relativity (which has been validated nine ways out of Sunday) is wrong.
But according to a new study by the International Dark Energy Survey (DES) Collaboration, the nature of gravity has remained the same throughout the entire history of the universe.
The findings come shortly before two next-generation space telescopes (Nancy Grace Roman and Euclid) are sent into space to make more precise measurements of gravity and its role in cosmic evolution.
The DES Collaboration includes researchers from universities and institutes in the United States, United Kingdom, Canada, Chile, Spain, Brazil, Germany, Japan, Italy, Australia, Norway and Switzerland.
Their results were presented for the third year at the International Conference on Particle Physics and Cosmology (COSMO’22), which took place in Rio de Janeiro from August 22 to 26.
It was also shared in a paper titled “Results of the Year 3 Dark Energy Survey: Constraints on Lambda CDM Extensions with Lens Weakening and Galactic Clusters” which appeared in the Journal of the American Physical Review D.
Einstein’s general theory of relativity, which he finalized in 1915, describes how the curvature of spacetime changes in the presence of gravity.
For more than a century, this theory has accurately predicted almost everything in our universe, from Mercury’s orbit and gravitational lenses to the existence of black holes.
But between the 1960s and 1990s, two contradictions were discovered that prompted astronomers to question whether Einstein’s theory was correct. First, astronomers note that the gravitational effects of massive structures (such as galaxies and galaxy clusters) do not correspond to their observed mass.
This gave rise to the theory that space is filled with invisible mass that interacts with “normal” (also known as “luminous” or visible) matter through gravity. Meanwhile, the observed expansion of the universe (and how it undergoes acceleration) gave rise to the theory of dark energy and the Lambda Cold Dark Matter Model (Lambda CDM).
Cold dark matter is an explanation where this mass consists of large, slow-moving particles while lambda represents dark energy. In theory, these two forces make up 95 percent of the total mass energy content in the universe, yet all attempts to find direct evidence for them have failed.
The only possible alternative is that relativity needs to be modified to take account of these contradictions. To find out if that was the case, DES members used the 4-meter Victor M. Blanco telescope at the Cerro Telolo Inter-American Observatory in Chile to observe galaxies up to 5 billion light-years away.
They hoped to determine whether gravity has changed over the past five billion years (since the acceleration began) or at cosmic distances. They also consulted data from other telescopes, including the European Space Agency’s Planck satellite, which has been mapping the cosmic microwave background (CMB) since 2009.
They paid close attention to how the images they saw contained subtle distortions due to dark matter (gravitational lenses). As the first image from the James Webb Space Telescope (JWST) showed, scientists can infer the force of gravity by analyzing how much the gravitational lens distorts spacetime.
So far, the DES Collaboration has measured the shapes of more than 100 million galaxies, and all observations match what general relativity predicts. The good news is that Einstein’s theory still stands, but it also means that the mystery of dark energy still remains for now.
Fortunately, astronomers won’t have to wait long before new, more detailed data becomes available. First, there is the European Space Agency’s Euclid mission, which is scheduled to launch by 2023 at the latest. This mission will map the geometry of the universe, looking 8 billion years in the past to measure the effects of dark matter and dark energy.
And by May 2027, it will be joined by NASA’s Nancy Grace Roman Space Telescope, which will go back more than 11 billion years. These will be the most detailed cosmic surveys ever conducted, and are expected to provide the most convincing evidence for (or against) the Lambda-CDM model.
As study co-author Agnès Ferté, who conducted the research as a postdoctoral researcher at the Jet Propulsion Laboratory, said in a recent NASA press release, “There is still room to challenge Einstein’s theory of gravity, as the measurements become more accurate. But we still have a lot to do.” Before we are ready for Euclid and Roman. It is therefore imperative that we continue to collaborate with scientists around the world on this issue as we did with the Dark Energy Survey.”
In addition, Webb’s observations of the oldest stars and galaxies in the universe will allow astronomers to map the evolution of the universe from its earliest periods. These efforts have the potential to answer some of the universe’s most pressing mysteries.
This includes how relativity, observed mass, and the expansion of the universe match, but can also provide insight into how gravity and other fundamental forces of the universe (as described in quantum mechanics) interact – the theory of everything (ToE).
And if there is one thing that characterizes the current era of astronomy, it is the way in which long-range surveys and next-generation tools combine to test what has been theoretical material up until now.
The potential hacks it could lead to are sure to delight and confuse us. But in the end, they will revolutionize the way we look at the universe.
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