For more than a century, Einstein’s general theory of relativity has been a cornerstone of modern physics, offering the most comprehensive explanation of gravity on cosmic scales. This framework became especially significant following the 1998 discovery of the Universe’s accelerated expansion. However, this revelation has posed a major challenge to the theory. Astronomers have uncovered phenomena that conflict with some of Einstein’s predictions, particularly the observation that the Universe is not just expanding faster but is doing so in a way that suggests an increasingly rapid separation of cosmic structures over time.
This acceleration has been found in various ways, including measuring the distances of quasars and galaxies using a technique called cosmic microwave background (CMB) imaging. The CMB is the electromagnetic radiation left behind by the Big Bang, a massive explosion that shook the Universe and created everything we see today.
In other experiments, astronomers have watched how a star orbits a supermassive black hole. The stars do not follow a simple elliptical orbit but dance forward in a rosette pattern. This behavior has also been observed in other black holes and explains how they pull the stars and other matter surrounding them.
The accelerated expansion has prompted many astrophysicists to question the validity of Einstein’s theory, although other discoveries have lent support. These include discovering dark energy, a mysterious substance speeding up the Universe’s expansion.
Dark energy may be created when massive objects collide, such as two neutron stars, a neutron star, and a black hole. Their gravity should cause a ripple in space-time that we can measure. By studying how the Universe expands over time, we can also create a model for the effects of dark energy.
In 1915, Einstein published his general relativity theory, which explained how massive objects distort the fabric of space-time around them. It’s like setting a trampoline in the center of a large house. The weight of the house would press down into the trampoline, making it “dimple” in places. The shape of the Universe as we observe it today results from how that distortion affects it.
Einstein, along with mathematicians David Hilbert and Emmy Noether, worked with several scientists to develop his theory. In 1916, he published the final version of his equations.
That year, German physicist Karl Schwarzschild solved those equations and discovered that the curvature of space-time varies by how much mass is concentrated in a small area. The smaller the area, the more it curves. He also found a limit to how small the mass can be, as measured by the gravitational radius of a star. This was confirmed by a 1919 experiment, which involved tracking the apparent position of stars during a solar eclipse. The results were hailed by newspapers worldwide.
