Read Aloud the Text Content

This audio was created by Woord's Text to Speech service by content creators from all around the world.

Text Content or SSML code:

In the early 20th century, Albert Einstein used Riemann’s metric tensor to develop General Relativity: a four-dimensional theory of spacetime and gravitation. An illustration of gravitational lensing showcases how background galaxies – or any light path – is distorted by the presence of an intervening mass, but it also shows how space itself is bent and distorted by the presence of the foreground mass itself. When multiple background objects are aligned with the same foreground lens, multiple sets of multiple images can be seen by a properly-aligned observer. (Credit: NASA, ESA & L. Calçada) In straightforward terms, Einstein realized that thinking of space and time in absolute terms — where they didn’t change under any circumstances — didn’t make any sense. In special relativity, if you traveled at speeds close to the speed of light, space would contract along your direction of motion, and time would dilate, with clocks running slower for two observers moving at different relative speeds. There are rules for how space and time transform in an observer-dependent fashion, and that was just in special relativity: for a Universe where gravitation didn’t exist. But our Universe does have gravity. In particular, the presence of not only mass, but all forms of energy, will cause the fabric of spacetime to curve in a particular fashion. It took Einstein a full decade, from 1905 (when special relativity was published) until 1915 (when General Relativity, which includes gravity, was put forth in its final, correct form), to figure out how to incorporate gravity into relativity, relying largely on Riemann’s earlier work. The result, our theory of General Relativity, has passed every experimental test to date. What’s remarkable about it is this: when we apply the field equations of General Relativity to our Universe — our matter-and-energy filled, expanding, isotropic (the same average density in all directions) and homogeneous (the same average density in all location) Universe — we find that there’s an intricate relationship between three things: the total amount of all types of matter-and-energy in the Universe, combined, the rate at which the Universe is expanding overall, on the largest cosmic scales, and the curvature of the (observable) Universe. A photo of Ethan Siegel at the American Astronomical Society’s hyperwall in 2017, along with the first Friedmann equation at right. The first Friedmann equation details the Hubble expansion rate squared as the left-most term on the left hand side, which governs the evolution of spacetime. The further-right terms on that side include all the different forms of matter and energy, while the right-hand side details the spatial curvature, which determines how the Universe evolves in the future. This has been called the most important equation in all of cosmology, and was derived by Friedmann in essentially its modern form back in 1922. (Credit: Harley Thronson (photograph) and Perimeter Institute (composition)) The Universe, in the earliest moments of the hot Big Bang, was extremely hot, extremely dense, and also expanding extremely rapidly. Because, in General Relativity, the way the fabric of spacetime itself evolves is so thoroughly dependent on the matter and energy within it, there are really only three possibilities for how a Universe like this can evolve over time. If the expansion rate is too low for the amount of matter-and-energy within your Universe, the combined gravitational effects of the matter-and-energy will slow the expansion rate, cause it to come to a standstill, and then cause it to reverse directions, leading to a contraction. In short order, the Universe will recollapse in a Big Crunch. If the expansion rate is too high for the amount of matter-and-energy within your Universe, gravitation won’t be able to stop and reverse the expansion, and it might not even be able to slow it down substantially. The danger of the Universe experiencing runaway expansion is very great, frequently rendering the formation of galaxies, stars, or even atoms impossible. But if they balance just right — the expansion rate and the total matter-and-energy density — you can wind up with a Universe that both expands forever and forms lots of rich, complex structure. This last option describes our Universe, where everything is well-balanced, but it requires a total matter-and-energy density that matches the expansion rate exquisitely from very early times. The intricate balance between the expansion rate and the total density in the Universe is so precarious that even a 0.00000000001% difference in either direction would render the Universe completely inhospitable to any life, stars, or potentially even molecules existing at any point in time. (Credit: Ned Wright’s cosmology tutorial) The fact that our Universe exists with the properties we observe tells us that, very early on, the Universe had to be at least very close to flat. A Universe with too much matter-and-energy for its expansion rate will have positive curvature, while one with too little will have negative curvature. Only the perfectly balanced case will be flat. But it is possible that the Universe could be curved on extremely large scales: perhaps even larger than the part of the Universe we can observe. You might think about drawing a triangle between our own location and two distant galaxies, adding up the interior angles, but the only way we could do that would involve traveling to those distant galaxies, which we cannot yet do. We’re presently limited, technologically, to our own tiny corner of the Universe. Just like you can’t really get a good measurement of the curvature of the Earth by confining yourself to your own backyard, we can’t make a big enough triangle when we’re restricted to our own Solar System. Thankfully, there are two major observational tests we can perform that do reveal the curvature of the Universe, and both of them point to the same conclusion.