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Get the Popular Science daily newsletter💡 Everything on Earth, in our solar system, our galaxy, and beyond is contained within the universe. So how much does science tell us about the all-encompassing, four-dimensional cradle that holds all of space time? A lot. Philosophers, mathematicians, and astronomers across cultures and centuries have long debated and theorized about the night sky. But in the early 1920’s, building on the work of Henrietta Swan Leavitt and others, astronomer Edwin Hubble produced the first clear evidence that the swirling clusters visible through telescopes were actually distant galaxies, comparable to our own Milky Way. By capturing detailed, long-exposure images of space features like pulsing, Cepheid variable stars, Hubble confirmed the true nature of the Andromeda Nebula and others. These weren’t just nearby gas clouds, but far away islands of worlds and stars. In the century since, our ability to see clearer and farther out into space has dramatically improved. The James Webb Space Telescope (JWST) is the most advanced ever launched, and it routinely provides remarkable imagery from across the universe. Using data from space telescopes and other instruments, astronomers, cosmologists, and astrophysicists are able to deduce and predict many things about the universe’s shape, rate of change, and character. Here’s what we know, and what we don’t. [ Related: The universe isn’t just expanding—it may be spinning. ] How big is the universe? Let’s get the disappointment out of the way early: “There is physically, absolutely zero way that we will ever know,” how large the universe is, Sara Webb, an astrophysicist at Swinburne University of Technology in Australia, tells Popular Science. However, we do know that the universe is larger than 93 billion light-years across. This is the diameter of the sphere of the “observable universe” that we find ourselves at the center of. Our ability to look out and measure the stars is limited by the age of the universe and the speed of light. The only light we can see is light that’s been able to travel to us in the time since the big bang, which happened about 13.8 billion years ago. Therefore, light that’s traveled 13.8 billion light-years is the oldest we can see. However, the observable universe extends farther than 13.8 billion light-years in every direction because, for all the time space has existed, it’s also been expanding. That expansion means that light from 13.8 billion years ago has actually traversed 46.5 billion light-years to reach our eyes and telescopes. This composite image features one of the most complicated and dramatic collisions between galaxy clusters ever seen. Known officially as Abell 2744, this system has been dubbed Pandora’s Cluster because of the wide variety of different structures found. Data from NASA's Chandra X-ray Observatory (red) show gas with temperatures of millions of degrees. In blue is a map showing the total mass concentration (mostly dark matter) based on data from NASA's Hubble Space Telescope, the VLT (Very Large Telescope), and the Subaru telescope. Optical data from Hubble and VLT also show the constituent galaxies of the clusters. Astronomers think at least four galaxy clusters coming from a variety of directions are involved with this collision. CREDIT: NASA, ESA, J. Merten (Institute for Theoretical Astrophysics, Heidelberg/Astronomical Observatory of Bologna), and D. Coe (STScI) “It means, in theory, that space is actually expanding faster than the speed of light, when we add it all up– which really conceptually hurts your brain,” says Webb. “The nothingness of space and time doesn’t really abide by the laws for matter and physical things.” And though we don’t have firm evidence of the universe’s total size, Webb thinks it’s quite possibly infinite. “There’s no reason that it should be bounded. There’s no reason why there should be an edge here or there,” she says. The existence of edges remains a question mark, but astrophysicists generally agree on the universe’s shape: it’s flat, though perhaps not in the way you’d imagine. Flat doesn’t mean our universe is two-dimensional (space-time exists in 4D, after all). However, it does mean that traveling forward without changing direction in the universe will never get you back to where you started. Instead of a doughnut, a sphere, or a Pringle, the universe is most probably a four-dimensional sheet of paper, says Webb. How can we know the universe is expanding? Using theories and measurements about light coming from distant stars, multiple astronomers in the early 1900’s suggested that the universe was expanding. In 1924, Swedish astronomer Knut Lundmark, found the first observational evidence for universe expansion. Hubble’s work confirmed these findings in 1929. These early observations relied on a phenomenon called red shift, which is the visual version of the Doppler effect. Think about how sound waves from a passing ambulance siren change pitch with the vehicle’s position and speed: sounding higher on approach and lower once the ambulance is speeding away. Similarly, our perception of light waves is also impacted by the light’s movement and velocity. A light moving towards you will appear more blue, and one moving away will appear redder as the peaks and troughs of the wave are compressed and stretched respectively. Hubble and others noted that the galaxies they were discovering all appeared red from Earth, with more distant galaxies exhibiting the greatest red shift. This suggests that all galaxies are moving away from us. The more distant galaxies appear to be speeding off into space faster because there is more nothingness between us and them to expand. The history of the universe is outlined in this infographic. CREDIT: NASA In addition to red-shift observations, astronomers past and present also rely on “standard candles” to assess the size and speed of the universe. Standard candles are nifty cosmological markers of known brightness that can be used to observe how light is traveling and changing through space and time, says Abigail Lee, an astronomer and PhD candidate at the University of Chicago. The first type of standard candles discovered were Hubble’s Cepheid variables, pulsating stars that emit bright light in a regular, periodic pattern, which can be used to deduce their distance from Earth. Lee explains it with an illuminating analogy. Imagine a 40 watt incandescent lightbulb. All lightbulbs of shared wattage are the same intrinsic brightness. However, if you look at the lightbulb from 100 feet away, it will appear dimmer than it does at a distance of 10 feet. That relative dimness can be used to calculate how far away the bulb is. It’s the same with Cepheids in space. Other standard candles used for the same purpose include certain types of supernovae (i.e. exploding stars), “tip of the red-giant branch” stars, and carbon stars. “We know that these stars have the exact same intrinsic luminosity, and so we can use that property to measure distance,” Lee tells Popular Science. We can approximate the distance between Earth and other galaxies by looking for nebulae that contain these standard candles. In 2011, three scientists were awarded the Nobel Prize in Physics for demonstrating that not only is the universe expanding, but dark energy is accelerating that expansion. Dark energy is a mysterious and repulsive force pushing space matter and objects apart. The expansive forces of dark energy are generally thought to be uniform across the entire universe, pushing against all objects equally. However, expansion itself is not uniformly observable. Within our planet, solar system, and galaxy, the attractive force of gravity keeps things relatively bound and less subject to dark energy. And the expansion rate itself is not fast enough to be readily observed on the small-scale. To detect it, you have to observe very distant objects. How fast is the universe expanding? Based on his early observations, Hubble first proposed that the universe was expanding at a rate of about 500 kilometers per second per megaparsec (Mpc), where a megaparsec is equal to 3.26 million light-years. The speed of universe expansion came to be known as the Hubble Constant (H0), despite the fact that the titular astronomer’s initial estimate turned out to be pretty far off. We now have a clearer sense of the expansion rate. Scientists generally agree H0 is between 65-75 km/sec/Mpc. If that sounds complicated, it’s because it is. The rate of universal expansion is dependent on both time and distance. It’s larger across bigger areas of space and longer durations. And the question of the exact speed remains unresolved. Depending on who you ask and how one measures, calculations for the true H0 vary. Broadly, two different approaches to quantifying the H0 routinely yield different results. This discrepancy is known as the “Hubble Tension”. By one set of measurements,

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