Big Bang Theory CER

The Story of the Universe: How the Big Bang Shaped Everything

Long ago—about 13.8 billion years in the past—there was nothing. No stars. No planets. No space between things. Then, in a moment that still amazes scientists today, something extraordinary happened: the universe began. It didn’t begin with an explosion like fireworks in the sky, but instead with a sudden, rapid expansion from a point smaller than an atom. In the blink of an eye, that tiny point stretched into something vast, setting the stage for everything that would come after. This idea is called the Big Bang Theory, and it's the leading explanation for how our universe began.

But how can anyone possibly know what happened billions of years ago? We weren’t there to see it, and we can’t go back in time. Instead, scientists look for clues—evidence left behind in space like cosmic fingerprints. Over many years, they have found powerful evidence that supports the Big Bang Theory, and that evidence is written in the light, motion, and matter of the universe.

One of the first big clues came from a surprising observation: galaxies are moving away from us. In the 1920s, a scientist named Edwin Hubble studied the light from distant galaxies and noticed something strange. The light coming from these galaxies looked stretched, almost like it had been pulled apart. This is called redshift, and it means that the galaxies are moving away from Earth. Even more interesting, the farther away a galaxy is, the faster it seems to be moving. This told scientists that the universe isn't staying the same size—it’s expanding. That expansion today means that everything was once much closer together in the past, which supports the idea that the universe started from one incredibly small, dense point.

As scientists studied the universe more deeply, they also looked at what it’s made of. After all, if the Big Bang really happened, it would have created certain types of matter in the very beginning. The universe would have been extremely hot just after the Big Bang, and this heat would have created simple elements. Scientists predicted that in those first few minutes, most of the matter formed would be hydrogen, with some helium and tiny traces of lithium. Billions of years later, when we look at stars, galaxies, and gas clouds, we find exactly what was predicted: about 75% hydrogen and 25% helium by mass. This is called the abundance of light elements, and it’s another major clue that helps confirm the Big Bang Theory. The match between the predicted and observed amounts is almost too perfect to be a coincidence.

But scientists didn’t stop there. They had one more important question: could they find more direct evidence of what the early universe looked like? One way to answer that was by studying the light spectra from distant stars and galaxies. When light passes through a special tool called a spectroscope, it breaks into a rainbow of colors, much like sunlight through a prism. But this rainbow isn’t smooth—it has dark lines, called absorption lines, which are like a barcode that tells us what elements are in a star. When scientists compared the light from nearby stars to light from faraway galaxies, they saw the same patterns. The same elements—mainly hydrogen and helium—were everywhere. That told them that the universe has been following the same physical rules since the beginning. The light from ancient galaxies shows that the universe was behaving just like scientists thought it would if the Big Bang really happened.

Each of these discoveries—redshift, the abundance of hydrogen and helium, and the patterns found in light spectra—tells a part of the story. They come from different types of observations and different kinds of scientists working across many generations. Yet, they all point to the same idea: our universe had a beginning. The Big Bang Theory gives us a way to understand not just how the universe started, but also why it continues to expand and how it formed the stars, planets, and galaxies that we see today.

Even though we may never be able to witness the beginning of time, the evidence is all around us—in the light from distant galaxies, in the makeup of the stars, and in the very atoms that make up our bodies. The story of the Big Bang is, in many ways, the story of everything.

How do scientists know that the Big Bang is the best explanation for the origin of the universe?

Write a complete Claim–Evidence–Reasoning (CER) response using what you learned from the passage "The Story of the Universe: How the Big Bang Shaped Everything."