The Big Bang — Reading Comprehension

The Big Bang theory is the leading scientific explanation for the origin of the universe. According to this theory, the universe began about 13.8 billion years ago from an extremely hot and dense state. Scientists study the universe’s beginning to understand how matter, energy, and even time itself came to exist. The Big Bang is not described as an explosion but as an expansion of space itself, where everything was much closer together than it is now. This concept helps explain the large-scale structure and behavior of the universe we observe today.

How Did the Universe Begin?
Instead of a traditional explosion, the Big Bang was an event where space itself began to expand rapidly. At the start, the universe was smaller than an atom, filled with energy and particles packed tightly together. As expansion began, the universe cooled, allowing fundamental forces like gravity and electromagnetism to separate. In the first few minutes, simple particles called protons and neutrons formed. These particles joined to create hydrogen and helium nuclei, which are the simplest and most common elements in the universe. About 380,000 years later, it became cool enough for atoms to form, and light was able to travel freely. This released a faint glow called the cosmic microwave background (CMB), which scientists can still detect today.

Evidence for the Big Bang
Scientists use several lines of evidence to support the Big Bang theory. First, telescopes show that distant galaxies are moving away from us, meaning the universe is still expanding. This observation, made by Edwin Hubble in the 1920s, matches predictions from the Big Bang theory. Second, the CMB is a uniform glow found in all directions in space. The temperature of this background radiation is about 2.7 Kelvin, just above absolute zero, and it is a "snapshot" of the early universe. Third, the amounts of hydrogen and helium found throughout the universe match what the Big Bang predicts—hydrogen makes up about 75% of normal matter, and helium about 25%. These lines of evidence connect observations to scientific models, showing how scientists build understanding from data.

Timeline and Misconceptions
The Big Bang theory describes a timeline of key events. In the first fraction of a second, fundamental forces separated. Within three minutes, protons and neutrons combined to form hydrogen and helium nuclei. After 380,000 years, atoms formed, and the universe became transparent to light, releasing the CMB. The first stars appeared about 100–200 million years later, and the first galaxies formed around 400 million years after the Big Bang. Some people mistakenly believe the Big Bang was an explosion into empty space or happened at a single point, but actually, space itself expanded everywhere at once. There was no "outside" to the universe, and time itself began with the Big Bang, so asking what happened "before" may not make sense.

The Big Bang theory unites many scientific ideas, including physics, chemistry, and astronomy. It helps us understand not just how the universe began, but how it continues to change. Ongoing research, from satellites that measure the CMB to telescopes studying distant galaxies, adds new details and tests our theories. The story of the Big Bang is a powerful example of how science uses evidence to answer big questions about our place in the universe.

Interesting Fact: The cosmic microwave background is so faint that if you tune an old television to static, about 1% of the noise comes from this ancient light left over from the Big Bang!

What does the Big Bang theory explain?

According to the passage, what is the cosmic microwave background (CMB)?

What two elements were mainly formed in the first minutes after the Big Bang?

What did Edwin Hubble discover that supported the Big Bang theory?

What does the term 'expand' mean in the context of the Big Bang?

What are 'fundamental forces' as described in the passage?

Why does the passage say it may not make sense to ask what happened 'before' the Big Bang?

How are the cosmic microwave background and the abundance of hydrogen and helium connected to the Big Bang theory?

True or False: The Big Bang was an explosion into empty space.

True or False: The first stars formed about 100–200 million years after the Big Bang.