The Expanding Universe — Reading Comprehension

The universe is not fixed in size. For many years, people believed that the universe was unchanging. This idea began to shift when astronomers started to study distant galaxies. The discovery that the universe is expanding changed our entire understanding of space and time. Today, scientists know that everything—from galaxies to clusters of stars—is moving farther apart over time. This expanding universe shapes how we study the cosmos and our place within it.

How Scientists Discovered the Expansion

In 1929, astronomer Edwin Hubble made a breakthrough. He observed that most distant galaxies appeared to be moving away from Earth. He used a phenomenon called redshift, where the light from these galaxies was stretched into longer, redder wavelengths. This stretching is similar to the Doppler effect you hear when an ambulance siren lowers in pitch as it moves away. Hubble found that the farther a galaxy is, the faster it is receding. This relationship is called Hubble's Law. The rate of expansion is measured by the Hubble constant. These findings proved that the universe is expanding, not just the objects within it.

What Expanding Universe Really Means

The expansion does not mean galaxies are flying through empty space. Instead, space itself is stretching. A useful analogy is to imagine dots on the surface of a balloon as it inflates. As the balloon grows, each dot moves away from every other dot. There is no special center; every galaxy sees others moving away. Another example is raisin bread rising in the oven, where raisins move apart as the bread expands. This helps us understand that the universe has no edge and no center. The observable universe is about 93 billion light-years across, even though it is about 13.8 billion years old, because space has been expanding during that time.

Evidence and Implications

One major piece of evidence for the expanding universe is the cosmic microwave background. This is faint radiation left over from the Big Bang, detected in all directions. In 1965, scientists Arno Penzias and Robert Wilson discovered this afterglow, supporting the idea that the universe began as a hot, dense state and has been expanding ever since. In 1998, astronomers found that the universe's expansion is speeding up, not slowing down. This acceleration is caused by a mysterious force called dark energy. Understanding the expanding universe helps scientists estimate its age, predict its future, and explore fundamental questions about the cosmos.

The concept of an expanding universe connects to larger scientific principles, such as the nature of space and time, and the evolution of matter. Ongoing research into dark energy and the universe's structure continues to shape our understanding of the cosmos.

Interesting Fact: If you could see the universe from any galaxy, it would look like all other galaxies are moving away from you, showing that there's no true center to the universe.

What discovery did Edwin Hubble make in 1929?

What is redshift?

How does the balloon analogy help explain the expanding universe?

What is the cosmic microwave background?

According to the passage, what causes the universe's expansion to speed up?

What does 'observable universe' mean as used in the passage?

What is the main reason scientists believe the universe has no center?

True or False: The universe is about 13.8 billion years old.

True or False: The expansion of the universe means galaxies are moving through space, not that space itself is stretching.

How did the discovery of the cosmic microwave background support the idea of the expanding universe?