The Expanding Void: Hubble’s Law Explained Quiz

In 1929, Edwin Hubble observed that the recessional velocity of a galaxy is directly proportional to its distance from Earth. This means that if Galaxy A is twice as far away as Galaxy B, it will appear to be moving away from us at twice the speed.

By showing that almost all distant galaxies are moving away from us (and from each other), Hubble's Law serves as the observational foundation for the Big Bang theory, implying that in the past, all matter in the universe was much closer together.

Just as a siren changes pitch as it passes you, light waves from a receding galaxy are "stretched" out. Because longer wavelengths of visible light are red, this phenomenon is called "redshift." The faster a galaxy moves away, the greater the redshift we observe.

The Hubble Constant is defined by the formula $v = H_0d$. To determine this value, astronomers must measure how far away a galaxy is (usually using "standard candles" like Cepheid variables) and how fast it is moving (using redshift).

While measurements vary slightly between different methods, most modern observations (from the Hubble Space Telescope and Planck satellite) place the value around 70 kilometers per second per megaparsec. This value represents the current rate of the universe's expansion.

This is a common misconception. From any point in an expanding universe, every other point appears to be moving away. An observer in a distant galaxy would see the Milky Way moving away from them in exactly the same way we see their galaxy moving.

Because galaxies are so far apart, astronomers use megaparsecs (Mpc) to describe intergalactic distances. The Hubble Constant describes how much faster a galaxy moves for every additional megaparsec of distance from the observer.

Distances are measured using a "Cosmic Distance Ladder." Parallax works for nearby stars; Cepheids work for nearby galaxies; and Type Ia Supernovae, which are incredibly bright and consistent, allow us to measure distances to the furthest reaches of the observable universe.

While the vast majority of distant galaxies are redshifted, a few nearby galaxies, like Andromeda, show a blueshift. This means they are moving toward the Milky Way due to local gravitational attraction, which is strong enough to overcome the general expansion of space at short distances.

If we assume the expansion rate has been constant, we can "rewind" the clock. By taking the inverse of the Hubble Constant, we get a time value of roughly 13.8 billion years, which aligns closely with other independent measurements for the age of the universe.

In general relativity, it is not that galaxies are "flying through" space like bullets; rather, the fabric of space-time between the galaxies is expanding. As the "grid" of the universe grows, the distance between objects increases, even if the objects themselves are not "moving" in a traditional sense.

Hubble's Law implies that if we go back in time, the universe becomes smaller, denser, and hotter. This leads directly to the conclusion that the universe originated from a single point (the Big Bang) and has been evolving ever since.

This is known as the "Hubble Tension." Measurements taken by looking at the early universe (the Cosmic Microwave Background) give a slightly lower value than measurements taken by looking at the modern universe (Supernovae). Solving this discrepancy is one of the biggest challenges in modern physics.

Because light has a finite speed, it takes time to reach us. When we observe a galaxy 100 million light-years away, we are seeing the light that left it 100 million years ago. This allows Hubble's Law to help us "see" the history of the universe's expansion over time.

While similar in result, cosmological redshift happens because the light waves are literally stretched as they travel through space that is expanding. By the time the light reaches Earth, its wavelength has been lengthened by the growth of the universe during its journey.

The Hubble Flow refers to the general, large-scale movement of galaxies away from each other due to the expansion of the universe. It is the "average" motion of the universe once you ignore the small-scale "peculiar velocities" caused by local gravitational pulls between neighboring galaxies.

In the late 1990s, observations of Type Ia Supernovae showed that the expansion of the universe is not slowing down—it is actually speeding up. Scientists attributed this acceleration to a mysterious "Dark Energy" that acts as a repulsive force against gravity.

Expansion only occurs on the largest scales (between galaxy clusters). On smaller scales, like inside a person, a planet, or even a galaxy, forces like electromagnetism and gravity are much stronger than the expansion of space, keeping these objects "bound" together.

Because Type Ia Supernovae occur when a white dwarf reaches a specific mass (the Chandrasekhar limit), they always explode with roughly the same energy. If we know how bright they "should" be, we can determine exactly how far away they are by how faint they appear.

A larger Hubble Constant would mean that for every megaparsec of distance, galaxies would be receding even more quickly. This would imply a more violent expansion and potentially a younger age for the universe, as it would have reached its current size in less time.