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Understanding the Theory of the Expanding Universe

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  1. The discovery of Hubble's law

    In 1929, Edwin Hubble observed that the farther away a galaxy is, the greater its redshift β€” the stretching of light's wavelength toward the red end of the spectrum β€” and discovered Hubble's law (v=Hβ‚€d), showing that the speed at which galaxies recede from us is proportional to their distance. This was the first direct observational evidence that the universe is not static but expanding.

  2. The basic concept of the Big Bang theory

    The Big Bang theory holds that the universe began about 13.8 billion years ago in an extremely hot, extremely dense, point-like state, and has been expanding and cooling ever since. One of the core pieces of evidence supporting this theory is that when Hubble's observations are run backward in time, all galaxies appear to converge as though they started from a single point.

  3. Why cosmic microwave background radiation is evidence for the Big Bang

    Cosmic microwave background radiation is a faint radio signal observed almost uniformly across the entire sky, at a temperature of about βˆ’270Β°C (roughly 2.7K on the absolute scale), interpreted as the leftover trace of light from when the hot early universe expanded and cooled. When Penzias and Wilson accidentally discovered this signal in 1965, it provided powerful observational support for the Big Bang theory.

  4. Accelerating expansion and dark energy

    Supernova observations in 1998 revealed that the universe's rate of expansion isn't slowing over time but is actually speeding up. To explain this accelerating expansion, scientists named the unidentified energy filling the universe and driving this expansion "dark energy," though its true nature remains an unconfirmed, hypothetical concept to this day.

  5. An analogy for understanding the expansion of the universe

    The expansion of the universe is often compared to raisin bread dough rising. As the dough (space) itself expands, every raisin (galaxy) inside it appears to move farther away from every other raisin β€” this analogy shows that galaxies aren't actually moving across space, but rather moving apart because space itself is stretching.

  6. The extent of the observable universe

    Just because the universe is about 13.8 billion years old doesn't mean the radius of the observable universe is 13.8 billion light-years. Because space itself has continued expanding even after light set out on its journey, the radius of the actually observable universe is estimated to reach about 46.5 billion light-years.

What does it mean that the universe is expanding?

The universe expanding doesn't mean galaxies are moving across an empty void called the universe β€” it means space itself is continuously growing larger over time. This discovery led to the Big Bang theory, the idea that the universe had a beginning, and became the starting point of modern cosmology.

Explore the diverse galaxies scattered throughout the expanding universe

Galaxies of many different shapes are scattered throughout the expanding universe. If you're curious about the difference between spiral and elliptical galaxies, take a look at our complete overview of galaxy types as well.

Frequently Asked Questions

Is dark energy an actual substance that has been discovered?

No. Dark energy is a concept introduced to explain the accelerating expansion of the universe, and its true nature has not yet been directly observed or confirmed β€” it remains a hypothetical concept. That said, it is actively studied, since several independent observations support the reality of accelerating expansion.

Was the Big Bang an explosion that scattered matter throughout space?

Contrary to a common misconception, the Big Bang was not an event in which matter exploded and scattered within an already-existing empty space β€” rather, it refers to the process by which space and time themselves began in an extremely small, hot state and have been expanding ever since.