Edwin Hubble helped change astronomy from a study of nearby stars into a science of the whole universe. Using the 100-inch Hooker Telescope at Mount Wilson Observatory, he showed that many fuzzy nebulae were actually galaxies far beyond the Milky Way. His work gave strong evidence that the universe is much larger than people had believed.
It also led to one of the most important discoveries in modern science, the expansion of the universe.
Hubble compared galaxy distances with their spectra and found that farther galaxies usually have larger redshifts. A redshift means the light from a galaxy is stretched toward longer, redder wavelengths, which is evidence that the galaxy is moving away from us. This relationship became known as Hubble's law, v = H0d, where recession speed increases with distance.
The discovery became a foundation for Big Bang cosmology and for measuring the age and scale of the universe.
Understanding Edwin Hubble, Discoverer of the Expanding Universe
Measuring a galaxy distance is difficult because galaxies are too far away for ordinary parallax measurements. Hubble used a step in what astronomers now call the cosmic distance ladder. Certain pulsating stars brighten and dim in a regular pattern.
Their pulse period reveals their true brightness. By comparing true brightness with how faint they appear from Earth, astronomers can calculate distance. This works because light spreads out as it travels.
A similar lamp looks dimmer when it is moved farther away. These stars gave Hubble a reliable way to place some galaxies on a much larger map of space.
Redshift comes from examining light with a spectrograph. This instrument separates incoming light into a spectrum, much like a prism makes a rainbow. Atoms in stars and gas produce dark or bright lines at known colors.
When the same lines from a galaxy appear shifted toward the red end, astronomers can measure the shift precisely. For nearby galaxies, a larger shift indicates a greater recession speed.
The effect resembles the changing pitch of a passing ambulance siren, though light is not sound. The important evidence lies in matching many spectral lines, not in judging a galaxy by its visible color.
The expansion is best understood as increasing distance between widely separated galaxies. It does not mean that galaxies are racing through empty space from one central point. Space itself changes on very large scales.
A useful picture is dots drawn on the surface of an inflating balloon. Every dot sees other distant dots move away, yet the two dimensional surface has no center.
The balloon picture has limits, but it explains why observers in other galaxies would find a similar pattern. Gravity holds together nearby systems such as the Solar System, the Milky Way, and many galaxy groups, so they do not expand in the same way.
Real observations contain scatter. Galaxies have their own local motions caused by gravity. The Andromeda Galaxy, for example, is approaching the Milky Way even though the universe expands overall.
These local motions can be a large part of the measured speed for close galaxies. Astronomers therefore study many distant galaxies and look for the overall trend. They must correct for the motion of Earth, the Sun, and the Milky Way before comparing results.
Dust, uncertain star brightness, and telescope limits can affect distance estimates. Good science means reporting these uncertainties rather than treating one measurement as final.
The rate of expansion, called the Hubble constant, connects observations to the history of the universe. A faster present rate suggests a shorter simple estimate of the time since everything was much closer together. That estimate is not the full age calculation because the expansion rate has changed over time.
Gravity slowed the early expansion, while a mysterious effect called dark energy now makes expansion speed up. Modern astronomers measure the rate in several ways, including nearby variable stars, exploding stars called supernovae, and faint radiation left from the early universe. Different methods do not yet agree perfectly, which is an active and important problem in astronomy.
Key Facts
- Hubble showed that the Andromeda Nebula is a separate galaxy outside the Milky Way.
- Cepheid variable stars were used as standard candles to measure distances to nearby galaxies.
- Hubble's law: v = H0d, where v is recession speed, H0 is the Hubble constant, and d is distance.
- Redshift formula for small speeds: z = Δλ/λ0, where z is redshift and λ0 is the rest wavelength.
- For relatively low speeds, recession speed can be estimated by v ≈ cz, where c = 3.00 x 10^5 km/s.
- A rough estimate for the age of the universe is t ≈ 1/H0, after converting H0 into units of 1/s.
Vocabulary
- Galaxy
- A galaxy is a huge collection of stars, gas, dust, and dark matter held together by gravity.
- Redshift
- Redshift is the stretching of light to longer wavelengths, often caused by an object moving away or by the expansion of space.
- Hubble's law
- Hubble's law states that a galaxy's recession speed is proportional to its distance from us.
- Cepheid variable
- A Cepheid variable is a star whose regular brightness changes can be used to find its true luminosity and distance.
- Hubble constant
- The Hubble constant is the proportionality value that relates a galaxy's distance to its recession speed in the expanding universe.
Common Mistakes to Avoid
- Thinking Hubble discovered galaxies by looking at their shapes alone is wrong because his key evidence came from measuring Cepheid variable stars and distances.
- Treating redshift as only a change in color is wrong because it is a measurable shift in spectral lines across the whole spectrum.
- Using Hubble's law for nearby stars inside the Milky Way is wrong because local gravitational motions dominate over cosmic expansion at small scales.
- Forgetting units in v = H0d is wrong because H0 often uses km/s/Mpc, so distance must be in megaparsecs to get speed in km/s.
Practice Questions
- 1 A galaxy is 50 Mpc away. If H0 = 70 km/s/Mpc, what is its recession speed using v = H0d?
- 2 A spectral line normally has wavelength 500 nm, but it is observed from a galaxy at 510 nm. Find the redshift z = Δλ/λ0, then estimate the recession speed using v ≈ cz with c = 3.00 x 10^5 km/s.
- 3 Explain why Hubble's discovery that distant galaxies are redshifted supports an expanding universe rather than a universe with galaxies sitting still in space.