Hubble's Law: The Universe on the Move
The Great Discovery: From a Static to an Expanding Universe
For centuries, many scientists, including Albert Einstein, believed the universe was static and unchanging on a large scale. This view was completely overturned in the 1920s. Astronomer Edwin Hubble, using the powerful Hooker telescope at Mount Wilson Observatory, made a series of observations that would change science forever. He was studying special stars called Cepheid variables[1] in distant nebulae[2]. By measuring the properties of these stars, he could calculate the distance to their host galaxies. At the same time, other astronomers like Vesto Slipher were measuring the light from these galaxies and noticed it was shifted towards the red end of the spectrum, a phenomenon called redshift.
When Hubble combined these two sets of data—the distances to galaxies and their redshifts—he found an astonishing pattern. The farther away a galaxy was, the greater its redshift. A redshift in light is interpreted as a recessional velocity, meaning the galaxy is moving away from us. Hubble's 1929 paper revealed a direct, linear relationship: the velocity of a galaxy's recession is proportional to its distance from us. This discovery was the first solid evidence that our universe is not static but is actively expanding.
Understanding the Key Concepts: Redshift and Recessional Velocity
To understand Hubble's Law, we first need to understand redshift. Imagine a police car passing you with its siren on. As it moves away, the sound waves stretch out, and the pitch of the siren sounds lower. This is the Doppler Effect. Light waves behave in a very similar way.
The recessional velocity ($ v $) is not because galaxies are physically moving through space like rockets. Instead, it is the space between galaxies that is stretching. Think of the universe as a raisin cake baking in an oven. As the cake rises, the dough (representing space) expands. Every raisin (representing a galaxy) moves away from every other raisin. No raisin is the "center"; the entire cake is expanding. This analogy helps explain why more distant galaxies recede faster—there is simply more expanding space between us and them.
The Hubble Law Equation and the Hubble Constant
The relationship discovered by Hubble is expressed in a beautifully simple mathematical formula:
Where:
$ v $ = Recessional velocity of the galaxy (usually in km/s).
$ H_0 $ = Hubble Constant (the rate of expansion of the universe).
$ d $ = Distance to the galaxy (usually in Megaparsecs, Mpc).
The Hubble Constant ($ H_0 $) is one of the most important numbers in cosmology. It tells us how fast the universe is expanding today. A higher value means a faster-expanding universe. Pinpointing its exact value has been a major challenge. Different methods have yielded slightly different results, but the current best estimate from the Planck satellite is around $ 67.4 $ km/s per Megaparsec. This means that for every million parsecs (about 3.26 million light-years) of distance, a galaxy moves away from us 67.4 kilometers per second faster.
A Concrete Example: Calculating Galactic Recession
Let's use Hubble's Law in a practical example. Suppose astronomers measure the distance to a galaxy and find it to be $ 150 $ Mpc (Megaparsecs) away. Using the Hubble Constant value of $ H_0 = 70 $ km/s/Mpc, we can calculate its recessional velocity.
We plug the numbers into the formula:
$ v = H_0 \\times d = 70 \\ \\text{km/s/Mpc} \\times 150 \\ \\text{Mpc} $
$ v = 10,500 \\ \\text{km/s} $
This means the galaxy is receding from us at a staggering speed of 10,500 kilometers per second! The following table shows how velocity increases with distance for a few hypothetical galaxies, using $ H_0 = 70 $ km/s/Mpc.
| Galaxy Name | Distance (Megaparsecs) | Recessional Velocity (km/s) |
|---|---|---|
| Galaxy A | $ 10 $ | $ 700 $ |
| Galaxy B | $ 50 $ | $ 3,500 $ |
| Galaxy C | $ 200 $ | $ 14,000 $ |
| Galaxy D | $ 500 $ | $ 35,000 $ |
Common Mistakes and Important Questions
Q: Does Hubble's Law mean Earth is the center of the universe?
No, this is a very common misunderstanding. From the perspective of any galaxy in the universe, it would appear that all other galaxies are moving away from it. In our raisin cake analogy, every raisin sees all the other raisins moving away. There is no central point; the expansion is happening everywhere at once. We are not in a special location.
Q: Are galaxies actually moving through space at these incredible speeds?
Not exactly. The recessional velocity ($ v $) in Hubble's Law is primarily due to the expansion of space itself, not galaxies moving through a static space. It's the "fabric" of the universe that is stretching, carrying the galaxies along with it. This is a key distinction in the theory of general relativity.
Q: If the universe is expanding, are we, and the things around us, also expanding?
No, objects that are bound together by fundamental forces—like atoms, planets, solar systems, and even galaxies—do not expand. The gravitational force holding the Earth together, or the electromagnetic forces holding your body together, are far stronger than the effect of the universe's expansion on such small scales. The expansion only becomes significant on the vast, intergalactic scales where these forces are weak and the amount of expanding space between objects is enormous.
Footnote
[1] Cepheid variables: A type of star whose brightness pulsates regularly. The period of this pulsation is directly related to its intrinsic brightness, making it a "standard candle" for measuring astronomical distances.
[2] Nebulae: (Singular: Nebula) A term historically used for any diffuse astronomical object, including what we now know to be other galaxies outside our Milky Way.
[3] Redshift (z): A measure of how much the light from an astronomical object has been stretched to longer, redder wavelengths, typically due to the expansion of the universe.
[4] Megaparsec (Mpc): A unit of distance used in astronomy, equal to one million parsecs. One parsec is approximately 3.26 light-years.