If you are wondering "What is Hubble's law?" or "What is the value of the Hubble constant?", the Hubble law distance calculator is the place to be! We will show you how to calculate the speed any galaxy is moving at, given its distance to Earth, using the Hubble's law equation.
We will also explain what redshift is, and the history and controversy behind Hubble's law definition.
What is Hubble's Law? – Hubble's law definition
When astronomers first saw all the galaxies around us, they discovered that all of them seemed to be moving away from us. In fact, the speed at which they "run away" from us is proportional to the distance between us and them. The first people to notice this were Edwin Hubble and George Lemaitre.
Hubble's Law definition is the empirical fact that galaxies (and everything) in the universe move away from us faster the further they are from us. This proportionality was thought to be constant (now we know that it is not), and the value of this constant speed was, understandably, named after Dr. Hubble.
The way to measure the speed at which galaxies move away from us is to look at their change in color or redshift. As weird as it may sound, when an object moves away from us, its color becomes slightly redder. For normal speeds, this is practically undetectable, but when we get close to the speed of light this phenomenon becomes more and more perceptible.
What does Hubble's Law tell us?
When you look around and see that EVERYTHING is moving away from you and that it does so faster the further you are from it... there is only one possible explanation: everything moves away from everything. The fact that it seems to move away from us is because we have our own point of reference, the center of our observable universe.
Imagine a piece of fabric, on which you have marked a series of dots in permanent marker. All of the dots are at the same distance from each other. When you start stretching that fabric, all the points will be further and further away from each other. If you kept your view centered on one dot, you would see that all of the other dots are "running away" from it.
In our universe, the same thing happens. The universe is expanding, and, as a consequence, all things are getting further away from each other. The reason this is not constant, but instead slightly accelerated (as far as we can tell), is dark energy.
How to use the Hubble law distance calculator?
The Hubble law distance calculator is very simple in principle. It is based on Hubble's law equation, which shows the proportionality of distance and speed for distant galaxies:
v = H₀ × dwhere:
- v – the recession velocity of the galaxy (how fast it moves away from us), usually in km/s;
- H₀ – the Hubble constant, expressed in km/s/Mpc; and
- d – the distance between us and the galaxy, in megaparsecs (Mpc).
To use the calculator, follow these steps:
- Choose what you want to calculate from: distance → velocity, velocity → distance, or redshift → velocity & distance.
- Enter the known value (distance, recession velocity, or redshift z) and pick its unit.
- Set the Hubble constant H₀, or pick one of the presets (Planck 67.4, Standard 70, SH0ES 73.0 km/s/Mpc).
- Select your preferred measurement system: Metric (km, m) or American / Imperial (miles).
- Click Calculate. The calculator instantly returns the recession velocity, distance (in Mpc, million light years, light years, and your chosen length units), and the redshift.
- Use the example buttons (Virgo, Coma, Hydra, and Hercules clusters) to try real galaxy-cluster distances.
The redshift is computed using the non-relativistic approximation z ≈ v / c, where c is the speed of light. This is accurate for velocities much smaller than the speed of light.
Measurement systems
This calculator supports both the Metric system and the American (Imperial) system:
- Metric: Velocity in km/s and m/s; distance also shown in kilometers (km).
- American: Velocity in miles per second (mi/s); distance also shown in miles (mi).
- Both systems always display the distance in megaparsecs (Mpc), million light years (Mly), and light years (ly).
Hubble constant and "Hubble trouble"
The Hubble constant H₀ tells us how fast the universe is expanding. The trouble is that different ways of measuring it give different answers. Measurements of the early universe from the Planck satellite's cosmic microwave background give about 67.4 km/s/Mpc, while measurements of the nearby universe using Cepheid variables and supernovae (the SH0ES team) give about 73 km/s/Mpc.
These two values disagree by more than their stated uncertainties, a discrepancy astronomers nickname the "Hubble tension" or "Hubble trouble". It may point to new physics beyond the standard cosmological model — which is exactly why a value of around 70 km/s/Mpc is often used as a convenient middle ground.
FAQs
What is the recession velocity?
The recession velocity is the speed at which a galaxy moves away from us due to the expansion of the universe. According to Hubble's law, it increases in direct proportion to the galaxy's distance from us.
What is redshift?
Redshift is the stretching of light to longer (redder) wavelengths as an object moves away from us. The faster a galaxy recedes, the larger its redshift. For low speeds, the redshift z is approximately equal to the recession velocity divided by the speed of light (z ≈ v/c).
What value of the Hubble constant should I use?
It depends on the method. Use about 67.4 km/s/Mpc for early-universe (Planck) values, about 73 km/s/Mpc for local-universe (SH0ES) values, or roughly 70 km/s/Mpc as a common middle-ground estimate.
Why does Hubble's law imply the Big Bang?
If everything is moving apart and the expansion has been going on for a long time, then in the past everything must have been much closer together. Running the expansion backwards leads to a hot, dense beginning — the Big Bang.