In the 1970s, NASA began creating the Hubble Space Telescope (HST). This amazing telescope was finally in 1990 and it's still taking fantastic pictures and collecting important data about deep space. But where did the name Hubble come from? Well, it's named after an American astronomer named Edwin Hubble.

Edwin Hubble was a really important scientist. He discovered that the light frequency of galaxies changes and he used this discovery to prove that galaxies are moving away from us. This discovery is called Hubble's Law, and we're going to explain it to you now.

Hubble's Law is all about how fast galaxies are moving away from us. It turns out that the universe is expanding, and as it expands, things move away from each other. So the recessional velocity is the speed at which things are moving away from us because of the expanding universe.

If you're interested in medical physics, you might want to know more about how Hubble's Law works. By using this law, we can learn a lot about the universe, including how it started and how it might end. It's a really important discovery and it's amazing that Edwin Hubble was able to figure it out!

Hubble’s law explained

Edwin Hubble was really interested in studying objects called nebulae. He focused on the ones that had a spiral shape and tried to figure out what they were. There were two theories at the time either they were part of the Way were galaxies away from us.

Hubble observed the light coming from the spiral nebulae and noticed something interesting. The further away the nebulae were, the more the light shifted to the red part of the spectrum. This shift is called Doppler shift. By studying the redshift of 20 spiral nebulae, Hubble figured out that galaxies are moving away from us at speeds that are proportional to their distance. This means that the further away a galaxy is from us, the faster it is moving away. This is called Hubble's law.

To understand Hubble's law, we need to know about something called the Doppler effect. When an object emits waves (in this case, light) and moves closer to an observer, the waves are blueshifted, which means the wavelength decreases. If the object moves further away from the observer, the waves are redshifted, which means the wavelength increases.

The accepted value of Hubble's constant is 73.8km/s/Mpc. This might seem like a really fast speed, but it's actually less than 0.1% of the speed of light. The velocity of galaxies increases with distance and time, which means that eventually, some galaxies might disappear from our view.

It's important to not get confused between Hubble's constant and how fast galaxies travel. Galaxies move at different speeds based on their distance from us. If you want to learn more about the Doppler effect and calculations, check out our explanation on it!

Hubble’s law graph

To find the relationship between the velocity of recession and the distance, Hubble analysed more than 20 galaxies. In his analysis, he plotted their distance from the Earth in parsecs against their velocity. You can see the results of this plot below:

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From this, Hubble concluded that an increasing linear relationship exists between the distance and the velocity.

A parsec (pc) is a length unit used to measure more considerable distances outside the Solar System (extrasolar distances). A parsec equals the distance covered by light in space during 3.26 years. Using this definition, Sirius, which is the brightest star in the Northern Hemisphere, is 2.64 parsecs away from Earth.

Hubble’s law equation

The linear relationship obtained by Hubble can be summarised with this simple equation:

Here, v is the recessional velocity in km/s, Ho is the slope of the line (also known as Hubble’s constant), and D is the distance from the galaxy to Earth measured in Mpc.

If you're interested, you should head on over to NASA's Hubble site to read up on how astronomers are able to measure accurately the universe’s expansion rate (including the use of a basic tool of geometry known as parallax).

Hubble’s law equation examples

Using the equation above, we can obtain the velocity of galaxies moving further away.

Calculate the velocity of the NGC 55 galaxy if it is at a distance of two megaparsecs from Earth.

First, we need to calculate the distance, and to do this, we must convert from parsecs to kilometres. We know a parsec is a distance covered by light in 3.26 light years. If the distance covered by light in a vacuum in one second is 300,000,000m, then we must multiply the distance by the number of seconds in one year, which is 31,556,926 seconds, and multiply this by 3.26.

The NGC 55 galaxy is at two megaparsecs, so we now need to multiply the parsec distance per the parsec to NGC 55.

Now, we can substitute this information into Hubble’s law equation and obtain the galaxy’s velocity. However, we must first convert the Hubble constant to metres per second per meter.

Hubble’s law: redshifted and blueshifted galaxies

Hubble made two critical observations: he discovered that galaxies are moving further away from our own and observed that the further away the galaxies are, the more they accelerate.

Almost all galaxies present a redshift in the visible light, but there are some that don’t. These galaxies present a blueshift instead, which is the result of gravitational forces.

Blueshift is the decrease in the emitted wavelength as the emitter (the object producing the electromagnetic waves) moves closer to the observer.

Andromeda is one of these galaxies that has a blueshift. The blueshift of Andromeda is a product of the gravitational attraction between our galaxy (the Milky Way) and Andromeda, and it has been predicted that both will collide after some millions of years.

Andromeda and the Milky Way have a blueshift because the dominant force between them is gravity (red arrows). Galaxies outside our group are not bound by gravity, so they follow the universal expansion (in green). Manuel R. Camacho – StudySmarter Originals

Did you know? Because Hubble’s law helps us to calculate (approximately) how much time has passed since galaxies started moving, researchers are able to gather information about the early stages of the universe and understand the Big Bang theory better.

Universe expansion

Hubble’s discovery of galaxies moving further away from our local group of galaxies was a groundbreaking discovery that helped us understand the universe better. His work laid the foundation for other theories on the accelerating expansion of the universe, which have been supported by later evidence such as the measurement of the redshift of supernovae.

By studying the light emitted by supernovae, scientists have been able to measure the distance and redshift of these objects. This has allowed them to confirm that the expansion of the universe is accelerating. Theories predict that the rate of expansion of the universe will continue to increase as time advances.

This discovery has led to new questions about the nature of the universe, such as what is causing this acceleration, and whether it will eventually lead to a "big rip" – a hypothetical end to the universe where the expansion will become so rapid that it tears everything apart.

Hubble’s discovery has laid the groundwork for much of our current understanding of the universe and continues to inspire new research to this day.

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Type 1a supernovae have been critical in helping us understand the expansion of the universe. Because the mass at which this explosion happens is fixed, the luminosity is similar every time it happens. This near-constant luminosity can provide a constant flux of radiation (light), and researchers can use this to measure the distance of the star system.

Observations of these binary systems were the ones that offered clues about the expansion of the universe. Type 1a supernovae were observed to be travelling further away with time, which supported the idea of an expanding universe.

Edwin Hubble's observation of a change in the light frequency of galaxies provided critical evidence on the recessional velocity, which led to the formation of Hubble’s law. This law is the observation that galaxies are moving away from us with a velocity that is proportional to their distance from us. In other words, further galaxies recede quicker than closer galaxies.

Hubble concluded that an increasing linear relationship exists between the distance and the velocity. Almost all galaxies present a redshift in the visible light, but there are some that present a blueshift instead, which is the result of gravitational forces.

Overall, the study of type 1a supernovae and Hubble’s law have provided us with a better understanding of the universe and its expansion. The ongoing research in this field continues to inspire new discoveries and theories about the nature of the universe.

Hubble’s Law

What is Hubble’s law?

Hubble’s law is the observation that galaxies are moving away from us with a velocity that is proportional to their distance from us. In other words, further galaxies recede quicker than closer galaxies.

How does Hubble’s law support the Big Bang theory?

Hubble’s law helps us to calculate (approximately) how much time has passed since galaxies started moving. This allows us to gather information about the early stages of the universe and understand the Big Bang theory better.

Do all galaxies follow Hubble’s law?

No, not all galaxies follow Hubble's law. Almost all galaxies present a redshift in the visible light, but there are some that don’t. These galaxies present a blueshift instead, which is the result of gravitational forces. 

How does Hubble's law depend on parallax?

Hubble researchers use trigonometric parallax to determine an object’s distance. This method detects the tiniest apparent shift in an object's location caused by a shift in the observer's viewpoint. 

How do you use Hubble's law?

We can use Hubble's law to calculate the velocity of galaxies. The equation is v=Ho·D where v is the recessional velocity measured in kilometres per second, Ho is Hubble’s constant, and D is the distance from the galaxy to Earth measured in megaparsecs.

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