Are distant galaxies really moving away from us at faster than light speed?

© 2025 Kieron Conway - All rights reserved.

This is a very common question. 

Physics tells us that the speed of light in a vacuum is the fastest speed that anything can travel and is reserved for electromagnetic waves or photons.

And yet, far-away galaxies appear to be doing a runner away from us at speeds that are faster the further away they are! So, some really far-off galaxies must be moving away at close to light speed and even beyond it! This is scientific fact and was first discovered by Edwin Hubble back in the early years of the last century. From measurements, he produced a straight line graph that showed that the apparent velocity of recession of a distant galaxy is proportional to the distance from us.

In other words, the further away a galaxy is, the faster it is doing a runner! Any galaxies high-tailing it beyond the speed of light, we'll never see!

So, what's going on? 

First, the universal speed limit only applies to particles or collections of particles like say, a space ship and even galaxies. It doesn't apply to the expansion of the universe, which is oh-so-special!

Second, at large distances, everything in the universe is moving away from everything else. Close to us, galaxies are pootling away from us or heading towards us at speeds that are tiny fractions of light speed. In our local back-yard of the universe, we don't sense the universe's expansion. It's only when we start to look way out into the distance do we see this 'running-away-from-us' effect.

There are two theories about the universe's expansion

One theory is centred around dark energy pushing things apart at the grand scale. The other is that dark energy is actually expanding space itself. The general consensus is that space is expanding as this theory provides a more accurate description when considering cosmological effects close to light speed. So we'll go along with the expansion of space and see if we can shed some light on things.

Most important point to make is that very distant galaxies are not actually moving at these insane speeds away from us: they only appear to be doing so, according to our measurements. It's the rate at which the space between two galaxies, unconnected by gravity, expands that gives rise to the measured effect.

Time for an analogy

To help understand the expansion of space and its effect on measuring distance and speed.

Imagine an almost infinitely, long, straight, road orientated north to south. At some location on this road there are two tortoises, one called Bertram and the other Marigold. Bertram is pointing north and sets off in that direction at a steady 1 metre per hour. Marigold is pointing south and heads off in that direction, also at 1 metre per hour. So, their separation is measured at 2 metres after one hour, each having travelled one metre in the opposite direction to the other.

However, in this hour, each metre of the road has expanded by, let's say 0.00000000000000000001 mm so, the real distance between them is 2.00000000000000000002 metres after the first hour. Suppose this tiny discrepancy is too small to measure, so it's probably got lots more zeroes in it! Every hour, each metre along the road expands another 0.00000000000000000001 mm, so for a long while, any measurement made to determine how far apart Bertram is to Marigold is only wrong by a minute amount, which is unnoticeable.

If our two intrepid tortoises are immortal, didn't need food, water or sleep, they could go on walking away from each other at 1 metre per hour for ever. Suppose each is able to measure the distance to the other tortoise, then the further apart they become, the greater the distance becomes taking into account that as well as their constant 1 metre per hour speed, separating them by another 2 metres every hour, the road is becoming longer by another 0.00000000000000000001 mm in each metre of their separation.

After billions of years, there are a great many metres between Bertram and Marigold and the expansion of each one between them pushes them even further apart. These tiny cumulative additional amounts to every single metre that they have travelled add up to massive distances that the road has stretched after billions of years.

Bertram and Marigold are not just walking apart—the road itself is helping to pull them away from each other faster and faster.

To sum up

Locally, Bertram and Marigold move slowly (like tortoises!).

But the road's expansion causes their separation to grow faster and faster each hour— even faster than light after they have travelled for billions of years!

Remember, each hour adds another 2 metres to their separation, but every single metre between them keeps expanding and the longer they continue the journey the faster this cosmic separation grows. Once their speed of separation passes the speed of light, they loose sight of each other for ever!

Back to reality - The physics bits

Mathematically, Hubble showed that the relationship between speed of separation and distance is;

v = H x d

Where 'v' is the velocity of separation (the speed at which the galaxy is doing a runner), 'd' is the distance between earth and galaxy and H is the Hubble parameter. It's often called a constant, but it will change over time as the universe's expansion is actually accelerating as astronomers have discovered.

Distance to galaxies is measured using red-shift of light

The further away a galaxy is, the greater the red-shift of its light. This is the stretching of the received light towards the red end of the visible spectrum, caused by an expanding universe. If the red-shift is relatively small and measured as z, it can be calculated from;

z = [ λ(observed) – λ(emitted) ] / λ(emitted)

where λ is the wavelength of the light and for small redshifts where z is much lower than 1, you can approximate the recession velocity to;

v = z x c (for z<<1)

Where, 'v' is the recessional velocity and 'c' is the speed of light. When the observed wavelength is close to the emitted wavelength (i.e. very low z value), the velocity of recession is well below c and the measurement involves standard red-shift of light due to relative velocities.

The value of z gets larger as the red-shift increases and cosmological versions of the equations are necessary as the effects of the universe's expansion on the red-shift must be taken into account

Type 1A supernovas provide an accurate means of checking the distance to a galaxy. First, spot a type 1A explosion in the target galaxy. The light intensity from a type 1A supernova is very consistent and they tend to last for a well defined duration, helping to identify them. By comparing the brightness of a target with the maximum intensity possible, distance can be determined. The brighter the target supernova, the closer the galaxy: the dimmer the target supernova, the further away the galaxy.

So, there you have it: the speed of recession of distant galaxies is all about the rate at which the distance from us to them is expanding and the further away a galaxy is, the faster it appears to be doing a runner. Astronomers have techniques for measuring the distances and speeds of recession based on cosmological red-shift and standard candle comparisons help to verify results.

There are two profound outcomes to take away from all this.

1) It is not possible to provide a value of speed to define the expansion of the universe as it varies according to distance.

The best we can do is to define a speed for a fixed distance and this is H, the Hubble parameter. So, H is a measure of how fast galaxies are receding, due to cosmic expansion, per unit of distance, at this moment in time.

2) Currently, H indicates that a galaxy recedes at about 70 Km per second for every 3.26 million light years of separation. (3.26 million light years is referred to as a mega-parsec.) For a galaxy 10 mega-parsecs distant, the speed of recession is approximately ten times 70 Kps or 700 Kps, for a galaxy 100 mega-parsecs distant, the speed is 7,000 Kps and so on. The following table indicates the range of recessional speeds.

Distance to galaxy Speed

Million light years Kps

3.26 70

  32.6 700

  326 7,000

     3,260 70,000

   32,600 700,000

The speed of light is approximately 300,000 Kilometres per second, so once a galaxy is about 14,000 million light years from earth, it appears to be moving at light speed. For any galaxy further away than this, we will never see its light.

What's worse is that many galaxies that we can see now, will disappear from our telescopes once the rate at which they separate from us goes FTL, (a term used in Sci-Fi to mean 'Faster Than Light').

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You can read more about all this in a Journey into Modern Physics, from which the table above was extracted. What a type 1A supernova is, can also be found in the the journey. What you won't find is the tale of Bertram and Marigold as they are unique to this blog!

 Look out for more interesting blogs on physics from this site.

But Wait - January 2026 Update on Dark energy

Something may be stirring in the astro-physics community. Recent research, unverified at present, indicates that the density of dark energy may not be constant as has been determined since the 1990s and it may even be decreasing! This does not change the fact that distant galaxies are moving away from us at speeds beyond that of light, but the effect may be slowing-down.

Theoretically, if these new results are verified independently, we could be looking at a universe that might start collapsing in on itself if the matter in the universe has sufficient oomph to overcome the effects of dark energy. If gravity could be able to create a collapse in the future, the universe would contract back to a super dense state. This is known as the "big Crunch" ending of the universe and would take billions of years.

All this is pure conjecture until the current research has been verified or thrown out.

To learn more, have a look at the blog article from 8th January 2026, titled: "What is Dark Energy up to?"

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