The Universe is as far as we know infinite. The size is increasing, and space itself is expanding. Physicists have known this for decades, yet it is still very hard to grasp.
How can something be infinitely long in all directions and at the same time increase in size in all directions? If the Universe is already infinite, how can it grow? If the Universe is an infinite number of meters long, how long is it then after increasing with 1 additional meter?
The Size Of The Observable Universe
The observable Universe is the ‘limit’ defined as the maximum distance we can see into deep space, as allowed by the laws of nature. It corresponds to the part of the Universe, which has been revealed to us over the past ~14 billion years. This ‘limit’ is called a horizon, and the horizon is set by how far light has travelled during the age of the Universe.
When you hear people on the bus saying that the Universe is expanding, what they often really mean is, that the size of the Universe is increasing, i.e. horizon is moving further away from us. This effect is something very different from what is known as the expansion of the Universe, which we will get back to shortly.
The size of the Universe increases because light from further points in space are reaching us as time goes by. It is like waiting at a bus station. After 5 minutes the city bus will arrive, but if you want for three hours, the neighbor county bus will arrive, because it had more time to travel and hence could cover a larger distance. Light rays in the Universe work exactly the same way.
As the size increases and more light rays reach us, we get a better view of the Universe. This is imagined most intuitively, if you imagine standing under a light cone. To the best of your knowledge, the world is the size of the green grass you see around you. The world is as big as you can see with your own eyes!
Now imagine this light cone being pulled further and further up. What happens now is that the round lit area around you increases in size, making it appear as if the world is expanding. However, the world is not expanding – more of it is just revealed to you. The grass lawn could be infinite in all directions, but you only see the part of it which is lit.
This is how the Universe ‘grows’ in size. It does not actually grow – it just reveals more of its infinity to us via traveling light rays.
The Infinite Universe has no Size – because it is Infinite
The infinite Universe is the entire Universe, regardless of what we can see or measure. The infinite Universe is everything in front of and behind the observable Universe’s horizon.
To continue the above analogy (where we imagine that the Universe is a grass lawn), this corresponds to the grass lawn itself being infinite. It does not have an end, nor does it have a beginning. The grass simply covers an infinitely large area. But the observable area, i.e. the area you can see, observe and validate, is finite and expanding.
As the lit area expands, you notice that your area just gets more green grass. No pink elephants or blue lakes. You only see more of the same.
If we now jump out of the analogy and back into reality, we notice that as the Universe expands, we similarly continue to see more of the same: galaxies, radiation, dust, etc. This indicates that the Universe is the same everywhere. Even though we have not seen what is beyond the horizon, everything we have seen so far, points towards the fact that there is nothing new to be revealed behind the horizon.
The Expansion Of The Universe
In the first section we elegantly avoided putting numbers on the size of the Universe. Instead we concluded that the observable Universe is defined by how far light has traveled since Big Bang ~14 billion years ago.
Intuitively this would make most people assume, that the horizon of the Universe is around 14 billion light years away from us. But in reality, the horizon is much further away: a staggering 45 billion light years away from us, which seems counter-intuitive to anyone with semi-decent math skills. How can light travel 45 billion light years in just 14 billion light years? This would correspond to a car driving 100km/hour but instead of covering 100 km in 60 minutes, it covers 300 km! It simply makes no sense.
The above can at a first glance appear to violate the laws of physics. But it does not. The reason is found in the expansion of the Universe. This mechanism is very different from mechanism that causes observable Universe to increase in size, described by the light cone analogy above, where we found that the increasing Universe is due to ‘more and more Universe space being lit up’.
The expansion is very different. It is a mechanism in the Universe, that no one really understands. The mechanism causes space to expand everywhere all the time. This means, that unlike the horizon of the observable Universe, the expansion is not something that takes place at ‘the border of the Universe’.
Expansion is something that happens in. Every. Single. Square. Meter. Of. Space. Everywhere. In. the. Universe.
The Universe Expands Like Chewing Gum
Expansion is tricky to understand. To get a better feeling of what it is, you can compare it to when you chew on a gum and then take it out of your mouth to play with it (not because you are gross but because you are a scientist). If you now stretch the gum, you will notice how the gum stretches everywhere and not only at the edge, where your fingers are holding the rubbery piece.
To properly understand this expansion, we need to understand Hubble’s Law. This law applies to galaxies with very large distance between them – much larger than the distance between us and our neighboring galaxy, Andromeda. In numbers, Hubble’s Law applies to objects that are located more than tens of millions of light years (more than ~10 Mpc) away from each other.
Hubble’s Law says, that expansion apparently gets faster with distance. For every Mpc of distance between two galaxies, space expands with a velocity of ~70 km/s. This means that the apparent expansion velocity between two galaxies is “70 times the distance to the galaxy”.
Let us look into what that means in a specific example: Galaxies located, say, 5.000 Mpc away from us is apparently receding with a velocity of 350.000 km/s (because 70 x 5.000 = 350.000). Knowing that the speed of light is less than 300.000 km/s, this then gives us a receding velocity higher than the speed of light. This is why light from galaxies present at this distance will never be able to reach us in the future.
In other words, the reason why we can see objects (e.g. galaxies), is because expansion happens in the parts of space between the galaxies. It therefore follows that the more space there is between us and a galaxy, the faster the galaxy will appear to be moving away from us.
It is important to note, that the high speed of the faint galaxy is just an apparent effect. There is no physical motion, and nothing is actually travelling anywhere. It is just space itself that expands, while light rays from the faint galaxies are trying to reach us with their finite speed. If the light ray was a car driving with finite speed, this light-through-the-Universe travel would correspond to the car driving on a road made of gum, that is constantly stretching to become longer and longer. Quite an unfair race, huh?
Eventually, as time passes and the Universe’s expansion increases, it will result in light rays from faint galaxies eventually being unable to reach us and therefore leaving the night sky pitch black. Fortunately for us, there is still a very long time until that happens.