Is the universe expanding faster than the speed of light

Is the universe expanding faster than the speed of light
Is the universe expanding faster than the speed of light

You can often hear statements by astrophysicists and cosmologists that extremely distant regions of the Universe are moving away from us faster than the speed of light. But what exactly does this mean? Do they mean that there are objects in the Universe that can exceed one of the most fundamental values.

expansion of the universe

The most fundamental law of special relativity at one time led Einstein to the realization of the most breakthrough idea in physics - that nothing can move faster than light. Massless particles in a vacuum move at the speed of light, while everything else - a particle with a mass somewhere or a massless particle in a medium - will always move slower than the speed of light. But when it comes to the expansion of the universe, thoughts often arise that this is happening faster than the speed of light. Let's try to figure out if this is so.

The universe as we see it today has been around for about 13.8 billion years - since the hot Big Bang. But if you ask how far we can look in any direction, the answer is not 13.8 billion light years, but much more. If you think about it, you can imagine the distance twice as large: if an object emitting light was 13.8 billion light years from “us” 13.8 billion years ago, then it most likely emitted light, moving away from us - perhaps even at a speed close to light. If a bright object existed for so long and was constantly moving away from us at a speed of 299,792 kilometers per second, its light would reach us only now, although the object itself would already be 27.6 billion years from us. This all sounds reasonable, but it might lead us to the not-so-good assumption that space itself is static.

The space in which we live is not static - it expands. Moreover, we can measure the current rate of expansion, as it was in the distant past and as it was in all "intermediate" eras. It turns out that the light of an object that was only 168 meters away from us at the time of the Big Bang (okay, 10-33 seconds after the Big Bang), would have reached us only today, after 13.8 billion years, after an incredible travel and an unreal degree of stretching, and the object itself would now be 46.1 billion light years away.


“Aha! you exclaim. "So space has expanded faster than the speed of light!"

Is not it? Just in order for something to move faster than light, it must have speed: something that can be measured, for example, kilometers per second. But that's not how the universe expands.

On the contrary, it expands at a speed per unit distance. This is usually measured as kilometers per second per megaparsec, where a megaparsec is about 3.26 million light years. If the expansion speed is 70 km / s / Mpc, this means that, on average, an object located 10 Mpc from us moves away at a speed of 700 km / s from our point of view, at 200 Mpc - 14,000 km / s, and at In the case of an object of 5000 Mpc, it will seem to us that it is moving away at a speed of 350 thousand km / s.

However, does it follow from this that some objects move faster than light? Let's go back to Einstein's Special Relativity and think about what we mean when we say that nothing can travel faster than light. This means that if you have two objects in the same space-time event - occupying the same space at the same time - then they cannot move relative to each other faster than the speed of light. Even if one of them moves north at 99% of the speed of light, and the other moves at the same speed to the south, their speed will not be 198% of the speed of light relative to each other, but will be equal to 99.995% of the speed of light. No matter how fast each of them moves, they will never exceed the speed of light relative to each other.


That's why it's called relativity: it measures the relative motion between two objects at the same point in space and time. But this type of relativity - Special Relativity - sets the rules in your area of ​​non-expanding space. General relativity adds another layer to this: the fact that space itself is expanding. By measuring the amount of ordinary matter, dark matter, dark energy, neutrinos, radiation and other things in today's universe, as well as how light that reaches us from different distances in the universe is shifted to the red spectrum as a result of expansion, we can recreate how large was the universe at any moment in the past.

When the universe was about 10 thousand years old, its observable portion was already 10 million light years across. When she was only a year old, the observable universe was 100,000 light-years across. When she was only one second, she was already 10 light years across. Yes, it all sounds like it is expanding faster than light. But at no moment in time did not a single particle move faster than light in relation to another particle with which it interacted.


On the contrary, the very space between the particles expanded, in the process of which the distance between them increased, and the radiation wavelength in this space was stretched. This went on for many billions of years during cosmic history and continues today. Despite the fact that we can never reach any objects that are more than 15.6 billion years old at the moment, even if we move at the speed of light (which by definition is impossible), it is not because they are moving away faster than light, but because the space between different points continues to expand.

The main takeaway is that space does not expand at any particular speed, but rather at a certain rate: at a speed per unit distance. As a result, the further away the object you are looking at, the more the expansion affects the distance between you. The farther away an object is from you, the redder it will appear and the faster it will move away from your point of view. But is it faster than light? In order to measure this, you need to be in the same area. Nothing moves faster than light in relation to your location, and this can be said about any place in the universe at any time. Space expands, but not faster than light; moreover, this expansion has no speed.

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