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Ever since the prominent astrophysicist Edwin Hubble discovered in the mid-20s that the universe is not static but is constantly expanding, and the objects whose light reaches us are constantly moving away from us, this has been one of the central themes of modern science.
In 1998, two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team, discovered that the Universe was expanding at an accelerating rate. To do this, they observed the same object for a long time. The results of observations of distant galaxies confirmed that they are moving away. The speed of their movement was measured. After some time, astrophysicists returned to observing these objects and recorded that they began to move away even faster.
But how exactly is this expansion happening? Astrophysicists often say that regions of the Universe far from us continue to move away at FTL speeds. But does this mean that some objects in space are capable of violating one of the key principles of the Special Theory of Relativity formulated by Albert Einstein, that no object in the Universe can exceed the speed of light.
Thus, particles with no mass can move at the speed of light in a vacuum. Whereas all other objects with a certain mass elsewhere, or the same particles without mass, but in a certain environment, will always move at a speed lower than the speed of light.
According to astrophysicists, the age of the Universe is about 13.8 billion years. Today, scientists can measure not only the current rate of expansion of the Universe, but also what it was in the distant past and in all intermediate periods. For example, the light from an object that was 100 meters away at the time of the Big Bang would have reached us only after 13.8 billion years, while the object itself would have been well over 46 billion light-years away.
But does this mean that space has expanded at FTL speed? Actually, no, because the expansion of the Universe does not occur at any particular speed, but rather at a certain rate, measured in terms of speed per unit distance. Usually, this expansion is measured in kilometers per second per megaparsec (km/(s-Mpc)), where megaparsec — is about 3.26 million light years. In this case, if the expansion rate is 70 km/s/Mpc, then an object at a distance of 5 Mpc is moving away from us at a speed of 350 km/s, 100 Mpc — at a speed of 7 thousand km/s, and in the case of an object located at a distance of 10 thousand Mpc, it will seem to us that it is moving away at a speed of 700 thousand km/s.
This does not imply that some objects can move at FTL speeds. The special theory of relativity postulates that two objects in the same spacetime, i.e., occupying the same space at the same time, cannot move relative to each other at FTL. Even if one of them moves north at 99% of the speed of light, and the other — south at the same speed, their speed will not be equal to 198% of the speed of light relative to each other, but will be equal to 99.995% of the speed of light. It doesn’t matter how fast each of them moves, they can never exceed the speed of light relative to each other.
This is what is called relativity. It measures the relative motion between two objects at the same point in space-time. However, Special Relativity describes rules for static, non-expanding space, and only cases of straight, uniform motion.
At the same time, General Relativity already contains the fact of space expansion and all nonlinear cases of motion. By measuring the amount of ordinary (baryonic) matter, dark matter, dark energy, neutrinos, radiation, and light coming to us from different parts of the visible Universe, shifting into the red spectrum as space expands. Scientists can reconstruct the size of the Universe at any point in the past.
Within a second after the Big Bang, the Universe was 10 light-years across, and a year later it was —100 thousand light-years across. However, no particle has ever moved at FTL relative to another.
The space itself between the particles expanded, the distance between them increased, and the wavelength in space stretched. And all of this has been going on for billions of years and is still going on, even with acceleration. Even if we move at the speed of light, we will never reach objects that are further away than 15.6 billion light-years because the space between objects continues to expand. According to many physicists, the driving force behind this expansion is the vacuum energy, which seems to push space apart and accelerate galaxies moving away from us.
We will never be able to reach most of the Universe.
The farther away the object is, the more it will shift into the red spectrum and the more it will appear to be moving away faster and faster relative to you. Relative to the observer, nothing moves faster than light, and this is true for any place in the Universe at any given time.
The farther astronomers look into outer space, the faster these galaxies and galaxy clusters move away. And at large distances —, we can see that the further they are from us, the faster they will move away, which is reflected in Hubble’s law.
This also means that there are areas of the Universe so far away from us that light radiation from there will never reach us. In particular, these are the parts of the Universe that are 46.1 billion light-years away. Thus, any object at a distance of 15.6 billion light-years from us becomes forever inaccessible. And this is about 95% of the volume of the visible part of the Universe.
If humanity started exploring space right now, moving at the speed of light, it would be able to reach all the galaxies within these limits. However, this is no more than 4-5% of the current visible Universe. Every year, about 160 billion stars, enough to form one small galaxy, move away from the Earth at an unattainable distance.
