Five questions about expanding the universe that you hesitated to ask

When we look at the distant Universe, we see galaxies everywhere – in all directions, in millions and even billions of light years. Since there are two trillion galaxies that we could observe, the sum of everything that is behind them is bigger and steeper than our most daring representations. One of the most interesting facts is that all the galaxies that we have ever observed obey (on average) the same rules: the further they are from us, the faster they are removed from us. This discovery, made by Edwin Hubble and his colleagues in the 1920s, led us to the picture of the expanding universe. But what if it expands? Science knows, and now you will know.

The farther we look, the farther back in time we look, seeing the yet undeveloped Universe. But – only if the general theory of relativity is applied to the expanding universe

In what (where) is the universe expanding?

At first glance, this question may seem robust. Because everything that expands, usually consists of matter and exists in the space and time of the universe. But the Universe itself is space and time, containing matter and energy in itself. When we say that “the universe is expanding,” we mean the expansion of the space itself, as a result of which individual galaxies and galaxy clusters move away from each other. It would be easiest to imagine a ball of dough with raisins inside, which is baked in the oven, says Ethan Siegel.

The model of the expanding “bun” of the universe, in which the relative distances increase as space expands

This dough is a fabric of space, and raisins are related structures (like galaxies or clusters of galaxies). From the point of view of any highlights, all the other raisins will depart from it, and the further they go – the faster. Only in the case of the Universe of the furnace and air outside the dough does not exist, there is only dough (space) and raisins (substance).

Redshifts are not created simply by receding galaxies, but rather by the space between us

How do we know that this space expands, and not the galaxies are removed?

If you see that objects are being deleted from you in all directions, there is only one reason that can explain this: the space between you and these objects is expanding. You could also assume that you are near the center of the explosion, and many objects are simply located further away and are removed faster because they have received more explosion energy. If this were so, we could prove this in two ways:

  • At large distances and high velocities, there will be fewer galaxies, since in time they would strongly spread in space
  • The ratio of redshift and distance will take a specific shape at large distances, which will be different from the shape if the tissue of space

When we look at great distances, we find that the density of galaxies is higher in the Universe than the closer to us. This is consistent with the picture in which space expands, because looking further is the same as looking at the past, where less expansion took place. We also find that distant galaxies have a relationship of redshift and distance corresponding to the expansion of space, and there is none at all – if the galaxies simply moved away from us quickly. Science can answer this question in two different ways, and both answers support the expansion of the universe.

Did the universe always expand at the same speed?

We call it the Hubble constant, but it is constant only in space, not in time. The universe is currently expanding more slowly than in the past. When we talk about the speed of expansion, we are talking about speed per unit distance: about 70 km / s / Mpc today. (Mpc is a megaparsec, approximately 3 260 000 light-years). But the speed of expansion depends on the densities of all the different things in the universe, including matter and radiation. As the universe expands, matter and radiation in it become less dense, and together with the decrease in density, the velocity of expansion also falls. The universe expanded faster in the past and slowed down since the Big Bang. The Hubble constant is the wrong name, it should be called the Hubble parameter.

The distant fates of the universe offer different possibilities, but if the dark energy is really constant, as the data show, we will follow the red curve

Will the universe expand forever or will it ever stop?

Several generations of astrophysicists and cosmologists puzzled over this question, and it is possible to answer it only by determining the speed of expansion of the universe and all types (and quantities) of energy present in it. We have already successfully measured how much ordinary matter, radiation, neutrinos, dark matter and dark energy exist, as well as the speed of expansion of the universe. Based on the laws of physics and what happened in the past, it seems that the universe will expand forever. Although the probability of this is not 100%; if something like a dark energy behaves differently in the future compared to the past and the present, all of our conclusions will have to be reviewed.

Are galaxies moving faster than light? Is not it forbidden?

From our point of view, the space between us and the remote point is expanding. The further it is from us, the faster, as it seems to us, it is removed. Even if the expansion speed was tiny, a distant object would once cross the threshold of any limiting velocity, because the expansion speed (speed per unit distance) would multiply multiple times at a sufficient distance. The UTO approves this scenario. The law that nothing can move faster than the speed of light, applies only to the movement of the object through space, and not to the expansion of space itself. In reality, the galaxies themselves move at a speed of only a few thousand kilometers per second, well below the 300,000 km / s limit set by the speed of light. It is the expansion of the universe that causes recession and redshift, not the true movement of the galaxy.

Within the observable universe (yellow circle) is approximately 2 trillion galaxies. Galaxies that are closer than a third of the way to this boundary, we will never be able to catch up because of the expansion of the universe. To master the forces of people, only 3% of the volume of the Universe is open

Expansion of the universe is a necessary consequence of the fact that matter and energy fill space-time, which obeys the laws of the general theory of relativity. While there is matter, there is also a gravitational attraction, so either gravity will win and everything will shrink again, or gravity will lose and the expansion will win. There is no center of expansion and there is nothing outside of space that expands; it is the very fabric of the universe that expands. What is most interesting, even if we left the Earth at the speed of light today, we could visit only 3% of the galaxies in the observed universe; 97% of them are already out of our reach. The universe is complex.

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