Five questions about the expansion of the universe that you were embarrassed to ask (6 photos). When dark energy was found

Everyone knows that the universe is expanding. But where? What is this extension? By watching the nuclear fungus grow, we can definitely limit the space in which it grows. The question can be very stupid, on the one hand, but on the other hand, very interesting.

So, the Universe is expanding or contracting (respectively, showing red and blue shift) from the moment big bang. But where will the end of its expansion come? It can't be in infinity, after all. Why should we talk about the universe expanding as if it were the most normal and natural thing in the world?

Let's start with a few simple truths.

1. You are not expanding right now. The earth doesn't either. Neither solar system, nor the Milky Way. The expansion of the universe depends on gravity, which means only that in regions with high density local effects of gravity dominance are observed. It turns out that not all galaxies move away from Milky Way. Our nearest neighbor, the Andromeda galaxy, is rushing towards us at 80 km/s and will collide with us within a few billion years.

2. Don't trust metaphors. You may think that the universe is expanding like balloon which is filled with air. “Look, just like the Universe!” a fashionable British scientist will tell you. But you, being smart, will notice that there is space outside of the balloon, and that the 2-dimensional surface of the balloon expands into 3-dimensional space. However, our universe has three dimensions.

3. The universe has no end or edge. We're not really sure if the universe is infinitely big or just very big, but even if it is, traveling in one direction long enough will still bring you back to square one. Remember Pacman, but without fruits and ghosts. As far as the center of the universe is concerned, this is where the balloon analogy will help us. It seems to us that all galaxies are moving away from us, but from their point of view they will also be the center of the universe. It's just an illusion.

So where is the universe actually expanding? Yes, nowhere. There is no space closet filled with things. But to understand this, let's look at what general relativity has to say about spacetime.

In general relativity (as professionals say), the most important property space (and time) is the distance (and time interval) between two points. In fact, distance fully defines space. The evolution of the distance scale is determined by the amount of matter and energy in space, and as time goes on, the scale increases and so does the distance between galaxies. However - and this is odd - this happens without the actual movement of galaxies.

Perhaps at this point your intuition failed. But that won't stop us from figuring out the oddities.

We have already said that galaxies are moving away from us. Not really. It's just easier for scientists to explain what is really happening. They are deceiving you.

“But wait!” the most scientifically savvy of you will say. - "We measure the Doppler shift of distant galaxies." This so-called "redshift" that you know about is fixed on the Earth, and like the siren of a passing ambulance, lets us know that there is movement. But this is not what happens on a cosmological scale. It's just that since the distant galaxies emitted light and it got to us, the scale of space has seriously changed, grown. As space has expanded, so has the wavelength of the photons, so the light is red.

Another question arises from this approach: “Is the Universe really expanding? faster speed Sveta?". It is absolutely true that most distant galaxies increase their distance from us faster than the speed of light, but so what? They don't move faster than light (they generally stand still). Moreover, knowing this will not help you in any way: the information is not transmitted. If you send a package of food to another galaxy, faster than the speed of light, this will not be done (and even here, in principle, you will have to try). The speed of light remains the universal speed limiter.

We have given the most common (well, or established in the field of relativists) opinion about the cosmological expansion, but it will be logical to end with what we do not understand at all. All of the above works great as long as you have room to step forward and stretch. But what happened at the very beginning, what caused the space to form literally from nothing? Physics has no answer to this question. And we will have to wait until the theory of quantum gravity appears and sheds light on this issue.

In accordance with the standard cosmological model, as well as the results spectral measurements galaxies, the universe is expanding. But where does it expand? This is how many people who are interested in space and the universe put the question. But it is precisely in the question itself that the widespread misunderstanding or misunderstanding of this phenomenon lies.

Space is expanding - at least between galactic clusters. A photo from the Hubble Space Telescope shows Cluster SDSS J0333+0651. The spiral galaxy at the bottom left is far in the foreground Photo: ESA/Hubble & NASA

Albert Einstein's general theory of relativity is a superbly measured theory of gravity, cosmic time, and the universe as a whole today. It follows from this that space on a large scale cannot be static. It would have to be unstable, Einstein concluded in 1917 (but didn't want to believe it at first). That is, the space must shrink or expand. measurements spectral lines distant galaxies and galaxy clusters, as well as other observational data, confirmed that the universe is still expanding. And what is remarkable is that it has been doing this for six billion years, and the rate of such expansion is getting faster.

Three mistakes in understanding the process

There are three common misunderstandings:

The universe is expanding relative to some central point at which the Big Bang occurred.

Since then, the universe has been expanding uniformly in all directions.

The universe is expanding from its "outer boundary" outward.

All of these statements are fundamentally wrong.

Four cosmological conclusions

The following four conclusions of cosmology, which are based on the principles of the General Theory of Relativity, are correct:

The Big Bang was not an explosion in space and time, but an explosion of space and time. The spatial time of our universe arose precisely with big bang. That is, there is no center of the universe, and the Big Bang happened, although it is hard to realize, everywhere.

Space expands only where the gravity of matter and energy is limited. Therefore, space does not expand within galaxies or complex galactic groups, but only between galactic clusters and superclusters. The multiplicity of the expansion (which is often erroneously called the Hubble constant) is not a constant, changing in space-time coordinates. (Therefore in this case it is best to talk about the local Hubble parameters and the global, time-dependent Hubble parameter as an average.)

There is no outer boundary or outer edge of the universe - whether it is finite or infinite in its size. True, at the same time, for each point of the universe, there are fundamental limits to possible observation: cosmic horizons. They are the result of the finite speed of light in a vacuum. That is, light takes time to travel distances. Therefore, any look into space is a look into the past. Since our universe has a finite age and is expanding, we can only observe a certain fragment of the universe.

The specific state of affairs is that the universe is about 13.8 billion years old and its expansion rate is currently estimated to be about 70 kilometers per second per megaparsec (Hubble's constant). This means that we could theoretically look back 13.8 billion years ago - the first light still measurable in the microwave range appeared 380,000 years after the Big Bang. But in fact, we are able to look deep into space only 13.8 billion light years, but even further. Because it (space) is not static, but expanded and continues to do so. How far we can actually look depends on the expansion history of the universe. According to the latest data, our cosmic horizon is around us at a distance of 46 billion light years.

The Universe is expanding not only on the cosmic horizon (moreover, such a horizon is different for each observer in different places), but everywhere between galactic superclusters. That is, the space does not expand "outward", or, as some theorists suggest, in general, inside the space on the other side of the universe, due to some additional "surrounding space". Evidence and even minimal signs of such special space does not exist, and it is not necessary for the cosmology of the theory of relativity. In significantly more the universe expands "inwardly". This means that it expands everywhere where gravity does not hold it back, and such an expansion is summed up by increasing distances. And at the same time, gravitationally weak regions increase: a distance of one megaparsec (ie 3.26 million light years) increases with an expansion speed of about 70 kilometers per second.

When we look at the distant Universe, we see galaxies everywhere - in all directions, for millions and even billions of light years. Since there are two trillion galaxies that we could observe, the sum of everything beyond them is bigger and cooler than our wildest imaginations. One of the most interesting facts is that all the galaxies we have ever observed obey (on average) the same rules: the farther they are from us, the faster they move away from us. This discovery, made by Edwin Hubble and his colleagues back in the 1920s, led us to a picture of an expanding universe. But what if it expands? Science knows, and now you will too.

At first glance, this question may seem reasonable. 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 space itself, as a result of which individual galaxies and clusters of galaxies 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.

Model of an expanding "bun" of the universe, in which relative distances increase as space expands

This dough is the fabric of space, and the raisins are connected structures (like galaxies or clusters of galaxies). From the point of view of any raisin, all other raisins will move away from it, and the further they are, the faster. Only in the case of the universe of the oven and the air outside the dough does not exist, there is only dough (space) and raisins (substance).

Redshift is created not just by receding galaxies, but rather by the space between us.

How do we know that this space is expanding and not the galaxies are receding?

If you see objects moving away 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 further away and moving away faster because they received more energy from the explosion. If this were the case, we could prove it in two ways:

• At greater distances and at higher speeds, there will be fewer galaxies, since over time they would spread out in space a lot.
• The ratio of redshift and distance will take specific form at large distances, which will differ from the shape if the fabric of space expanded

When we look at great distances, we find that the density of galaxies farther in the Universe is higher than closer to us. This is consistent with the picture in which space is expanding, because looking further is the same as looking into the past, where there has been less expansion. We also find that distant galaxies have a redshift-to-distance ratio corresponding to the expansion of space, and not at all - if the galaxies were simply rapidly moving away from us. Science can answer this question in two ways. different ways, and both answers support the expansion of the universe.

Has the universe always expanded at the same rate?

We call it the Hubble constant, but it is only constant in space, not time. Universe in this moment expanding more slowly than in the past. When we talk about the expansion rate, we are talking about the speed per unit distance: about 70 km/s/Mpc today. (Mpc is megaparsec, approximately 3,260,000 light years). But the rate of expansion depends on the densities of all the different things in the universe, including matter and radiation. As the Universe expands, the matter and radiation in it become less dense, and as the density decreases, so does the rate of expansion. The universe has expanded faster in the past and has been slowing down since the Big Bang. The Hubble constant is a misnomer, it should be called the Hubble parameter.

The distant fates of the universe offer different possibilities, but if dark energy is indeed constant, as the data suggests, we will follow a red curve.

Will the universe expand forever or will it ever stop?

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

Do galaxies move faster than the speed of light? Isn't it forbidden?

From our point of view, the space between us and the remote point is expanding. The farther it is from us, the faster it seems to us that it is moving away. Even if the expansion rate were tiny, a distant object would one day cross the threshold of any velocity limit, because the expansion rate (speed per unit distance) would be multiplied many times over with sufficient distance. The OTO favors such a scenario. The law that nothing can move faster than the speed of light only applies to the movement of an object through space, not to the expansion of space itself. In reality, the galaxies themselves move at 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 true movement galaxies.

Within the observable universe ( yellow circle) contains approximately 2 trillion galaxies. Galaxies that are closer than a third of the way to this border, we will never be able to catch up due to the expansion of the universe. Only 3% of the volume of the Universe is open for development by human forces

The expansion of the universe is a necessary consequence of the fact that matter and energy fill space-time, which obeys the laws general theory relativity. As long as there is matter, there is gravitational attraction, so either gravity wins and everything contracts again, or gravity loses and wins the expansion. There is no center of expansion and there is nothing outside of space that expands; it is the very fabric of the universe that is expanding. What is most interesting, even if we left the Earth at the speed of light today, we would be able to visit only 3% of the galaxies in the observable universe; 97% of them are already out of our reach. The universe is complex.

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Where is the universe expanding?

I think everyone has already heard that The universe is expanding, and often we imagine it as a huge ball filled with galaxies and nebulae, which increases from some smaller state and the thought creeps in that at the beginning of time Universe was generally pinned down.

Then the question arises, what is behind border , and where the universe is expanding ? But what is the limit? Is Universe not endless? Let's try to figure this out though.

Expansion of the Universe and the Hubble sphere

Let's imagine that we are observing in a super-huge telescope, in which you can see anything in Universe . It is expanding and its galaxies are moving away from us. Moreover, the more spatially they are relative to us, the faster the galaxies are moving away. Let's look further and further. And at some distance it turns out that all bodies are moving away relative to us at the speed of light. Thus, a sphere is formed, which is called, Hubble sphere . Now it's a little less 14 billion light years , and everything outside of it flies faster than light relative to us. It would seem that this contradicts Theories of Relativity because the speed cannot exceed the speed of light. But no, because here we are not talking about the speed of the objects themselves, but about the speed expansion of space . But this is completely different and it can be anything.

But we can look further. At some distance, objects are receding so quickly that we will never see them at all. Photons emitted in our direction will simply never reach the Earth. They are like a person walking against the movement of an escalator. Will be swept back by rapidly expanding space. The boundary where this happens is called Particle horizon . Now before him about 46.5 billion light years . This distance increases The universe is expanding . This is the so-called border observable universe . And everything beyond this border, we will never ever see.

And here is the most interesting thing. And what about her? Maybe this is the answer to the question? It turns out everything is very prosaic. In fact, there is no border. And there the same galaxies, stars and planets stretch for billions and billions of kilometers.

But how?! How does it happen?!

Universe expansion center and particle horizon

Just Universe shatters quite cleverly. This happens at every point in space in the same way. As if we took a coordinate grid and increase its scale. From this, it really seems that all the galaxies are moving away from us. But, if you move to another Galaxy, we will see the same picture. Now all objects will move away from it. That is, at every point in space it will seem that we are in expansion center . Although there is no center.

So if we get close to Particle horizon , neighboring galaxies will not fly away from us faster than the speed of light. After all Particle horizon move with us and again it will be very far away. Accordingly, the boundaries will shift observable universe and we will see new galaxies, previously inaccessible to observation. And this operation can be done indefinitely. You can move to the particle horizon over and over again, but then it will shift itself, opening up new vistas to your gaze. Universe . That is, we will never reach its borders, and it turns out that Universe and true endless . Well, only the observed part of it has borders.

Something similar happens to globe . It seems to us that the horizon is the border earth's surface, but it is worth moving to that point and it turns out that there is no border. At Universe there is no limit beyond which there is no space-time or something like that. Just here we come across infinity which is unusual for us. But you can say this Universe has always been infinite and stretches while continuing to remain infinite. She can do it because space has no smallest particle. It can stretch for as long as you like. The universe, for expansion, does not need borders and areas where to expand. So it just doesn't exist anywhere.

So wait, how Big Bang ?! Wasn't everything that exists in space compressed into one tiny dot?!

Not! It was compressed into a dot only observable boundary of the universe . And the whole as a whole, she never had boundaries. To understand this, let's imagine universe billionths of a second after , when the observed part of it was the size of a basketball. Even then we can move to Particle horizon and all visible Universe will move. We can do this as many times as we like, and it turns out that Universe really endless .

And we can do the same before. Thus, moving back in time, we will find ourselves closer to big bang . But at the same time, each time we will find that The universe is infinite in every period of time! Even in the moment of the Big Bang! And it turns out that it happened not at any particular point, but everywhere, at every point of the infinite Cosmos.

However, this is only a theory. Yes, quite consistent and logical, but not without flaws.

What state was the substance in at the moment? big bang ? What happened before it and why did it happen at all? So far, there are no clear answers to these questions. But scientific world does not stand still, and maybe even we will become eyewitnesses of the solution of these mysteries.