What is the edge of the universe like?

There is a threshold for which we can not get out, there are things that we will never know. But we know something, and we have powerful tools: science, imagination, analysis. 13.8 billion years ago, the universe, as we know it, was born in a hot Big Bang. Over time, space expanded, matter passed through gravitational attraction and it turned out what happened. But to everything that we see, there is a limit. At a certain distance the galaxies disappear, the stars fade and no signals of the distant universe can be seen. What lies beyond this limit? If the universe is limited in volume, does it have a boundary? Is it achievable? What is the edge of the universe like?

To answer this question, we need to start from where we are now, and try to look as far as we can.

The universe is full of stars literally at our side. But if you pass more than 100,000 light years, you will leave the Milky Way. Behind him will be a sea of ​​galaxies: perhaps two trillion galaxies in total can be found in our observable universe. They are represented in a wide variety of types, shapes, sizes and masses. But when you look farther and farther, you start to notice something unusual: the further the galaxy, the more likely it will be smaller, lighter and its stars will be bluish.

This makes sense in the context of the fact that the universe had a beginning: birth. The birthday of the universe is the Big Bang. The galaxy, which is relatively close to us, will be close in age to the universe itself. But if we look at the galaxy for billions of light years, the light from it must have passed billions of years to reach our eyes. The galaxy, whose light will go to us 13 billion years, will be less than a billion years old, so the farther we look, the farther back in time we look.

The above image is a Hubble eXtreme Deep Field (XDF), the deepest image of a distant universe. In this picture, thousands of galaxies, located at a great distance from us and from each other. But what you do not see with the usual glance is that each galaxy has an associated spectrum in which a gas cloud absorbs light of a certain wavelength depending on the physics of the atom. With the expansion of the universe, the wavelengths are stretched, so distant galaxies seem redder than they really are. This physics allows us to determine the distance to them, and when we determine distances, the most distant galaxies turn out to be the youngest and smallest.

In addition to galaxies, we expect to find there the first stars, and then nothing but neutral gas, because the universe did not have enough time to bring down matter in a sufficiently dense state for the formation of stars. Millions of years ago, the radiation in the universe was so hot that neutral atoms could not form, and photons continuously bounced off charged particles. When neutral atoms were formed, the light simply flowed along the straight line forever, not dependent on anything other than the expansion of the universe. The discovery of this afterglow – the cosmic microwave background – more than 50 years ago became the final confirmation of the Big Bang.

Where we are now, we can look in any direction that we choose, and see there the same unfolding cosmic history. Today, 13.8 billion years after the Big Bang, we have stars and galaxies in their current form. Previously, the galaxies were smaller, blue, younger and less developed. Before them were the first stars, and even earlier – just neutral atoms. Neutral atoms were ionized plasma, and even earlier – free protons and neutrons, spontaneous creation of matter and antimatter, free quarks and gluons, all unstable particles of the Standard Model and, finally, the moment of the Big Bang itself. Looking further into space means to look further back in time.

Although this determines our observable universe – with the theoretical boundary of the Big Bang located in 46.1 light years from our current position – this is not the real boundary of the cosmos. Instead, we have just a boundary in time; There is a limit to what we can see, since the speed of light allowed information to advance only to this distance for 13.8 billion years. This distance exceeds 13.8 billion light years, because the tissue of the universe expanded (and continues to expand), but is still limited. But what about what happened before the Big Bang? What would you see if you somehow looked at a tiny fraction of a second before the Universe was at the peak of its highest energy, hot and dense, full of matter, antimatter and radiation?

You would see that there was a state of cosmic inflation: when the universe expanded very quickly and the energy inherent in the space itself prevailed in it. The space expanded exponentially at this time, when it was stretched flat, when it had the same properties everywhere, when fluctuations of the quantum fields inherent in space pervaded the entire universe. When inflation was over, the hot Big Bang filled the Universe with matter and radiation, giving rise to that part of the universe – the observable universe – that we see today. 13.8 billion years later we are here.

But it is worth noting that there is nothing special about our place, neither in space, nor in time. The fact that we can see for 46 billion years does not make this boundary or place something special; This is just the limit of what we can see, on its own. If we could somehow make a "snapshot" of the whole universe, go beyond the observable part, we would see everything the same as our universe has. We would see a large cosmic web of galaxies, clusters, filaments and cosmic voids that extend far beyond the relatively small region that we can see. Any observer in any field would see exactly the same Universe as us.

The individual details will, of course, be different. There will be another solar system, a galaxy, a local group and so on. But the Universe itself is not limited in scope; Only the observed part is limited. It is the boundary in time – the Big Bang – that separates us from everything else. We can approach this boundary only with the use of telescopes (which can see the early Universe) and theories. Until we figure out how to circumvent the stream of time that is striving forward, this will be our only approach, a way to see the "edge" of the universe. But the cosmos is unlimited.

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