Short answer:
We do not know , but most likely there is no edge, and there is no 'outside' .
Foreword to long answer:
I answer based on the best explanation of the world we have today, based on thousands of years of systematic and / or scientific exploration and observations with eyes, binoculars and telescopes (both on Earth and in space).
The currently accepted (among thousands of physics scientists who work with this full time, and therefore are the experts in society that the rest of us should trust in this) current theory is called the ' Standard Model of the Universe'.
The standard model describes everything from the smallest building blocks as quarks and electrons to how the stars were composed of these right after the beginning of the universe, the 4 forces that act between the various components (gravity, electromagnetic force, weak and strong nuclear force) to how the stars put together new elements when they explode and what has happened later during the approx. 13.8 billion years that the universe has existed [ Age of the universe - Wikipedia]. The various specializations in physics that together contribute to the Standard Model are called e.g. particle physics, quantum physics, classical field theory, general relativity, cosmology and astronomy.
When you read that scientists disagree or have new theories, it does not mean that the Standard Model is completely wrong, or that physicists do not know what they are doing, or that anyone can come up with alternative facts about how the universe is connected.
It is incredibly important to remember that the Standard Model itself is not a text like the one I have written over - it consists of a large number of mathematical equations that describe how everything in the universe is connected. Without understanding these equations, one cannot suggest how they can be changed to adapt to new theories or observations. The standard model is not just a story about what one believes, such as religious or other myths about the evolution of the universe.
One example (of several, but these are better to write about in another answer) of how physics develops without previous theories being 'wrong' is how Einstein improved Newton's theory. Newton's description of how gravity works is perfect for us here on earth, but for stars, planets, spacecraft and GPS satellites that go much faster, we need a more accurate description.
Also remember that the Standard Model does not describe how the universe came into being. There are a handful of different descriptions of what the universe may have looked like in the very first fraction of a second, such a short period of time that we can not fathom it - because we have not so far managed to build an experiment that can show this (then we need something much bigger and stronger than the largest current particle accelerator LHC at CERN; the largest and most expensive machine ever built).
Nor can we see what happened at the beginning of the universe, since the universe was not transparent in the first approx. 375,000 years - then it was as hot as inside the center of a star, where quarks, electrons and photons whiz around in a soup that we call plasma. Only when it got cold enough could the quarks come together three by three to make protons and neutrons which in turn could form hydrogen and helium, and gather to the first stars, with visible light (photons) whizzing around between them.
| Size of universe |
Thus, there is nothing in state-of-the-art physics that indicates that the universe in its very first moment could not have been created by a supernatural force such as. a god, but it is very thoroughly explained what happened then, when it literally became light after approx. 375,000 years, the elements were made of 'star dust' and these gathered in new stars and planets that contained all the building blocks for life to arise and then evolve through evolution. The details of this are studied in separate subjects such as chemistry and biology, which have many specializations in the same way as in physics, with associated mathematical equations.
I have written this whole preface only because, unfortunately, an increasing number of people are choosing to believe in their own inventions about the universe, rather than relying on the best explanation humanity in collaboration over hundreds of years has managed to produce.
Long answer:
- The universe is not 'inside anything'.
- The universe is this 'something' that other things are inside.
So there is nothing outside the universe.
Researchers have different hypotheses about what may be beyond our horizon. Maybe the universe is infinite in the same way as the earth's surface - if you go far enough to the east you will return to where you started, from the west.
Since the earth has 3 dimensions while the earth's surface is almost flat and therefore has only 2 dimensions, the earth's surface can curve around the earth so that you can sail around the whole earth and reappear where you started.
In the same way, we can imagine that it is theoretically possible with a 'universe bypass' if the universe has more than 3 space dimensions.
But this is just a speculation; we can not and will never be able to find out what lies beyond the horizon.
We will never be able to travel that far, not even far enough to look further away, behind the current horizon. We will not be able to do that even if we spend a hundred times as long as the universe has existed so far (ie 1380 billion years).
We also have no opportunity to see things that are further away than as far as light has been able to move since the universe became transparent about 13.8 billion years ago.
The universe is already expanding so fast that the horizon is moving away from us faster than the speed of light. One way to imagine this is to run on a treadmill that goes faster than you can run - so that you move backwards. This treadmill moves away from us in all directions, just as if we were in the middle of a giant balloon that is constantly inflated and getting bigger and bigger (but we can only see a part of what is inside the balloon ).
This means that the stars furthest from us (of those we can see) have already disappeared behind the horizon. The reason we can still see them is that the light they sent out when they were still on the horizon has only reached us now.
It is important to remember that when we use telescopes to see galaxies far away, we do not see them as they look now, but as they were when they emitted the light we see now.
Since then, these galaxies have moved much further away from us. This means that the observable part of the universe (as we can see) is not 13.8 billion light-years in diameter, even though light has traveled at the speed of light for 13.8 billion years from the horizon of our telescopes. As of today, the horizon is approx. 46 billion light-years away [ Size of the universe - Wikipedia].
| Size of universe |
The observable universe is therefore a sphere with a diameter of approx. 92 billion light-years. The light from each end has spent 'only' 13.8 billion years in the middle (where we are) because this whole sphere has stretched outwards during the same time.
Over the next 100 to 1000 billion years, the observable part of the universe (ie what we can see, within the horizon) will shrink noticeably and more and more galaxies will disappear from view.
Remember that the sun will expand into a red giant and then explode in approx. 5 billion years. So we must leave the solar system well in advance if we intend to observe the universe for longer than that.
So there is no 'edge' to see, and we will never be able to see so far that we can examine how the universe really is connected at such great distances.
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