Everything that happens outside of what can be observed leaves us with more questions than answers.
One of the greatest discoveries of the 20th century is that the universe is expanding on its own. When Einstein proposed the theory of general relativity, he quickly realized there was a consequence of it that made him uncomfortable. The universe, filled with matter in all directions, could not be stable and would succumb to gravitational collapse, where all objects pull on each other and ultimately perish.
In 1917, a year after the theory of general relativity was published, Einstein introduced a concept known as the cosmological constant. This constant was responsible for an invisible force that was preventing the universe from collapsing.
However, if the cosmological constant did not exist, scientists could still demonstrate that the universe was not collapsing in other ways. Theoretical and observational evidence from the 1930s showed that the universe is indeed expanding.
The universe continues to expand after the Big Bang and has not stopped.
Back in the 1910s, Vesto Melvin Slipher, an American astronomer, noticed that the “spiral nebulae” in the sky were exhibiting redshift. This was evidence that they were moving further away from us.
The following decade, another American astronomer, Edwin Hubble, demonstrated that the “spiral nebulae” observed by Slipher were actually galaxies. He also determined their distances from Earth.
If you combine these two pieces of information, you will realize that the galaxies are moving further away from us. There is another interesting trend: the farther away the galaxies are, the faster they seem to be moving away.
To explain this, many factors have been considered by scientists. For example, light from distant galaxies may be “tired”, losing energy as it travels through space. Or, it could be the result of a massive explosion that pushed the galaxies far away a long time ago.
However, only one hypothesis has been confirmed by both general relativity and all astronomical observations available at that time: The space of the universe is expanding on its own.
Hubble’s research data shows the retreat of the nebulae.
This is a puzzling conclusion for most people. They might wonder, “If it’s expanding on its own, what is it expanding into and how? Does the universe inflate like blowing up a balloon, or does it stretch out like a rubber band?”
These are very reasonable questions. Our life experiences show that a balloon inflates in all directions, and a soccer ball does the same when we blow air into it. But consider this: Do you really need to blow air to expand the balloon?
Perhaps not. For instance, you could stretch it with your hands. And when another person joins in, four hands can stretch it outward in all directions. There are many, many ways to make something expand.
Is the universe expanding like a balloon, with galaxies resembling dots on it?
But now imagine that you are no longer large enough to see a balloon in that way. You have shrunk to the point where the balloon becomes as big as the universe. And your view is limited to a circle with a radius of half a millimeter.
At this point, regardless of how the balloon is being stretched, if you look at all areas within that half-millimeter radius, you will see them moving away from you. Furthermore, the areas farther away are indeed moving faster. This is exactly what Edwin Hubble observed in the early 20th century, and it continues to occur in the universe today.
But if the universe is expanding, what is causing it? Is the cosmic balloon being stretched by a gigantic hand from the outside? Is there still space beyond the universe, and has someone blown air into it to make it inflate? Is there something inherent to the balloon that causes it to expand voluntarily? Or was the initial state of the balloon already one of expansion?
Is there something outside the universe pulling it apart?
Unfortunately, the most complete and honest answer we have right now is: We don’t know. Even worse, the next assertion is: We cannot know. From our position, we can only observe the balloon that is expanding.
Today’s science can hypothesize that there are more balloons than the one we inhabit. Then we can track exactly how the balloon expands over time. We can measure every property affecting that process.
Indeed, as long as the issue remains within the balloon, science can do that.
However, beyond our balloon, we have no information to say anything. Our universe now resembles a three-dimensional version of the surface of a cake. The galaxies are like raisins scattered on it.
The universe is like a cake, with galaxies resembling raisins.
We can measure the raisins. We can hypothesize that there are more raisins and even more cakes beyond our sight. But that’s all we know. We can determine the history of the universe’s expansion, and we have even discovered that it is expanding at an accelerating rate. But everything that happens outside of what can be observed leaves us with more questions than answers.
Can a two-dimensional deflated balloon expand in a three-dimensional space, or can a three-dimensional universe expand in a four-dimensional space? Is there a bakery outside of where we live that we can observe and sense? Is our cake infinite, or is it just that we are too small to see beyond it? Or is it expanding in a mold and overflowing from a microwave?
Scenarios for the expansion of the universe.
Humanity not only does not know these things. In fact, we don’t even have a clue to answer these questions. But that’s also part of the excitement of science. Until we learn about what lies outside the universe, let’s just accept the seemingly absurd answers that science provides.
The universe does not need to expand within anything. Because it is itself the greatest and largest entity. The universe itself expands on its own.
Because a man conceived the theory of general relativity, and it makes sense in this context. But humanity, with its intelligence, is not just represented by one man. If we are fortunate in the future, we will have clearer answers.
