By Nicole Granucci, Quinnipiac University December 18, 2024
Collected at: https://scitechdaily.com/expanding-into-what-the-universes-infinite-growth-explained/
What is the universe expanding into if it’s already infinite?
The universe expands by itself, not into any external space, with galaxies continuously moving away from the Milky Way. Initiated by the Big Bang, this expansion is accelerating due to dark energy, which comprises most of the universe’s total energy. The complexities of expansion challenge both foundational physics and lead to theories like the multiverse, aiming to reconcile quantum mechanics with gravity.
Expanding Without Boundaries
When you bake a loaf of bread or a batch of muffins, the dough goes into a pan. As it bakes, the dough expands, pushing against the sides of the pan. If there are chocolate chips or blueberries in the batter, they spread farther apart as the dough rises.
The expansion of the universe works similarly — but with one key difference. While dough expands into the pan, the universe doesn’t expand into anything. It expands within itself.
This idea can feel like a brain teaser because the universe includes everything that exists. There’s no external “pan” holding it. If such a container existed, it would be part of the universe too — and would expand right along with it.
Understanding Cosmic Expansion
Even for someone like me, a teaching professor in physics and astronomy who has studied the universe for years, this concept is hard to wrap my head around. It’s as tricky as asking what lies farther north than the North Pole.
A helpful way to picture the universe’s expansion is to consider how galaxies move. Scientists know the universe is expanding because they observe galaxies drifting away from the Milky Way. They define expansion by how fast these galaxies move apart. This approach helps describe the universe’s growth without needing an external space for it to expand into.
Origins of the Universe
The universe started with the Big Bang 13.8 billion years ago. The Big Bang describes the origin of the universe as an extremely dense, hot singularity. This tiny point suddenly went through a rapid expansion called inflation, where every place in the universe expanded outward. But the name Big Bang is misleading. It wasn’t a giant explosion, as the name suggests, but a time where the universe expanded rapidly.
The universe then quickly condensed and cooled down, and it started making matter and light. Eventually, it evolved to what we know today as our universe.
Discovery of Expansion
The idea that our universe was not static and could be expanding or contracting was first published by the physicist Alexander Friedman in 1922. He confirmed mathematically that the universe is expanding.
While Friedman proved that the universe was expanding, at least in some spots, it was Edwin Hubble who looked deeper into the expansion rate. Many other scientists confirmed that other galaxies are moving away from the Milky Way, but in 1929, Hubble published his famous paper that confirmed the entire universe was expanding, and that the rate it’s expanding at is increasing.
The Mystery of Accelerating Expansion
This discovery continues to puzzle astrophysicists. What phenomenon allows the universe to overcome the force of gravity keeping it together while also expanding by pulling objects in the universe apart? And on top of all that, its expansion rate is speeding up over time.
Many scientists use a visual called the expansion funnel to describe how the universe’s expansion has sped up since the Big Bang. Imagine a deep funnel with a wide brim. The left side of the funnel – the narrow end – represents the beginning of the universe. As you move toward the right, you are moving forward in time. The cone widening represents the universe’s expansion.
Scientists haven’t been able to directly measure where the energy causing this accelerating expansion comes from. They haven’t been able to detect it or measure it. Because they can’t see or directly measure this type of energy, they call it dark energy.
According to researchers’ models, dark energy must be the most common form of energy in the universe, making up about 68% of the total energy in the universe. The energy from everyday matter, which makes up the Earth, the Sun, and everything we can see, accounts for only about 5% of all energy.
Outside the Expansion Funnel
So, what is outside the expansion funnel?
Scientists don’t have evidence of anything beyond our known universe. However, some predict that there could be multiple universes. A model that includes multiple universes could fix some of the problems scientists encounter with the current models of our universe.
One major problem with our current physics is that researchers can’t integrate quantum mechanics, which describes how physics works on a very small scale, and gravity, which governs large-scale physics.
Bridging Theories at Different Scales
The rules for how matter behaves at the small scale depend on probability and quantized, or fixed, amounts of energy. At this scale, objects can come into and pop out of existence. Matter can behave as a wave. The quantum world is very different from how we see the world.
At large scales, which physicists call classical mechanics, objects behave how we expect them to behave on a day-to-day basis. Objects are not quantized and can have continuous amounts of energy. Objects do not pop in and out of existence.
The quantum world behaves kind of like a light switch, where energy has only an on-off option. The world we see and interact with behaves like a dimmer switch, allowing for all levels of energy.
But researchers run into problems when they try to study gravity at the quantum level. At the small scale, physicists would have to assume gravity is quantized. But the research many of them have conducted doesn’t support that idea.
Theoretical Implications and Future Outlook
One way to make these theories work together is the multiverse theory. There are many theories that look beyond our current universe to explain how gravity and the quantum world work together. Some of the leading theories include string theory, brane cosmology, loop quantum theory, and many others.
Regardless, the universe will continue to expand, with the distance between the Milky Way and most other galaxies getting longer over time.
Written by Nicole Granucci, Instructor of Physics, Quinnipiac University.
Adapted from an article originally published in The Conversation.
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