When you bake a loaf of bread or a batch of muffins, you put the batter into a pan. As the batter bakes in the oven, it expands within the pan. Any chocolate chips or blueberries in the muffin batter will move farther apart as the muffin expands.

The expansion of the universe, in some ways, is similar. But this analogy has one flaw — while the batter expands within the baking pan, the universe has nothing to expand into. It simply expands within itself.

It might sound like a brainteaser, but the universe is considered to be everything in existence. In an expanding universe, there is no pan. There’s only the batter. Even if there were a pan, it would be part of the universe and thus expand along with the batter.

Even for me, a professor of physics and astronomy who has studied the universe for years, these ideas are hard to grasp. You don’t encounter anything like this in everyday life. It’s like asking which direction is farther north of the North Pole.

Another way to think about the expansion of the universe is by considering how other galaxies are moving away from our galaxy, the Milky Way. Scientists know the universe is expanding because they can observe other galaxies as they drift farther from our own. They define expansion by the rate at which other galaxies move away from us. This definition allows them to conceptualize expansion without requiring something for the universe to expand into.

The Expanding Universe

The universe began with the Big Bang 13.8 billion years ago. The Big Bang describes the origin of the universe as an extremely dense and hot singularity. This tiny point suddenly underwent a rapid expansion process called inflation, during which every part of the universe expanded outward. However, the name “Big Bang” is misleading. It wasn’t a massive explosion, as the name suggests, but rather the moment when the universe rapidly expanded.

The universe then quickly condensed and cooled, starting to form matter and light. Over time, it evolved into what we now know as our universe.

The idea that our universe is not static and could be expanding or contracting was first proposed by physicist Alexander Friedman in 1922. He mathematically demonstrated that the universe was expanding.

While Friedman proved mathematically that the universe was expanding, at least at certain points, it was Edwin Hubble who delved deeper into the rate of this expansion. Many scientists had already observed other galaxies moving away from the Milky Way, but in 1929, Hubble published his famous paper confirming that the entire universe was expanding and that its rate of expansion was increasing.

The Mystery of Expansion

This discovery continues to puzzle astrophysicists. What phenomenon allows the universe to overcome gravity, which pulls it together, while still expanding by pulling objects in the universe farther apart? On top of that, why is the rate of expansion accelerating over time?

Many scientists use an image known as the “expanding funnel” to describe how the universe has expanded more rapidly since the Big Bang. Imagine a deep funnel with a wide rim. The narrow end of the funnel represents the beginning of the universe. As you move to the right, you’re moving forward in time. The expanding cone represents the universe’s expansion.

The expanding funnel visually illustrates how the rate of the universe’s expansion has increased over time. The left side of the funnel represents the Big Bang, and since then, the universe has been expanding at an ever-accelerating rate.

Scientists have yet to directly measure the source of the energy driving this accelerated expansion. They haven’t been able to detect or quantify it. Because they cannot directly observe or measure this form of energy, they refer to it as dark energy.

According to researchers’ models, dark energy must be the most prevalent form of energy in the universe, accounting for about 68% of the total energy in existence. The energy from ordinary matter, which makes up the Earth, the Sun, and everything we can see, constitutes only about 5% of the total energy.

Outside the Expanding Funnel

So, what lies outside the expanding funnel?

Scientists have no evidence of anything beyond the universe as we know it. However, some speculate that there might be multiple universes. A model that includes many universes could potentially address some of the issues scientists face with the current models of our universe.

One major challenge with our current physics is that researchers cannot integrate quantum mechanics, which describes how physics works on very small scales, with gravity, which governs physics on large scales.

The rules for how matter behaves at small scales depend on probabilities and quantized or fixed amounts of energy. At this scale, objects can appear and disappear, and matter can behave like a wave. The quantum world is very different from how we perceive the world.

On large scales, which physicists call classical mechanics, objects behave in ways we expect in daily life. Objects are not quantized and can have continuous amounts of energy. They don’t appear or disappear.

The quantum world works like a light switch, where energy has only the option to turn on or off. The world we see and interact with operates more like a dimmer switch, allowing for any level of energy.

However, researchers encounter problems when they try to study gravity at the quantum level. On small scales, physicists would have to assume gravity is quantized. But much of the research they’ve conducted does not support that idea.

One way to make these theories work together is through the multiverse theory. There are many theories that look beyond our current universe to explain how gravity and the quantum world could coexist. Some leading theories include string theory, brane cosmology, loop quantum theory, and others.

Regardless of what lies beyond, the universe will continue to expand, with the distance between the Milky Way and most other galaxies growing over time.

Nicole Granucci, Lecturer of Physics, Quinnipiac University