The universe began its cosmic journey with the Big Bang nearly fourteen billion years ago and has been expanding ever since. But what exactly is it expanding into? This question is more complex than it seems. Let’s explore why.
Einstein’s theory of general relativity describes space and time as a kind of interconnected fabric that makes up the universe. This means that space and time exist only within the universe and not beyond it. So, when we talk about the universe expanding, it’s not expanding into something else like a balloon inflating into the air around it. Instead, the universe is expanding within itself.
In 1929, astronomer Edwin Hubble made a groundbreaking discovery. He observed that all distant galaxies are moving away from Earth, and the farther away a galaxy is, the faster it seems to be receding. This observation led to the understanding that the universe is expanding.
To visualize this, imagine a loaf of raisin bread rising in the oven. As the dough rises, the space between the raisins (representing galaxies) expands. Similarly, as the universe expands, the space between galaxies stretches, causing them to move away from each other. The farther apart the galaxies are, the faster they seem to move away from each other.
According to general relativity, there is a cosmic tug-of-war between gravity and expansion. In the vast emptiness between galaxies, expansion wins, causing space to stretch. Thus, the universe is expanding within itself.
Cosmologists are exploring mathematical models to understand what might exist beyond our spacetime. These are not mere guesses but hypotheses that address gaps in the Big Bang theory. For instance, the Big Bang suggests that matter should be evenly distributed, yet galaxies and stars formed. The inflationary model proposes a brief period of rapid expansion, linking quantum fluctuations in the early universe to the formation of gas clumps that eventually became galaxies.
This model suggests our universe might be one region in a larger cosmic reality undergoing eternal inflation. This speculative reality could be driven by an unstable quantum energy state. Occasionally, this energy might stabilize, forming bubble universes, each with its own Big Bang and physical laws. Our universe could be one such bubble in a vast multiverse, where the rapid rate of eternal inflation makes it unlikely for us to encounter another universe.
Mathematical string theories propose a unified description of fundamental forces and the structure of sub-atomic particles. In these models, tiny vibrating strings are the universe’s building blocks. These strings might interact with massive, higher-dimensional surfaces called branes. Our universe could be contained within one such brane, floating in a higher-dimensional space known as “the bulk” or hyperspace. Other branes, possibly containing different universes, might coexist in this hyperspace.
Both eternal inflation and brane theories describe a multiverse, but while universes in eternal inflation are isolated, brane universes might interact. A collision between branes could leave an echo in the cosmic microwave background, a radiation remnant from the Big Bang. However, no such echo has been found yet.
These multiverse hypotheses are speculative and based on mathematical models. While inspired by scientific experiments, there are few direct experiments to test them. Until new discoveries are made, scientists will continue to debate and dream about what lies beyond our universe.
Join a lecture where you will delve into Einstein’s theory of general relativity. Participate in discussions and solve problems that illustrate how space and time form the fabric of the universe. This will help you understand why the universe expands within itself rather than into something else.
Conduct a hands-on experiment to simulate Hubble’s discovery. Use a balloon with dots to represent galaxies and measure how the distances between them change as the balloon inflates. This activity will give you a tangible understanding of the universe’s expansion.
Create a model using raisin bread dough to visualize the expansion of the universe. Observe how the raisins (galaxies) move apart as the dough rises. Discuss how this analogy helps explain the movement of galaxies in our expanding universe.
Engage in a structured debate on the multiverse concept. Explore different perspectives, including eternal inflation and string theory. This will challenge you to think critically about the speculative nature of these theories and their implications for our understanding of the universe.
Prepare a presentation on string theory and higher dimensions. Investigate how these theories propose the existence of branes and their potential interactions. Present your findings to the class, fostering a deeper understanding of these complex concepts.
The universe began its cosmic life in a big bang nearly fourteen billion years ago and has been expanding ever since. But what is it expanding into? That’s a complicated question. Here’s why: Einstein’s equations of general relativity describe space and time as a kind of interconnected fabric for the universe. This means that what we know of as space and time exist only as part of the universe and not beyond it.
When everyday objects expand, they move out into more space. But if there is no such thing as space to expand into, what does expanding even mean? In 1929, Edwin Hubble’s astronomical observations gave us a definitive answer. His survey of the night sky found that all faraway galaxies recede, or move away, from the Earth. Moreover, the further the galaxy, the faster it recedes.
How can we interpret this? Consider a loaf of raisin bread rising in the oven. The batter rises by the same amount in between each and every raisin. If we think of raisins as a stand-in for galaxies and batter as the space between them, we can imagine that the stretching or expansion of intergalactic space will make the galaxies recede from each other. For any galaxy, its faraway neighbors will recede a larger distance than the nearby ones in the same amount of time.
The equations of general relativity predict a cosmic tug-of-war between gravity and expansion. It’s only in the dark void between galaxies where expansion wins out, and space stretches. So there’s our answer: the universe is expanding unto itself. That said, cosmologists are pushing the limits of mathematical models to speculate on what, if anything, exists beyond our spacetime.
These aren’t wild guesses, but hypotheses that tackle kinks in the scientific theory of the Big Bang. The Big Bang predicts matter to be distributed evenly across the universe as a sparse gas — but then, how did galaxies and stars come to be? The inflationary model describes a brief era of incredibly rapid expansion that relates quantum fluctuations in the energy of the early universe to the formation of clumps of gas that eventually led to galaxies.
If we accept this paradigm, it may also imply our universe represents one region in a greater cosmic reality that undergoes endless, eternal inflation. We know nothing of this speculative inflating reality, save for the mathematical prediction that its endless expansion may be driven by an unstable quantum energy state. In many local regions, however, the energy may settle by random chance into a stable state, stopping inflation and forming bubble universes.
Each bubble universe — ours being one of them — would be described by its own Big Bang and laws of physics. Our universe would be part of a greater multiverse, in which the fantastic rate of eternal inflation makes it impossible for us to encounter a neighboring universe. The Big Bang also predicts that in the early, hot universe, our fundamental forces may unify into one super-force.
Mathematical string theories suggest descriptions of this unification, in addition to a fundamental structure for sub-atomic quarks and electrons. In these proposed models, vibrating strings are the building blocks of the universe. Competing models for strings have now been consolidated into a unified description and suggest these structures may interact with massive, higher-dimensional surfaces called branes.
Our universe may be contained within one such brane, floating in an unknown higher-dimensional place, playfully named “the bulk,” or hyperspace. Other branes — containing other types of universes — may co-exist in hyperspace, and neighboring branes may even share certain fundamental forces like gravity. Both eternal inflation and branes describe a multiverse, but while universes in eternal inflation are isolated, brane universes could bump into each other.
An echo of such a collision may appear in the cosmic microwave background — a soup of radiation throughout our universe that’s a relic from an early Big Bang era. So far, though, we’ve found no such cosmic echo. Some suspect these differing multiverse hypotheses may eventually coalesce into a common description or be replaced by something else. As it stands now, they’re speculative explorations of mathematical models.
While these models are inspired and guided by many scientific experiments, there are very few objective experiments to directly test them yet. Until the next Edwin Hubble comes along, scientists will likely be left to argue about the elegance of their competing models and continue to dream about what, if anything, lies beyond our universe.
Universe – The totality of known or supposed objects and phenomena throughout space; the cosmos; macrocosm. – The study of the universe involves understanding the fundamental forces and particles that govern its behavior.
Expansion – The increase in the distance between any two given gravitationally unbound parts of the observable universe with time. – The expansion of the universe was first observed by Edwin Hubble, leading to the formulation of Hubble’s Law.
Galaxies – Massive systems consisting of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravity. – The Milky Way and Andromeda are two of the most well-known galaxies in our local group.
Gravity – A natural phenomenon by which all things with mass or energy are brought toward one another, including planets, stars, and galaxies. – Gravity is the force responsible for the formation of stars and planets from clouds of gas and dust.
Spacetime – The four-dimensional continuum in which all events take place and all things exist, consisting of three dimensions in space and one in time. – Einstein’s theory of general relativity describes gravity as the curvature of spacetime caused by mass.
Cosmologists – Scientists who study the origin, evolution, and eventual fate of the universe. – Cosmologists use observations of the cosmic microwave background radiation to understand the early universe.
Inflation – A theory of exponential expansion of space in the early universe, occurring just after the Big Bang. – The inflationary model helps explain the uniformity of the cosmic microwave background radiation observed today.
Multiverse – A hypothetical set of various possible universes, including the one we live in, that together comprise everything that exists. – The concept of the multiverse arises from different interpretations of quantum mechanics and string theory.
String – A theoretical one-dimensional object that is the fundamental building block in string theory, replacing the point-like particles of particle physics. – String theory suggests that particles are actually tiny vibrating strings, each with its own frequency and mode of vibration.
Dimensions – Independent directions in which movement or extension is possible; in physics, often referring to the fundamental aspects of space and time. – In string theory, additional spatial dimensions beyond the familiar three are proposed to exist at very small scales.
