Key Takeaways:

  1. The expanding Universe concept challenges intuition but aligns with observational evidence, including redshifts observed in distant galaxies.
  2. Analogies like the balloon and raisin bread models help conceptualize the expansion, although they have limitations in capturing the full complexity of space-time dynamics.
  3. The Universe’s expansion isn’t into any external medium; it’s self-contained, evolving within itself.
  4. Speculation about a multiverse or higher dimensions adds depth to the discussion but lacks empirical evidence.
  5. Understanding the essence of spacetime is crucial; the Universe encompasses everything and persists independently of external context.

For almost six decades, the Big Bang theory has stood as the foremost explanation for the origins of our universe. Commencing from a dense, hot state filled with matter and radiation, the universe has continuously expanded, cooled, and gravitated since its inception. Throughout its evolution, it generated protons, neutrons, basic elements, stable atoms, and eventually, celestial bodies like stars, galaxies, planets, and intricate chemical compounds capable of supporting life. Now, 13.8 billion years later, we find ourselves observing the ongoing expansion of the universe, endeavoring to understand its origins and the processes that shaped it into its current state.

However, amidst this continual expansion, a common query arises: what exactly is the universe expanding into? James Nastelli recently posed this question, seeking clarity:

“One question comes to mind, just exactly what is the universe expanding into?”

The succinct yet somewhat unsatisfying answer to this query is: itself. Rather than expanding into any external or definable medium, the universe expands within itself. This concept challenges common intuition, rooted in the principles of General Relativity. Let’s delve deeper into the scientific underpinnings of the expanding universe to shed light on this enigma.

Hubble’s discovery of a Cepheid variable in the Andromeda galaxy, M31, opened up the Universe to us, giving us the observational evidence we needed for galaxies beyond the Milky Way and leading us, in short order, to the discovery of the expanding Universe. Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Illustration via NASA, ESA, and Z. Levay (STScI)

In the early 20th century, two significant pieces of observational evidence emerged. Firstly, astronomer Vesto Slipher utilized spectroscopy to study various nebulae in the sky. He observed that many of these nebulae exhibited spectral lines indicative of known elements but shifted towards longer wavelengths, suggesting a Doppler-like effect implying they were moving away from us.

Simultaneously, starting in 1923, Edwin Hubble and Milton Humason identified individual stars within these nebulae, inadvertently discovering periodic variable stars instead of novae as initially intended. By understanding the nature of these stars and measuring their brightness, they could determine their distances.

Combining these observations revealed a crucial correlation: galaxies farther away exhibited greater redshifts, indicating they were receding from us at higher velocities. Had these discoveries preceded Einstein’s General Relativity, alternative explanations might have been considered.

Edwin Hubble’s original plot of galaxy distances, from 1929, versus redshift (left), establishing the expanding Universe, versus a more modern counterpart from approximately 70 years later (right). Many different classes of objects and measurements are used to determine the relationship between distance to an object and its apparent speed of recession that we infer from its light’s relative redshift with respect to us. As you can see, from the very nearby Universe (lower left) to distant locations over a billion light-years away (upper right), this very consistent redshift-distance relation continues to hold. Earlier versions of Hubble’s graph were composed by Georges Lemaître (1927) and Howard Robertson (1928), using Hubble’s preliminary data.
Credit: E. Hubble; R. Kirshner, PNAS, 2004

One could have speculated about a “lossy medium” in space causing energy loss in light as it traversed the universe. Another hypothesis might involve intergalactic matter absorbing light, especially at shorter wavelengths. Alternatively, the idea of galaxies being closer together in the past, with faster-moving ones now farther away due to increased speeds over time, could have been entertained.

However, the sequence of events unfolded differently, aligning with our understanding of physics. The confirmation of General Relativity’s predictions during the 1919 solar eclipse highlighted the intertwined nature of space and time, leading to the realization that the universe’s fabric, spacetime, is not static but dynamic.

We often visualize space as a 3D grid, even though this is a frame-dependent oversimplification when we consider the concept of spacetime. In reality, spacetime is curved by the presence of matter-and-energy, and distances are not fixed but rather can evolve as the Universe expands or contracts. Prior to Einstein, space and time were thought to be fixed and absolute for everyone; today we know this cannot be true. If you place a particle on this grid and allow the Universe to expand, the particle will appear to recede from you.
Credit: Reunmedia/Storyblocks

Incorporating these insights, it became evident that a universe filled with matter and energy throughout could not remain static. Instead, it must either expand or contract. Observational evidence overwhelmingly supported expansion, emphasizing the need to reconcile theoretical possibilities with empirical reality. Thus, while the universe theoretically could have contracted, its observed expansion solidifies this as the prevailing reality.

There exist two common analogies frequently employed to provide a tangible understanding of the expanding Universe, although both possess their inherent limitations.

One analogy involves conceptualizing the Universe akin to a balloon, particularly, a balloon adorned with coins affixed onto its surface. In this analogy, the expanding Universe resembles the act of inflating or blowing up this balloon, with the coins representing galaxies dispersed throughout space.

For an observer residing within a galaxy — within one of these coins — the expansion of the Universe results in the apparent movement of coins away from each other. The nearby coins exhibit a relatively gradual recession, while those situated farther away appear to recede at an increasingly rapid pace. It’s crucial to note that this apparent motion does not entail the coins physically moving relative to the fabric of space; rather, it reflects the expanding nature of space itself, propelling them further apart over time.

As a balloon inflates, any coins glued to its surface will appear to recede away from one another, with ‘more distant’ coins receding more rapidly than the less distant ones. Any light will redshift, as its wavelength ‘stretches’ to longer values as the balloon’s fabric expands. This visualization solidly explains cosmological redshift within the context of the expanding Universe. If the Universe is expanding today, that means it was smaller, hotter, and denser in the past: leading to the picture of the hot Big Bang. It also explains why all quanta lose kinetic energy as the Universe expands. Credit: E. Siegel/Beyond the Galaxy

However, this analogy is not without its shortcomings. The fundamental issue lies in the fact that while a balloon’s surface is two-dimensional, our Universe encompasses three dimensions. The inflation of a balloon occurs due to the introduction of air into an additional spatial dimension, unbeknownst to the inhabitants on its surface.

Unlike the balloon, there is no evidence indicating the existence of a fourth or higher spatial dimension in our Universe. Consequently, one must confine their considerations solely to the surface of the balloon, disregarding its internal space or the forces responsible for its inflation.

A slightly more accurate analogy involves envisioning a three-dimensional object undergoing expansion, such as a ball of raisin bread dough, with raisins dispersed throughout its entirety. As the dough rises, it expands, causing the raisins within to move farther away from each other in all three dimensions.

From the perspective of an observer within a raisin, the nearby raisins exhibit a slow recession, while those at intermediate distances recede more rapidly, and the most distant raisins recede at the highest velocity, despite their stationary nature relative to the expanding dough. If the dough were transparent, it would simulate the behavior of the fabric of space, while the raisins would represent individual galaxies within the expanding Universe.

redshift distance raisin bread
The ‘raisin bread’ model of the expanding Universe, where relative distances increase as the space (dough) expands. The farther away any two raisins are from one another, the greater the observed redshift will be by the time the light is received. The redshift-distance relation predicted by the expanding Universe is borne out in observations and has been consistent with what’s been known since the 1920s.

Although this analogy aligns more closely with the correct number of dimensions and does not rely on the notion of extra dimensions or external impetus for expansion, it still possesses certain drawbacks:

  • The expansion of the dough is constrained by the biochemical reactions of yeast cells, dictating the extent to which it can expand.
  • The properties of the dough undergo changes as it expands, becoming less dense and lower in mass density.
  • The dough still expands into its surrounding environment, whether it be the air or empty space.

It is this last point, regarding the expansion of the dough into “something,” that presents the greatest challenge in analogizing the expanding Universe. While the dough exists as an object within a larger reality — the entirety of three-dimensional space — the Universe itself constitutes the entirety of reality, or at the very least, all that is necessary to consider.

To overcome this challenge, one may contemplate an alternative scenario: instead of envisioning a ball of dough embedded with raisins within our three-dimensional Universe, imagine a three-dimensional Universe entirely filled with this raisin-embedded dough.

In a Universe that comes to be dominated by dark energy, there are four regions: one where everything within it is reachable and observable, one where everything is observable but unreachable, one where things will someday be observable but aren’t today, and one where things will never be observable. The labeled numbers correspond to our consensus cosmology as of 2024, with boundaries of 18 billion light-years, 46 billion light-years, and 61 billion light-years separating the four regions.Credit: Andrew Z. Colvin/Wikimedia Commons; annotations: E. Siegel

It is widely believed that the observable Universe we perceive, delineated by factors such as the expansion rate, the speed of light, and the time since the Big Bang, is just a fraction of a much larger, concealed Universe. Over time, our visibility and access to this Universe have increased and will continue to do so. This is because light emitted in the distant past, en route to us, will eventually reach us despite the ongoing expansion of space. With time, it’s projected that more than double the current volume of the Universe will become visible to us.

There’s speculation, albeit uncertain, that this unobservable Universe might be truly infinite. In such a scenario, questioning what the Universe expands into becomes nonsensical, as it already encompasses every conceivable inch of space. Consider these analogies:

  • Adding infinity to infinity still yields infinity.
  • Doubling, tripling, or quadrupling infinity still results in infinity.
  • Multiplying infinity by infinity, even across multiple dimensions, only produces infinity.

This illustrates that if the Universe is indeed infinite, its expansion is akin to these mathematical concepts, which even those without a background in physics or mathematics can grasp.

expanding universe
This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them. The expanding Universe allows for galaxies up to 15 billion light-years beyond our present cosmic horizon to eventually become visible, even while fewer and fewer galaxies become reachable.

However, it’s not guaranteed that the Universe is infinite or that the unobservable Universe represents its entirety. According to the theory of cosmic inflation, our Universe might be just one region where a Big Bang occurred among many. Beyond our region, inflation persists indefinitely, creating additional pockets where Big Bangs occur. This concept is known as a multiverse, with distinct “baby Universes” existing independently.

Does this imply that our Universe expands into this inflationary multiverse? Surprisingly, no. The significant disparity lies not in what lies inside or outside the Universe, nor in its boundaries, but in the concept of space, spacetime, and the fabric of the Universe itself. The Universe encompasses everything, serving as the stage for cosmic events. While there are rules governing this stage, including the behavior of particles and the laws of physics, there is no external context beyond space and time. The Universe is all-encompassing, past, present, and future.

In theory, there could be more than three spatial dimensions to our Universe, so long as those “extra” dimensions are below a certain critical size that our experiments have already probed. There is a range of sizes in between ~10^-19 and 10^-35 meters that are still allowed for a fourth (or more) spatial dimension, but nothing that physically occurs in the Universe can be allowed to rely on that fifth dimension.

Understanding this is crucial. The Universe isn’t expanding into anything because there is nothing beyond it. It is simultaneously everything and nothing: containing all existence yet persisting even in the absence of matter and energy. This is the essence of spacetime, an intrinsic component of reality that expands and contracts autonomously.

While it’s conceivable that there may be a higher-dimensional perspective where our Universe expands into something akin to the surface of a balloon expanding in three dimensions, there’s no empirical evidence supporting this notion. Presently, it remains an intriguing possibility without substantiation. Ultimately, the Universe doesn’t necessitate an external context for its expansion; it’s self-contained and capable of evolving independently.

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