It’s been 100 years since we discovered that the Universe was expanding. But if it’s expanding, then what is it expanding into?

Key takeaways

  • The Universe began 13.8 billion years ago from a hot, dense state and has expanded and cooled ever since.
  • Vesto Slipher and Edwin Hubble’s observations showed that galaxies are moving away, proving the Universe is expanding.
  • The Universe expands within itself, not into an outside space, a concept explained by General Relativity.
  • Visualize the Universe’s expansion like a balloon or raisin bread dough to grasp the idea, but remember these analogies have limitations.
  • If the Universe is infinite, it remains infinite as it expands, and doesn’t need to grow into any larger space.

For almost 60 years, the Big Bang has been the most effective idea for understanding our cosmic beginnings. The Universe began as a hot, dense, matter-and-radiation-rich state, and has subsequently expanded, cooled, and gravitated. As it evolved, it produced protons and neutrons, the first light elements, stable atoms, and, eventually, stars, galaxies, planets, and complicated chemistry capable of supporting life. We’re here, 13.8 billion years later, viewing the still-expanding Universe and trying to figure out where it all came from and how it got to be the way it is today.

But, if the Universe has been expanding for all of time, what exactly is it growing into? That is a typical question that individuals have, and this week, it was posed by James Nastelli, who just wants to know.

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

The quick, one-word response to this question is as concise as it is unsatisfactory: itself. The Universe grows inside itself rather than into a definite “outside” medium. This is another example of how General Relativity violates our ordinary experience and intuition, so let us try if we can help you grasp it a little better by delving into the physics underlying the expanding Universe.

Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Illustration via NASA, ESA, and Z. Levay (STScI)

Two pieces of observational evidence arrived about the same time, in the early twentieth century. First, astronomer Vesto Slipher saw a variety of nebulae in the sky using spectroscopy, a revolutionary technology at the time. Slipher discovered that many of these things had the same spectral lines we were familiar with from known elements, but they were systematically shifted, as if someone had stretched them to longer wavelengths. One probable explanation was a Doppler shift, which indicated that these objects were traveling away from us.

Second, beginning in 1923, Edwin Hubble (with his helper, Milton Humason) began recognizing individual stars in spiral and elliptical nebulae. Originally looking for novae, Hubble discovered that some of these “flares” were really periodic, brightening-and-fainting variable stars. Knowing the nature of these objects and watching how brilliant they looked might help establish their distance.

When these two pieces of information were joined, a new image emerged: one in which, on average, the further a galaxy was from us, the more its light looked to move toward the redder, longer-wavelength section of the spectrum.

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

If this finding had been discovered before to Einstein’s General Relativity, we would have considered a variety of “alternative” hypotheses for why this was happening.

  • Perhaps there existed a “lossy medium” in space that caused light to lose energy as it propagated throughout the Universe.
  • Perhaps there was some intergalactic matter present, which extinguished the light, especially at shorter wavelengths.
  • Perhaps these galaxies were all closer together in the past, and the ones that traveled faster compared to each other ended up farther apart, as higher speeds result in bigger distances over time.

However, that was not the order of events, at least according to our knowledge of physics. We had previously proven General Relativity’s predictions in dramatic form during the 1919 total solar eclipse, and we had recognized that “space” and “time” were not absolute quantities in our Universe, but were woven into the fabric of spacetime.

Putting all of this together meant that, once we learned that the Universe was full of matter and other types of energy, we couldn’t have a static, stable Universe. Instead, the Universe must grow or decrease.

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

The observations clearly indicated that expansion, not contraction, was taking place; as is often the case in physics, when the equations reveal multiple possible solutions, you must look to the Universe itself to determine which solution corresponds to your physical reality. Although the Universe might theoretically be growing or contracting, it turns out to be expanding.

There are two typical analogies we employ to make physical sense of the expanding Universe, but each has limits.

One approach is to view the Universe as a balloon, especially one with coins glued (or otherwise adhered) to its surface. The expanding Universe is equivalent to inflating (or blowing up) this balloon, with the coins on the balloon’s surface representing the galaxies in space. If you live in a galaxy, inside one of these coins, you’ll notice that as the Universe expands, all of the coins move apart. ones close to you appear to retreat slowly, and ones further away appear to recede faster. The coins are not “moving” relative to the fabric of space, but rather that the expansion of space forces them farther and farther apart with 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, and why photons have their wavelengths lengthen as the Universe expands. Credit: E. Siegel/Beyond the Galaxy

However, this parallel is not without problems. The difficulty is that a balloon’s surface is two-dimensional, but our Universe is three-dimensional. A balloon inflates when someone or something — generally a person — blows air into it in an additional spatial dimension unbeknownst to those on the balloon’s surface. And the balloon is definitely growing into that (additional) third dimension, although there is no proof in our Universe of the existence of a fourth (or higher) extra spatial dimension. You must focus just on the balloon’s surface, ignoring the “inside” or the forces that cause it to expand.

A somewhat better comparison is to imagine a fully three-dimensional item that expands, such as a ball of raisin bread dough, which is bread dough with raisins evenly dispersed throughout. As the dough rises, it expands, but the raisins inside do not. The raisins just move apart from each other in all three dimensions. If you are inside a raisin, you will see:

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For nearly 60 years, the Big Bang has reigned supreme as our most successful theory for explaining our cosmic origins. Beginning from a hot, dense, matter-and-radiation-rich state, the Universe has expanded, cooled, and gravitated ever since. As it evolved, it formed protons and neutrons, the first light elements, stable atoms, and eventually, stars, galaxies, planets, and complex chemistry capable of giving rise to life. Some 13.8 billion years after it all began, here we are, observing the still-expanding Universe and working to figure out exactly where it all came from and how it came to be the way it is today.

But if the Universe has been expanding for all this time, what is it that the Universe is expanding into? That’s a common question that people have, and this week, it was asked by James Nastelli, who simply wants to know:

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

The short, one-word answer to this question is as pithy as it is unsatisfying: itself. The Universe expands into itself, rather than into any definable “outside” medium. This is another example where the science of General Relativity defies our common experience and intuition, so let’s see if we can help you understand it a little bit better by unpacking the science behind the expanding Universe.

Hubble discovery cepheid andromeda

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)

There were two pieces of observational evidence that came in at about the same time: in the early part of the 20th century. First, astronomer Vesto Slipher had been observing a variety of nebulae in the sky, leveraging a novel technique (at the time) known as spectroscopy. By breaking up the light from these objects into their various wavelengths, Slipher noted that many of them had the same spectral lines we were familiar with from the known elements, but were systematically shifted: like something had stretched them to longer wavelengths. One possible explanation was that of a Doppler shift, as though these objects were moving away from us.

Second, starting in 1923, Edwin Hubble (with his assistant, Milton Humason) began — quite by accident — identifying individual stars in these spiral and elliptical nebulae. Originally seeking to observe novae, Hubble realized that some of these “flares” were actually periodic, brightening-and-faintening variable stars. By knowing the nature of these objects and observing how bright they appeared, the distance to them could be determined.

Lo and behold, when you combined these two pieces of information, a new picture emerged: one where, on average, the farther away a galaxy was from us, the greater its light appeared to be shifted toward the redder, longer-wavelength part of the spectrum.

hubble plot expanding universe

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

If this discovery had been made before Einstein’s General Relativity had come along, perhaps we would have resorted to a number of “alternative” explanations for why this was occurring.

  • Perhaps there was some sort of “lossy medium” in space as light traveled through it, causing the light to lose energy as it propagated through the Universe.
  • Perhaps there was some sort of intergalactic matter present, extincting the light, particularly at shorter wavelengths.
  • Perhaps these galaxies were all closer together in the past, and the ones that moved faster relative to each other now wound up farther away, as higher speeds, over time, lead to greater distances.

But that wasn’t the order of events, at least, as far as our understanding of physics was concerned. We had already confirmed the predictions of General Relativity in spectacular fashion during the total solar eclipse of 1919, and had already realized that “space” and “time” were not absolute quantities in our Universe, but were woven together into the fabric of spacetime.

Putting all of these things together meant that, once we realized that the Universe was filled with matter and other forms of energy all throughout it, we couldn’t have a static-and-stable Universe. The Universe, instead, must be either expanding or contracting.

static spacetime grid cartesian

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

The observations pretty clearly indicated that it was expansion, not contraction, that was occurring; as is often the case in physics, when the equations reveal that there are multiple possible solutions, you have to look to the actual Universe itself to pick out which solution corresponds to your physical reality. Although, in theory, the Universe could have been either expanding or contracting, expansion is what it turns out it’s actually doing.

There are two common analogies we use to make physical sense of the expanding Universe, although each one has its limitations.

One is to treat the Universe as a balloon, and specifically, as a balloon with coins glued (or otherwise affixed) onto its surface. The expanding Universe, then, is like inflating (or blowing up) this balloon, and the coins on the surface of the balloon are analogous to the galaxies all throughout space. If you yourself are living in a galaxy — inside one of these coins — then as the Universe expands, you’ll see all the coins moving away from one another. The coins nearby you recede relatively slowly; the coins farther away appear to recede more and more quickly. It isn’t that the coins are “moving” relative to the fabric of space, but rather that the expansion of space drives them farther and farther apart with time.

balloon expanding Universe

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, and why photons have their wavelengths lengthen as the Universe expands.
Credit: E. Siegel/Beyond the Galaxy

But this analogy has its flaws, for certain. The problem is that a balloon’s surface is only two-dimensional, while our Universe is three-dimensional. A balloon inflates because someone or something — usually a human — is blowing air into it in an extra spatial dimension that is unknown to inhabitants on the balloon’s surface. And the balloon, indeed, is expanding into that (extra) third dimension, whereas, in our Universe, we have no evidence indicating the presence of a fourth (or higher) extra spatial dimension. You have to restrict yourself to simply thinking about the surface of the balloon and you must ignore the “inside” of the balloon or the forces that actually cause it to inflate.

A slightly better analogy, therefore, is to consider a fully three-dimensional object that expands: like a ball of raisin bread dough, i.e., bread dough with raisins distributed all throughout it. As the dough leavens, it expands, but the raisins within it don’t. The raisins simply move farther away from one another in all three dimensions. If you’re within a raisin, you see:

  • nearby raisins recede slowly,
  • intermediate-distance raisins receding more rapidly,
  • and the most distant raisins receding most quickly of all,

even though the raisins are actually stationary relative to the expanding dough. If the dough were transparent, it would behave like the fabric of space, while the raisins behave like individual galaxies within the expanding Universe.

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. Credit: NASA/WMAP Science Team

This comparison is somewhat better since it addresses the proper number of dimensions and does not rely on the presence of any “extra” dimensions, nor does it rely on an additional, external push to “blow up” or “expand” the Universe.

However, there are still a few flaws:

  • there’s a limit to how much the dough can expand, governed by the biochemical reactions of yeast cells and how they convert sugars into alcohols and gas,
  • the dough itself changes in properties as it expands, becoming more rarified and lower in mass density,
  • and the dough still expands into something: into the air (or empty space) surrounding it at its boundary.

The last statement, about the dough developing into “something,” presents the most challenging connection with the expanding Universe. Whereas the bread is a thing embedded in a broader reality (the entirety of three-dimensional space), the Universe is simply all that exists. At the absolute least, that is all that is required.

Instead of imagining a ball of dough (embedded with raisins) in our three-dimensional Universe, picture a three-dimensional Universe completely 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, communicable and observable, one where everything is observable but unreachable and incommunicable, 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. On scales of ~10 billion light-years and larger, the Universe is almost perfectly uniform. Credit: Andrew Z. Colvin/Wikimedia Commons; annotations: E. Siegel

We have every reason to believe that our visible Universe—whose cosmic horizon is determined by a combination of the expansion rate, the speed of light, and the period of time since the Big Bang—is merely a small fraction of a bigger, more complete unobservable Universe. The quantity of light that we can see and access has grown throughout time and will continue to do so, since light that was released long ago and is already on its way to us will ultimately arrive for the first time, despite the ongoing expansion of space. Given enough time, more than double the current volume of “Universe” will become visible to humans.

And it is feasible, though no one is positive, that the unobservable Universe is actually boundless in scope. If this were the case, asking “What’s it expanding into?” would be pointless because it is already limitless, spanning every cubic inch of “space” possible. Think about the following questions:

  • What’s infinity plus infinity? It’s still infinity.
  • What do you get if you double, triple, or quadruple infinity? It’s still infinity.
  • And what happens if you multiply infinity by infinity, even if you do it in multiple dimensions? You still only get infinity.

That’s what the expanding Universe is like if it’s truly infinite, and that’s something that even many non-physicists (or non-mathematicians) can see for themselves.

While many independent Universes are predicted to be created in an inflating spacetime, inflation never ends everywhere at once, but rather only in distinct, independent areas separated by space that continues to inflate. This is where the scientific motivation for a Multiverse comes from, and why no two Universes will ever collide. The Universe doesn’t expand into anything; it itself is expanding. Credit: Ozytive/Public Domain

Now, there is no certainty that the Universe is limitless, or that the “unobservable Universe” encompasses everything out there. It’s feasible — as the theory of cosmic inflation strongly predicts — that our Universe, no matter how huge it is, is merely one place where the Big Bang occurred. Outside of that region is more space where inflation hasn’t finished but continues on indefinitely (perhaps for an eternity), punctuated by other pockets where inflation ended and Big Bangs occurred. This is usually referred to as a multiverse, and no two “baby Universes” ever meet, collide, or overlap.

Does this imply that our universe is growing toward the inflationary multiverse?

The answer, maybe unsurprisingly, is no. The primary distinction between a balloon, a ball of dough, and our actual Universe is not what is within or outside of it, nor whether it has limits. When we refer about space, spacetime, or the fabric of the Universe, we mean everything: it’s the stage on which the Universe’s performance takes place. There are rules, of course.

  • the players are the particles and antiparticles and other quanta of energy,
  • the governing rules are the laws of physics and the values of the fundamental constants,
  • and the stage itself is not fixed, but rather is an evolving entity,

but there’s no “outside” or “beyond” to the cosmos; it is simply all there is and ever was and ever will be.

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. Credit: Rob Knop

And it is the most difficult concept to comprehend. The expanding Universe does not grow into anything since there is nothing above, beyond, outside, before, or beyond space-and-time. It is both everything and nothing at the same time: everything since it encompasses everything that exists, and nothing because even if all of the matter, radiation, and quanta that exist inside it are removed, the emptiness of empty space remains. That is what the fabric of spacetime is, and it cannot extend “into” anything since it is a basic component of our reality.

That’s why the greatest response to the question “What is the Universe expanding into?” is “itself.” The crucial understanding is to cease thinking of the Universe as something that evolves in some broader, grander context; it’s completely logical to conceive of it as everything, and to simply recognize that expansion and contraction are intrinsic in space itself.

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. Credit: Public Domain/retrieved from Fermilab Today

It is still plausible that there is some higher-dimensional viewpoint into which our Universe is expanding, similar to how a two-dimensional surface of a balloon expands in three dimensions, although there is no evidence to support this theory. It’s merely one exciting option that can’t be ruled out right now. But that isn’t essential; the Universe doesn’t need anything else to grow into. It can just exist and get less dense on its own.

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