Ask Ethan: What drives the expansion of the Universe?

Even though it’s been nearly 100 years since its initial discovery, the expanding Universe still puzzles almost everyone who thinks about it. What causes the Universe to expand? Why did it start off expanding in the first place? What determines the rate of expansion, and how does that rate translate into something that we can actually go and observe? What do we mean when we say “the expansion is accelerating,” and why is that such a profound, revolutionary statement that we only started making in the 1990s? And, behind the scenes, what is the ultimate cause of each aspect of our cosmic expansion, and how confidently can we state such things?

There’s a remarkable scientific story to be told here, and even seasoned scientists and science communicators frequently make mistakes when trying to provide answers to these questions. It makes educating the general public, especially young people, a particular challenge for father-of-a-curious-13-year-old, Philip Gee, who writes in to ask:

“I read everything you write, absorb 5% and talk to my kids like I’m an expert haha… I so often read this sort of thing:

‘Well, our cosmological model predicts an expanding universe, and as a consequence, the existence of an event that we call Hot Big Bang. Yet, the current state of the expansion is not constant in time, instead is increasing; thus this growing rate in the expansion has to be driven by a different factor, something that wasn’t predominantly acting during the early stages of the Universe or at times where galaxies formed.’

The expansion of space isn’t accelerating right? Well, it is, but not because the Hubble Constant is increasing? Shouldn’t we just be saying: ‘the Hubble Constant, which is a measure of expanding space over time, has to be driven by something?'”

Let’s break down and answer the different parts of this question, because even though it’s complicated, we can come to an understanding by unpacking it one step at a time. Let’s check it out, one step at a time.

A photo of Ethan Siegel at the American Astronomical Society’s hyperwall in 2017, along with the first Friedmann equation at right. The first Friedmann equation details the Hubble expansion rate squared on the left hand side, which governs the evolution of spacetime. The right side includes all the different forms of matter and energy, along with spatial curvature (in the final term), which determines how the Universe evolves in the future. This has been called the most important equation in all of cosmology and was derived by Friedmann in essentially its modern form back in 1922.

The theoretical background

Imagine yourself as an astrophysicist just over 100 years ago. Einstein has just published his theory of general relativity, and with the next well-measured solar eclipse, it’s validated, as light bends and deflects according to his theory’s predictions, not Newton’s. Then, you come along and want to apply these equations to the entire Universe, in an attempt to figure out what sort of predictions it has in store.

This is exactly what Soviet scientist Alexander Friedmann did back in 1922, approximating the Universe as any spacetime that’s (roughly) uniformly filled with matter, radiation, and any other form of energy you can imagine.

Not only did he derive what many have called the most important equation(s) in cosmology, but he showed that any spacetime that was:

• uniformly filled with matter, radiation, and/or any other species of energy,
• and that possessed the same physical properties in all directions in three-dimensional space,

could not be both static and stable; it must either expand or contract. Furthermore, the rate of expansion or contraction would be determined exclusively by the combination of the energy densities of (the sum of) all the different species of energy present within the Universe, as well as by the spatial curvature of the Universe. That profound realization, borne out over the past 101 years in the form of the Friedmann equations, has been a cornerstone of physical cosmology ever since.

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.

Early observations and the discovery of cosmic expansion

The evidence for cosmic expansion began with three observations:

1. Henrietta Leavitt’s discovery of the period-luminosity relation for Cepheid variable stars. Just by measuring how long it takes one of these stars to go from bright-to-faint-to-bright again, you can know how intrinsically bright it is. Then, by measuring its apparent brightness, you can infer how distant it is from you, enabling you to measure cosmic distances wherever you can identify and measure these variable stars.
2. Vesto Slipher’s discovery and measurement of the spectral line shifts of these spiral and elliptical “nebulae” in the skies. Whereas stars and other objects within the Milky Way appear to be moving — and hence, have their emission and absorption lines shifted dependent on their relative motion with respect to ourselves — at tens or even a couple hundred km/s relative to us, these objects moved at thousands of km/s, and were almost all “redshifted,” corresponding to motion away from us.
3. And finally, Edwin Hubble (and his assistant, Milton Humason) measured Cepheids in those same spiral and elliptical nebulae, measuring their distances and confirming their extragalactic nature.

When you combine “How far away are these objects?” with “How quickly do we see these objects appearing to recede from us?” and put them on the same graph, you find precisely what Friedmann had predicted: there’s a direct relationship between the two. It could be ignored no longer: the Universe was expanding.

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.

The big question for 20th century cosmology… and a surprising answer

But how quickly was the Universe expanding, and moreover, how was the expansion rate changing over time? Throughout the 20th century, it was often noted that cosmology was a hunt to measure two parameters:

1. H₀, or the Hubble parameter today, which tells us how fast the Universe is expanding right now: at present.
2. q₀, sometimes called “the deceleration parameter,” which is a measure of how the Hubble parameter is changing with time.

Once we came to realize that the hot Big Bang describes the early stages of our Universe, we quickly understood that the expanding Universe was a race: between the initial expansion rate, which worked to drive everything apart, and the gravitational effects of all the matter and energy within our Universe, which worked to bring everything back together. Depending on, relative to the initial expansion rate, there was more matter-and-radiation, less matter-and-radiation, or exactly the same amount of matter-and-radiation as some particular critical value would tell us something no less profound than the ultimate fate of the Universe.

• More matter-and-radiation than expansion: in this scenario, the Universe expands for a time, but gravitation not only slows that expansion down, but eventually overcomes it. Things reach a maximum size/separation, then expansion stops and reverses, and everything eventually recollapses, with our Universe ultimately ending in a Big Crunch.
• Less matter-and-radiation than expansion: in this scenario, the Universe expands and gravitation works to slow it down, but never stops it entirely. The Universe keeps expanding, forever and ever, with only isolated, gravitationally bound clumps persisting within it. This Universe eventually ends in a “Big Freeze” fate.
• Exactly enough matter-and-radiation to balance the expansion: in this final, balanced on a knife’s edge scenario, there’s exactly enough matter-and-radiation to slow the initial expansion down and cause it to approach, but never quite reach zero. If there were one more atom in this Universe, it would recollapse, but instead it just coasts forever.

The expected fates of the Universe (top three illustrations) all correspond to a Universe where the matter and energy combined fight against the initial expansion rate. In our observed Universe, a cosmic acceleration is caused by some type of dark energy, which is hitherto unexplained. All of these Universes are governed by the Friedmann equations, which relate the expansion of the Universe to the various types of matter and energy present within it.

It took many decades to finally reveal what the Universe was actually doing, and to practically everyone’s surprise, the answer was none of these scenarios matched the data. Instead, when we measured the Universe’s expansion history as a function of time, we found that the “deceleration parameter,” q₀, was actually NEGATIVE, meaning that the Universe wasn’t decelerating right now, but was accelerating instead!

In all three of the above scenarios, if you were to start off at one galaxy and measure how fast a distant galaxy receded from you over time, you would find that its recession speed started off fast, and then slowed down over time. The rate at which it slowed down would tell you which scenario described your Universe, and would allow you to infer your Universe’s fate and, ideally, its composition as well.

But what the observations showed, instead, was that if you were to measure a distant galaxy’s recession over time, it would have started off fast, then slowed down for a time, and then, about ~6 billion years ago, stopped slowing down and started speeding up again. The deceleration parameter, q₀, was positive for those first ~7.8 billion years of cosmic history, but then switched signs, passing through 0, and has been negative ever since.

The far distant fates of the Universe offer a number of possibilities, but if dark energy is truly a constant, as the data indicates, it will continue to follow the red curve, leading to the long-term scenario frequently described on Starts With A Bang: of the eventual heat death of the Universe. If dark energy can strengthen, weaken, or reverse sign over time, however, all bets are off, and alternative possibilities suddenly abound.

Our consensus model of what’s happening today

How did we get it so wrong, for nearly all of the 20th century? It was because of our erroneous underlying assumptions.

We had assumed that the Universe started off expanding at a certain rate, and then everything within the Universe would work to gravitationally bring it all back together again. That’s because we had assumed that everything within the Universe was (or behaved as) matter and radiation, including:

• normal, atom-based matter,
• black holes,
• photons and all forms of light,
• gravitational waves,
• neutrinos,
• and dark matter, whatever it may be ultimately composed of.

What we most often didn’t consider — at least, not until the 1990s came around — is that there might be some exotic form of energy that didn’t get less dense as the Universe expanded. But such a thing is clearly possible: there could be energy inherent to the fabric of space itself. Two possible and compelling theoretical origins for this include:

1. Einstein’s cosmological constant, which can be written down in any spacetime,
2. and the zero-point energy of the quantum vacuum, which may not be zero, but which might instead have a positive, non-zero value everywhere.

Both of these explanations are still 100% valid and consistent with all of the data, and the phenomenon of the accelerated expansion of the Universe is most typically described as being driven by some form of dark energy, which is the generic term for any species of energy that would lead to the types of cosmic accelerations we observe.

Various components of and contributors to the Universe’s energy density, and when they might dominate. Note that radiation is dominant over matter for roughly the first 9,000 years, then matter dominates, and finally, a cosmological constant emerges. (The others, like cosmic strings and domain walls, do not appear to exist in appreciable amounts.) However, dark energy may not be a cosmological constant, exactly, but may still vary with time by up to ~4% or so. Future observations will constrain this further.

What’s accelerating, what isn’t, and what does it mean?

This is where the biggest points of confusion often arise: when it comes to the question of what it is, exactly, that’s accelerating.

In cosmology, we normally talk about the expansion rate as being described by the Hubble parameter: H, or the Hubble parameter today, H₀. This is normally expressed and measured in units of km/s/Mpc, which is to say that a distant object recedes as though it has a recession speed of a certain amount (a certain value of km/s) for every megaparsec (Mpc, or about 3.26 million light-years) of distance it has from us today.

This value — the expansion rate — does not accelerate (or increase), not even with dark energy. Without dark energy, it always drops to approach zero (and reverses in the “Big Crunch” scenarios), but with dark energy, it only drops to and approaches some finite, positive, non-zero value. According to our best measurements, the current expansion rate is around 70 km/s/Mpc, but will someday drop to around 45 km/s/Mpc, but no lower, in the Universe we inhabit. The expansion itself is accelerating, but that doesn’t mean the expansion rate is accelerating. Ever since the hot Big Bang, it’s been decreasing, and it’s still decreasing today; the fact that the Universe is accelerating just tells us that the final, ultimate value that it approaches won’t be zero, but a positive, greater-than-zero value.

The relative importance of different energy components in the Universe at various times in the past. Dark matter has been largely important for extremely long cosmic times, and we can see its signatures in even the Universe’s earliest signals. Meanwhile, radiation was dominant for the first ~10,000 years of the Universe after the Big Bang. Note that in the future, when dark energy reaches a number near 100%, the energy density of the Universe (and, therefore, the expansion rate) will remain constant arbitrarily far ahead in time.

What is accelerating, though, is the recession speed that you measure for any individual object within the expanding Universe. If a distant galaxy, today, is located about 1 billion light-years (around 300 Mpc) away, then it’s receding at around 21,000 km/s. At some point in the future, it will be twice as far away: 2 billion light-years (around 600 Mpc), and when it is, even though the expansion rate will have dropped by a little (maybe to 60 km/s/Mpc), it will be receding at a faster rate of around 36,000 km/s. In the even more distant future, it will reach a distance of around 21.7 billion light-years (6,667 Mpc), and even though the expansion rate will now be at its minimal value of ~45 km/s/Mpc, this object will now be receding at 300,000 km/s: greater than the speed of light.

This implies that there’s only a limited amount of time we have — or that anyone, anywhere has — to reach a distant galaxy that’s not bound to the same galaxy group-or-cluster that they are. As objects get pushed to greater and greater distances, their recession speed will appear to progressively increase with no upper bound, surpassing even the speed of light at some point. Once that occurs, no signal, spaceship, or message that is sent off can ever reach that destination, implying that there’s a “reachability” limit as well as a “visibility” limit for every object in the Universe beyond one’s own Local Group.

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.

And finally, what’s ultimately driving the expansion of the Universe?

So what, then, is the ultimate cause of the Universe’s expansion? It turns out that there are two things responsible, that a large number of things that we previously thought could be responsible aren’t, and that the two things that are responsible are only possibly related to one another: the initial expansion and the onset of dark energy. We have to consider both independently, and then, and only then, the possibility that they might be related.

Where did the initial expansion rate, the rate at which the Universe began expanding at the start of the hot Big Bang, come from?

This arises from the end of cosmic inflation: the period that preceded and set up the hot Big Bang. During inflation, the Universe was expanding relentlessly — at a constant rate — as though there were a large amount of energy inherent to space itself during this epoch. With every ~10^-35 seconds or so that elapsed, the Universe would double in size in all three dimensions: in length, in width, and in depth. The energy density of space would remain constant, even as this expansion constantly created new space. When inflation came to an end, practically all of this energy was transformed into matter-and-radiation, with the matter-and-energy density at that time determining the expansion rate. That’s why the Universe, our Universe, began expanding so rapidly right at the start of the hot Big Bang, and also why the expansion rate and the matter-and-energy densities balanced out so perfectly.

While matter and radiation become less dense as the Universe expands owing to its increasing volume, dark energy is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant.

For billions of years, as the matter and radiation densities dropped, the expansion rate dropped along with it: in direct proportion to the square root of the overall energy density, just as Friedmann’s equations predicted. And then, those densities dropped by a great enough amount so that a new form of energy began to influence the expansion rate: dark energy, which behaves indistinguishably from either

• a cosmological constant,
• the zero-point energy of space,
• or to energy inherent to the fabric of space itself.

The value of this energy density is incredibly small: a factor of ~10²⁵ smaller than it was during inflation, but its presence at all makes it inevitable that it will eventually come to dominate the expansion of the Universe. It only took several billions of years, and now here we are: living in a Universe dominated by dark energy, where it’s responsible for driving the expansion rate.

Many other things could have driven the expansion of the Universe: spatial curvature, topological defects, exotic forms of energy, etc. Yet it appears that, other than a period where the expansion rate and the matter-and-radiation densities were balanced, it’s always been some form of energy that behaves like it’s inherent to space that drives our cosmic expansion. It brings up a speculative but tantalizing possibility: that the early period of inflation and the current period of dark energy-domination are related. It’s possible, but no one knows how — or even whether — there’s a relationship at all. We know these things exist and observe their effects, but an underlying explanation for “how” or “why” still eludes us at present. Perhaps some young, creative, ambitious person reading this right now will be the one to discover the answers!

Send in your Ask Ethan questions to startswithabang at gmail dot com!