Ask Ethan: Is dark energy just leftover momentum from the Big Bang?

Ever since the late 1990s, astrophysics has had a big puzzle to reckon with: one that still remains unsolved. When we try to solve Einstein’s field equations for General Relativity as they apply to our actual Universe — our Universe that, on large scales, is both isotropic, or the same in all directions, and homogeneous, or the same in all locations — we find that there’s a specific relationship between two things:

• the rate of the expansion of space, as well as how that rate changes over time,
• and the full suite of “stuff” that’s present within the Universe: matter, radiation, and any and all other forms of energy.

It was only in the late 1990s that we finally measured the expansion rate, and its change over time, well enough to conclude what type of “stuff” was present in the Universe, and what we found was shocking. While about ~30% of the Universe could be in the form of matter, the majority of it, around ~70%, was neither consistent with matter nor radiation. It needed to behave very differently: as a new form of energy.

In all the time since, we’ve called it “dark energy” and recognize that it’s what drives and dominates the expansion of the Universe today, just as it has for the past six billion years. But must it truly exist? That’s the question of Alan Finkel, who asks:

“How do we know dark energy exists as the thing driving expansion of the universe, and that it isn’t just the leftover momentum of everything blowing apart during the hot big bang?”

This is a great question: one that’s deep, and compels us to look at the foundations of the expanding Universe. Let’s find out together!

A mural of the Einstein field equations, with an illustration of light bending around the eclipsed Sun: the key observations that first validated general relativity four years after it was first theoretically put forth: back in 1919. The Einstein tensor is shown decomposed, at left, into the Ricci tensor and Ricci scalar, with the cosmological constant term added in after that. If that constant weren’t included, an expanding (or collapsing) Universe would have been an inevitable consequence.

What we first need to know is what this key, underlying relationship is for our Universe: the one between the rate of expansion and the “stuff” that’s present within the Universe. This goes all the way back to the fundamental relationship that Einstein first put forth within General Relativity: the relationship between mass-and-energy, on one side, and the curvature and evolution of spacetime itself, on the other side. That’s the key relationship encoded in the Einstein Field Equations: the equation(s) shown above. On the left-hand side is what’s known as the Einstein tensor, which is determined by the matter and energy in the Universe (as well as how it’s distributed), decomposed into its three allowable parts: the Ricci tensor, the Ricci scalar, and a cosmological constant. On the right-hand side is the stress-energy tensor, which details the curvature and evolution of spacetime.

To paraphrase the famed relativist John Wheeler, who described this relationship years later: mass (and other forms of energy) tell spacetime how to curve (and expand/contract), and then that curved (and expanding/contracting) space tells mass (and other forms of energy) how to move and evolve. Originally, Einstein and his contemporaries only considered the solutions for simple configurations of matter and energy: point masses, empty Universes, a Universe with a cosmological constant alone, etc.

But after only a few years, a new type of solution was found: that for a Universe that was uniformly filled with matter, or radiation, or some other form of energy, or some combination of these forms of energy.

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, an exact solution in general relativity, 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.

This solution — or what the Einstein equations tell you is happening to a Universe with this particular set of mass/energy conditions and distributions — was first derived way back in 1922: by Soviet physicist Alexander Friedmann. What he found was something remarkable and unexpected: that if you had a Universe that was uniformly filled with matter, radiation, or any form of energy at all, that it absolutely could not be both static and stable over time. Instead, either:

• the gravitation from all the “stuff” in the Universe will cause it to contract,
• some type of initial expansion will be opposed by gravitation, causing the Universe to expand initially but to do so more slowly (and potentially reverse) over time,
• or some exotic form of energy, such as a cosmological constant (or a different form of dark energy), actually drives the Universe to not only expand forever, but for distant objects to recede faster and faster over time.

Although the idea of a non-static Universe was initially rejected by Einstein (and many of his contemporaries), the decisive data began to pour in throughout the 1920s and 1930s. We discovered that the spiral and elliptical nebulae in the skies were actually galaxies far beyond the extent of the Milky Way, and that (on average) the farther away a galaxy was from us, the faster it appeared to recede from us. With this data, combined with Friedmann’s equations, we couldn’t escape the conclusion: 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.

With this picture as our new foundation:

• the Universe was uniform on the largest of cosmic scales,
• was expanding,
• and was governed by Einstein’s General Relativity as our law of gravity,

the next question became one about our cosmic origins. After all, if we want to know “how our Universe got to be the way it is today,” then we need to be able to answer the question of, “what was our Universe like long ago?” Many ideas initially abounded, including that of an empty Universe, one where light from distant objects loses energy and gets “tired” in some sense, one where the Universe wasn’t just homogeneous in space but unchanging in time as well, and one where the Universe was large and sparse today because it arose from a state that was smaller, denser, and hotter in the past.

This final scenario, as worked out in the 1940s and thereafter, became known as the Big Bang, and predicted a total of three new cornerstones in addition to the already-observed expanding Universe. Specifically, it predicted that:

1. the Universe gravitates, clumps, and clusters together over time, meaning that the structures in it should be younger, smaller, and less evolved the farther away (i.e., the deeper into the distant past) we look,
2. the Universe transitioned from a hotter, denser state of an ionized plasma to one where neutral atoms could stably form, leaving a near-uniform background of relic radiation behind (i.e., the CMB) at just a few degrees above absolute zero,
3. and that the Universe, even earlier, began fusing bare protons and neutrons into atomic nuclei, leading to a population of pristine elements that existed even before any stars could form within it.

The second prediction was the one that was validated first: back in the mid-1960s. (All three have since been robustly confirmed.) In all the time since, the Big Bang has reigned supreme as the story of our Universe’s ancient past.

According to the original observations of Penzias and Wilson, the galactic plane emitted some astrophysical sources of radiation (center), but above and below, all that remained was a near-perfect, uniform background of radiation. The temperature and spectrum of this radiation has now been measured, and the agreement with the Big Bang’s predictions are extraordinary. If we could see microwave light with our eyes, the entire night sky would look like the green oval shown.

But what, exactly, is the Universe made out of?

That was the big question facing cosmology for the second half of the 20th century: to learn the answer to that question. It’s not necessarily the easiest, most straightforward thing to measure directly. Sure, we know how to measure normal matter, and we know how to measure radiation, but what if there are other species of energy in the Universe? What if there was some type of unseen matter or radiation: dark matter or dark radiation? What if there were exotic forms of energy: cosmic strings, domain walls, or other topological defects? What if there were new fields or other forms of energy that influenced our Universe? How could we tell?

Going all the way back to Friedmann’s work, we can find that the answer was put forth way back in 1922. If you want to know what the Universe is made out of, you just use the relationship between how the Universe expands, and how that expansion changes over time, and all the different types of matter and energy combined that exist within the Universe. Specifically, if you can measure:

• what the expansion rate of the Universe is today, at the present time,
• and how the expansion rate has evolved over cosmic history,

then you can infer what all the different types of matter and energy are in the Universe throughout its history, including right up to the present day.

A plot of the apparent expansion rate (y-axis) vs. distance (x-axis) is consistent with a Universe that expanded faster in the past, but where distant galaxies are accelerating in their recession today. This is a modern version of, extending thousands of times farther than, Hubble’s original work. Note the fact that the points do not form a straight line, indicating the expansion rate’s change over time. The fact that the Universe follows the curve it does is indicative of the presence, and late-time dominance, of dark energy.

So that became what astronomers, astrophysicists, and cosmologists set out to do: to measure both the expansion rate of the Universe at present — known as the Hubble constant — and to measure what the expansion rate was in the distant past: to determine how the Universe’s expansion rate has changed over time. The way to do this, observationally, was to find objects that simultaneously:

• were very far away, so that it took light billions of years to journey through the expanding Universe before reaching our eyes (in order to be able to see those objects as they were long ago, back when the Universe was younger),
• and for which we had a way of knowing an intrinsic property (something that’s inherent to the object itself) about them, such as their intrinsic brightness or intrinsic size.

If you know how intrinsically bright (or large) an object is, then when you measure how bright (or large) it appears through your instruments, you can then infer how far away that object is from you. If you can also measure how severely the light emitted from that object has been shifted due to the Universe’s expansion — i.e., the light’s redshift — then you can measure how much the Universe has expanded since that object’s light was emitted.

There are many such objects whose intrinsic properties, like brightness, we can know and use to determine their distances: RR Lyrae stars, Cepheid variable stars, certain red giant and asymptotic giant branch stars, rotating galaxies, galaxies whose surface brightness fluctuates, etc. But the brightest such object, and hence the ones that we can observe at the greatest distances, are type Ia supernovae, or white dwarfs that explode in a brilliant cataclysm.

This animation of the double detonation scenario shows two white dwarfs in close orbit around one another. When material accumulates onto one member, it can cause a surface thermonuclear reaction, which can then propagate around the star until it triggers a core detonation. This scenario could be responsible for up to 100% of observed type Ia supernovae.

Back in the late 1990s, scientists on two independent, competing teams — the high-Z supernova search team and the supernova cosmology project — were the first to point out something incredible. Based on the type Ia supernova data they had collected, they determined that the expansion history of the Universe couldn’t be described by a Universe containing matter and radiation alone. Even if you added in dark matter, there was still an unexplained puzzle: the light from the most distant supernovae seen was too faint, and the inferred distances to them was too great, to be reconciled with a matter-only Universe. Even if you allowed for large amounts of spatial curvature, that still didn’t fit the data very well.

Instead, both teams proposed, there must be some new kind of energy present in the Universe: one that didn’t cause the expansion rate to continue to slow over time, but rather one that caused the apparent recession speeds of any distant-enough galaxy to increase over time. This is where the idea of the accelerating expansion of the Universe came from, and the evidence for it has only strengthened over time.

But this wasn’t always the case. In the early days of dark energy, there was a lot of skepticism about these results, and whether there could be another explanation for them. In fact, the default position was actually that of our question-asker this week: that the only “expansion” that’s occurring today is the leftover “coasting” of expanding space from the Big Bang all those billions of years ago, with the gravitational effect of matter working to slow that expansion down.

Measuring back in time and distance (to the left of “today”) can inform how the Universe will evolve and accelerate/decelerate far into the future. By linking the expansion rate to the matter-and-energy contents of the Universe and measuring the expansion rate, we can come up with an estimate for the amount of time that’s passed since the start of the hot Big Bang. The supernova data in the late 1990s was the first set of data to indicate that we lived in a dark energy-rich Universe, rather than a matter-and-radiation dominated one; the data points, to the left of “today,” clearly drift from the standard “decelerating” scenario that had held sway through most of the 20th century.

Assuming that was the case, a number of ideas came out to attempt to explain these supernova observations.

• People wondered whether the supernovae themselves evolved over time, and whether modern supernovae were inherently brighter than their more ancient counterpart. They studied these supernovae in depth, and determined that no, they were indistinguishable.
• They brought up the idea that maybe supernovae didn’t evolve, but the environments they were in could have been different. Maybe they were dustier in the past. It was only with the advent of multiwavelength studies — studies that showed the same faintening of distant supernovae at all wavelengths — that we learned that dust in the environments of these supernovae couldn’t be to blame.
• People wondered if something novel, like photon-axion oscillations (where the light from distant supernova was “switching” into a species of dark matter), could cause these distant supernovae to appear fainter. But observations of evermore distant supernovae showed that the oscillation scenario was ruled out.
• And finally, people wondered if some new, color-independent (or gray) dust, could be faintening these distant supernovae. It wasn’t until 2004 that enough distant type Ia supernovae had been observe that these “gray dust” scenarios could be ruled out as well.

This is part of the power of how science works: you look at all the possible explanations, regardless of how you feel about them, and subject them to impartial, robust scientific scrutiny. If you do it right, the last idea left standing will be the only one that’s viable.

This graph shows the type Ia supernova data from the high-z supernova search team in 2004, compared with the dashed-line of a dark-energy rich Universe, or models with gray dust (dot-dash line) or evolving supernovae (yellow line). It was with this study that the notion of gray dust could be experimentally ruled out.

By 2005, we had other, independent lines of evidence — such as from the large-scale structure of the Universe and from the cosmic microwave background — that even if you ignored the supernova data entirely, there would still be very compelling evidence that was not only in favor of the existence of dark energy, but that showed that dark energy must be some new kind of energy that caused the recession speed of distant-enough objects to speed up over time. In other words, the Universe’s expansion really was accelerating, and there were multiple, independent lines of evidence that showed this to be the case.

This, after all, is how we answer questions about the Universe: by putting the question to the Universe itself, and listening to whatever it is that it tells us about the answer. It was already over 100 years ago that Max Planck issued his most famous quote:

“A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.”

It’s often paraphrased as “physics advances one funeral at a time,” but in the case of dark energy, it didn’t take that at all. The notion that there would be a fundamentally new form of energy in the Universe was almost unheard of idea in the literature in 1997, save for a few weakly suggestive studies in the 1980s and 1990s. Dark energy was introduced into the mainstream in 1998, and by the mid-2000s, there was no escape from it. Practically everyone working in the field had been convinced by an overwhelming, convincing suite of evidence.

Three different types of measurements, distant stars and galaxies (from supernovae), the large-scale structure of the Universe (from BAO), and the fluctuations in the CMB, tell us the expansion history of the Universe and its composition. Constraints on the total matter content (normal+dark, x-axis) and dark energy density (y-axis) from three independent sources: supernovae, the CMB (cosmic microwave background) and BAO (which is a wiggly feature seen in the correlations of large-scale structure).

Today, the situation is even more interesting, as in each of the three main lines of evidence describing the Universe’s evolution over time:

• from large-scale structure data, and particularly the baryon acoustic oscillation feature that appears in galaxy clustering data,
• from the CMB, and particularly the temperature and polarization fluctuation patterns that appear in the all-sky data,
• and from ultra-distant type Ia supernovae, of which there are now more than ~1500 in the largest data sets,

we see this overwhelming evidence for dark energy and an accelerating Universe. However, the exact rate of expansion, amount of dark energy vs. dark matter, and how effective dark energy is at accelerating the Universe, vary in a small but significant way between each of these data sets.

We can confidently take away from the full suite of data that dark energy is real, and that it represents the presence of an additional species of energy in our Universe: beyond normal matter, beyond radiation, beyond dark matter, and that it’s a special type of energy that causes the expansion to accelerate, not decelerate, over time. We can furthermore conclude that the expansion we observe today is due to more than just the leftover expansion of space from the early stages of the hot Big Bang; there is something “driving” it, and that something is what we call dark energy. But exactly what the nature of dark energy is, what its properties are, and whether it evolves over time are questions we’re all still working on figuring out.

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