Two of the greatest mysteries in cosmology are the nature of dark energy and the apparent
conflict in our measurements of the expansion rate of the early versus the modern universe
that even dark energy can’t account for.
Could both of these be explained by looking to a part of the universe that we’ve largely
ignored so far?
Could cosmic voids be driving the universe?
Einstein completed his general theory of relativity in 1915, and it didn’t take long for other
physicists to figure out some pretty astonishing stuff.
Maybe the most famous is how Karl Schwarzschild solved the Einstein equations to describe
the nature of spacetime near a compact mass and in the process found the equation for
black holes.
Not long after, Alexander Friedmann solved the Einstein equations to discover the nature
of spacetime on the scale of the entire universe.
Along with George Lemaitre, Friedmann’s solution showed that the universe must be
a dynamic place—either expanding or contracting.
The former as it turns out, as revealed soon after by Edwin Hubble’s observation of the
recession of the galaxies.
That expansion seemed to perfectly match the uniform expansion of space predicted by Friedmann
and Lemaitre’s model.
At first we thought that expansion had to be slowed down under the inward gravitational
pull of its contents.
But fast forward to the late 90s to the final piece of the puzzle when observations of supernovae
revealed that the universe is not just expanding, but that expansion is accelerating.
The discovery of dark energy was added into our cosmological model with a simple modification
to the Einstein equations—this lambda factor—the cosmological constant—ironically the same
anti-gravitational modification that Einstein had made as an erroneous attempt to thwart
Friedmann and Lemaitre’s prediction of a non-static universe.
But the addition of the cosmological constant completed our modern LCDM cosmology.
The CDM stands for cold dark matter—by far the largest source of inward-pulling gravity
and the main competitor to the outward-pushing dark energy.
LCDM seems to work very well.
When we use it to model the expansion of the universe and the formation of structure, our
predictions mostly match reality.
But they don't perfectly match.
We talked about some mild issues with Lambda CDM's prediction of the finer details of galaxy
formation.
But there’s a much bigger concern—LCDM may be failing to give the right expansion
rate of the universe.
When we measure that rate of expansion in the modern universe we get one number, expressed
as the Hubble constant.
But then we can also calculate what the current expansion rate should be by looking at the
early universe through the cosmic microwave background radiation and then calculating
how the expansion rate should have changed between the early and modern universe under
the LCDM model.
When we do that we get a pretty different value for the Hubble constant.
The modern universe appears to be expanding around 10% faster than it should be based
on LCDM, and that’s including the accelerating effect of dark energy.
This is the so-called Hubble tension, and it is actually making some people pretty tense.
It seems like one of two things must be true: either the Hubble constant measurement in
the modern universe is wrong, or the model used to extrapolate that constant from the
early universe is wrong.
LCDM could well be wrong.
And actually, either of these issues could have stemmed from one subtle assumption made
right back at the beginning of this story.
When Friedmann and Lemaitre solved the Einstein equations for the universe they had to make
some simplifications.
One was that matter is evenly distributed everywhere.
That’s largely true according to our observations—as long as you look to the largest scales.
But as you go to smaller scales this breaks down.
Just as a smooth surface of the Earth gets bumpier as you descend from space, first with
hills and mountains resolving, then finer structures all the way down to pebbles and
grains of dirt.
As you zoom in on the universe, the smooth sprinkling of galaxies reveals clumpiness—superclusters
of galaxies, clusters, then galaxies themselves.
LCDM doesn’t account for this lumpiness.
But according to some recent research, the lumpiness might have a huge effect.
It could explain the Hubble tension, or make it worse.
It may even explain away the existence of dark energy.
To understand the effect of cosmic lumpiness, let’s start with the local lump.
The Milky Way lives in something called the Laniakea supercluster.
It’s this rather beautiful confluence of around 100,000 galaxies stretching a half
a billion light years, all moving under a vast, mutual gravitational influence.
Unlike simple galaxy clusters, superclusters are not gravitationally bound.
They’re loose collections of galaxies and clusters of galaxies that will eventually
dissolve under the accelerating expansion of the universe.
It’s like gravity tried its best to form something this big, but at that scale dark
energy won the battle.
Nice try, gravity—but at least you slowed down nearby galaxies a little bit.
And because the outward velocities are slower than what you’d expect from pure expansion,
if we measure the Hubble constant based on Laniakea galaxies we should get a number that’s
too small.
But wait—the number we measure in the modern universe is even higher than predicted by
LCDM—the galaxies are moving away faster.
So does that means the Hubble tension is even worse than it seems?
Well, that was the finding of Leonardo Giani and collaborators.
They figured out how much lower our local measurement of the Hubble constant should
be based on the gravitational influence of Laniakea.
They found that the measurement should be a bit more than a percent too low due to the
supercluster’s influence.
That’s exciting because it makes it harder to explain away the Hubble tension as some
local effect of lumpiness, perhaps increasing the chance that we’re seeing interesting
physics in the dark energy.
By the way, Dr. Becky has a deeper dive into this paper that I’ll link below.
OK, let’s zoom a little further out.
So the Milky Way is in an overdense region on the scale of 500 million light years.
But looking further afield, Laniakea seems to form a higher density bump in a much bigger
underdense region.
It seems we’re in a cosmic void that forms a rough sphere around 2 billion light years
across.
This is the Local Hole, or the Keenan-Barger-Lenox Void, and the Milky Way is pretty close to
its center.
This underdense region should have exactly the opposite effect of an overdense region.
The gravitational pull for points inside such a region should be outwards towards all that
extra mass beyond its edge.
In this case the velocities we measure inside such a region should be higher than expected
for pure expansion.
You’d have the expansion velocity plus a bit more from the outward gravitational tug.
The effect of that would be to make the Hubble constant look bigger than it really is—perhaps
explaining away the Hubble tension and saving LCDM.
And there are indeed a couple of studies that claim just this.
Let’s look at this one: Shanks, Hogarth and Metcalf from 2019.
They combine a djustments in our distance measurements to Cepheid variables from the
Gaia satellite with measurements of outflow velocities within the Local Hole to calculate
that the real modern Hubble constant should be around 69 rather than 73.5, which would
put it in fair agreement with the values based on the cosmic microwave background and LCDM.
So does cosmic lumpiness fix or worsen the Hubble tension?
Are our measurements skewed more by Laniakea or the Local Hole?
And why is this so difficult to figure out?
Well, because it’s not easy to make an accurate 3-D atlas of positions and velocities of all the matter
out to these insane distances.
Remember, our giant telescopes only really see faint smudges of light on the night sky.
We have to infer distances from a chain of potentially biased steps, and we only measure
the component of these galaxies's velocities towards or away from us.
The astronomers trying to do this are very, very careful, but they don’t always agree with each other.
For example, a team including one of the discoverers of dark energy, Adam Reiss, claims that the
Local Hole is barely even there based on the atlas they made with supernova distance measurements.
And to be fair, it’s weird if the Local Hole exists at all.
It’s just stupidly large at two billion light years across.
Some argue that the standard LCDM model shouldn’t allow for lumps that big—at least not starting
with the miniscule lumps we see in the cosmic microwave background.
So maybe the hole is an illusion, or maybe its existence points to another issue with
LCDM.
And that is certainly the stance of the authors of a paper that came out just last month,
who argue that they can explain the local hole with a model that uses modified Newtonian
dynamics—a variant gravitational model that purports to also explain dark matter.
I should add that MOND has more and more points against it than for it.
And there’s another Dr. Becky link below for the latest on that.
OK, let’s zoom out one more time—now to the entire universe—to look at the effect
of lumpiness on the largest scales.
There’s at least one proposal for structure not just changing the apparent effect of dark
energy, but for structure being the entire cause of dark energy.
The idea, published by a team of Iranian scientists, is that dark energy is just the sum total
effect of all cosmic voids.
To see how THAT works let’s have a quick refresher on dark energy—and we have a whole
playlist if you want the gory details.
Dark energy is usually thought of as the energy of the vacuum—the faint buzz of space itself.
For complicated reasons, space having a non-zero energy density should cause accelerating
expansion.
In our boy Alexander Friedmann’s equations, dark energy ends up having the same physics
as a gas with negative pressure.
That’s an inward pulling pressure, which, counterintuitively, leads to accelerating
expansion if you fill the universe with the stuff so the inward-pulling aspect cancels
out.
But you know what else has negative pressure?
Bubbles.
Both bubbles and droplets occur when a pocket of fluid is constrained by a surface that
has a surface tension.
Surface tension results when the molecules of the surface are attracted to each other,
holding the bubble together and forming a sphere as they try to minimise surface area.
While the inside of the bubble may push outwards—so positive pressure—the surface resists that
push—which results in an effective negative pressure.
So maybe dark energy is due to space bubbles.
AKA cosmic voids.
In the early universe matter was spread out pretty smoothly.
Gravity started pulling matter into the first lumps that would become galaxy clusters.
The universe was expanding rapidly at the same time, throwing these dense regions apart.
Material that was still falling towards these regions pulled itself together into great
sheets and filaments flowing towards the clusters.
The result is the cosmic web—an interlocking network of clusters and intercluster filaments.
But all this clumping of matter left vast regions that are almost empty—cosmic voids.
The team behind this recent paper argue that you can treat these voids as growing bubbles
whose surfaces are made of these sheets and filaments of galaxies.
Those galaxies are really moving under the gravitational pull of each other, falling
towards the highest density regions, and they’re also moving apart due to the expansion of
space.
But you can also think of them as moving apart from each other due to the expansion of these
giant void bubbles.
They are sort of pushed apart against their mutual gravitational pull, which creates something
like a surface tension, and so functionally produces negative pressure.
The study in question argues that the resulting negative pressure of these void bubbles is
enough to explain all of dark energy.
Now I haven’t found a lot of commentary on this result and haven’t thought hard
enough about it myself to know whether there’s merit here.
But in general it’s a good idea to wonder if the crude predictions of the Friedmann
and Lemaitre cosmological model might break down if you don’t assume that matter is
perfectly smooth.
Others have thought a lot about the idea of this “back-reaction” of detail structure
on the global effect of gravity, so it’s not silly to wonder about this stuff.
And a fun implication if this hypothesis is right is that it means dark energy should change
over time, first increasing and then decreasing, as the void bubbles first form and then in
the future dissolve.
This is all pretty speculative—there are good reasons why dark energy might be a simple
vacuum energy, which means it’s not caused by cosmic voids, and which means the Hubble
tension probably has to be some local bias in our measurement of the Hubble constant—perhaps
due to our local cosmic void.
Either way, if you want to understand the universe on the largest scales it turns out it's important
to also understand the details—especially of the voids—the least populated and least
studied, but perhaps most consequential regions of spacetime.