PBS Space Time

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 cant account for.

Could both of these be explained by looking to a part of the universe that weve largely ignored so far?

Could cosmic voids be driving the universe?

Einstein completed his general theory of relativity in 1915, and it didnt 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, Friedmanns solution showed that the universe must be a dynamic placeeither expanding or contracting.

The former as it turns out, as revealed soon after by Edwin Hubbles observation of the recession of the galaxies.

That expansion seemed to perfectly match the uniform expansion of space predicted by Friedmann and Lemaitres 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 equationsthis lambda factorthe cosmological constantironically the same anti-gravitational modification that Einstein had made as an erroneous attempt to thwart Friedmann and Lemaitres prediction of a non-static universe.

But the addition of the cosmological constant completed our modern LCDM cosmology.

The CDM stands for cold dark matterby 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 theres a much bigger concernLCDM 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 thats 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.

Thats largely true according to our observationsas 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 clumpinesssuperclusters of galaxies, clusters, then galaxies themselves.

LCDM doesnt 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, lets start with the local lump.

The Milky Way lives in something called the Laniakea supercluster.

Its 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.

Theyre loose collections of galaxies and clusters of galaxies that will eventually dissolve under the accelerating expansion of the universe.

Its like gravity tried its best to form something this big, but at that scale dark energy won the battle.

Nice try, gravitybut at least you slowed down nearby galaxies a little bit.

And because the outward velocities are slower than what youd expect from pure expansion, if we measure the Hubble constant based on Laniakea galaxies we should get a number thats too small.

But waitthe number we measure in the modern universe is even higher than predicted by LCDMthe 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 superclusters influence.

Thats exciting because it makes it harder to explain away the Hubble tension as some local effect of lumpiness, perhaps increasing the chance that were seeing interesting physics in the dark energy.

By the way, Dr. Becky has a deeper dive into this paper that Ill link below.

OK, lets 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 were 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.

Youd 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 isperhaps explaining away the Hubble tension and saving LCDM.

And there are indeed a couple of studies that claim just this.

Lets 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 its 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 dont 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, its weird if the Local Hole exists at all.

Its just stupidly large at two billion light years across.

Some argue that the standard LCDM model shouldnt allow for lumps that bigat 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 dynamicsa 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 theres another Dr. Becky link below for the latest on that.

OK, lets zoom out one more timenow to the entire universeto look at the effect of lumpiness on the largest scales.

Theres 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 lets have a quick refresher on dark energyand we have a whole playlist if you want the gory details.

Dark energy is usually thought of as the energy of the vacuumthe faint buzz of space itself.

For complicated reasons, space having a non-zero energy density should cause accelerating expansion.

In our boy Alexander Friedmanns equations, dark energy ends up having the same physics as a gas with negative pressure.

Thats 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 outwardsso positive pressurethe surface resists that pushwhich 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 weban interlocking network of clusters and intercluster filaments.

But all this clumping of matter left vast regions that are almost emptycosmic 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 theyre 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 havent found a lot of commentary on this result and havent thought hard enough about it myself to know whether theres merit here.

But in general its a good idea to wonder if the crude predictions of the Friedmann and Lemaitre cosmological model might break down if you dont 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 its 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 speculativethere are good reasons why dark energy might be a simple vacuum energy, which means its not caused by cosmic voids, and which means the Hubble tension probably has to be some local bias in our measurement of the Hubble constantperhaps 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 detailsespecially of the voidsthe least populated and least studied, but perhaps most consequential regions of spacetime.