Cosmic void
Cosmic Void.
Huge empty patches of the Universe could help solve,
some of the greatest mysteries in the cosmos.
Cosmic voids can stretch from tens to hundreds of millions of light years across.
Scientists have known since the 1980’s that these voids exists.
Inadequate observational data and insufficient computing power,
kept them from being the focus of serious research.
Now an increasing number of scientists are convinced, that the study of these voids,
could offer important clues to help solve the mysteries of dark matter, dark energy,
and the nature of the enigmatic subatomic particles called neutrinos.
Voids are shown that Einstein’s general theory of relativity probably operates,
the same way at very large scales as it does locally.
This has never been confirmed.
The discovery of cosmic voids in the late 1970’s,
came as something of a shock to astronomers.
They were startled to learn that the Universe did not look the way they always thought.
They knew that stars were gathered into galaxies, and galaxies often clumped together,
into clusters of dozens are even hundreds.
If you zoomed out far enough, they figured this clumsiness would even out.
They thought at the largest scales the cosmos would look homogeneous.
It wasn’t just an assumption.
The so called cosmic microwave background, CMB,
- the electromagnetic radiation emitted about 380,000 years of the big bang -
is extremely homogeneous.
This reflects the smoothness in the distribution of matter when it was created.
Even though that was nearly 14 billion years ago,
the modern Universe should presumably reflect that structure.
But we cannot tell whether that’s the case just by looking up.
The night sky appears two dimensional, even through a telescope.
To confirm the presumption of homogeneity,
astronomers needed to know not only how galaxies are distributed across the sky,
but how they are distributed in the third dimension of space - depth.
So they needed to measure the distance from Earth to many galaxies near and far,
to figure out what’s in the foreground, what’s in the background, and what’s in the middle.
In 1978, scientists did just that and discovered the first hints of cosmic voids.
This shook the presumption that the Universe was smooth.
In 1981, scientists discovered a huge void, about 400 million light years across,
in the direction of the constellation Bootes.
It was so big and so empty that if the milky way had been in the centre of the Bootes void,
we would not have known that their were other galaxies in the Universe.
In 1986, scientists confirmed that the void’s discovered earlier were no flukes.
They painstakingly surveyed the distance to many hundred’s of galaxies,
spread over a wide swath of the sky, and found that void’s appear to be every where.
Scientists made a map of the galaxies’s locations.
They found big circular voids, and sharp walls full of galaxies.
These findings posed a serious challenge for current models,
for the formation of large scale structure.
Research confirmed that galaxies and cluster of galaxies,
are concentrated into a gigantic web of concentrated regions of matter,
connected by streaming filaments, with gargantuan voids in between.
In other words the cosmos today vaguely resembles Swiss cheese,
whereas cosmic background radiation resembles cream cheese.
The question was: what forces made the Universe,
evolve from cream cheese into Swiss cheese?
One factor was almost certainly dark matter.
Dark matter was accepted by scientists only in the 1980’s.
It was more massive than ordinary matter by a factor of about six.
That would have made the gravitational pull of slightly over dense regions,
in the early Universe stronger than anyone had guessed.
Stars and galaxies would have formed preferentially in these areas of high density,
leaving low density regions largely empty.
Scientists continued to explore what would come to be known as the cosmic web,
but very few concentrated on voids.
It wasn’t for lack of interest : the problem was there wasn’t much to look at.
Voids were important not because of what they contained but because of their very existence.
Their shapes and sizes and distances from one another,
had to be the result of the same forces that gave structure to the Universe.
To use voids to understand how those forces worked,
scientists needed to include many examples in statistical analysis of void’s,
average size and shape and separation.
Too few had been found to draw useful conclusions from them.
It was analogous to the situation with exoplanets in the 1990’s.
The first few discovered were proof that planets did indeed orbit stars beyond the Sun.
It wasn’t until the Kepler’s space telescope began raking them in by the thousands in 2009, that scientists could say anything meaningful,
about how many and what kind of planets populated the Milky way.
Another issue with studying voids, is that they are conducted in redshift space,
and not actual space.
To understand what they meant, consider that as the Universe expands,
light waves are stretched from their original wave lengths and colours into to longer,
redder wave lengths.
The further away something is from an observer, the more its light is stretched.
The James Web Space Telescope was designed to be sensitive to infrared light,
partly so that it can see the earliest galaxies,
whose light has been stretched all the way out of the visible spectrum - it’s redder than red.
And the Cosmic Microwave Background or CMB, the most distant light we can detect,
has been stretched so much, that we now perceive it in the form of microwaves.
Measuring the physical distance to galaxies is difficult.
It is much easier to measure redshifts.
Redshifts can distort the actual distances to galaxies that enclose a void,
and thus give a misleading idea of their size and shape.
As the void expands, the near side is coming towards us, and the far side is streaming away.
That differential subtracts from the redshift on the nearside, and adds to it on the far side,
making the void look artificially elongated.
Despite the difficulties scientists began to feel more equipped to tackle voids by the 2000s.
The Sloan Digital Sky Survey probed much more deeply into the cosmos,
and confirmed that voids were every where you looked.
Two teams of scientists, mean while, had revealed the existence of dark energy,
a kind of negative gravity that was forcing the Universe to expand faster and faster,
rather than slowing down from the mutual gravitational attraction of trillions of galaxies.
Voids seemed to offer astronomers a promising way of studying,
what might be driving dark energy.
One of the aspects of voids, was that, under dense regions are much quieter in some ways,
and more amenable to modelling, then the clusters and filaments that separate them.
Galaxies of gases are crashing into each other in non linear and complicated interactions.
There is a chaos that erases information about their formations.
Further complicating things the gravitational attractions between galaxies,
is strong enough on smaller scales that it counteracts the general expansion of the universe,
and even counteracts the extra oomph of dark energy.
Andromeda, for example, the nearest galaxy to our own,
is actually drawing closer to the Milky way.
In about 4 billion years they will merge.
Voids in contrast are dominated by dark energy.
The biggest ones are actually expanding faster than the rest of the universe.
That makes them ideal laboratories for getting a handle,
of this still puzzling force of dark energy.
Voids could also cast light on the nature of dark matter.
Although voids have much less dark matter in them,
than the clusters and filaments of the cosmic web do, there is some.
Unlike the chaotic web, with its swirling hot gases and colliding galaxies,
the voids are calm enough, to make scientists think that dark matter might be detectable.
They would not show up directly, because they neither absorb or emit light.
But the particles could occasionally collide, resulting in tiny bursts of gamma rays.
They would also probably decay eventually, releasing gamma rays as well.
A sufficiently sensitive gamma ray telescope in space,
would theoretically be able to detect their collective signal.
Scientists believe that dark matter produces gamma rays, the signal should be in there.
Voids could even help to nail down the nature of neutrinos.
Neutrinos are elementary particles, once thought to be massless,
that pervade the Universe while barely interacting with ordinary matter.
If you sent a beam of neutrinos through a slab of lead, one light year,
or about six trillion miles thick, about half of them would sail through effortlessly.
Scientists have confirmed that the three known types of neutrinos do have masses,
but they aren’t sure why or exactly what those masses are.
Voids could help find the answer.
They are places that have a lack of both luminous matter and dark matter.
But they are full of neutrinos, which are almost uniformly distributed, through the Universe,
including in voids.
That is because neutrinos zip through the cosmos at nearly the speed of light.
This means they don’t clump together under their mutual gravity,
or under the gravity of dark matter concentrations,
that act as the scaffolding for the cosmic web.
Although the voids always contain a lot of neutrinos the particles are only passing through.
The particles that fly out are constantly replenished by more neutrinos streaming in.
Their combined gravity can make the voids grow more slowly over time,
than they would otherwise.
The rate of growth, determined through the comparison of the average size of voids,
in the early Universe, to those in the modern Universe,
can reveal how much mass neutrinos actually have.
We now know about 6000 voids.
That’s huge, but still not enough for the comprehensive statistical analysis,
necessary for voids to be used for serious cosmology.
There is one exception.
In 2020 scientists showed that general relativity behaves at least the same way,
on very large scales, as it seems to do in the local Universe.
Voids can be used to test this question because astrophysicists think they result,
from the way dark matter clusters in the Universe.
Dark matter pulls in ordinary matter,
creating the cosmic web and leaving empty spaces behind.
What if general relativity, our best theory of gravity,
breaks down somehow over very large distances?
A few scientists expect that to be the case,
but it has been suggested as a means to explain away, the existence of dark matter.
By looking at the thickness of the walls of matter surrounding voids,
scientists have determined that Einstein’s theory is safe to rely on.
To understand why, imagine a void as a circle,
whose radius increases with the expansion of the Universe.
As the circle grows, it pushes against the boundaries of galaxies and clusters at the perimeter.
Over time these structures aggregate, thickening the wall, that defines the void’s edge.
Dark energy and neutrinos affect the thickness as well,
but because they are smoothly distributed both inside and outside the voids,
they have a much smaller effect overall.
Scientists plan to use voids to learn more about the Universe soon,
because they expect to rapidly multiply the number of known voids in their catalogue.
The next 5 to 10 years hundreds of thousands of voids are likely to be discovered.
Machine learning will make it easier to analyse void properties.
The European space agency’s Euclid mission, launched in 2023,
will create a 3D map of the cosmic web with unprecedented breath and depth.
NASA’s Nancy Grace Roman Space Telescope will begin its survey in 2026,
looking in infrared light.
In 2024, the ground based Vera C. Rubin observatory will launch a 10 year study,
of cosmic structure among other things.
Combined, these projects should increase the inventory of known voids,
by two degrees of magnitude.
Void’s science won’t answer all of astrophysicist’s big questions about the Universe by itself.
But it could do something even more valuable in a way.
It could test ideas about dark matter, dark energy,
neutrinos and the growth of cosmic structure,
independently of other strategies that scientists use.
If the results match, it would be great.
If not astrophysicists will have to reconcile their differences,
to find out what’s actually going on in the cosmos.
It is poetic that looking into voids where there is nothing,
might yield information about some of the outstanding mysteries of the Universe.