In 2013, astronomers reported that they’d found what was, at the time, the biggest thing
in the known universe.
It’s a collection of quasars about nine billion light-years away -- appropriately
named the Huge Large Quasar Group.
And some scientists say that it’s impossibly big.
Like, three times larger than anything that could ever exist in our universe.
When you look up at the night sky, it probably looks like there are about the same number
of stars in all directions.
Sure, some regions are a little denser or a little sparser.
But in theory, you could divide the sky into squares so that each square had about the
same number of stars in it.
The cosmological principle applies that idea to the whole universe: if you broke the universe
up into big enough cubes, each of those cubes would have about the same amount of matter
in it.
Mathematicians have used the cosmological principle to predict that there shouldn’t
be any collections of matter -- or spaces between matter -- bigger than about one and
a half billion light-years wide.
So if you made your cubes with sides one and a half billion light-years long, they should
all have about the same amount of matter in them.
And for the most part, this seems to be true: galaxies tend to be grouped together into
structures smaller than about five hundred /million/ light-years.
So everything seemed to line up: mathematicians said there shouldn’t be anything bigger
than one of the cubes -- because otherwise one would look different than the rest -- and
astronomers hadn’t seen anything that big.
But in the 1980s, astronomers started conducting surveys of quasars -- huge jets of light let
out by matter as it fell into the central black holes of galaxies billions of years
ago.
By looking at where quasars were, they could get an idea of how galaxies in the early universe
were spread out.
And they noticed that quasars were often bunched together -- just like galaxies are today -- into
what are known as Large Quasar Groups, or LQGs.
Some researchers think that these LQGs could be the roots of today’s superclusters, which
can contain thousands of galaxies.
Large Quasar Groups could have anywhere between two and twenty quasars in them, and they could
be up to hundreds of millions of light-years across.
And in the 1990s and early 2000s, a couple LQGs were discovered that were about two billion
light-years wide, making them a little big bigger than the limit imposed by the cosmological
principle.
Then, in January 2013, astronomers announced they’d found the /Huge/ Large Quasar Group,
or Huge-LQG.
Its 73 quasars are an average of nine billion light-years away.
And it’s raised a lot of eyebrows, because it’s a whopping four billion light-years
across.
That’s three times larger than mathematicians said anything in the universe could get!
Some scientists think that the discovery of the Huge-LQG means that there really is some
large-scale structure in the universe that we can’t ignore any longer.
But many aren’t so quick to jump ship, since the cosmological principle underpins a lot
of our understanding of the evolution of the universe.
Some question the methods of the original team and say that there just isn’t yet enough
evidence to know for sure that the Huge-LQG is actually a collection of quasars and not
some sort of statistical blip.
Others say that the quasars might be farther apart than the original researchers thought,
which might split up the Huge-LQC into a bunch of smaller, plain-old Large Quasar Groups.
But earlier this year, one group of scientists -- including some of the discoverers of the
Huge-LQG -- claimed that the it isn’t necessarily a violation of the cosmological principle,
after all.
They say that that one and a half billion light-years isn’t some sort of hard limit.
Instead, there’s just a low probability of finding anything so large.
But it’s not impossible.
There might be as much as a 19% chance of finding something even bigger than the Huge-LQG
in our universe.
And those odds aren’t that bad.
In fact, astronomers might have already found something much bigger than the Huge-LQG -- something
called the Hercules-Corona Borealis Wall, a group of galaxies that was discovered just
a few months after the Huge-LQG.
And it might be twice as big!
Just like with the Huge-LQG, scientists are still debating whether this structure really
exists or whether it too might be a statistical blip.
But that’s a story for another episode.
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