Okay so this is a paper entitled
"A structure in the early universe at z~1.3 that exceeds the homogeneity scale of the R-W concordance cosmology"
I think that's quite a mouthful for a title
That is not a catchy title
It isn't really a catchy title, no
Um but, the physics and the astronomy behind it is rather interesting
Give me a better title
Um, "The discovery of an implausibly large structure in the universe"
Wow, that's better! You should have written the title.
There's some big structure out there, a long way away from Earth, and it's- they're saying it's the biggest structure that there is in the universe
So this is basically a collaboration which are looking at quasars in the universe, or these very bright active nuclei in galaxies
Not to study the quasars themselves, but because where there's a quasar there's a galaxy
So they're a good way of figuring out where galaxies are in the universe, where the galaxies themselves are too faint to see
And they're mapping out these quasars in three dimensions to figure out whether they're randomly spread in space
Or whether they're clustered together, and if they are clustered together how they're clustered together
Such a large structure is incompatible with a universe which is homogeneous on very large scales
Homogeneous means you're in no special place
That what we see around us is representative of everywhere else in the universe
And so, um, you don't expect, that the argument goes, I'm not so sure I buy it yet
You don't expect to see such a big structure in a universe that is meant to be, on average, homogeneous
So a quasar is a very, very bright nucleus of a galaxy
It's so bright it actually outshines the whole galaxy around it
The picture we have of these things is, we believe that pretty much every galaxy has one of these massive black holes in the middle
And probably what's happening where you see a quasar, it just means that that black hole is particularly active
Which means stuff is falling into it, which is making it light up
And so when these things were first discovered, they were called quasars
Because they were called "quasi-stellar objects"
Which means they look basically like stars because all you see is this bright nucleus of the galaxy
It's only when you look very carefully you see actually there's a whole galaxy around it
But they're very useful as sort of lighthouses
Because you can see a quasar at enormous distances all the way across the universe
Even if you couldn't see a normal galaxy
What is the scale above which you would say everything looks fairly uniform,
And below which you expect there to be structures?
As you said, you know, we are, you and I are quite different (laughs)
In many ways, and then you can just go up
Planets are different
Stars and then galaxies, they're different from one another
And you keep going to bigger and bigger scales, and you think, "Well, there's nothing homogeneous here"
"When I look around, things are looking quite different"
But the idea is that if you go on to large enough scales
And by "large enough," we're really talking on scales of more than 100 megaparsec
A parsec is about 3 lightyears, so we're talking about 300 million lightyears
You have to go before you begin to sort of suggest that above that scale things should start looking very similar
So what these people have found is that looking at the distribution of quasars in space
They're not randomly distributed. That's not news
But what they've actually found is that they form these incredibly large structures
When you start kind of mapping out the quasars, you find they form these enormous filaments
The latest thing that they've found, which this paper is all about
is a structure where the largest dimension of it is fairly elongated
and the longest dimension of this collection of quasars is about 4 billion lightyears long
And bearing in mind that the entire universe is only about 13 billion lightyears across
So this is almost a third the size of the universe, this structure
So it's a huge collection of these things
That sounds like the ultimate elephant in the room!
That sounds like it's the dominant beast of the universe.
And actually it causes a problem because the picture we have
is that when you go to large scales, the universe is supposed to be homogeneous
There's this thing called the Cosmological Principle, which says that
one bit of the universe is really very much like another bit of the universe
And of course if you've got a structure this big
it really means that one bit of the universe isn't really like another bit of the universe
because one bit's got this massive structure in it, and another bit hasn't got this massive structure in it
The big question is whether that is
A) Is it real? Is it a real feature? I can't really comment on that
I just find it mindblowing that the idea you can have 73 quasars linking across a distance of 4 billion lightyears
It just blows my mind
So how did they determine that they've got this structure? But let's assume it is, let's assume it's a real structure
The next question is: Is it compatible with an idea that the universe, on large scales, is homogeneous?
It could be that, we know there are fluctuations in a universe that's homogeneous
There has to be because without those fluctuations, we wouldn't form, you wouldn't form structures
So it could be that what we're seeing is just a rather extreme fluctuation, which is perfectly consistent
And in fact there is a slightly smaller structure that was observed a couple of years ago
called the Sloan Great Wall
This was also seen in the Sloan survey
This is a structure of 200-300 megaparsecs, so it's not quite as big as this one
And even then that was pushing the idea of homogeneity
So here's this structure that they've identified
Which, as I say, is about, as I say, from end to end here, is about 4 billion lightyears long
And so they've sort of played this game of join the dots
They've found where all the quasars are and found where all their nearest neighbors are
and used that to kind of span between them to say it looks like there's a structure here
You have to be very careful when you're playing this game because
If you just threw down a bunch of quasars at random, once in a while they'd start forming things that looked like structures
And so the exercise that you have to go through once you've found something that looks like a structure
is to say, "Is this consistent with just part of a random collection of things and I just happen to have seen them arranged in this way?"
And the statistics they've done in this paper show that what they've found
is very unlikely to be just a random distribution of things
It really looks like they've found a real structure in that sense
A collection of these quasars which have formed into this arrangement of quasars
which is not consistent with them just being randomly spattered around the place
The model that we're talking about, just to give this, trying to put some flesh to these bones
is the universe is very smooth on very large scales
so if I think of the matter distribution in the universe, if I think of it as a mill pond
And that matter distribution is perfectly flat, that mill pond is perfectly flat
that's your homogeneous bit
but on top of it there are these little ripples, right?
coming from fish, some of those are fish
And um, so these ripples are there, and those are the small fluctuations
So you've got the background, homogeneous. Everything's smooth.
And then you've got the small fluctuations,
And where you've got slight excess fluctuations, that means the gravitational pull
You've got slightly more matter, so from Newton's law
Where the force of attraction is proportional to the mass, you've got slightly more here, that will cause things to attract to it
And then it will leave regions where it's attracting matter from. They'll be devoid of matter.
They actually will create voids.
The force that we think shapes the large-scale structure of the universe is gravity
It forces things to collapse in certain dimensions
and actually you end up forming these sort of sheets of things and filaments and those kinds of things
which we've seen all the time when we look at galaxies in the nearby universe
We find that they're not randomly spread. There are some places where there are clusters of galaxies
Some places where there are none at all, some places where they form into these sort of sheet-like structures or filament-like structures
So presumably the physics is the same on these scales
When forming these very large structures, this gravity's forcing things to collapse in certain directions
The problem is that that's not what theory predicts, right?
The picture we have of the universe is that gravity shouldn't have formed structures on these kind of scales.
So that's why it's starting to kind of start to challenge our picture of cosmology
is that it seems to be there but we're struggling a bit to come up with an explanation as to how it might have formed
which of course, that's when science starts getting interesting
when you start finding things that you weren't expecting to find.
In that analogy you made with the fish or whatever it was making little ripples on our mill pond
What's the reality? In the real universe, what's causing these fluctuations that allow large structures to form?
Okay, so this is where it gets really fun
Now we're moving back to the very early universe
How early? About 10^-35 seconds into the universe
That's really...Point zero zero zero zero zero zero, 35, 34 zeroes and a one, okay? Seconds into the universe
And this is a period known as, where the universe expanded exponentially quickly
And so the space expanded exponentially rapidly
And in that universe was a thing called a scalar field, a bit like the Higgs field
permeating the whole of the universe, and it fluctuated because of quantum mechanics
just like the Higgs field fluctuates. (Randomly?) Randomly
And those fluctuations, the big difference that goes on here, is that those fluctuations don't just remain small
Because the universe is expanding so rapidly, they quickly get--they--get grabbed hold of
I'm getting too excited and I'm speaking too quick--slow down
They, they (Ed, I will never accuse you of speaking too quickly)
These fluctuations get grabbed hold of by the expansion of the universe, and stretched onto big, big scales
and those are the cosmological scales
and it's there that the initial seeds came from.
So what we're seeing when we look at the large-scale structures in the universe
When we look at the microwave background fluctuations in the W map
and see these hot and cold spots
What you're actually seeing is an imprint of those very early moments,
way before a microsecond, way before a nanosecond into the universe's history
when these fluctuations from the field emerged.
(A big, blown-up projection of just a funny little wobble in a field) Wow