The night sky. People have studied it for millennia, seeking to unlock its secrets.
We've come a long way in our understanding and in the process we've uncovered a surprising truth.
Everything that we see: the stars, planets, life makes up only about five percent of the stuff in the universe.
Roughly 25% is what we call dark matter, holding galaxies together through gravity.
The last 70% is known as dark energy, accelerating the expansion of the universe.
Building on Einstein's theory of general relativity, scientists have ideas about how dark matter
and dark energy affects the universe. And they can test their predictions without knowing
the identities of these mysterious substances.
The way that structures form in our universe
is very sensitive to the proportions of and characteristics of dark matter and dark energy.
Without a certain amount of dark matter there wouldn't have been enough mass in the
early stages of the universe to form structures like galaxies that are very comfortable hosts that we live in.
But without a specific percentage of dark energy, there might have been too much structure.
So we need a very precise balance to be able to make the universe that we observe.
There seem to be some interesting tensions between observations that we've made of the early light
in the universe, like the Cosmic Microwave Background, and observations that we make
of the local universe, so what's really near to us. We have a hint however that the
local measurements give slightly different values. So the question is 'is that real?'
Now, new results from the Dark Energy Survey take us one step closer to
figuring out how the universe got to be the way it is and what the future has in store.
The Dark Energy Survey is an international collaboration that aims at observing hundreds of
millions of galaxies, and taking pictures of them. The primary science goal of the project is to
study the distribution of galaxies in space and that helps us better understand the expansion of
the universe, and that's related to dark energy.
The multinational project led by Fermilab
is based at the National Science Foundation's Blanco telescope at
Cerro Tololo Inter-American Observatory in Chile.
Over more than 300 nights, the survey charted about 1/8th of the whole sky
and includes the largest sample of galaxies ever used for cosmology: over 226 million of them.
The lens of the dark energy camera is more than three feet
and it can take pictures with a whopping 570 megapixels.
This is far beyond the capabilities of our iPhone which takes at most 12 megapixels.
One of the primary reasons we have our telescope in Chile is because
they have these very tall mountains where the atmosphere is, relatively speaking, more stable.
So the light as it passes through the atmosphere is going to be deflected less.
In the Dark Energy Survey we use several probes, but the most powerful of them are galaxy
clustering and weak gravitational lensing.
Galaxy clustering, it's sort of just a name for the fact
that galaxies they aren't distributed sort of uniformly randomly in space, they exist in a web.
And that's what we observe, we observe this web.
And more clustered galaxies allude to more dark
matter in that region. So the galaxies serve to illuminate the dark matter scaffolding.
Weak gravitational lensing, it's a name for an effect that we can use to help understand
dark matter and dark energy.
I have a wine glass here and the wine glass bends the light that
passes through it. If I put like the base this wine glass on the camera you see that the image
is going to be distorted, and importantly that as I move the wine glass away you see the amount of
distortion changes. And that's exactly what we're studying with the Dark Energy Survey.
Figuring out those patterns is a complicated process.
It's a bit like looking at an abstract painting
and figuring out the order in which the layers were painted.
To do weak lensing and clustering we need to understand how far away the galaxies are,
so the distance is from earth to the galaxies. And we call this the redshift. So the higher a redshift
a galaxy has the further away it is from us.
So redshift refers to the change in the color of light.
So light has been shifted to be more red. When a galaxy is moving towards you it would be,
it would appear bluer for this reason and when it's moving away from you it appears redder.
Redshift then is a measure of the velocity of an object relative to you.
In the Dark Energy Survey, we make photometric observations. So we observe each galaxy in
different filters on our camera and that allows us to pick up different features
and measure the colors of these galaxies which we use to determine their distances.
We need to measure how far these are with extreme precision otherwise we can get
biased results and then it won't matter.
The Dark Energy Survey gives us a clearer picture of our universe than ever before. The results of
our analysis are as impressive as the road to get there. We found that our measurements are well
described by predictions that we make from the Standard Model of Cosmology.
Measurements of the early universe are an exquisite fit to this model
but they probe the universe at an unrecognizable stage when it's just a plasma of particles,
very different to the one that we observe with the Dark Energy Survey
which is teeming with galaxies and dark energy. So it's a beautiful
test and a remarkable feat that one theory can describe billions of years of cosmic evolution.
Now, our analysis best measures how clumpy the matter in the universe is and it is intriguing
that our measurements find slightly less clumpiness than what the early universe dictates.
So although our standard model survives a more stringent test than ever before,
there is enough ambiguity to keep us looking up.
Now the Dark Energy Survey has doubled the amount
of data still to be analyzed, so we're really just at the beginning of learning what we can
from the darkness and that's exciting.
The Dark Energy Survey has already done a lot of great science.
Now, with the three-year analysis completed, the best is still to come.
The survey lasted six years in total. To unpack the full data set, the
team will harness all the sophisticated methods that they've developed so far. In the process,
they'll nail down the distribution of dark matter and dark energy with even greater precision.