(Image courtesy of the LSST Gallery–construction as of October 24th, 2019)

*LC: I read that there are four primary scientific goals of the Large Synoptic Survey Telescope (LSST) project, and I was wondering if you could kind of run over each. So the first one is like dark matter or dark energy, right? *

ZI: So you could joke and ask how many Nobel prizes we would get. That one is our highest chance of a Nobel prize. There’s something mysterious when you write an equation describing how the universe is expanding. When you put normal stuff in, and by normal stuff I mean dark matter and the matter we are made of, you conclude that universe should either be uniformly expanding or slowing down, but definitely not accelerating.

We frequently draw an analogy with a stone. If you take a stone and you throw it up, it will reach some altitude and it will be slowing down because of gravity. It will stop and then it will come back. Now if you are super strong like babe ruth, you throw it very fast it can basically go to infinity. If you give it enough energy to overcome potential energy of earth it can go to infinity and it can even have some residual velocity when it gets to infinity or in other words super far away from earth.

*Never accelerating.*

But it will never accelerate unless you put little rocket engines and that’s what it seems like with the universe. About twenty years ago people starting realizing with different kinds of measurements that the universe is accelerating, which is a totally crazy finding. Actually the guys who discovered it got a nobel prize. But then the question is how to explain it.

One way to explain it is to postulate that there is some mysterious fluid in the universe called dark energy with super weird properties, but then you can explain everything. What it takes is to add this weird component. The other way to explain it is to say “Well, when I derive conclusion about dark energy I assumed that relativity is correct in describing how gravity behaves.” But if you say maybe the theory of relativity is not entirely correct, that we have to modify a little bit, then you don’t need dark energy. Everything is still okay, it is only a slightly different theory of gravity.

*So how does dark energy explain the property of acceleration.*

So the weird property is that it’s got negative pressure, meaning the more you squish it the more it will try to go in the same direction, unlike normal gas that will push back. So when you have the whole space permeated by this dark energy, as gravity is pushing it in one direction, dark energy will repel this. It will want to accelerate in the opposite direction. So it’s a theoretical construct, and it was originally put in equations by Einstein himself because without that term the universe needs to be expanding or collapsing, it cannot be stable. But when Einstein developed the theory of relativity, astronomers didn’t realize that the universe was expanding. So he added a term that cancels other terms to make everything stable. And then he later called it the greatest mistake he ever made. For about eighty some years people forgot about it. But if the theory of relativity is wrong then there is no need for dark energy.

And to distinguish the two, you need to measure two things at the same time. You need to measure the expansion of the universe, how fast exactly it is accelerating, and you need to measure the growth of structure. At the beginning of the universe everything was pretty much smooth. There were no galaxies, there were no stars. All the matter in the universe had density that was pretty smooth—maybe ten to the minus five variation across the universe. But today we have huge variation. If you take our cells, the density between us and the air in this room is a huge contrast. And if you compare the density that we are made of to the density of the vacuum between the stars, it would be even more. So today we have this structure in the universe. How it grew from the beginning of the universe to today and how exactly it grows depends on the properties of the matter. And that’s why you can distinguish dark energy existence from problems with the theory of gravity if you measure both of them simultaneously.

*So you need a lot of data about the structures of the universe and its expansion. *

Right, and you have to measure it very precisely with lots of data so that statistical noise is very small, and that’s why LSST is so unique. Nobody could have done that at the same precision level as LSST.

*Great, in ten years we’ll know. *

Yes, thirteen fourteen.

Negative pressure — yikes! I’ve always told my students that it’s impossible to achieve a negative concentration. I wonder if I’ll have to revise that…

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