The Force: Dark energy and dark matter

12 December 2017

Interview with

Professor Ben Allanach - Cambridge University

The Force pervades all of the Star Wars story, and the Jedi are masters of it - they perform mind tricks, move heavy objects with the power of thought and even feel the effects of events happening many light years away. So is there any sort of equivalent to The Force based in reality? Chris Smith spoke to Ben Allanach - a theoretical physicist from Cambridge University...

Ben - Every Star Wars fan knows about the “dark side” of the force and, indeed, there are dark forces out there in the universe, and there is definitely dark matter, which makes galaxies rotate at a wierd speed. So if you try and work out how fast the galaxy should rotate from laws of gravity, you get the wrong answer, basically, compared to observing how fast they go round the outside...

Chris - Because [Fritz] Zwicky spotted this...?

Ben - That’s right.

Chris - A number of years ago and made the first observations of the stars spinning at the wrong speed compared with how much mass we knew was there?

Ben - That’s right. If you add a lot of extra mass that you can’t see - that’s the dark matter, then that makes it rotate faster on the outside and agree with observations.

Chris - So we think there is this bizarre force, which we’re calling dark matter because we don’t know what it is. It’s in a big halo or at least some kind of distribution around the outsides of galaxies and it’s gravitationally influencing stuff in the galaxy but, beyond that, we don’t know what it is?

Ben - Exactly. We know it’s a particle because if it’s something that’s big you should be able to see it gravitationally lens light in the sky. So you look for these “massive compact halo objects” they’re called, and you look for a lensing effect that moves across the sky, and that you don’t see that enough to be a big thing.

Chris - So, it has to be some small particle, and it won’t interact with anything, it won’t talk to us in light because it’s dark, so how do we find it then and how do we try and interrogate it?

Ben - It might not have interactions with light, but it might have interactions with the weak force, which is responsible for radioactive decay, and in that case you could conceivably produce it in the Large Hadron Collider in the collisions there between protons.

Chris - How would you know you’d made it though?

Ben - The protons have equal and opposite momentum, and then you look for a final state after the collision which is unbalanced in one direction. You know from school physics that momentum’s conserved and so if it’s unbalanced, there’s been something invisible going in the opposite direction. The invisible stuff is precisely dark matter which takes the momentum off - sneaks if off like a thief in the night through the detector.

Chris - We know that we’ve got a dark side. There’s the good side as well, isn't; there, so is there a counterbalancing force?

Chris - Unfortunately, not that we know of, but there is another nefarious force, which is dark energy. That’s something which is even more mysterious than dark matter. It’s making space accelerate across the universe weirdly. When we look at other supernovi, a long way away from us, they are accelerating more than they should do. We know, okay, that everything’s getting further apart. Space itself is growing because of the big bang, but there’s an extra effect on top of that, this extra acceleration and it’s consistent with a small negative energy density of space itself and it’s making every accelerate even further away.

Chris - But what’s bizarre about what you’re saying is that space is making more space, accelerated by this notional thing, dark energy, and when it makes more space, the space it makes has more dark energy to make more space, and make more space expand more quickly, so it’s like it’s getting energy from nowhere then?

Ben - Well, yeah. The energy’s kind of hidden in there from the word go. Space itself, there’s a saying, has an energy. It’s a negative energy, right. So, in fact, you’ve got to be careful about whether you’re taking energy away or giving it to the system. You’re taking energy away by producing more space, but then that’s pumped into the acceleration of planets and whatever various galaxies on the edges of the universe.

Chris - You gave us a suggestion as to how we might be able to find dark matter, how are scientists trying to interrogate dark energy to discover its nature?

Ben - We look at the afterglow of the big bang; it’s called the “cosmic microwave background;” it’s all around us in the sky. And small variations in its temperature tell you about what was happening in the very early universe because that light has travelled since just a 100 thousand years after the big bang, so since a long time ago. That light encodes some of the secrets of the universe.

There’s a technical thing, you can look at particular bits of the light that have gone through a gravitational potential and then come out. You can do very precise measurements to check whether this acceleration was happening. But actually getting to the nub of really what’s causing it, I don’t think we’re very close to that yet. The work’s all theoretical and there aren’t very many observational ways of telling the different theories apart.

Chris - As one theoretical physicist said “you have to be very cautious of maths and theoretical physics because you can prove anything you like on paper.” Actually it’s really whether or not it’s manifest up there in the sky that’s a different matter. Wouldn’t you agree Katie?

Katie - I certainly would, yeah. We’ve got a few quick fire questions for you:

In episode 4, Luke Skywalker blows up the evil Death Star by shooting proton torpedos from his X-wing fighter. Are proton torpedos a real thing?

Ben - Yeah. Proton torpedos are absolutely a real thing, and they exist in the Large Hadron Collider. You have lots of these bunches of about a billion protons; there are 8 thousand in each beam - in counter-rotating beams. I don’t know what you call the torpedo, but there’s a length of them a few metres long, and they’re sent round at very close to the speed of light. You can’t put your hand in the beam because it would burn a hole through it.

Katie - So would it be possible to use this as a weapon?

Ben - Absolutely, yeah. You could do. If you fired this thing out in space the protons would travel a long distance. The thing is, if you use it in the atmosphere, protons are going to lose energy pretty quickly, but it would still have a range. If you could get enough power into the beam it would be pretty formidable.

Chris - People are using this clinically, aren’t they, to do cancer therapy because you can fire a beam of protons into a tumour and they can be predicted in terms of how fast they’ll slow down? And if they slow down and stop, and dump all of their energy just in the cancer they do a lot more harm to the cancer than the tissue around them.

Ben - That’s right, yeah. It’s got advantages over conventional radiotherapy where you get extra damage on the entry and exit of the body. It’s not indicated for all cancers, but I think particularly for deep tumours it’s proved very useful.

Katie - We’ve just got time for one more quick question for you:

Can I ever get my own droid like R2D2 or BB8 to help me out with tasks around the house?

Ben - That’s coming, I’m sure. We’re getting self-driving cars. I think in the near future there’s going to be self-loathing cars. AI is all over everywhere. That’s science fact, that’s not science fiction.

Comments

Can't u measure dark matter and dark energy by firing light into the universe using points of reference IE planets or solar systems ?

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