The search for clues to the mysterious dark matter has been going on for almost half a century, which has motivated researchers to think outside the box.
But it’s hard to be more creative than Lisa Randall, a particle physicist at Harvard University, who hypothesized that dark matter killed the dinosaurs.
Hey, wasn’t that a bolide crashing into Earth 66 million years ago? Calm down, yes it was. That part is not up for debate. But Randall suggests that this only happened because of a fortuitous and periodic crossing of the Solar System through the Milky Way’s disk.
When crossing the plane of the galaxy, gravitational interactions of the dark matter contained in it with some of the most distant objects orbiting the Sun would have thrown several of them towards the interior of the planetary system, promoting violent collisions, including the one that killed the dinosaurs.
There are a ton of “ifs” attached to this hypothesis.
It works if in fact there is a periodic increase in large impacts every 32 million years (which not everyone agrees with).
If only some of the dark matter behaves differently from the usual models, concentrating in the galactic disk, instead of just forming a more or less homogeneous spherical halo around it.
And if there is enough of it in the disk to produce the gravitational effects on objects in the so-called Oort cloud, the furthest repository of remnants of the Solar System’s formation. That’s just to name the three biggest “ifs”.
The conversation, of course, is complex, and American particle physics has written an entire book to break it down. “O Universo InvisÃvel”, originally published in the US in 2015, is now out in Brazil by Companhia das Letras. Read the best excerpts from the interview with the author below.
One of the great ideas of the book is the notion that everything, even if it seems to be far from everyday reality, makes up a picture that explains what we are doing here. It is possible to make a parallel with what Carl Sagan used to call cosmic consciousness. Is it around?Let me say that there are very different ways of thinking that everything is connected. There’s that kind of, I don’t know, like “The Secret” or something, and I want to make it very clear that I’m not talking about that. I’m not talking about mystical ways in which things can be connected.
I don’t think of it as consciousness, but as the fact that if you seek to understand what you are at a fundamental level, what you are made of, and then you see that without these particular ingredients, without these specific laws of physics, you cannot would have the world you see today, that’s amazing.
I mean, to think that some things that seem so irrelevant to us actually contributed to our existence. These connections are not always completely obvious, or even close to obvious. Sometimes the answers are right under your nose, but other times, to really have a deep understanding, you need to get back to the fundamentals.
In “The Invisible Universe”, you present the notion of interacting dark matter and the possibility of a second disk made of it in the Milky Way, commenting that the data from the Gaia satellite would help to put some restrictions on this hypothesis. The book was written in 2015, and now there is data from Gaia. How has the hypothesis fared since then?There have been several scientific articles where they look for the disk. Two claim to rule out certain parameters, and one sees evidence of a disk that has a little less density than we would like.
I would not decisively conclude that it has been discarded, nor that it is there either.
The evidence is not conclusive, but there is some suggestion of a disk that is slightly less dense than what would be displayed to cause such a disturbance in the Oort cloud, in the outer parts of the Solar System. So I wouldn’t call it a spectacular success, but I wouldn’t necessarily rule it out either.
By putting together dark matter and dinosaurs, you bring this notion that there is a periodicity in mass extinctions…Let me say, a periodicity in large comet impacts. Not all of them lead to mass extinction, the last one was caused by such an object… I just want to distinguish real physics, which is some object crashing into Earth, from mass extinction, which is another step.
In the book, you present this as a not very firm notion. Some people think that this periodicity exists, others don’t, that it would be noise resembling periodicity. I wonder if, since the publication of the book, you have been following this area as well, if there are any new developments.No, I don’t, but you should keep in mind that what we’re looking at as evidence is craters from potential comet impacts. And those are there. Unless they find out more about them, statistics are what they are.
We look for craters from the last 500 million years. I mean, it’s really cool that we have this evidence, these big craters that came from big objects, bigger than a kilometer. And the fact that there’s only 20 or 25 of them that we know of, the stats are poor, you’d rather have better stats.
We show that the hypothesis is statistically favored, but not by an enormous amount that necessarily convinces everyone. Including ourselves.
We still want to know if it’s true or not. But what we liked about this project was promoting that debate.
And, of course, the big elephant in the room is dark matter. Seven years have passed since you wrote the book and it seems we are still at the same point. There are experiments, some intriguing results, but no definitive conclusions. What has changed since then?On the frontier of science you are always doing things that are difficult to study. I mean, if they were easy, someone would have done it in the last century and you’d be out of a job. So you study things that are difficult.
In the case of dark matter, just remember, it’s something that we know through its gravitational effects, but we don’t know at a fundamental level.
Is it a particle? What is your mass? What are their interactions, if any? What we do know is that it interacts extremely weakly with our matter.
Other than gravity, we don’t know of any other interactions, so what that tells us is that it’s not the stuff we’re familiar with. It’s something new, which is exciting.
And people have looked for it under what I would say is an optimistic assumption that it is somehow connected to extensions of the standard model of particle physics. And they didn’t find it.
So I think the biggest change, although it might be behind the scenes and you might not see it out there, was people realizing that there are other possibilities for what dark matter could be and thinking creatively about new ways to look for it. Of course, we still don’t know what it is, but I wanted to say that it’s not entirely shocking that we don’t know what it is.
By definition, it’s something that doesn’t interact much with matter, which means it’s really hard to see. So the fact that we didn’t see something that’s hard to see actually isn’t incredibly surprising.
You defend in the book the idea that astrophysics has more to contribute at this point in the understanding of dark matter than particle experiments. Is that still your opinion?It depends on what you call particle experiments and it depends on what dark matter is. I think we shouldn’t underestimate any of them.
We don’t know what it is and we don’t know how we’re going to find it, so we want to think of all possible ways…
I think we would be very lucky if we found it in particle physics experiments, because it would mean that it has interactions or connections with the standard model in ways that we don’t know to be true.
So I would say that the ways of astrophysics are somewhat more general in some cases. But that doesn’t mean we should do one or the other. We should do both.
