Connecting Ideas in Astrophysics & Cosmology

There are many areas of scientific research that resonate strongly with the general public, from genetics to neuroscience to particle physics, but few can compare with the appeal of astrophysics and cosmology.

Perhaps it boils down to a question of accessibility. Appreciating the finer points of the structure of DNA or a proton can be daunting even for highly-trained professionals, but everyone knows what it’s like to look up at the night sky in awe and wonder.

It’s not that astrophysics is easy, of course — far from it — it’s that, somehow, its fundamental allure is so universal as to almost not need stating. Tell someone at a party that you’re studying geology and you’ll likely be asked, “So, what are you going to do with that?” Tell people that you’re studying astronomy, on the other hand, and you’ll probably simply hear, “Cool!

Even the most practically-minded person, it seems, can’t help but feel a deep sense of personal fascination at the prospect of uncovering some secrets of the universe. And over the past few decades, things have only got more fascinating still.

Cosmology, once regarded as little more than speculative hand-waving, has been stunningly transformed into one of the most rigorous, data-driven fields in all of science. Dark matter has gone from being viewed as some conceptual or observational mistake that astronomers are one day going to sort out a mysterious entity that accounts for more than one quarter of the total energy of the universe. Black holes have moved from theoretically possible structures to relatively common astrophysical objects, with startlingly large ones, of masses millions or billions times that of our sun, now being thought to lie at the heart of most, if not all, galaxies.

And, perhaps most significant still, rather than the expansion rate of the universe slowing down due to gravity as most people had envisioned, a number of rigorous observations have confirmed that the universe is, in fact, accelerating in its expansion, with the associated force responsible — the so-called “dark energy” — for a shocking 70% of the energy of the universe.

Combining “dark energy” and “dark matter”, then, puts us in the decidedly unexpected position where we’re forced to admit that no less than 95% of the total energy of the known universe lies in stuff we really, to all intents and purposes, are entirely ignorant of.

Talk about awe and wonder.

I’d like to be able to assure you that the answers to all of these questions are contained within this collection of conversations with scientific experts, but of course I can’t.

What I can tell you however, is that each of the five conversations offers a remarkably unique opportunity to get a vivid, first-hand perspective of what it’s like to be at the very forefront of this riveting, rapidly-evolving scientific enterprise.

Justin Khoury, University of Pennsylvania, reveals the often-considerable gap between formal scientific positions and personal scientific desires (Cosmological Conundrums).

“There’s a funny discrepancy between what people talk about at the conferences and the formal talks. We give talks that are rather conservative, because that’s how we’re trained. That’s a good trait to have, actually, to be conservative: we’re not completely wild.

“But at the end of conferences, sometimes somebody will want to have some fun and say, ‘Let’s have a survey, let’s write down some options for what we think dark energy is’.

“And it’s always startling that the way people vote on these surveys is far from conservative: they vote for the most radical option. Now, why is this? Well, maybe it’s because we are hopeful: we don’t want to put our names on a paper that’s crazy, but deep in our heart, we hope that it’s a radical answer so we have more stuff to do for the next 20 years. Or it could be a real belief: we don’t have an idea for how to actually explain it and write papers about it, but we actually believe that it’s true.”

Scott Tremaine, Institute for Advanced Study, illustrates how the process of discovery in astrophysics is typically very different from other domains of physics (Astrophysical Wonders).

“In a certain sense, astrophysics is more like a detective story than other branches of physics, because in other branches of physics if you have something you don’t understand, you try to design an experiment that’s going to allow you to understand it.

“In astrophysics, however, you often can’t design any experiments. You have a much more incomplete set of clues; and, like Sherlock Holmes, you’re trying to deduce what must have happened from partial evidence.

“Also, like in Sherlock Holmes, the game of knowing when you’ve got the answer — because one answer is so much more compelling or simple or beautiful than all the other possibilities — is often more sophisticated in astrophysics than in other branches of physics.

“You have to have the imagination to ask if there are other things that are allowed by the laws of physics that we haven’t detected which might be quite different; and, if so, should we have seen them already, and are there techniques for detecting them?”

Rocky Kolb, University of Chicago, admits how, dark energy, in stark contrast to dark matter, simply drives him nuts (A Universe of Particles: Cosmological Reflections).

“Dark matter doesn’t really bother me. I don’t lose sleep over it. My attitude is, ‘Oh, this is a great opportunity to use this idea. Maybe it’s this, maybe it’s that.’ Dark energy, on the other hand, to me is like fingernails on the chalkboard. It just drives me nuts. I don’t like it. I don’t like it; I admit that it’s a prejudice, but there it is.

I don’t have a good explanation. It’s not a logical thing. It’s not that I say the observations are wrong — although I did for at least a couple of years. I kept saying, ‘There must be some other effect responsible for these observations’, until I was finally convinced otherwise. I just can’t swallow it.”

Paul Steinhardt, Princeton University, describes his profound bemusement at how many of his colleagues flatly deny the multiverse problem of inflationary cosmology (Inflated Expectations: A Cosmological Tale).

“I’ve often had this discussion where I’ll say, ‘Well, what do you think about the multiverse problem?’ and they reply, ‘I don’t think about it.’

“So I’ll say, ‘Well, how can you not think about it? You’re doing all these calculations and you’re saying there’s some prediction of an inflationary model, but your model produces a multiverse, and so it doesn’t, in fact, produce the prediction you said: it actually produces that one, together with an infinite number of other possibilities, and you can’t tell me which one’s more probable.’

“And they’ll just reply, ‘Well, I don’t like to think about the multiverse. I don’t believe it’s true.’

“So I’ll say, ‘Well, what do you mean, exactly? Which part of it don’t you believe is true? Because the inputs, the calculations you’re using — those of general relativity, quantum mechanics and quantum field theory — are the very same things you’re using to get the part of the story you wanted, so you’re going to have to explain to me how, suddenly, other implications of that very same physics can be excluded. Are you changing general relativity? No. Are you changing quantum mechanics? No. Are you changing quantum field theory? No. So why do you have a right to say that you’d just exclude it?’”

Roger Penrose, University of Oxford, relates his longstanding frustration with the fact that so many of his colleagues fail to appreciate the importance of why the universe started out in such a remarkably smooth state, maintaining that the answer usually proposed to account for it — cosmic inflation — actually does no such thing (The Cyclic Universe).

“The original theorem that I had on black holes showed that singularities came about no matter how irregular the collapse was, or whatever kind of matter you had, just so long as the energy densities weren’t negative. And then Stephen Hawking picked up on this and applied these techniques to cosmological situations and we collaborated on a paper that encompassed most of these results.

“But there is a sort of irony there too, because at the time I was thinking, ‘Why are we limiting ourselves to these simple models of the universe? We could have all sorts of complicated things. That’s why you need the singularity theorems.’ But the point is that the universe is not like that.

“I remember being in a car near Princeton going to some conference, and in this car was Jim Peebles, one of the world’s most distinguished cosmologists. And I was saying, ‘Surely people should have considered all these complicated things that might happen…’ And he replied, ‘But it’s not like that. The universe is this very uniform state.’

“That is what started me thinking that this was the real puzzle: why is the universe so smooth and uniform instead of such a great big mess?

“Almost all of the calculations that people do are in a background of a very smoothed out universe. They put in a little perturbation here or there, but that doesn’t really come to terms with the problem. If we had been in this unbelievable messy situation to begin with then inflation wouldn’t do anything for us at all. The ‘unbelievable mess’ would have been a state of enormously high entropy — in terms of the gravitational degrees of freedom — and inflation, being a time-reversible dynamical process acting in accordance with the Second Law of Thermodynamics, wouldn’t be any use at all: it would just spread out the clumps. So it’s really no explanation to the question of why our universe is so uniform.”

Fully appreciating all the subtleties associated with today’s deep cosmological mysteries is unquestionably difficult. But getting a genuine taste of what the world’s top cosmologists are grappling with turns out to be startlingly easy.

Howard Burton, howard@ideasroadshow.com

This is the preface of Conversations About Astrophysics & Cosmology, a five-part Ideas Roadshow Collection which includes enhanced books that have been developed from in-depth conversations between Howard Burton and Roger Penrose, Paul Steinhardt, Scott Tremaine, Justin Khoury and Rocky Kolb.

This book is now available in electronic format and in paperback format, visit this page for further details: https://ideasroadshow.com/conversations-about-astrophysics-and-cosmology/.

Howard Burton holds a PhD in theoretical physics and an MA in philosophy. He was the Founding Executive Director of Perimeter Institute for Theoretical Physics.

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