The Big Unanswered Questions

Science, in its effort to unravel the rules governing the workings of nature, is all about asking the right questions. These questions, whose answers may be forever elusive, nevertheless frame the direction of scientific research, sometimes for decades or longer. In the process, new unexpected discoveries are made that refine or even change what the questions are. The process has continued successfully for over 400 years, and shows no signs of abating.

At the same time, it is important to distinguish between those questions that are answerable in principle and those that are not, and also between those questions whose answers can be practically obtained in the near or medium term. Graduate students in physics, for example, often enter graduate school with grand goals of discovering the Theory of Everything. But, as my friend and Nobel Laureate Frank Wilczek likes to say, what we really need is a Theory of Something!

With these issues as a guide, at the invitation of the editors of The Huffington Post, I list below a few of the burning questions that are driving the fields of cosmology and particle physics. The first two are being addressed by ongoing experiments that might shed significant light within the next decade. The last two are foundational questions whose resolution may be around the corner, but only if we are extremely lucky, or may take centuries if at all, depending on the kindness of nature as we probe it experimentally. Good ideas are much harder to come by than good experiments, so if a new good idea is required to resolve these foundational issues, all bets are off. It took a long time between Newton and Einstein to refine the theory of gravity after all.

What is the nature of Dark Matter?

Since the 1970's, when the evidence that the mass of our own galaxy, and indeed essentially all galaxies we can see, was dominated by some material other than stars and hot gas, the question of the nature of this 'dark matter' has played a central role in both cosmology and particle theory. As time progressed it became clear that dark matter dominates not only galaxies, but clusters of galaxies, and is over 10 times more abundant than all visible matter in the Universe.

With this abundance, arguments stemming from our understanding of the origin of light elements in the Big Bang imply that this material cannot be made of normal matter, i.e. matter comprised of protons, neutrons and electrons, the building blocks of all atoms. If instead, it is made from a new type of elementary particle that doesn't interact with electromagnetically, dark matter would exist as a diffuse gas or particles permeating throughout galaxies, including our own. As a result, it is not just "out there," it is "in here," going through you and me, the whole earth, and the computer I type this on.

This possibility provides both a challenge and an opportunity. Without knowing the identity of dark matter, attempts to detect it directly on earth require making some educated guesses about what it might be. However, there is at least the possibility of detecting it directly! Such detection could reveal not only the nature of what makes up the dominant matter in the universe, but also could tell us something fundamental about elementary particles and forces.

It is therefore particularly appropriate that there are two different approaches to detecting dark matter: (1) deep underground detectors hoping to detect minuscule signals from the rare dark matter particles that might actually scatter off an atomic nucleus and deposit energy, and (2) The Large Hadron Collider, which has turned on again, and may recreate briefly the conditions in the very early universe in which these new elementary particles were created, producing enough of them to be detected in collisions.

There is thus a race on right now, between direct detection of primordial dark matter underground, and the LHC, to see who might discover it first. Either set could easily announce a discovery this decade... Or, we may be wrong about its nature and need to go back to the drawing board.

Why is the Weak force Weak?

The Large Hadron Collider of course already has done more than search for dark matter. It did, after all, discover the Higgs particle, the last jewel in the crown that is the Standard Model of particle physics. Nevertheless, each new discovery in physics generates more questions. The Higgs endows the particles that convey the weak force with their masses. These in turn determine the nature of that force. But why does the Higgs exist at the scale it does? Why is the weak force so much weaker than, say, the strong force, and why are these forces, including electromagnetism, so much stronger than the force of gravity?

It is these questions that we hope the LHC will shed light on as it probes further, following its recent upgrade in energy and beam intensity. And interestingly, dark matter may play a role here as well. Perhaps the most interesting possible explanation of why the weak scale is what it is posits the existence of a new symmetry in nature, called Supersymmetry, that predicts a whole new set of elementary particles that have not yet been seen. The lightest of these could be absolutely stable, and is a prime candidate for dark matter. So, if the LHC discovers this particle it could not only unravel the mystery of dark matter, but also perhaps shed light on Supersymmetry, and beyond that, on the unification of all forces. Thus, we are waiting with great anticipation to what the LHC will report after its next year or two of operation.

Is Our Universe Unique?

Perhaps one of the most fundamental questions in physics, and indeed the question that Einstein himself mused about when he questioned whether 'God' had any choice in the creation of the universe (where of course he was speaking metaphorically and not literally), is whether our universe is unique, and whether the laws of physics are themselves unique and fixed. Would a small change in even one of the fundamental constants cause the whole edifice to crumble?
This question, while fundamental, may also seem completely inaccessible. After all, we only have access to our universe, so speculating about other universes may seem like pure metaphysics. This of course has not caused such speculation to disappear, and in fact, most extensions of the Standard Model of Particle physics suggest that our universe is not likely to be unique at all, and the perhaps the nature of elementary particles and fields that we observe may be due to pure chance.

What makes this question potentially more interesting is that we might get some indirect experimental hints of the existence of other universes, even if we may never directly observe them. Recently the BICEP2 experiment at the South Pole claimed to detect gravitational waves from the very early universe. Unfortunately it appears that the signal was probably due to foreground noise from our own galaxy. Nevertheless, if future experiments definitively detect such a background it would provide evidence of a process in the very early universe called Inflation, which, besides explaining many features of our observed universe at large scales, generically creates many other universes in the process as well. If we could measure these waves precisely, we could probe the possible nature of Inflation precisely, and in so doing explore the physics that led to the generation of our observable universe, and possibly others. In that way, while we might never have direct access to other such possible universes, we might have strong indirect evidence of their existence.

What is the nature of Nothing?

I couldn't resist saving this for last, as it is, after all, the subject of my most recent book. But I don't want to get hung up here with the contentious definitions of nothing. Here I simply refer to empty space, and to the remarkable discovery 15 years or so ago that empty space contains most of the energy in the universe, for reasons we don't understand at all. This energy is causing the observed expansion of the universe to accelerate, and will ultimately determine the future of our universe. There are a host of astrophysical observations now underway to try and shed light on the mystery of this 'dark energy' as it has become known, but at present we are no closer to understanding its origin than we were when it was first discovered. Is it truly the 'quantum energy of the vacuum', or is it associated with some new invisible field permeating all of space, or perhaps to something even more exotic?

I expect that without a full theory of quantum gravity we won't be able to fully resolve this problem, and that may take centuries. But I have been wrong before, and perhaps one of the upcoming probes of the expansion of the universe will reveal a new wrinkle that will point us in the right direction. That is why we simply have to keep trying. You never know in advance.

This post is part of a series commemorating The Huffington Post's 10 Year Anniversary through expert opinions looking forward to the next decade in their respective fields. To see all of the posts in the series, read here.