While astronomy and cosmology have made amazing advances in the last century, many deep mysteries remain. The staff of Science magazine consulted among themselves and spoke to professional researchers. The result: they identified eight areas where no clear answers exist—and where the questions may continue to bother us for some time.

New observatories and experiments are coming online over the next few years, but budget cuts for science around the world threaten much fundamental research—and many of these questions may remain too challenging for the time being. Some of the questions are deep and relate to fundamental details about our Universe, while others may be tractable as new observations provide more data. We'll look at each question individually.

Question 1: What is Dark Energy?

The 2011 Nobel Prize in physics was awarded to three cosmologists—Brian Schmidt, Saul Perlmutter, and Adam Riess—for their discovery that the Universe is expanding at an ever-accelerating rate. While this cosmic acceleration has been labeled "dark energy", that name is a placeholder for our ignorance: we don't have a good idea of what it is, despite 14 years of effort.

That's not to say cosmologists are completely at a loss. According to data from the Wilkinson Microwave Anisotropy Probe (WMAP) and other observations, we know that dark energy comprises about 74 percent of the total energy density in the Universe today. Through measurements of white dwarf supernova explosions (known as "type Ia") and studies of the distribution of galaxies on the largest scales, astronomers determined that dark energy's effects either have not changed over time, or have changed only very slightly. In other words, we know how much dark energy there is and have some ideas about its behavior.

Nevertheless, we are still no closer to identifying what dark energy is. New instruments (including the Dark Energy Camera (DECam) at Cerro Tololo in Chile) will survey vast numbers of galaxies, probing the large-scale structure of the Universe and locating more supernovae. The hope is to distinguish between the major candidates for dark energy, including the possibility that the accepted theory of gravity—Einstein's general theory of relativity—may need modification on the largest distance scales.

Question 2: How Hot is Dark Matter?

Dark matter, which makes up 80 percent of all matter in the Universe, is an even more persistent mystery than dark energy. The first hint about dark matter's existence came in the 1933, from Fritz Zwicky's study of galaxy clusters. Its presence was cemented in the early 1970s by Vera Rubin's measurement of galactic rotation. Countless other observations in subsequent decades have confirmed dark matter's role in the cosmos, ranging from spiral galaxies to WMAP's measurement of temperature fluctuations in the early Universe.

As with dark energy, we don't know what dark matter is (though we have some solid leads), but that's not the question posed in Science. Instead, they focus on another aspect: the temperature of dark matter. Temperature is a measure of the average speed of particles; dark matter doesn't interact readily, so it's very hard to heat or cool, which means its speed is dictated by its mass. The consensus in cosmology is that dark matter is cold: relatively heavy particles moving at slow rates. If dark matter is too light—too warm—then galaxy clusters can't form: the rapid motion of the particles would prevent them from collecting in one place.

However, if dark matter is a little warmer than permitted by cold dark matter (CDM), then it might solve another difficulty. While CDM is an excellent model for large-scale structure formation and cosmology, the simplest version runs into trouble within galaxies, where it predicts too much dark matter. While this isn't a deal-breaker for CDM (the interplay between ordinary matter and dark matter at these densities makes the math complicated), it's possible warmer dark matter might help. It would still be heavy enough to allow for structure formation, but light enough that it wouldn't form cusps, the sharp increases in density near galactic centers that plague CDM models. Interestingly, particle physicists are already discussing this idea, as I found out at Pheno 2012.

Question 3: Where Are the Missing Baryons?

As if missing mass in the form of dark matter isn't enough, about 50 percent of ordinary matter—known as baryonic matter— is also unaccounted for. Using cosmic microwave background measurements from WMAP, cosmologists have determined how much baryonic matter there must be. But we've added up everything seen in galaxies, clusters, and gas, and come up short of the WMAP result.

The current consensus is that the missing atoms lie in between galaxies, but the density of this warm-hot intergalactic medium (WHIM, one of many whimsical acronyms in cosmology) is too low. The predicted temperature of the WHIM is high enough to emit ultraviolet and X-ray light, but none of our current telescopes are able to pick up light from such a diffuse source. However, even though it's an abiding mystery, the missing baryon problem seems (to this writer at least) to be of a different character than the dark matter or dark energy problems: at least astronomers know what to look for, and dedicated searches are likely to achieve success.

Question 4: How Do Stars Explode?

Supernovas from the deaths of very massive stars are, for a brief time, the brightest objects in the Universe. However, astronomers lack a clear model to describe the explosions, known as core-collapse supernovas. Part of the problem lies with the nature of stars: we cannot observe inside an exploding star; we can't even see inside the Sun—and that's the only star close enough to see in detail. Therefore, astronomers have to resort to detailed computer simulations, but as yet none of those reproduce the explosions that are observed.

The second important type of supernova, known as type Ia, involves white dwarfs, which are the cores of stars like our Sun. White dwarfs can reach a maximum mass, after which they become unstable, so they explode in a predictable way. Despite their usefulness, some fundamental facts about them are uncertain: do type Ia supernovas require one white dwarf or two, or do both of these scenarios occur? White dwarfs are Earth-size, even though their masses are comparable to the Sun's, so the situation is worse than with regular stars: we can't see them in detail, even with our most powerful telescopes. As with core-collapse supernovas, computer simulations fail to replicate the explosions we actually observe.

Question 5: What Reionized the Universe?

The cosmic microwave background (CMB), which provides some of the best data for the contents and structure of the Universe, was created when the first atoms formed, about 380,000 years after the Big Bang. However, between the formation of the CMB and the earliest galaxies we see, those atoms were broken apart again. This process, known as reionization, is still mysterious. Ionization requires energetic light—ultraviolet or higher frequency—which in turn likely requires a lot of high-mass stars in bright young galaxies.

Current observations don't show enough galaxies from when the Universe was so young, though the data simply isn't good enough—yet. A number of instruments are either online or in production, dedicated to the task of finding and studying the first stars, along with their environment. These include LOFAR (the Low Frequency Array), the James Webb Space Telescope (to be launched in 2018), and the Square Kilometer Array (SKA). Looking specifically at the epoch of reionization should reveal the solution to the mystery.

Question 6: What's the Source of the Most Energetic Cosmic Rays?

Cosmic rays are massive particles, mostly protons, that impact Earth's atmosphere at appreciable fractions of the speed of light. Most of these come from the Sun, but many—including the highest energy particles—are of mysterious origin. The most dramatic of these was the "Oh My God Particle," a cosmic ray proton with energy equivalent to a Major League fastball, but there are a lot more traveling at less frightening speeds. Their distribution, as measured by cosmic ray observatories, indicates the highest-energy particles come from outside the Milky Way.

Researchers using the Pierre Auger Observatory in Argentina have potentially identified supermassive black holes at the center of galaxies (known as active galactic nuclei or AGNs) as sources for cosmic rays. However, the number of events was small and the spatial resolution of the observatory wasn't good enough to pinpoint the origin of the particles. Worse, recent results from the IceCube neutrino observatory in Antarctica cast doubt on whether gamma ray bursts (GRBs) associated with these black holes could be responsible for high-energy cosmic rays, so the mystery continues.

Question 7: Why Is the Solar System So Bizarre?

Thirty years ago, our entire catalog of known planets orbited just one star: the Sun. Today, exoplanets—planets orbiting other stars—dominate the census. One major effect is that planetary formation models have had to change, since these new exoplanets don't fit with the Solar System-centric ideas of old. Even so, our Solar System can seem weird. The terrestrial planets (Mercury, Venus, Earth, and Mars) are chemically similar, but vary a lot in terms of atmosphere, magnetic fields, and internal structure. Uranus is tipped on its side, Venus is flipped completely over so it rotates the opposite way from its orbital direction, and so forth.

On the other hand, we don't know what directions exoplanets rotate or anything about their magnetic fields. Some guesses about composition can be made, and under special conditions, astronomers can glean some atmospheric data, but it's a far cry from the detailed view we have of the Solar System. In other words, weird as our Solar System is, it may yet turn out to be no weirder than any other star system. My guess (as a cosmologist rather than a planetary scientist) is that the detailed history of each system may turn out to be as important as the formation, making the specific weirdness a contingent rather than predetermined property.

Question 8: Why Is the Sun's Corona So Hot?

We know the Sun better than any other star, but a few aspects still nag at researchers. While the Sun's interior is well modeled, something odd happens in the atmosphere. The temperature at the surface (a mere 5800 Kelvin) transitions to millions of degrees in the corona, the Sun's billowy crown that's visible mainly during total eclipses or using X-ray telescopes.

Many astronomers suspect the excessive coronal heating is magnetic in origin, but how that works is still mysterious. Perhaps waves within the Sun's magnetic field accelerate the electrons in the corona, or maybe tiny flares are created when loops of the magnetic field collapse back onto the surface. Both numerical simulations of the Sun and new instruments should help resolve the question.

Questioning the Questions

These questions are all undoubtedly mysteries, though they are not all of the same level of complexity. Speaking for myself, I would rephrase Question 7 to "How do planetary systems form?", which covers another issue: many exoplanetary systems contain very massive planets close in to their host stars. Weird variations like Venus and Uranus may not be classifiable in a broader scheme, but knowing why we have one big Jupiter relatively far from the Sun seems to be a bigger puzzle. Similarly, I would alter Question 2 to become "Where are all the small galaxies?", since structure formation models predict a lot of low-mass galaxies as well as large ones we see. One possible solution may be warm dark matter, but it seems (again to this cosmologist) other options should be left on the table.

However, these are relatively minor quibbles, and it's obvious answering any of these questions would constitute a significant advance in the knowledge of our marvelous Universe.

Science, 2012. DOI: 10.1126/science.336.6085.1090-a (About DOIs).