The June 1, 2012 issue of Science magazine presented a list of 8 astronomy questions. These questions are not the type that could be answered by the next Mars Rover or the next release of Kepler data. Rather, these questions are cosmic in every sense of the word. Some may not be answered for centuries but most will at least be studied and perhaps some answers will be presented in our lifetime. Here are the questions:
1. What is Dark Energy? There are some possibilities. Maybe it is just a property of empty, the cosmological constant that Einstein considered. Or maybe it is a unknown force sometimes called the fifth essence of “quintessence”. Or maybe it is an illusion, gravity and general relativity create artifacts that we don’t understand. How can we solve this? The Dark Energy Survey will use a Chilean telescope to study hundreds of millions of galaxies, 100,000 galaxy clusters and thousands of supernovae. This should help astronomers determine if Dark Energy is truly constant or if it diminishes as space expands which would bolster the case for a quintessence. Other efforts to study dark energy include the ESA’s planned space observatory called Euclid. There are plans in the U.S. to build an 8.4 meter instrument called the Large Synoptic Survey Telescope. Current measurements of dark energy have error bars that make it impossible to separate the quintessence theory from the cosmological constant theory. And if it turns out to be illusory, we might not know that for sure for a long time.
2. How Hot is Dark Matter? Notice this question is not “What is Dark Matter?”. Astronomers think an answer about that is not far off. If dark matter is cold that predicts that the Milky Way should be surrounded by thousands of dwarf galaxies. The count so far: 20. Also, cold dark matter suggests that the dark matter should be concentrated tightly in the center of a galaxy but it appears to be more spread out though that could be due to some interactions within galactic centers. Another problem, cold dark matter says the Milky Way galaxy should have about 10 large satellite galaxies such as the Magellanic clouds. The count so far: 3. And this is a big problem because it seems real unlikely that 7 Magellanic clouds could have escaped our attention this long. But let’s get back to what dark matter is. If it turns out to be WIMPs (Weakly Interacting Massive Particles), and the LHC might create them, that would bolster the case for Cold Dark Matter. But not finding WIMPs means dark matter may be hard to pin down anytime soon.
3. Where are the Missing Baryons? If it is embarrassing that we don’t know what 96% of the universe is made of, what word do you use to characterize the fact that half of the rest of the matter is unaccounted for as well? Baryonic matter is the matter that is mostly protons and neutrons. Not only is much of it missing, it seems to have disappeared recently. Looking at the list of distant quasars, it looks like the expected number of baryons was still around 10 billion years ago. But when looking at nearer (and thus newer) objects, the baryon count falls off. The breakdown goes like this: 10% of the baryons are in galaxies; another 10% is in the warm gas between galaxies; another 30% is in cold globs of matter outside of galaxies. The remaining 50% might be nothing but a WHIM (Warm-Hot Intergalactic Medium). The temperature of this medium might be as hot as 10 million K which means it would emit electromagnetic radiation in the high UV or low X-ray wavelengths. The medium would be so thin that light passing through it would not show up as spectral lines. Well, almost. Astronomers think they should be able to detect highly ionized Neon VIII (neon minus 8 electrons) or Oxygen VI (O stripped of 6 electrons). They may have made such a detection but others are not convinced. Instead they look forward to new, more sensitive X-ray space observatories.
4. How Do Stars Explode? While a lot about supernovae is known, questions remain. For example, in the case of Type 1A supernovae, a binary star system heads toward destruction when a white dwarf star pulls material from its companion. Since the supernovae will occur when a distinct amount of material (1.38 solar masses) has been built up onto the white dwarf, these supernovae have consistent properties making them useful as standard candles for measuring the accelerating expansion of the universe. But how long does it take from the start of this mass stealing process until the star explodes? What are the exact final events of such a star system? The answer might lie in studies of the supernova as soon as it starts, triggered by detection of a gamma-ray burst.
5. What Reionized the Universe? According to the standard model of cosmology, the universe appeared dark for the first 400,000 years after the big bang. The cosmic microwave background is from this time when photons were finally allowed to move. Move forward a few hundred million years when something stripped the electrons off of the atoms. This is called ionization. Why did that ionization occur? The answer might be that early stars emitted UV radiation and early galaxies filled with these stars would practically burp out ionized hydrogen, swamping the existing neutral hydrogen. The problem is that the number of such galaxies should not be enough to do the job. Hubble can look back to galaxies when the universe was 800 million years old but that isn’t far enough. The James Webb Space Telescope, now slated for a 2018 launch, should be able to find galaxies from when the universe was 700 million years, maybe close enough. Another idea is to find the signature of neutral hydrogen which would be so red-shifted that it would appear in long radio waves. Remember stringing out a wire for your crystal receiver?
6. What’s the Source of the Most Energetic Cosmic Rays? Some cosmic rays are easily accounted for. These particles, mostly single protons, travel at almost the speed of light and have energies of around 10 to 10 power electron volts, if they come from the sun. More powerful energies come from supernovae, up to 10 to 15 power eV. But some of these particles have energies of 10 to 20 power eV and models of supernovae can’t account for them. Recent measurements suggest that these higher energy particles become active galactic nuclei (such as quasars) that are relatively close (250 million light-years). More information may come from the Extreme Universe Space Observatory. This spacecraft was designed by the ESA but dropped for budgetary reasons. Japan has picked up the idea and plans to launch it in 2016 and add it to the ISS.
7. Why is the Solar System So Bizarre? Why are Venus and Earth so different from each other? They have the same mass and density. But one has an ocean, a magnetic field, a large moon, rotates 365 times a year, has a breathable atmosphere, and it has plate tectonics. Venus isn’t close on any of those and doesn’t even rotate in the same direction. Why does Mercury have a stronger magnetic field than the much larger Mars? Also, the MESSENGER spacecraft is telling us that Mercury is made out of different stuff than the other terrestrial planets. Jupiter and Saturn have magnetic fields that seem more or less aligned with their axis of rotation, that makes sense. But Uranus and Neptune have magnetic fields that are off by more than 45 degrees. Why the diversity? It might come down to nothing but randomness. If we could study exoplanets for these same attributes we might see stronger patterns.
8. Why is the Sun’s Corona So Hot? The sun is really hot at the core, 16 million K. It is cooler on the surface, 5780K. But then the sun’s “atmosphere” heats up to 1 million K. The answer as to why this should be usually includes magnetic fields transporting energy. But just how does that work? One idea involves magnetic waves, possibly the Alfve’n wave, an oscillation traveling along magnetic field lines. Another idea is nanoflares, produced when magnetic field lines break and reconnect. This probably does happen but does it really account for the high heat? Three new instruments might shed some light on the problem. NASA’s Interface Region Imaging Spectrograph is set to launch in December. ESA has a Solar Orbiter scheduled for 2017 which will give us a good view of the poles. And the Advanced Technology Solar Telescope is under construction in Hawaii - it will be twice the size of the current largest solar telescope. Studies will concentrate on the chromosphere, the thin area between the solar surface and the corona best seen during a total eclipse as a thin red line.
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