Astronomy, being an enormous field of science that study the Universe, the celestial objects that make up the Universe and the processes that govern the life cycle of these objects, provides no shortage of material for Astronomers/Scientists to reflect deeply on the subject.
Though, there is no shortage of materials to study our universe, the vastness and size of the universe and the celestial objects hinder the scientists to fully understand or to provides a detail explanation on some circumstances in the universe and on some astronomical entities processes.
Some of the unsolved problems in astronomy are theoretical, meaning that the existing theories are incapable of explaining a certain observational result of a phenomenon or experiments. The others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail. In fact, while some of the topics to be discuss may one day be solved through astronomical observations, others may never be solved.
The paragraphs below are some of the most compelling mysteries of astronomy, as presented in no particular order:
1 - The State of Dark Matter
Dark matter is just as real as the many exoplanets discovered in orbit around stars other than the Sun, discovered solely through their gravitational influence on their host stars and not from direct measurement of their light.
There is no clue what exactly Dark matter it is, it’s kind of annoying. But the scientists desperately need it in calculations to arrive at an accurate description of the universe.
The problem of Dark Matter as it was initially called “missing mass”, was first fully analyzed in 1937 by the Swiss-American astrophysicist Fritz Zwicky. Zwicky studied the movement of individual galaxies within a titanic cluster of them, located far beyond the local stars of the Milky Way. Using the motions of a few dozen galaxies as tracers of the gravity field that binds the entire cluster, Zwicky discovered that their average velocity had a shockingly high value.
Just as astrophysicists had come to accept dark matter in galaxy clusters as a mysterious thing, the problem reared its invisible head once again. In 1976, the late Vera Rubin, an astrophysicist at the Carnegie Institution of Washington, discovered a similar mass anomaly within spiral galaxies themselves. Studying the speeds at which stars orbit their galaxy centers, Rubin first found what she expected: within the visible disk of each galaxy, the stars farther from the center move at greater speeds than stars close in. The farther stars have more matter (stars and gas) between themselves and the galaxy center, enabling their higher orbital speeds. Beyond the galaxy’s luminous disk, however, one can still find some isolated gas clouds and a few bright stars. Using these objects as tracers of the gravity field exterior to the most luminous parts of the galaxy, where no more visible matter adds to the total, Rubin discovered that their orbital speeds, which should now be falling with increasing distance out there in obscure location, in fact remained high.
These largely empty volumes of space - the far-rural regions of each galaxy - contain too little visible matter to explain the anomalously high orbital speeds of the tracers. Rubin correctly reasoned that some form of dark matter must lie in these far-out regions, well beyond the visible edge of each spiral galaxy. The Rubin’s work, lead the scientists to now call these mysterious zones “dark matter haloes”.
The dark matter doesn’t simply consist of matter that happens to be dark. Instead, it’s something else altogether. Dark matter exerts gravity according to the same rules that ordinary matter follows, but it does little else that might allow scientists to detect it. Of course, scientists are left in ocean of this analysis by not knowing what the dark matter is in the first place. If all mass has gravity, does all gravity have mass? The scientists don’t know yet. Maybe there’s nothing wrong with the matter, and it’s the gravity they don’t understand up to now.
2 - Dark Energy
In 1929, the American astrophysicist Edwin P. Hubble discovered that the universe is not static, as oppose been in a static before, by the astrophysicists. He had found and assembled convincing evidence that the more distant a galaxy, the faster the galaxy recedes from the Milky Way. In other words, the universe is expanding. Now, embarrassed by the cosmological constant Lambda Λ, in Einstein’s General Relativity equation, which corresponded to no known force of nature, and by the lost opportunity to have predicted the expanding universe himself, Einstein discarded lambda entirely, calling it his life’s “greatest blunder.” By yanking lambda from the equation, he presumed its value to be zero, such as in this example: Assume X = Y + Z. If you learn later that X = 10 and Y = 10, then X still equals Y plus Z, except in that case Z equals 0 and is rendered unnecessary in the equation.
In 1998, astrophysicists dig-up cosmological constant, lambda again. Early that year, remarkable announcements were made by two competing teams of astrophysicists: one led by Saul Perlmutter of Lawrence Berkeley National Laboratory in Berkeley, California, and the other co-led by Brian Schmidt of Mount Stromlo and Siding Spring observatories in Canberra, Australia, and Adam Riess of the Johns Hopkins University in Baltimore, Maryland. Dozens of the most distant supernovas ever observed appeared noticeably dimmer than expected, given the well-documented behavior of this species of exploding star. Reconciliation required that either those distant supernovas behaved unlike their nearer brethren, or they were as much as fifteen percent farther away than where the prevailing cosmological models had placed them. The only known thing that “naturally” accounts for this acceleration is Einstein’s lambda Λ, the cosmological constant. When astrophysicists dusted it off and put it back into Einstein’s original equations for general relativity, the known state of the universe matched the state of Einstein’s equations.
Here was the first direct evidence that a repulsive force permeated the universe, opposing gravity, which is how and why the cosmological constant rose from the dead. Lambda suddenly acquired a physical reality that needed a name, and so “dark energy” took center stage in the cosmic drama, suitably capturing both the mystery and the associated ignorance of its cause.
The most accurate measurements to date reveal dark energy as the most prominent thing in town, currently responsible for 68 percent of all the mass-energy in the universe - dark matter comprises 27 percent, with regular matter comprising a mere 5 percent.
Even though, the force remains elusive and has yet to be directly detected, scientists remain optimistic that nature will cooperate and that they can determine the origins of dark energy.
3 - Missing Baryonic Matter
Baryon is a classification for types of particles – sort of an umbrella term – that encompasses protons and neutrons, the building blocks of all the ordinary matter in the universe. Everything on the periodic table and pretty much anything that you can think of as “stuff” is made of baryons.
In the late 1990s, cosmologists made a prediction about how much ordinary matter there should be in the universe. About 5%, they estimated, should be regular stuff with the 95% a mixture of dark matter and dark energy. But still, when cosmologists counted up everything they could see or measure at the time, they came up short, by a lot.
The sum of all the ordinary matter that cosmologists measured only added up to about half of the 5% that was supposed to be in the universe. This is known as the “missing baryon problem” and for over two decades, cosmologists looked hard for this matter without any success.
The traditional law of nature held that matter can be neither created nor destroyed, raised by the problem of missing baryon, creates two possible explanations: Either the matter didn’t exist and the math was wrong, or, the matter was out there hiding somewhere.
The number for the assumed amount of baryons were successfully estimated by the cosmologists, but the comprehensive map up of the missing baryons was still remained a mystery.
Remnants of the conditions in the early universe, like cosmic microwave background radiation, gave scientists a precise measure of the unverse’s mass in baryons.
Image credit: NASA
4 - Explosion of Stars
When a star similar to the Sun in mass dies, it casts its outer layers into space, leaving its hot, dense core to cool over the eons. But some other types of stars expire with titanic explosions, called supernovae. A supernova can shine as brightly as an entire galaxy of billions of "normal" stars. Some of these explosions completely destroy the star, while others leave behind either a super-dense neutron star or a black hole.
Over the years, scientists have studied supernovas and recreated them using sophisticated computer models, but what goes on inside a star and how these gigantic explosions occur remains an enduring astronomical puzzle.
X-ray & Optical Images of SNR E0519-69.0 - When a massive star exploded in the Large Magellanic Cloud, a satellite galaxy to the Milky Way, it left behind an expanding shell of debris called SNR 0519-69.0. Here, multimillion degree gas is seen in X-rays from Chandra (blue). The outer edge of the explosion (red) and stars in the field of view are seen in visible light from Hubble.
Image credit: By X-ray: NASA/CXC/Rutgers/J.Hughes; Optical: NASA/STScI - http://astropix.ipac.caltech.edu/image/chandra/587b
5 - The Hotness of the Sun’s Corona
The corona is the outer atmosphere of the Sun. It extends many thousands of kilometers (miles) above the visible "surface" of the Sun. The corona is above the Sun's lower atmosphere, which is called the chromosphere. A relatively narrow area called the transition region separates the corona from the chromosphere. Temperatures raise sharply in the transition region, from thousands of degrees in the chromosphere to more than a million degrees in the corona.
The temperature in the corona is more than a million degrees, surprisingly much hotter than the temperature at the Sun's surface which is around 5,500° C (9,940° F or 5,780 kelvins).
So how does the corona get so hot, despite extending far beyond the surface of the sun? Well, scientists are not exactly sure. Astrophysicists failed to give the exact details on how the temperature of Sun’s corona was raised to several multiples of the Sun’s surface temperature.
Sun in extreme ultraviolet "light" with a wavelength of 17.1 nanometers (171 Ångstroms).
Image credit: Credit: Solar Dynamics Observatory/NASA
6 - Galaxy rotation problem
The rotation curve of a disc galaxy, also known as velocity curve, is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's center. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived by applying gravity theory to the matter observed in a galaxy.
Scientists are unable to provide a detail explanation on either a dark matter is responsible for the discrepancies in the observed and theoretical speed of stars revolving around the center of galaxies, or was it something else?
Schematic representation of rotating disc galaxies in the early Universe (right) and the present day (left). Observations with ESO's Very Large Telescope suggest that such massive star-forming disc galaxies in the early Universe were less influenced by dark matter (shown in red), as it was less concentrated. As a result, the outer parts of distant galaxies rotate more slowly than comparable regions of galaxies in the local Universe.
Image credit: By ESO/L. Calçada - http://www.eso.org/public/images/eso1709a/ , CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=57133864 | plain
7 - Gravitational singularities Mystery
Gravitational singularities, spacetime singularity or simply singularity is a region where the spacetime curvature becomes infinite. For a non-rotating black hole, this region takes the shape of a single point and for a rotating black hole, it is smeared out to form a ring singularity that lies in the plane of rotation. In both cases, the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution.
Does general relativity break down in the interior of a black hole due to quantum effects, torsion, or other phenomena, considering the extremely intense gravity of a black hole from which nothing, not even light, can escape?
8 - No-hair theorem:
Do black holes have an internal structure? If so, how might the internal structure be probed, since any matter that falls into the event horizon of black hole disappears, and is therefore permanently inaccessible to the external observers?
9 - Size and shape of the universe:
The diameter of the observable universe is about 93 billion light-years, but what is the size of the whole universe? What is the 3-manifold of comoving space, i.e. of a comoving spatial section of the universe, informally called the "shape" of the universe? Neither the curvature nor the topology is presently known, though the curvature is known to be "close" to zero on observable scales. The cosmic inflation hypothesis suggests that the shape of the universe may be unmeasurable, but, since 2003, Jean-Pierre Luminet, et al., and other groups have suggested that the shape of the universe may be the Poincaré dodecahedral space. Is the shape unmeasurable; the Poincaré space; or another 3-manifold?
10 - Origin and future of the universe:
How did the conditions for anything to exist arise? Is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch, or a Big Bounce?
11 - Extraterrestrial life
The Fermi paradox, named after Italian-American physicist Enrico Fermi, is the apparent contradiction between the lack of evidence for extra-terrestrial civilizations and various high estimates for their probability.
The following are some of the facts that together serve to highlight the apparent contradiction:
- There are billions of stars in the Milky Way similar to the Sun.
- With high probability, some of these stars have Earth-like planets.
- Many of these stars, and hence their planets, are much older than the sun. If the Earth is typical, some may have developed intelligent life long ago.
- Some of these Intelligence may have developed interstellar travel, a step human are investigating now.
- Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.
- And since many of the stars similar to the Sun are billions of years older, the Earth should have already been visited by extraterrestrial civilizations, or at least their probes.
- However, there is no convincing evidence that this has happened.
There have been many attempts to explain the Fermi paradox, primarily suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but (for various reasons) we see no evidence.
Then, is there other life in the Universe? Especially, is there other intelligent life? If so, what was the stance for the Fermi paradox explanation listed above?
REFERENCES
- Ultraviolet Sun with Coronal Loops (April 2012): CENTER FOR SCIENCE EDUCATION. https://scied.ucar.edu/image/sun-uv-171-sdo-coronal-loops-april-2012. Retrieved: August 10, 2020.
- The Sun's Corona (Upper Atmosphere): CENTER FOR SCIENCE EDUCATION. https://scied.ucar.edu/solar-corona#:~:text=The%20corona%20is%20the%20outer,hot%20but%20very%20tenuous%20plasma.. Retrieved: August 10, 2020.
- Fermi paradox: Wikipedia, the free encyclopedia. https://en.wikipedia.org/wiki/Fermi_paradox. Retrieved: August 09, 2020.
- Exploding Stars: StarDate. https://stardate.org/astro-guide/exploding-stars#:~:text=When%20a%20star%20like%20the,billions%20of%20%22normal%22%20stars.. Retrieved: August 11, 2020.
- Galaxy rotation curve: Wikipedia, the free encyclopedia. https://en.wikipedia.org/wiki/Galaxy_rotation_curve. Retrieved: August 11, 2020.
- Half the Matter in The Universe Was Missing – we found it hiding in the cosmos: The Conversation, Academic rigour Journalistic flair. https://theconversation.com/half-the-matter-in-the-universe-was-missing-we-found-it-hiding-in-the-cosmos-138569. Retrieved: August 11, 2020.
- Paul S. (August, 2017). The Sun's Corona, A Fiery Halo, Is Still a Mystery to Scientists: SPACE.COM https://www.space.com/37832-corona-of-the-sun-is-a-mystery.html. Retrieved: August 09, 2020.
- NEIL D. T. (May, 2017.) Astrophysics for People in a Hurry