Although it has been more than 200 years since astrophysics formally began, what we know is just a drop. The vast ocean of knowledge lies undiscovered before us. Yet, one of the most beautiful aspects of science is that discoveries lead us to new mysteries and a deeper understanding of the universe. In this article, I have compiled a list of 10 unsolved problems in astrophysics that are way too interesting. This article is a part of our’ Basics of Astrophysics’ series. You can read all the articles of the series here.

Unsolved Problems In Astrophysics

10. Tabby’s Star

Tabby’s star, formally known as KIC 8462852, is an F-type main-sequence star in the constellation of Cygnus. What brought Tabby’s star into the limelight is its anomalous dimming. One of the graphs that shows the maximum dimming of around 22% is given below.

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28 February 2013 − day 1519
22% max dip (Kepler)

Several hypotheses have been put forward to explain these fluctuations. The first one says that an uneven ring of dust surrounds the star. According to NASA, researchers found less dimming in the star’s infrared light than its ultraviolet light. Any object larger than dust particles would dim all wavelengths of light equally when passing in front of Tabby’s Star. The second hypothesis says that Tabby’s star is surrounded by a cloud of disintegrating comets orbiting the star elliptically. The fact that a cloud of a comet can cause dimming up to 22% has cast doubt on this theory.

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Artist’s impression of the theory of comet dust around Tabby’s star

Another interesting hypothesis says that the dimming might be a result of an alien megastructure. That alien megastructure could be the Dyson swarm, built by civilizations to intercept their star’s light for their energy needs. However, the likelihood of extraterrestrial intelligence being the cause of the dimming is very low.

9. Origin of Magnetar Magnetic Field

Magnetars are neutron stars with a tremendous magnetic field. The magnetic field of a typic magnetar lies between 1-100 billion teslas. To bring it to perspective, the maximum magnetic field that can be generated under special laboratory conditions is just a few hundred teslas. The neutron stars are about 20 Km wide and have a mass of 2-3 times that of the Sun. This implies they are quite dense. A tablespoon of its material will weigh 100 million tonnes.

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An artist’s impression of a magnetar (mage: © ESO/L. Calçada)

The origin of such a strong magnetic field is hypothesized to be a magnetohydrodynamic process in the turbulent, extremely dense conducting fluid that exists before the neutron star settles into its equilibrium position. The magnetic field of a magnetar would be lethal even at a distance of 1000 km due to the strong magnetic field distorting the electron clouds of the subject’s constituent atoms, rendering the chemistry of life impossible. A magnetar could strip information from the magnetic stripes of all credit cards on Earth at a distance of halfway from Earth to the moon. As of 2010, they are the most powerful magnetic objects detected throughout the universe.

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8. Fast Radio Bursts

Fast Radio Bursts (FRBs) are unresolved (point source-like), broadband (spanning a large range of radio frequencies), millisecond flashes found in parts of the sky. The physical phenomenon that causes these bursts is still a mystery. Possible sources of FRBs are neutron stars, black holes, or extraterrestrial intelligence.

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An artist’s impression of an FRB

Although the exact origin and cause are uncertain, most are believed to be extragalactic. The first Milky Way FRB was detected in April 2020. About six months later, after careful analysis, scientists confirmed that the source of this FRB was a magnetar. The phenomenon that produces these flashes is still one of the most intriguing unsolved problems in astrophysics.

7. Ultra-High Energy Cosmic Rays

The Ultra-High Energy Cosmic Rays (UHECR) are the cosmic rays with unimaginably high energy: greater than exa electron volt (1018 eV). The Oh My God particle by the University of Utah’s Fly’s Eye experiment on the evening of 15 October 1991 over Dugway Proving Ground, Utah, was a shock to astrophysicists.

They estimated its energy to be approximately 3.2×1020 eV (50 J) —in other words, an atomic nucleus with kinetic energy equal to that of a baseball (5 ounces or 142 grams) traveling at about 100 kilometers per hour (60 mph). The origin of such particles is still a hypothesis. It is one of the major unsolved problems in astrophysics.

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6. The Solar Cycle

The solar cycle is a repeated cycle of solar fluctuations that has a period of 11 years. Every 11 years, the magnetic field of the Sun flips completely. This means that the Sun’s north and south poles switch places. Solar activity is also influenced by this cycle. The beginning of the solar cycle is the solar minimum, with minimum sunspots. Then, in the middle of the cycle, the solar activity reaches its maximum, and the number of sunspots also increases.

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Images of the Sun during one solar cycle: Solar maximum occurred during 2001 while 1996 and 2006 were near solar minimum. Image credit: NASA

Giant eruptions on the Sun, such as solar flares and coronal mass ejections, also increase during the solar cycle. These eruptions send powerful bursts of energy and material into space. However, understanding the solar cycle is still a great mystery for scientists.

5. The Corona Mystery

Another unsolved problem in solar physics is the corona mystery. The corona is the outermost part of the solar atmosphere. It can be seen during a total solar eclipse. The problem is that the corona has a temperature of about a million K while the Sun’s surface, the photosphere, is at about 5,900 K. How can the heat flow from cold to the hot body? How can the most basic law of thermodynamics break down? Is there any other mechanism that is taking place in the corona? If yes, then what is it? Is it the Alfven Waves?

4. The Lithium Problem

The lithium problem is an astrophysical problem on the abundance of Li-7 isotope. Minutes after the big bang, the first elements formed were hydrogen, helium, lithium, and trace amounts of other elements. The observed composition of the universe is consistent with the big bang model for hydrogen and helium. However, there is a discrepancy when it comes to lithium. The most widely accepted models of the Big Bang suggest that three times as much primordial lithium should exist.

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Several solutions have been hypothesized to solve the missing lithium problem. First, there might be a need for a more accurate determination of the abundance of lithium in the universe. Second, the astrophysical solution says that there is a calculation error. Another possible way to find the remaining lithium is to make corrections in nuclear physics. Incorrect or missing reactions could give rise to the lithium problem.

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3. Hawking Radiation

The Hawking radiation is electromagnetic radiation that is predicted to be released by black holes due to quantum events near the event horizon.

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It is important to recall that for all discovered and localized black holes. Unfortunately, this effect is too small to be measured under experimentally achievable conditions. Because of that, Hawking radiation has not been detected yet. In the meantime, physicists are building and analyzing analogous systems.

In September 2010, it was thought that a laboratory-created body simulating a white hole’s event horizon radiated an optical analog to Hawking radiation. However, to this date, no official confirmation of the accuracy of this experiment exists. One of the latest predictions also links sonic black holes (for which sound perturbations are analogous to light in a gravitational black hole) to a form of perfect fluid flow that could simulate Hawking radiation. Read about Hawking radiation in our detailed article here.

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While it is unknown how and when Hawking radiation will be detected, scientists are diligently working towards this purpose. What happens after such radiation is discovered is also unknown, but one can only hope that it will open a new era for understanding black holes. 

2. Galaxy Rotation Curve And Dark Matter

A galactic rotation curve is the plot of the orbital rotational velocity of stars versus their distance from the center. Consider the solar system. Mercury orbits the Sun in 88 days, while for Neptune, it takes about 165 years. Also, the orbital velocity of planets decreases as we go from Mercury to Neptune. However, this is not true for disk galaxies. Stars revolve around their galaxy’s center at equal or increasing speed over a large range of distances.

Galaxy rotation curve
Rotation curve of spiral galaxy Messier 33 (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (gray line). The discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy.

This discrepancy has two implications. Either Newton’s classical mechanics’ laws aren’t universal, or there is an additional matter in the galaxy that isn’t visible to us. This invisible matter is known as dark matter. The discrepancy in the galaxy rotation curves is one of the first pieces of evidence of dark matter.

1. Black Holes

One of the most important unsolved problems in Astrophysics is black holes. Black holes first appeared in the solutions of the equations of general relativity. Though a lot of research is taking place in this field, the question is: Do the mathematical black holes predicted by General Relativity really exist, or are they the eternally collapsing objects?

The scientific community is divided into two sects: one which says that the black holes observed are the ones that are predicted by GTR, having a singularity. In contrast, others say that they aren’t mathematical black holes but are eternally collapsing objects. There are many observed facts that the former cannot explain. One of them is the strength of the magnetic field. How can black holes produce such a strong magnetic field when the only source is the particles of the accretion disk. You can find peer-reviewed research papers on the Eternally Collapsing Objects here.

The model of eternally collapsing objects instead of black holes accounts for many observed facts and is thought to be a better model. But still far away from being accepted worldwide.

These are some of the major unsolved problems in astrophysics.

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