Top 5 Unsolved Problems In Astrophysics

Finally we have reached the last article of our Basics of Astrophysics series. Exactly two months ago, we started our journey of learning the concepts of Astrophysics. We started with the most basic question: What is Astrophysics? We learnt some of the basic tools used in this subject: EM Spectrum, distances, concept of magnitude, Hertzsprung Russell Diagram and so on. Then we studied the structure of Sun, the atmosphere of stars, nuclear reactions in stars, stellar evolution, formation of black holes, types of black holes, quasars, galaxies, nebulae and CMB radiation. We shall now conclude this series by discussing top 5 unsolved problems in Astrophysics.

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5. Origin of magnetar magnetic field

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

Unsolved problems in Astrophysics-Magnetars
Illustration of Magnetars

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. At a distance of halfway from Earth to the moon, a magnetar could strip information from the magnetic stripes of all credit cards on Earth. As of 2010, they are the most powerful magnetic objects detected throughout the universe.

4. 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 centre at equal or increasing speed over a large range of distances.

Unsolved Problems In Astrophysics
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. (Image: Wikipedia)

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

Also Read: 5 Evidences That Prove The Existence of Dark Matter

3. 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 (10^18 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.

2. SOlar Corona Heating Problem

The Solar Corona during a total solar eclipse

The corona is the outer most 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 surface of the Sun, the photosphere, is at about 5,900 K. How can the heat flow from cold to 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?

1. Black Holes

One of the most important unsolved problems in Astrophysics is regarding 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 while others say that they aren't the 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.

Also Read: The 3 types of Black Holes

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 word-wide.

These were the top 5 unsolved problems, among other, in the field of Astrophysics.

Author's Message

This concludes the Basics of Astrophysics series. These 30 articles were one of the most challenging projects taken up by us. Squeezing such a vast topic into just 30 easy to understand articles was difficult. I am glad that the series received a great response worldwide. One of the basic purpose to bring such a series was to tell the importance of Physics in this field. I also wanted to shed light on the vastness of the subject. People think that Astrophysics is all about the glamorous topics, the ones popular in the science fiction. But no, there is much more and these 30 articles are a proof.

Though the series is ending, the journey will continue. Our team will be back with another educational series like this one. Keep visiting us.

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