At the end of the 19th century, scientific communities and physicists from all over the world reached a very peculiar conclusion that all the important laws of physics had already been discovered and that the future research and experiments should be concerned only to clear minor problems and for improvements in method of calculations. But, remember that was the time when we were unfamiliar with the theory of Relativity and Maxwell’s equation. Today, modern scientists are not even close to think that we have all the knowledge about the physics that the earth and nature has to offer. Even with all the prevalent knowledge about the physical nature, there are still some mysteries hidden and unanswered.

### 11. Entropy (Arrow of Time)

To successfully understand the entropy one has to get familiar with the 2nd Law of Thermodynamics, which states that during any spontaneous process, the total entropy change for an isolated system and its surroundings is positive. Entropy is a measure of the ways energy can be distributed in a system of molecules or a measure of microscopic configuration Ω that correspond in a thermodynamic system in a state specified by certain macroscopic variables.

Although entropy does increase in the model of an expanding universe, the maximum possible entropy rises much more rapidly, moving the universe further from the heat death with the increasing time, not closer. This results in an “entropy gap” pushing the system further away from the posited heat death equilibrium. There are other complicating factors, such as the energy density of the vacuum and macroscopic quantum effects, are difficult to reconcile with thermodynamical models, making any predictions of large-scale thermodynamics extremely difficult.

### 10. Baryon asymmetry

The baryon asymmetry problem in physics refers to the imbalance in the baryonic matter (matter) and antibaryonic matter (antimatter) in the physical universe. Now what is a matter and antimatter? A matter is anything that has mass and volume, which occupy space. In other words a matter is made up of atoms, positively charged protons, neutral neutrons and negatively charged electrons. On the other hand an antiparticle has the same mass as a matter but with opposite charges. Basically everything is made up of matter! Right?

Now just think of the first few moments after the Big Bang, we can assume a huge amount of both matter and antimatter were created which later generated energy that drove the evolution of the universe. But for some unknown reason there is more amount of matter than antimatter, which should have been equal. Neither the standard model of particle physics nor the theory of relativity provides an explanation on why this is the case. If matter and antimatter are created and destroyed together, the universe should contain nothing but leftover energy. Nevertheless, what we see today is a tiny portion of matter – about one particle per billion, which managed to survive.

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According to the scientist there is some possible explanation to understand this asymmetry. In particle physics violation of postulated C-symmetry (charge conjugation symmetry) and P-symmetry (parity symmetry) together known as the Charge Parity (CP). It is a discrete symmetry of nature given by the product of two components:

charge conjugation (C) and parity (P). Charge conjugation transforms a particle into the corresponding anti-particle. Parity is the transformation that inverts the space coordinates. But this symmetry is slightly violated during certain types of weak interaction, violation that could allow matter to be produced more commonly than antimatter in conditions immediately after the Big Bang.

### 9. Shape of the universe

Did it ever occur to you that what if our Earth has a different shape than a sphere? Well it is a reasonable question after all that what is the shape of the Universe. Is it a sphere? A torus? Is it open or closed, or flat? According to different mythologies the shape of the earth has been defined differently. Hindu texts describe the Universe as a cosmic egg, while the Jains believed it was human-shaped. Greeks saw the Universe as a single island floating in an otherwise infinite void, while Aristotle believed it was made up of a finite series of concentric spheres, or perhaps it’s simply “turtles all the way down”.

Scientists are considering mainly three possibilities; positively-curved, negatively-curved, and flat. Based on the Einstein’s theory of Relativity scientists have calculated the “critical density” of the universe. The critical density is proportional to the square of the Hubble constant, which is used in measuring the expansion rate of the universe. Comparing the critical density to the actual density can help scientists understand the cosmos.

If the actual density of the universe is less than the critical density, which is Ωο>1, then there is not enough matter to stop the expansion of the universe, and will expand forever. The resulting shape is curved like the surface of a saddle. This is known as an open universe. The second possibility is if the actual density of the universe is greater than the critical density (Ωο<1), then it contains enough mass to eventually stop its expansion. In this case, the universe is closed and finite, though it has no end, and has a spherical shape. Once the universe stops expanding, it will begin to contract. Galaxies will stop receding and start moving closer and closer together. Eventually, the universe will undergo the opposite of the Big Bang, often called the “Big Crunch.” This is known as a closed universe.

However, if the universe contains exactly enough mass to eventually stop the expansion, the actual density of the universe will equal the critical density (Ωο=1). The expansion rate will slow down gradually, over an infinite amount of time. In such a case, the universe is considered flat and infinite in size.

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### 8. What is Dark Matter?

What is the first thing comes to your mind when you read ‘Dark Matter’ in any science magazines or books? As the name suggests it is dark because it does not emit or interact with electromagnetic radiation that is light, and is thus invisible to the entire electromagnetic spectrum. It is not in the form of dark clouds of normal matter known as baryonic particles or baryons. Why? Because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Lastly, dark matter is not antimatter, simply because we do not see the unique gamma rays that are produced when antimatter annihilates with matter.

This is pretty much all we know about the Dark matter. Although there is no possible way to observe dark matter, its existence and properties are concluded from its gravitational effects such as the motions of visible matter, gravitational lensing, its influence on the universe’s large-scale structure, and its effects in the cosmic microwave background. There are certain possibilities regarding the actual state of the Dark Matter which scientists are currently working on.

### 7. What is Dark Energy?

By fitting a theoretical model of the composition of the Universe into the set of cosmological observations, scientists have come up with the composition of matter, that is 68% dark energy, 27% dark matter and 5% normal matter. In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to infiltrate all of space and held responsible to accelerate the expansion of the universe.

Dark energy is the most accepted hypothesis to explain the two decade long observations indicating that the universe is expanding at an accelerating rate. One explanation for dark energy is that it is a property of space. According to Albert Einstein it is possible for more space to come into existence. With the help of cosmological constant, we can predict that “empty space” can possess its own energy. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the Universe.

Another possibility is that Einstein’s theory of gravity is not correct. That would not only affect the expansion of the Universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. If it does turn out that a new theory of gravity is needed, what kind of theory would it be? There are candidate theories, but none are compelling. So the mystery goes on.

### 6. Magnetic Monopole

A magnetic monopole would be a magnet with only one pole (A North pole without South pole or vice versa). In other words, it would have a net “magnetic charge”. But magnetic monopoles have never been observed or created experimentally. When a magnet is cut in half, it becomes two magnets, each with its own north and south poles. There doesn’t seem to be a way to create a magnet with only one pole. Yet in particle physics theories like Grand Unified and string theory strongly predicts that the magnetic monopoles should exist.

In particle theory, a magnetic monopole arises from a topological glitch in the vacuum configuration of gauge fields in a Grand Unified Theory. The length scale over which this special vacuum configuration exists is called the correlation length of the system. A correlation length cannot be larger than causality would allow, therefore the correlation length for making magnetic monopoles must be at least as big as the horizon size determined by the metric of the expanding Universe. According to that logic, there should be at least one magnetic monopole per horizon volume as it was when the symmetry breaking took place.

### 5. Supersymmetry

To understand the supersymmetry one first has to grasp the basic knowledge of standard model of particle physics. The Standard Model is a theory concerning the electromagnetic, weak, and strong nuclear interactions, as well as a framework for particle physicists to describe the behavior of all known subatomic particles. Though the standard model has worked beautifully to predict what experiments have shown so far about the basic building blocks of matter, and has been a triumph of modern physics, it contains several looming unanswered issues.

One of the issues that physicists faces is why the four fundamental forces of the universe – gravity, electromagnetism, the weak force, and the strong force have such differing values, or why the weak force is approximately 10 quadrillion times as powerful as gravity. That’s where supersymmetry comes in. Essentially, it predicts that for every known particle there is super-partner of higher mass in the standard model. So the electron would be paired with a particle called a selectron, quarks would have corresponding squarks.

If these superpartners exist, they have the property of naturally canceling out the tiny quantum jiggles that would drive the weak force away from its observed range. The problem is that the LHC have not seen any strong evidence for new, heavy particles during their experiments. Though they keep searching at higher energy ranges, the particle accelerators don’t turn up any new superpartners.

### 4. Theory of Everything-Loop quantum gravity or M-theory

A theory of everything or master theory is a theoretical framework of physics that if proven will explain and links together all physical aspects of the universe. The theory in principle is actually based on uniting the four fundamental forces i.e. gravitational, electromagnetic, strong nuclear and weak nuclear forces. Finding a theory of this magnitude is one of the major unsolved problems in physics. Today, there are basically two universal theories that all of physics is based upon; theory of relativity and quantum field theory.

Scientist community has configured that integration of these two theories can solve the biggest problem in the realm of physics that is TOE. But with years of intensive research, they have also confirmed that these two theories are mutually incompatible–they cannot both be right at the same time. To solve this mystery, physicists have pursued the problem in two separate theories— string theory and loop quantum gravity. According to string theory, everything is made of tiny strings. The strings may be closed or have open ends. And in these millionth of a billionth of a billionth of a billionth of a centimeter of one dimensional objects lie the explanations for all phenomena we observe, both matter and space-time included.

Loop quantum gravity, on the other hand, is concerned less with the matter that occupies space-time than with the quantum properties of space-time itself. Moreover, quantum gravitational effects are extremely weak and therefore difficult to test at research labs. Since, all the other fundamental forces have one or more messenger particles, forces researcher to believe that gravitation also have one in the form of Graviton. If this mysterious particle is discovered, it is widely accepted that it will be a big step towards solving quantum gravity.

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### 3. Future of the Universe

The Fate of the universe depends upon the density and pressure of the matter in the universe. If the density of the universe is less than the critical density, then the universe will expand forever. This is also known as the “Big Chill” or “Big Freeze” because the universe will slowly cool as it expands until eventually it is unable to sustain any life. If the density of the universe is greater than the critical density, then gravity will eventually exceed and the universe will collapse back on itself, resulting into “Big Crunch”.

Recent observations of distant supernova have suggested that the expansion of the universe is actually accelerating, which implies the existence of a form of matter with a strong negative pressure, such as the cosmological constant or “dark energy”. Unlike gravity, which works to slow the expansion down, dark energy works to speed the expansion up. If dark energy, in fact plays a significant role in the evolution of the universe, then in all likelihood the universe will continue to expand forever.

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### 2. Problem with the Cosmological Constant

In Einstein’s gravitational equation, the cosmological constant is equivalent to an energy density in a vacuum, that is, a space without matter. By equating this density to the density of the zero point energy that is left in a volume after you remove all its particles, you obtain a number that is 120 orders of magnitude higher than what is observed. Such a high value would result a universe that would so rapidly expand that galaxies would have no time to form. This is the problem with the cosmological constant.

### 1. Yang–Mills existence and mass gap

The Yang–Mills theory seeks to describe the behavior of elementary particles using non-Abelian Lie groups or noncommutative group and is at the core of the unification of the electromagnetic and weak forces as well as quantum chromodynamics, the theory of the strong force. In early 1954, Chen Ning Yang and Robert Mills introduced the concept of gauge theory for Abelian groups, e.g. quantum electrodynamics, into the nonabelian groups to provide an explanation for strong interactions. The idea was put forward after the concept of Spontaneous Symmetry Breaking in Particle Physics was discovered by Jeffrey Goldstone, Yoichiro Nambu, and Giovanni Jona-Lasinio.

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This theory proved successful in the formulation of both electroweak unification and quantum chromodynamics (QCD). But the successful use of Yang-Mills theory to describe the strong interactions depends on a subtle quantum mechanical property known as “mass gap” (in quantum field theory, the mass gap is the difference in energy between the vacuum and the next lowest energy state) i.e. the quantum particles have positive masses, even though the classical waves travel at the speed of light. This property has been discovered by physicists from experiment time to time and confirmed by simulations, but it still has not been understood from a theoretical point of view.