dark energy and dark matter : the cosmic mystries

Dark energy and dark matter... the two cosmic unknowns...

By today, it is very well known that a our universe is expanding at lightning fast speed and all the constituents of the universe is falling apart. This expansion and separation has begun from the event of the big bang and its rate have grown higher as time passed by and still counting. It is expected that a day would come when our sky would not look like what we see today. The heavenly bodies, the distant stars from the far located galaxies would move away... there would be only a dark sky with few bright dots. That would definitely not be so pleasant at all... 

Since we know that the universe is not expanding at a uniform rate, we father the idea that there must be something which is causing the expansion and there must be something which is retarding the expansion.  The composition of these accelerating and regarding factors is very vital and a minor change in it can change the fate of the universe and its contents.

The universe is full of matter and attractive and repulsive forces from between the tiniest particles to the largest galaxies. In 1998, an observation made by the HST( Hubble space telescope) on some distant supernovae revealed that the universe is not is expanding on a accelerated rate, unlike what scientists thought. Before this discovery, it was believed that the expansion is gradually slowing Dow due to gravity and someday everything would collapse into singularity, the state just before the big bang. The fact that universe in expanding at a accelerated rate was surprising and no could put put forwarnd any proper reason for it... but its was proved to be true. Following this many attempts were made to explain this fact but none proved to be proper. At last a name was given to this mystery, dark energy.

The dark matter... :

From this point, the question was what is dark energy ? What are its properties ? Why and how it came into its existense?

Although at present no perfect answer exists for these questions but some suggestions still hold good.

The truth is that the only thing is known about dark energy is that it exists because we know that it is the factor responsible for the expansion of the universe. Observations hav and calculations have revealed that the dark energy comprises of approximately 68% , normal matter that our instruments can detect is comprising about 5%. So what is rest ? The rest matter is named as dark matter wchih our instruments can't directly detect but its gravitational activities can't be detected. It is surprising that the billions of galaxies including our earth and everything in the universe that can be directly detected is approximately only 5% of the total universe.

This composition may seem surprising but the truth is that it decides the fate of our universe.

Attempts to sole the mystery of dark energy... :

Albert Einstein tried to solve theyatery of dark energy.

He said that empty space is not actually empty but some characteristic energy is filled the space. In fact this energy is a property itself of the space. This just one among the never ending properties of the space. Einstein described the first property as " there is possibility that more space may come into existence." Confused? This means that space is limitless.

He made another prediction considering the cosmological constant that as energy is property of the space itself, so its value can not decrease with amount of space. Intact as more space is added, the energy would increase. So this energy cause the universe to expand farther and faster with time. But till today, it is a matter of confusion about the relation between the value of cosmological constant and expansion of space. Infact the value of the cosmological constant is doubted.

If the above explanation is not correct then what can be better than this ? A new assumption was that the space is filled with something called "quintessence" . Now what is this quintessence ? This is nothing but a name given by the scientists to new dynamic energy fluid or energy field. This energy was explained in a dramatical way but it couldn't hold its grip for long as it did not make a lot of sense. They said that is something which causes the expansion of the universe and acts opposite in case ofatter and dark matter. Now it is probably clear that why this idea was dropped soon. It was never explained what it is made up of and how and why it intersects with what and even why does it exists... 

The next hypothesis is bases on the famous quantum theory of matter. It sataes that empty space is full of matter which continuously appear and disappear or more precisely speaking, they are temporary matter. But when physists tried to calculate that how much energy is constitute empty space, the result was a disaster. The number was of order 10¹²° , which is not acceptable till today.

So this theory also short lived.

The last but not the least answer is that the present einstiens theory of gravity maybe wrong. Einstein's gravitational theory is found to be correct for galaxies , the motion of all the bodies in the universe including the motion of planets around the sun. But if we reject the present theory then what could be a better one ? The more important question is the even if the new gravitational theory is able to explain the expansion of universe, will it be able to explain the rest cases as einstiens theory did ? There is no answer to this question at present but it is well understood that rejecting represent theory of gravitation is not easy and safe and getting a better theory is still needs far more information.

The dark matter... :

Enough talking about dark energy ! Now we have another big question. What is dark matter ? Most of us may think that dark energy isore important topic to be concerned about as it is causing separation of constituents of the universe. But I would like to remind you that this is not correct. We have already said that the composition of matter, dark matter and dark energy is very crucial in  deciding the fate of our universe. So keep in mind, dark matter us is equally important.

Though not fully but dark matter is much more understood as compared to dark energy. Though indirectly, it is possible to detect and understand dark after. It does not emit or reflect light Luke ordinary matter, but its presence can be determined by its gravitational effect on its surroundings. Using a technique called gravitational lensing, it is found that the dark matter's gravitational pull bends and distorts light coming from distant galaxies. Some scientists say that huge black holes may constitute these darkatyer and some say that massive compact halo objects (MACHO) or weakly intersecting massive particles ( WIMP) constitute these dark matters. MACHOs are brown dwarfs, white dwarfs fans weak stars whereas WIMPs are radically different form of matter left over of big bang .

Although many suggestions and hypothesis hold good uoto some extent, no single argument is able to completely reveals the mystery. Researches are going on at full speed and we expect there'll be an answer soon. An perfect answer ! 

There are many more about matter, darrk matter and dark energy. Everything is not possible to put down here... I've tried to present the entire set of important points only.

Hope you've enjoyed this.

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all about supernovas

What are supernovae...?

Most of us might have heard the term " supernova ". The name itself suggest that it has something to do with light and explosion. Something really big and interesting and mysterious.



Let us first gather a basic idea : supernova is the name given to the phenomenon of explosion of supergiant huge massive stars after it has completed its lifespan.
A supernova throws out many heavy elements in all dimensions.
It is also found that supernovae emit audible sound waves before the explosion of the source star due to its own internal explosion.
The first supernova explosion was observed by the man kind in 2008.

The energy emitted by the supernovae may depend on the size and properties of the source star but the value revolves around the fourtieth power of ten (more than 10⁴³). This means it would emit energy that our own sun would emit in its entire lifetime. The brightness caused in a supernova may be observed at large distances throughout the space.

Now one may have the question in mind that whether our own sun result in a supernova or not ? The answer would be NO. Our sun does not have sufficient mass, size and energy to cause a supernova. Infact our sun would grow many times bigger than at present and gradually cool down. This is called a white dwarf. But before it become a white dwarf, due to sun's large grown shape, our earth and other planets would vapourise into the space.

It is expected that a supernova explosion occurs every second somewhere or else in the universe. This means that a galaxy like our own witnesses a supernova in every 50 years. This may sound a rare case when we talk about a period of 50 long years but we have to keep in mind that we are talking about the explosion of a huge star taking place in the space. So this rate can be considered quite often.

Now we have arrived at the point where the main question is why does some star explode to result a supernova ? Well... the answer is not simple...
There is a series of prosesses taking place through the surface to the depth of the stars. These complicated prosesses starts taking place at the very beginning of the star's existense and carry in till the event of its explosion. So we can say that supernova is the conclusion of the star's life.  Therefore a supernova is the event of the death of a giant and powerful star.

Now you may ask what actually causes the explosion ...? Fortunately this time there's a straight forward answer. The fusion or fission inside the star is responsible for it. An iron core is built up in the the centre of the star due to fusion. The nuclei of the iron group elements are the most tightly bound, so no more energy can be expected from nuclear fusion after it has operated for a period of millions of years. During this period, iron group elements has absorbed a large amount of energy. The star's own gravitation causes increase in temperature which in turn facilates the fusion of iron at the core. If the temperature is high enough , it'll be absorbed and there will be a reheat and restart of the process of collapse of the core. This process is repeated millions and millions of times during the whole lifetime of the star. The fusion of electrons and protons gives neutrinos which serves as the carrier of matter in all dimensions during the explosion.


Classification of supernovae :


Supernovae are classified and reclassified into two categories and and their sub-categories on the basis of the properties of their light spectrum and the properties of the source star and also on their shapes of light curves.

Classification as type-1 And type-2 supernovae : ...


Type-1 supernovae :

Type-1 supernovae are associated with the stars that are with low mass and low energy. This type of supernovae requires a binary system of stars where one is insufficient to cause supernova alone. Here one star is in the end of its lifespan and is in the state called white dwarf.

Here the hydrogen is not seen in its light spectrum. One star accumulates matter from the neighbouring ending star. As the gas and matter of the companion star is compressed, there starts a runaway nuclear reaction which eventually leads to a supernova explosion.

This type of supernovae have light curves exhibiting sharp maxima and then die gradually. This type of supernova occur in elliptical galaxies like our own.

Type-1 supernovae are sub-categorised as 1a, 1b and 1c supernovae :

Type-1a : if they have strong silicon lines in their light spectrum.

Type-1b : if they have strong helium lines in their light spectrum.

Type-1c : if they don't have any of the above.

Type 1b and 1c involves collapse of the core like type-2 supernovae.

Type-2 supernovae :

In type-2 supernovae, the stars run out of the nuclear fuel like hydrogen and helium. Here the collapse of the core of the star due to its own gravity is responsible for the explosion. This kind of supernovae are associated with one single giant massive star and unlike type-1 , no binary system of stars is needed. The star gradually runs out of hydrogen and helium after millions of years of its lifespan at the end but is left with a huge amount of mass and immense pressure. Here the carbon fission takes place inside the star due to the immense pressure. Gradually the star becomes layered, becoming more and more denser as we move towards the core of the star. Heavier elements occur at the core with immense pressure. Once the core crosses the chandrasekher limit, it starts imploding and the process of explosion begins. At last, after all the heavy elements are thrown away, a neutron star is left behind.

Type-2 supernovae are sub-categorised on the basis of their light curves as type-2L and type-2P :


Type-2L : this kind of supernovae decline uniformly after the explosion.

Type-2P : in this case, the light lasts for linger period of time.




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