Big Bang: The beginning of the Universe

The Big Bang theory is the prevailing cosmological model for the observable universe's earliest known periods and its subsequent large-scale evolution. It proposes that the universe began as a hot, dense state approximately 13.8 billion years ago and has been expanding and cooling ever since.

Here's a brief overview:

1. Initial Singularity: The Big Bang theory proposes that the universe began as a hot, dense point known as a singularity. At this moment, all the matter, energy, space, and time were contained within this infinitesimally small point. As a result, certain qualities became infinite or undefined at the Big Bang, hence the name singularity.

2. Rapid Expansion: Approximately 13.8 billion years ago, the singularity underwent a rapid expansion known as cosmic inflation. This expansion caused the universe to cool down and expand, leading to the formation of subatomic particles and eventually atoms.

3. Formation of Elements: As the universe expanded and cooled, subatomic particles such as protons, neutrons, and electrons combined to form the first hydrogen and helium nuclei. This process, known as Big Bang nucleosynthesis, occurred within the first few minutes after the Big Bang.

4. Formation of Stars and Galaxies: Over millions to billions of years, gravity caused regions of higher density in the early universe to collapse, forming stars and galaxies. Within galaxies, stars formed from clouds of gas and dust, and many stars went on to form planetary systems.

5. Cosmic Microwave Background Radiation (CMB): About 380,000 years after the Big Bang, the universe cooled enough for neutral atoms to form. This event, known as recombination, led to the release of photons that continue to travel through the universe today as the cosmic microwave background radiation. The CMB provides a snapshot of the universe at that early stage.

6. Observable Universe: The expansion of the universe continues to this day. The observable universe, the part of the universe we can see, includes galaxies, clusters of galaxies, and cosmic structures, all moving away from each other due to the ongoing expansion.

The Big Bang theory is supported by a wealth of observational evidence, including the cosmic microwave background radiation, the observed distribution of galaxies, the abundance of light elements, and the large-scale structure of the universe. However, it's important to note that there are still unanswered questions and areas of active research within cosmology, such as the nature of dark matter and dark energy, and the conditions of the universe before the Big Bang. This is probably the reason why various competing theories like the steady state theory, cyclic universe model, multiverse hypothesis, quantum cosmology, etc. find their place in the literature.

Prabir Rudra

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The explosion of Corona Borealis: Once in a lifetime sight in the sky

According to NASA, astronomers anticipate that a "new star" will emerge in the night sky at any moment between now and September, 2024. This celestial spectacle is expected to be once in a lifetime.

The Milky Way's Corona Borealis, or Northern Crown constellation, which is situated between the Boötes and Hercules constellations, is where the anticipated brightening event, known as a nova, will occur.

A nova is the quick, brief explosion from a collapsing star known as a white dwarf, whereas a supernova is the catastrophic death of a large star.

The "Blaze Star," or T Coronae Borealis, is a binary system in the Corona Borealis that consists of an aged red giant star and a dead white dwarf star. When stars run out of hydrogen for nuclear fusion and start to fade, red giants are created. NASA estimates that in roughly 5 or 6 billion years, our sun will turn into a red giant, sputtering and growing as it releases layers of material and probably vaporizing the inner planets of the solar system. In this context, Earth's future is yet unknown.

An explosive event occurs in T Coronae Borealis approximately every 79 years.

Because of their close proximity, the stars in the orbital pair interact furiously with one another. As the red giant heats up, it becomes more unstable, shedding its outer layers that fall as matter onto the white dwarf star.

According to the space agency, the interchange of stuff leads the white dwarf's atmosphere to steadily heat up until it undergoes a "runaway thermonuclear reaction," which results in a nova.

How to keep an eye on the event?

Astronomers are closely monitoring T Coronae Borealis once more after it last had a spectacular outburst in 1946.

In an email, NASA Meteoroid Environments Office lead William J. Cooke stated that "most novae happen unexpectedly, without warning." Nonetheless, T Coronae Borealis is among the galaxy's ten recurrent novae. From the last eruption in 1946, we know that the star will darken for a little more than a year before brightening up quickly. T Coronae Borealis started to dim in March of last year, and between now and September, some researchers anticipate that it may go nova. We are unable to predict with greater accuracy than the few months it will take for this to occur, given the current state of knowledge.

It is anticipated that the star system, which is 3,000 light-years away from Earth and usually too faint to be seen with the unaided eye, would become as bright as Polaris, or the North Star.

When the nova reaches its maximum brightness, it will appear as if a new star has emerged. It can be seen with binoculars for a little over a week and without any equipment for a few days, after which it will fade and be lost from view for another 80 years or more.

The nova will be visible from the Northern Hemisphere, appearing in a little arc between the constellations Hercules and Boötes.

The Neil Gehrels Swift Observatory, located in space, will be used by astronomers to analyze the celestial event through ultraviolet and X-ray light, while the Hubble Space Telescope will be used to observe the nova.

Cooke stated, "Repeated novae like T Coronae Borealis provide us insights into the thermonuclear runaway that occurs on the surface of the white dwarf when the star goes nova and help us understand the mass transfer between the stars in these systems."

Updates regarding the eruption and its appearance will be available from the NASAUniverse account on X, formerly known as Twitter.

Cooke remarked that the brightness of Nova Cygni, which he saw last in 1975, was comparable to that of T Coronae Borealis. It is not anticipated that Nova Cygni will encounter another explosion.

"On August 29, I was outside as a teenage astronomy geek who was about to start college," Cooke recalled. "Looking up at the sky, I saw that there was a star that shouldn't have been there in the Cygnus constellation. We discovered that we were staring at a nova after I persuaded some pals who thought I was insane to look at it! It was a really memorable event that confirmed my professional decision to pursue astronomy. I used to make jokes about how undergraduate physics had to blow out a star for me to have to endure it.

Prabir Rudra

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Wormhole: The missing link between us and our future home

 

The term "wormhole" usually describes a theoretical spacetime tunnel-like structure that may link two different points in space or time, possibly enabling instantaneous or faster-than-light travel between far-off places. The idea comes from theoretical physics, specifically from the perspective of general relativity.

Wormholes are frequently discussed as solutions to Einstein's field equations in astrophysics and cosmology. These equations control the gravitational effects of matter and energy on the curvature of spacetime. Although the equations of general relativity make wormholes theoretically viable, they have not been detected or verified to exist in reality, hence they are still considered speculative.

Scientists, authors, and filmmakers have all been captivated by wormholes, which are often depicted in science fiction as a way to travel through space or journey back in time. Wormhole stability, traversability, and the exotic stuff needed to stabilize them are only a few of the many unresolved concerns in the nascent scientific field of wormhole research.

More recently, the word "wormhole" has been used metaphorically to refer to systems or procedures that allow for unusually fast data transfer, financial transactions, or communication between distant sites or systems in domains like computer science and finance. A "wormhole" in the context of blockchain technology, for instance, might be a bridge that permits token transfers between several blockchain networks.

Given the challenges and uncertainties like existence, stability, need for exotic matter, traversability, time dilation, and paradoxes, interstellar travel via wormholes remains highly speculative at this point. While it's an intriguing concept that has captured the imagination of scientists and science fiction enthusiasts alike, practical implementation would require significant advancements in our understanding of fundamental physics and the development of technologies far beyond our current capabilities. But if this turns out to be a success then humanity can seriously think in terms of colonizing other habitable planets in the future, when the resources of our planet get completely depleted. 

It is said that the future of humanity lies in space and wormholes might just be the missing link between us and our destination (future home). But we have a long way to go!!

Prabir Rudra

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The rings of Neptune captured by James Webb Space Telescope after three decades

                         Rings of Neptune

Rings of Neptune!! Yes, you heard it correctly. We are all so familiar with the rings of Saturn, the countless small particles of different shapes and sizes that orbit around Saturn, giving a ring-like look. But very rarely do we find anybody speaking about the rings of Neptune. This is exactly what the James Webb space telescope has spotted. 

But this is not the first time that these rings have been visible to us. We knew about these rings earlier, but this is the first time we have received such a clear and crisp image of the rings of Neptune and this spotting has been possible after 30 years. 

                     James Webb Space Telescope

Neptune has always fascinated researchers since its discovery on 23rd September 1846 by Johann Galle. It is the only planet in the solar system which was discovered on the basis of mathematical prediction rather than by empirical observation. 

Alexis Bouvard observed that there were unexpected changes in the orbit of Uranus. From this, he hypothesized that the orbit of Uranus was subject to gravitational perturbations of an unknown planet, which was later found to be Neptune.

In 1989 Nasa's Voyager II spacecraft did a flyby past Neptune and during this time it captured the planet's rings. After that Voyager II continued its journey in interstellar space beyond the solar system, so no more images of these rings could be captured. 

The recent images captured by the Webb show Neptune's fainter dust bands in addition to several bright, narrow rings. It should be noted that Neptune is located 30 times farther from the Sun than Earth and its orbits are located in the remote darker regions of the solar system. Moreover, due to its chemical composition, it is generally referred to as an ice giant, in contrast to the gas giants like Jupiter and Saturn.

           

                    Hubble Space Telescope

The images of Neptune captured by the Hubble space telescope (images at visible wavelength) have a bluish tinge due to the presence of small amounts of gaseous methane. But the James Webb space telescope captured Neptune in its near-infrared range which gave the planet a purple tinge instead of blue. 

Apart from these, Webb has also captured 7 out of 14 moons of Neptune. There is a very bright point seen in the image captured by the Webb, which is Neptune's large and unusual moon Triton (it orbits Neptune in a strange backward orbit). Triton reflects more than 70% of the sunlight that is incident on it. Moreover, due to methane absorption, the planet's atmosphere is darkened at near-infrared wavelengths. The combined effect is that Triton quite easily outshines Neptune in the image. The image from Webb has also revealed a continuous band of high-latitude clouds surrounding the planet, which was never seen before.

By

Prabir Rudra

 

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The curious case of Pluto

 

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Prabir Rudra

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White dwarf: The death-bed of a star telling the story of its glorious past

 

A white dwarf is a compact stellar core remnant resulting from gravitational collapse. Dense electron degenerate matter fills up such astronomical objects. Since nuclear fusion ceases to take place in a white dwarf, it almost has no light of its own. But still, a white dwarf may appear faintly lit in the sky, due to the residual thermal energy that it carries forward from the fusion reaction during the star's lifetime. Sirius B at a distance of 8.6 lightyears is the nearest known white dwarf. Around 8 white dwarfs are known to us in the vicinity of our solar system.

Formation

When a star is on the verge of completing its life cycle, its thermonuclear fuel is completely depleted from the core. This results in the halt of the nuclear fusion occurring in its core and there is an imbalance of outward and inward forces in the star with the inward gravitational force dominating the outward thermonuclear force. So the star collapses due to its own mass, with its volume becoming smaller and smaller. This makes the inside material denser and denser. Now if the original star does not have enough mass (> 25 solar masses) to form a neutron star, the remnant generally goes on to form a white dwarf, which is the final evolutionary stage of such stars.

In the main sequence, a star fuses hydrogen to form helium, releasing an enormous amount of energy that counterbalances the inward gravitational attraction. But when all the hydrogen gets burned up, the star expands into a red giant phase, when it fuses helium into carbon and oxygen (true for low and medium mass stars). Now if the red giant cannot produce the necessary temperature (around 1 billion K) to fuse carbon, then the core will be filled with inert carbon and oxygen. After shedding off the outer layers, forming a planetary nebula, the final remnant core rich in inert carbon and oxygen will be the white dwarf. This is the most general scenario and most abundantly found in the universe. But in the case of some small stars, they are not able to fuse helium into carbon and oxygen, and so they form helium-rich white dwarfs.

Stellar Equilibrium

At the white dwarf stage, the further collapse of the star is hindered by the electron degeneracy pressure. But there is a maximum mass limit that can be supported by this degeneracy pressure. This limit is called the Chandrasekhar limit which is approximately equal to 1.44 solar masses. So if a white dwarf accumulates enough matter from its companion star to go past the Chandrasekhar limit, then the electron degeneracy pressure can no longer hinder further gravitational collapse. The resulting white dwarf explodes into a very violent and bright event known as a supernova and the core further collapses towards the formation of a neutron star.

Transformation into a black dwarf and the final fate

Since there is no nuclear fusion going on in a white dwarf, it slowly radiates out whatever remnant heat it has over time and cools down. So initially white dwarfs are bluish-white (hot) due to the radiations emanating from them, but with time they become redder due to cooling. Finally, these dwarf stars are supposed to totally stop radiating and turn into black dwarfs. But the time taken for a white dwarf to turn into a black dwarf is calculated to be greater than the age of the universe, and hence it is thought that there are no such black dwarfs existing in the universe today. The oldest white dwarfs exiting today still radiate out the heat of a few thousand kelvins, which is such a mind-blowing fact!! Even if a black dwarf forms, it will be a really challenging task to locate it due to its non-radiating nature. 

No matter how dark, cool or insignificant they are, such entities will continue to roam around in the eternal space and tell the stories of their glorious past!! Such death beds of once fierce and powerful stars will keep whispering through the ages!!

By

Prabir Rudra

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Neutron Star: the most extreme and violent object in the universe (Meme)

 

Read the full article at:   

https://www.spacetimerecipe.com/2022/08/neutron-star-most-extreme-and-violent.html

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Prabir Rudra

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Black Hole: The monster of the Universe (Meme)

 

Read the article at:  https://www.spacetimerecipe.com/2022/07/black-hole-monster-of-universe.html

by

Prabir Rudra

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Red dwarfs: The immortal entities of the universe (Meme)

 

Read the article at:   https://www.spacetimerecipe.com/2022/08/red-dwarfs-immortal-entities-of-universe.html

by

Prabir Rudra

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Life is just an accident from the astronomical point of view!! (Meme)

 

Read the article at:  https://www.spacetimerecipe.com/2019/02/life-is-just-accident-death-would-be.html

by

Prabir Rudra

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James Webb Space Telescope may have spotted the most distant and the oldest known galaxy till date

 

Image of Glass-z13 galaxy captured by JWST

Glass-z13 is the name of the galaxy that has been spotted by Nasa's James Webb Space Telescope (JWST), which scientists believe to be the most distant galaxy known to mankind to date. From a preliminary study, it is estimated that the galaxy was formed 13.5 billion years ago, which is around 300 million years after the Big bang (Big Bang occurred 13.8 billion years ago). This is an improvement of around 100 million years from the farthest known galaxy previously. 

Here we are gazing at the most distant starlight that any human has ever seen. The more distant the body, the more time it takes for the light to reach us. So looking farther deep into space means we are looking back in time. So we are not only looking at the farthest starlight but also these are the oldest photons to reach us. We are practically looking at the galaxy as it was 13.5 billion years ago!! Isn't it amazing!! Thus rightly said by Einstein, "The most comprehensible thing about the universe is that it is incomprehensible!!"

The observation was made in the "early release" data from the observatory's main infrared imager called NIRcam, but the result was not made public in the first set of publications of JWST. It needed some time to translate the data from infrared to visible spectrum, where the galaxy appeared as a blob of red with white in its center. The farther an object is situated from the observer, the farther the light has to travel, and due to the expansion of the universe, the light wavelength is stretched from the visible to the infrared region of the spectrum. This is where JWST specializes. It is well equipped with infrared detectors that are suitable to make observations of very distant objects right till the big bang with unprecedented clarity.

Two separate teams of astronomers, one headed by Mr. Naidu and the other by Marco Castellano have independently taken the observations, analyzed the data, and reached the same conclusions. This is why they are confident about the results they have obtained. Currently, the study is under peer-review in a scientific journal and posted publicly on a preprint server.

The team of scientists believes that they have to conduct a detailed spectroscopic analysis of the data they have obtained to confirm their claim and also extract more knowledge about the galaxy. As of now, they know that the galaxy has a mass of billion suns, which is really surprising considering how soon after the big bang it formed. They also need to confirm the distance of the galaxy that they have claimed. The spectroscopic analysis will also reveal some detailed properties of the galaxy. 

Along with GLASS z-13 they have also found the signature of another galaxy known as GLASS z-11, which is not as ancient as the former but around a similar time scale. JWST is expected to open a new era of astronomy, with records being broken with each observation. We have started to see the results already!! We hope that JWST outperforms its expectations and helps us to reveal the deep-lying secrets of the universe. As of now, our fingers are crossed!!

PC:  NASA/ESA/CSA

By

Prabir Rudra

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Black Hole: The monster of the Universe

 

A black hole is a region in spacetime where the gravitational pull is so strong, that no matter can escape from it. The gravity in such a region is of such magnitude that not even electromagnetic radiation (light) can escape from it. Theoretically, black holes were predicted from the solutions of the field equations of general relativity, as regions in spacetime with infinite curvature, such that gravity in the region becomes extremely large.

The boundary of a black hole is called the event horizon, which has great significance for a particle falling into the black hole. The event horizon is basically a point of no return for the infalling bodies, because once a particle crosses the event horizon it does not have any possibility of making it back, and is destined to be engulfed by the black hole. For an external observer, the infalling body will appear to fall into the black hole for eternity due to time dilation near the black hole. Time dilation is caused by the extreme gravity of the black hole and the flow of time is slowed down drastically. The object that is falling into the black hole will appear to fall very slowly to an external observer and the event of falling will seem to occur for eternity.

Using quantum field theory in curved spacetime famous cosmologist Stephen Hawking predicted that a particular type of radiation is emitted from the event horizon of a black hole. This radiation is called the Hawking radiation, whose temperature is inversely proportional to the mass. This temperature is of the order of a billionth of a kelvin for stellar black holes due to which it is almost impossible to detect it directly.

Black hole as a Mathematical entity

In 1916,  Karl Schwarzschild derived the first solution of the Einstein's equations of general relativity which directly showed the presence of a black hole theoretically. For a long time black holes just remained a mathematical curiosity with no physical reality associated with it. With the discovery of neutron stars by Jocelyn Bell Burnell in 1967, new interest was generated in the field of gravitationally collapsing bodies. 

Formation of a black hole

A star at the end of its lifetime undergoes gravitational collapse and forms a white dwarf which is a compact astronomical object. Further collapse from this state will occur if the mass of the white dwarf exceeds 1.4 solar mass (Chandrasekhar limit) and the resulting object will be a highly compact neutron star. If this state is not in equilibrium, then further collapse is a possibility, which will result in a singularity. If the singularity is shrouded by an event horizon, then it is a black hole. The first black hole known to us was Cygnus X-1 identified in 1971.

After the formation of a black hole

After a black hole is formed it can grow by absorbing the surrounding mass due to its immense gravitational pull. Supermassive black holes (millions of solar masses) can absorb stars or even other black holes. The presence of a black hole is generally inferred by the way it interacts with the surrounding matter and electromagnetic radiation. Any infalling matter in the black hole forms an accretion disk which is enormously heated due to friction. Stars which are in the close vicinity of a black hole will be shredded and engulfed by it, whereas stars which are at a considerable distance will orbit the black hole. The mass of the black hole and its location can be inferred from the orbit of these stars. It is believed that supermassive black holes exists at the centres of most galaxies. There is a radio source at the heart of our galaxy Milky Way known as the Sagittarius A*. It is believed that it houses a supermassive black hole of 4.3 million solar masses.

Recent Advancement

On February 11, 2016 gravitational waves due to a black hole merger was recorded by the LIGO and Virgo scientific collaboration. On April 10, 2019 the first direct image of a black hole was recorded by the Event Horizon telescope. It was situated in the M87 galaxy around 50 million lightyears away from us. Till date gravitational lensing is possibly the best method to infer the presence of a black hole. Although only around 20 black holes have been detected so far, it is expected that there are hundreds of millions of such monsters roaming in the universe. Having an encounter with such a thing is not going to be a pleasant experience at all!!

By

Prabir Rudra

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Dark Matter: An Unsolved Mystery

 

As the name suggests, dark matter is an invisible form of matter that is supposed to fill a substantial part of the universe and is responsible for driving some critical cosmological events. One thing is certain, it is not in the form of stars or planets that we can see around us. From the observations, it is estimated that 85 % of the total matter content of the universe must be dark matter. Along with dark energy, it forms 95 % of the total mass-energy content.

Why is it dark?

Dark matter seems to have no interactions with the electromagnetic field, which implies that it does not emit, reflect or absorb light. This is the reason why it is totally invisible to us and hence aptly termed dark matter.

Then how is it detected and what role does it play in the dynamics of the universe?

Observations from the evolution of galaxies suggest that the galaxy dynamics would be quite different if it is governed by the presence of the matter that we see in the universe (visible matter). The galaxy rotation curves (path of motion) would be quite different if gravitational interaction emerged from visible matter only. Many galaxies would never have formed if it was only due to the contributions of normal matter. Simply we need more matter to generate the necessary gravity such that the observed galaxy dynamics can be explained!! To put it in another way, we need around 27 % of matter to explain the observations, but the normal matter is far less than that.

So it is quite understandable that there is a missing link in the picture. Either our observations are faulty or there is some form of matter that is totally eluding our vision, but playing a significant role in the structure formation and evolution of galaxies. This invisible form of matter that generates sufficient gravitational interaction to sustain the galaxy dynamics is dark matter. We cannot see it, but we can feel its presence due to the role played by it. There are other scientific pieces of evidence of the presence of dark matter such as gravitational lensing (bending of light in a gravitational field), Cosmic microwave background radiation (which is a relic of the Big Bang), etc.

What is dark matter made of?  

It is now well known that dark matter has minimal interactions with visible matter & radiation, which is the basic reason behind its mysterious nature. It remains secluded without intermingling with other components of the universe but reveals its presence only through gravitational interaction. So naturally, a question arises regarding the chemical composition of dark matter. In simpler terms, one would like to know what is dark matter made of. This question is not yet answered by the scientific community, but extensive research is underway. It is believed that dark matter is made up of some new kind of elementary particle that is yet to be discovered. 

 

Speculations:

Dark matter is sometimes argued to be some form of antimatter (matter composed of antiparticles of the corresponding particles in the ordinary matter), but this is not correct. The reason is, that we do not see the unique gamma rays that are supposed to form when matter annihilates with antimatter. Some people argue that dark matter can be black holes, which are themselves mysterious invisible objects, and share some common properties. But this is not true either, because black holes are centers of gravitational lensing due to their enormous gravitational pull. If dark matter was present in the form of black holes we would have seen far more gravitational lensing phenomena than we actually see in the universe. As of now, we believe that dark matter must be made up of some exotic particles which are yet to find their places in the periodic table and, about which our knowledge is extremely limited.

By Prabir Rudra

#darkmatter #galaxy #galaxyrotationcurve #cosmology #blackhole #darkenergy #visiblematter #gravitationallensing #CMBR #bigbang #spacetime #spacetimerecipe #gravity #astrophysics

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Dark Energy: The greatest puzzle of modern cosmology

 

At the turn of the last century, observations from Supernova Type Ia confirmed the fact that our universe has entered a phase of accelerated expansion. Two separate teams respectively working on the Supernova cosmology project and the high-Z supernova search team reached this result independently in 1998. They were respectively headed by Saul Perlmutter and Brian P. Schmidt & Adam Riess.  This came as a shocking discovery to the scientific community since the outcome defied our orthodox understanding of the dynamics of the universe.

It is clearly known that gravitational interaction is attractive in nature. So an expanding universe that originated from the Big Bang would tend to slow down due to gravity interacting between the objects such as planets, stars, galaxies, etc. Thus a decelerated expansion of the universe will slowly bring the universe to a halt before the contraction phase sets in. But the discovery of accelerated expansion totally defies this idea, and we are left wondering about the mechanism that is responsible for this strange phenomenon.

It was clear to the scientific community that in order to explain this phenomenon theoretically, we will have to introduce a repulsive (anti-gravitational) interaction, which will counterbalance and even dominate the attractive gravitational interaction. But this repulsion will not jump out of nowhere, and there should be some source for it. So what is needed is basically a matter field that can sustain a repulsive interaction. This matter field is exactly what we term Dark Energy which is a hypothetical matter fluid with negative pressure.

Observations suggest that our universe is basically filled with three major components, namely normal or visible matter, dark energy, and dark matter. It is really very strange that normal matter (which includes all the visible entities around us) accounts for the smallest portion of the universe, which is around 6 %. Around 26 % of the universe is considered to be dark matter and the rest around 68% is dark energy. So it is evident that the present universe is dominated by dark energy and its queer exotic properties affect the universe at the cosmological scales, thus driving the accelerated expansion.

The density of dark energy is very low, estimated at around 7 x 10^ (-30) gm per cubic cm. This is considerably low compared to that of visible matter and dark matter, but still, it plays a major role in shaping the dynamics of the universe due to its uniformity across space. From the general relativistic point of view, Einstein’s field equations become inconsistent with the observations at the cosmological scales. This basically means that general relativity in its original form cannot account for the accelerated expansion of the universe in its framework. So we need to introduce modifications to the field equations in order to address this issue. The right-hand side of Einstein’s field equations comprises the matter fields, which can be modified by introducing dark energy. Such modifications have been attempted and found to be consistent with the observations.

There are basically two broadly proposed forms of dark energy, the cosmological constant and the scalar fields such as quintessence. The cosmological constant represents the constant vacuum energy density filling the space homogeneously, whereas scalar fields involve dynamic quantities, whose energy densities vary in space and time.

Cosmological inflation is an enormous and exponential expansion that took place just after the Big Bang. Dark energy can not only explain the dynamics of the late-time accelerated expansion of the universe but can also satisfactorily account for the early-time inflationary evolution of the universe. Alan Guth and Alexei Starobinsky in 1980 proposed that a matter field with negative pressure (similar to that of dark energy) can drive the cosmic inflationary mechanism in the early universe.

In physical cosmology, dark energy is studied extensively to explain cosmological observations and derive a proper cosmological model of the universe. This will ultimately help us get a description of the large-scale structure and the dynamics of the universe. Proper knowledge of dark energy will help us answer critical questions regarding the origin, structure, evolution, and ultimate fate of the universe. As of now we are still shrouded by the darkness of dark energy and trying to find our way out of the puzzle.

By Prabir Rudra

#darkenergy #cosmology #universe #cosmicexpansion #gravity #relativity #spacetime #spacetimerecipe #darkmatter #cosmicinflation

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Relativity for All Contd.!! (The General theory of Relativity)

On the 140^th birth anniversary of Albert Einstein it is our great pleasure to revisit one of his greatest masterpiece, the General theory of Relativity (GR) (arguably the best theory of Einstein). Today we discuss GR in his memory and ponder over the extraordinary genius of the man who redefined Physics in the 20^th century.

Albert Einstein

Background

After his miraculous year in 1905, when he laid the foundation of 4 groundbreaking theories of nature (Photoelectric effect, Brownian motion, special theory of relativity and “the world’s most famous equation, E= mc2”), Einstein came to the limelight. 

World’s most famous equation

From a patent clerk in Bern, Switzerland Einstein became a lecturer in the University of Bern in 1908. The following year he was appointed as an associate professor of theoretical physics in the university of Zurich. Einstein became a full professor in the Charles-Ferdinand university in Prague in April, 1911. In 1912 he returned to Zurich as a professor of theoretical physics in ETH, Zurich. He decided to move to Germany and on 1^st April, 1914, he joined the Berlin University.

During all these switch overs from one institute to the other Einstein kept nourishing his original idea of generalizing the theory of relativity that he proposed back in 1905. It was as situation of crisis to humanity. First world war was about to break out. But all these seemed to have minimal effect on the genius. A sense of incompleteness kept haunting him through days and nights.

General theory of Relativity

                                       The Problem 

If we recall our last discussion of Special theory of Relativity we will note that the theory was valid only for inertial (non-accelerated) frames of references. But in reality almost all frames are accelerated . A moving car needs to accelerate or de-accelerate. Similarly a cyclist has to use brakes while cycling. A football rolling on the ground de-accelerates due to friction. All these are non-inertial (accelerated) frames of references where special theory of relativity is not applicable. 

So we need a general theory of relativity that will be applicable to all frames of references (both inertial and non-inertial frames).

Why a theory of Gravity?

In the attempt to generalize his theory of relativity, Einstein knew that he has to incorporate acceleration in his theory. Now acceleration comes from force. In a great insight Einstein understood two things which he stated as the Principle of Equivalence.

1)  The equivalence of inertial and gravitational mass.    

2)  The gravitational force experienced locally while standing on a massive body like Earth is the same as the pseudo force experienced by an observer in a non-inertial frame of reference.

He had the feeling that this can really be a theory of gravity. He started to consider the problem of gravity seriously. He consulted Max Planck (who laid the foundations of Quantum mechanics in 1900) and told him about his intentions to work out the gravity problem. 

The legend says that Planck told him “There are two possibilities: (1) The gravity problem is so difficult that probably you will never succeed.  (2) Even if you are able to find a solution to the problem nobody will believe you.”

Max Planck

The weak link of the prevailing Newton’s theory of Gravitation

The first comprehensive theory of gravitation was stated by Isaac Newton in the 17^th century. It stated that:

Any two masses separated by a certain distance will attract each other by a force directly proportional to the product of the masses and inversely proportional to the square of the distance between them.

The theory seemed to be fine at that time with no flaws and was universally accepted. Newton became the God and very few people had the guts to question his intellect. Moreover the problem of gravity was very difficult to address and so only a few showed any further interest and believed in Newton.

Isaac Newton

But some natural questions arise:

What is the source or the origin of the force?

What is creating it?

Why is it present at all?

Newton was silent to these questions. He did not have any logical answer to it. 

The Theory that answered the above questions

Einstein’s theory of General Relativity is a geometric theory of gravitation that answered the prevailing questions of the critics.

Einstein argued that space-time (space and time together) is like a stretched fabric.  Now when we put a mass on such a fabric naturally it will create a curvature around itself. This is warping (curving, bending or twisting) of space-time that gives rise to a gravity well around the mass. Now when a second mass comes within the curvature quite naturally it moves towards the first body due to the gravity well produced. 

Curvature in space-time

Einstein argued this is what gravitation is!! It is not a force at all as Newton had proposed almost 250 years back. It is a phenomenon that is created by the geometry or curvature of the space-time.

Greater the mass greater the curvature on the space-time fabric

What a masterstroke indeed!! What a beautiful insight!! The common mind can only bewilder and get lost in its aura!! Such was the extraordinary genius of a curious mind.   

The Race to Glory

Einstein had developed the physics of the theory back in 1912, but he did not have any clue about the Mathematics. He needed sufficient knowledge about the geometry of curved space-time (Non-Euclidean geometry) before he could proceed towards developing any meaningful mathematics of the theory. 

Einstein was far from being an expert in such a geometry. But one man by the name of David Hilbert was. He was a mathematical giant of that era. Hilbert took up the problem and started developing the mathematics, which came as a terror to Einstein. His theory was in danger of being credited to somebody else. He had to act and act fast.

David Hilbert

Einstein’s friend Marcel Grossmann (a great mathematician) came to his rescue. With his help Einstein learnt the concepts of Riemannian (Non-Euclidean) geometry and started developing the mathematics of his theory. Hilbert on the other hand continued his effort silently but swiftly. 

As the race to glory became more and more intense the scientific community watched the battle of the two scientific giants (one physicist, the other mathematician) and waited eagerly for a result.

By the grace of God, it so happened that both these great minds reached their destination independently and almost simultaneously. By 1915 both had their own versions of the mathematics of GR ready. On careful scrutiny it was found that both were correct.

Hilbert accepted that the contributions to the theory was far more loaded in favour of Einstein and the theory of general relativity must go by the name of Einstein. This ended all confusions if at all there was any. Finally in 1916, Einstein published his theory of general relativity in the Annals of Physics.

It is difficult to contemplate the feelings of the great man. During his quest, at some point he must have felt that he was losing the battle. After all Hilbert was a great mathematician and matching him was a herculean task. Therefore after the publication, it could have felt like winning a battle once thought to be lost. But it should be remembered that the physical basis of the theory was solely laid by Einstein and probably that’s enough to retain the credit. 

The Equations of General Relativity

This is a relation between mass and spacetime geometry.

Equation of GR

The Reaction from the scientific community

Accepting this theory was not at all easy for the scientific community. The theory will shake the very foundations of physics. Shock-waves were abundant and far reaching. Scientists had many years of scientific career jeopardized. Isaac Newton’s theory of gravitation was in danger. So it happened as Max Planck had predicted some years back. Everybody demanded a proof for the theory.

The radical nature of the theory was criticized by many great minds. Some even thought that GR is not a theory of physics, but the whims and fantasies of Einstein, who is living in a fantasy world.

Experimental physicists like Philipp Lenard (Nobel Laureate, 1905) went into direct confrontations with Einstein and mocked him publicly. He said that physics is a subject of experiments and observations. We believe what we see. There is no place for dreamers like Einstein in the scientific community. Science got really ugly!!

Einstein retaliated but he understood that he had to give a proof to save his theory from being demolished by the critics (specially nazi).

The Proof of General Relativity

The paper was published. The theory was complete. But it didn’t matter to anybody because nobody believed it. Now the genius of Einstein faced a bigger challenge. How will he provide a proof for the theory for which he fought for over a decade. Moreover he was not an expert in experimentation at all. It was an uphill task for him.

But again his extraordinary genius came out with an extraordinary idea. He proposed the bending of starlight around the solar limb (gravitational lensing) as a possible proof for general relativity. 

He argued that the light from the distant stars that pass by the sun before reaching us on Earth will bend while their journey near the sun. The sun being a massive mass will create a huge curvature in the space-time around it. The starlight while passing by the sun will follow this curvature and get bent. As a result multiple images of the star will be formed in the sky. This is in complete compliance with general relativity.

So if Einstein’s theory is correct starlight will bend around the sun and if it is not then the light will pass unaffected. Einstein requested the astronomers all over the world to go and check it.

Many astronomers including Arthur Eddington, a renowned British astronomer took up the challenge. Watching the faint starlight in the presence of powerful sunlight was always going to be difficult. So they chose total solar eclipse for their observations.

In 1919, Eddington’s observations of a total solar eclipse provided concrete proofs in favour of Einstein’s general theory of relativity. 

A British proof to a German theory provided a thrust towards world peace in the post first world war era. Einstein dreamt of a peaceful world without any wars. Even his greatest scientific theory advocated his humane feelings.

In 1921, Einstein was awarded the Nobel prize, not for his general theory of relativity, but for his discovery of the photoelectric effect (may be to his disappointment).

General theory of relativity has been termed as ‘the most beautiful theory of physics’ ever. Till date it remains arguably the best theory of nature not to win a Nobel prize.

I am not a genius. I am just curious...I ask many questions and when the answer is simple, then God is answering.     

         --Albert Einstein.

By Prabir Rudra

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