The Big Bang Theory And The Origin Of Earth

Prepared by Dr. Abhijit Kar Gupta, Physics Department, Panskura Banamali College (e-mail: [email protected])

The Big Bang Theory and the Origin of Earth

General Introduction:

Where are we? How big are we?

Our planet Earth is around 150 million km away from the Sun. (Note: 1 million = 1000,000 = 10 lakhs) Light travels at nearly 300, 000 km in 1 second. So the light from the Sun takes a little more than 8 minutes to reach us. One of the furthest objects in our solar system is Pluto (a dwarf planet) which is around 6000 million km away from the Sun and the sunlight takes nearly 5 Hours and a ½ to reach Pluto. The time taken by light to come to Earth from other nearby stars is of the order of a few years. So the light from the stars that we see on the sky are really many years old. The distances among the stars are so great that it is useful consider the unit of ‘light year’ which is the distance covered by light in one year. Our Sun (and the solar system) is a member of a bigger system, the Milky Way Galaxy. There are approximately 100,000 million other stars in this galaxy. In fact, our Sun is only a very ordinary star; there are many massive stars, much bigger than the Sun in Milky Way. The Sun lies at a distance of about 30,000 light years from the centre of the Milky Way galaxy, around one third of the dimension of the galaxy. Our galaxy is not even a special one! It is estimated that there are 3000 million other galaxies in the universe that could be searched so far. The dimension of our universe, that could be ‘observed’, is around 10,000 million light years or more. Time scales and Masses are also equally enormous. Our earth takes a year to complete one trip round the sun. The Jupiter takes 12 years. The Sun and other stars are also orbiting around the centre of the galaxy. They take around 200 million years to complete one trip round the galactic centre. Mass can be so enormous that the masses of stars and even bigger objects are not measured in kg or other known units. The unit that is used is one solar mass. If a star is two times heavier than the Sun, for example, it is called 2 solar mass etc.

Everyone is born and everyone dies. 1

Prepared by Dr. Abhijit Kar Gupta, Physics Department, Panskura Banamali College (e-mail: [email protected])

Everything has a life cycle of its own; the living beings, the rocks, the elements, the planets, the stars and the galaxies. The life cycle of a star is amazingly interesting and this is of great concern to the scientists as the study of the universe and its structure is connected to this. Stars are born in a nebula, from collapsing clouds of interstellar gas and dust. The death of a star is inevitable. The thermonuclear reaction (nuclear fusion) in a star ends one day and the star approaches ‘death’. The nuclear fuel, mainly hydrogen and helium, in a star ends up sometime and then the inner core of it shrinks under gravitational pull and the outer core expands so much that it appears to be a giant and red coloured star – a ‘red giant’!

The force of attraction among the heavenly bodies like starts, planets, galaxies etc. is called GRAVITATION. Gravitational force acts between any two objects or particles (no matter how small or big) in the universe. However, the force of attraction is more if one of the bodies or both are big enough. For example, earth attracts us with a big force and that is why we fall down towards the earth and not go up easily. The force is less as the distance between two objects is more. According to Newton’s law of gravitation, Force between two objects is proportional to the product of the masses of two bodies and inversely proportional to the square of the distance

What will be the fate of our Sun? When the core of an ordinary star, like that of our Sun (mass less than 1.4 times the solar mass, called ‘Chandrasekhar limit’), runs out of hydrogen fuel, it will contract under the force of gravity. It cannot hold the force of gravity any more by generating heat and radiation pressure. However, some fusion (thermonuclear reaction) will still go on over the outer layers. As the core contracts, it heats up. This heat is supplied to the outer core, causing that to expand. As the outer layers expand, the radius of the star will increase and it will become a red giant. Our sun will also become a red giant one day! It will become so large that it will swallow the planets Mercury and Venus and possibly the Earth, including the life on this planet. This will of course take many thousand million years. After this, as the core contracts, this will become hot enough to cause the helium to fuse into carbon in it. When the helium fuel runs out, the core will expand and cool. The upper layers will expand and eject materials that will collect around the dying star to form a planetary nebula. Finally, the core will cool into a white dwarf and then eventually into a black dwarf. This entire process will take a few billion years. Sun Like Stars ⇒ Red Giant (Mass up to around 1.5 times Sun)

⇒

Planetary Nebula

⇒

White Dwarf

⇒ Black Dwarf 2

Prepared by Dr. Abhijit Kar Gupta, Physics Department, Panskura Banamali College (e-mail: [email protected])

The fate of Massive Stars: For a huge star (mass around 1.5 to 3 time the mass of Sun), as the core runs out of hydrogen, helium atoms fuse into carbon just like that in the Sun. However, after the Helium is gone, their mass is enough to fuse carbon into heavier elements such as oxygen, neon, silicon, magnesium, sulfur and iron. Once the core has turned to iron, it cannot burn any longer. The star collapses by its own gravity and the iron core heats up. The core becomes so tightly packed that protons and electrons merge to form neutrons. Here also the outer core expands and the star becomes a red supergiant! In less than a second, the iron core, which is about the size of the Earth, shrinks to a neutron core with a radius of about 6 miles (10 kilometers). The outer layers of the star fall inward on the neutron core, thereby crushing it further. The core heats to billions of degrees and explodes (supernova), thereby releasing large amounts of energy and material into space. The shock wave from the supernova can initiate star formation in other interstellar clouds. The remains of the core can form a neutron star. For even more massive stars, we call them Giant stars (over 3 times the mass of Sun), a similar fate happens but then it ends up in a black hole through supernova explosion. The massive stars shrink so much at the end of life; the density becomes so unimaginably high under its own gravitational pull that even the light cannot get out of it. Black holes are nearly impossible to detect as no electromagnetic radiation can come out them. Huge Stars (Mass 1.5 to 3 times the mass of Sun) ⇒ Red Super Giant ⇒ Supernova ⇒ Neutron Star Giant Stars (Over 3 times the mass of Sun) ⇒ Red Super Giant ⇒ Supernova ⇒ Black Hole

Supernova:

A massive star, when it reaches the end of its nuclear fuel, can become a ‘supernova’. A supernova arises when the core of the star collapses under its own gravitational attraction, releasing huge amount of energy (shock waves) which causes the outer core to explode. The inside core may become a Neutron star or a Black hole. But the tremendously exploding envelope carries huge amount of energies with very fast moving particles like electrons, protons etc. This is a possible source of Cosmic Rays.

3

Prepared by Dr. Abhijit Kar Gupta, Physics Department, Panskura Banamali College (e-mail: [email protected])

The beginning of the Universe: Big Bang Theory

The Big Bang theory is the scientific theory of the creation of the Universe. The Big Bang theory is an effort to explain what happened at the very beginning of our universe. It also tries to explain what happened during and immediately after the beginning. The study of the creation of Universe and the properties of early universe (immediately after the Big Bang) is called Cosmology. The Universe today, as we see, is different from what it was in the past and it will be different in the future. It is not static. Our universe, in fact, has a beginning as the discoveries in Astronomy and Astrophysics and mathematical calculations in Cosmology have established that truth beyond doubt. Prior to the moment of creation, there was nothing; during and after that moment there was something - our universe! The Big Bang Theory: Infinitely hot matter of infinite density was thought to be concentrated in an infinitesimally small volume (a point) at the moment of the creation of the universe. Around 14 billion years ago a tremendous explosion started the expansion of the universe. This beginning, the ‘explosion’ is known as Big Bang. [Note: 1 billion year = 1000 million year, 1 million year = 1000,000 year = 10 lakh years. Thus, 1 billion years = 1000,000,000 years = 100 crores years.] The Big Bang theory tells us that our Universe originated from a ‘singularity’. Singularity is a concept (a mathematical concept) which defies our understanding of physical reality and imagination! What existed prior to the moment of Big Bang is completely unknown and is a matter of pure speculation. The space, time, matter, or energy everything was created with Big Bang. Where did our Universe come from? We don't know. Why did it appear? We don't know. Where it is? We don’t know. All we know is that we exist inside the universe. The origin of the Big Bang theory can be credited to Edwin Hubble (1889-1953). Hubble made the observation that the universe is continuously expanding. Matter was created out of Big Bang and energy propagated at the speed of light (300,000 km/sec). The temperature of the universe, just after a tiny fraction of a second after the Big Bang explosion, was of the order of 1000 trillion degree Centigrade (1 trillion = 1000 billion) as estimated. As the universe quickly expanded, it had also undergone a rapid cooling enabling the creation of matter from energy. Universe back then was too hot for anything other than the most fundamental particles -4

Prepared by Dr. Abhijit Kar Gupta, Physics Department, Panskura Banamali College (e-mail: [email protected])

such as quarks and photons. About one ten-thousandth of a second after the Big Bang, protons and neutrons formed, and within a few minutes these particles stuck together to form atomic nuclei, mostly hydrogen and helium. Hundreds of thousands of years later, electrons stuck to the nuclei to make complete atoms. The evidences supporting the Big Bang theory: 







Experimental observations reveal that the galaxies appear to be moving away from us at speeds proportional to their distance (This is called "Hubble's Law," named after Edwin Hubble.). Farther they are, faster they travel! This observation supports the expansion of the universe at an accelerating speed and suggests that the universe was once compacted. Since the Big Bang, the universe has been continuously expanding and, thus, there has been more and more distance between clusters of galaxies. This phenomenon of galaxies moving farther away from each other is known as the red shift. If the universe was initially extremely hot as the Big Bang suggests, we should be able to find some remnant of this heat. In 1965, Arno Penzias and Robert Wilson discovered Cosmic Microwave Background radiation (CMB), which seems to be coming from the farthest reaches of the universe and that is supposed to be left over from the Big Bang. Even more recently, NASA’s COBE satellite was able to detect cosmic microwaves emanating from the outer reaches of the universe. These microwaves were remarkably uniform which illustrated the homogeneity of the early stages of the universe. Finally, the abundance of the "light elements" Hydrogen and Helium found in the observable universe are thought to support the Big Bang model of origins. .

How the Solar system was formed? The leading hypothesis to explain how the solar system was formed is called the condensation theory, which is based on a related explanation called the nebular theory. What is a Nebula?

A nebula is a large cloud of gas and dust that exists in the depths of interstellar space. These clouds typically form during the death of a giant star when it becomes supernova. This mighty explosion sends most of the star's mass outward into space as a massive wave of debris. The nebular cloud from which our solar system formed may have accumulated from one or more stars that went supernova billions of years ago. Astronomers have used the Hubble Space Telescope and other observatories to discover similar nebular clouds where new stars and possibly planets appear to be in the process of being created. Astronomers estimate that the nebular cloud from which our solar system was formed, contained about two to three times the mass of the Sun and was about 100 astronomical units (AU) across. An astronomical unit is defined as the average distance between the Sun and Earth, or about 150 million 5

Prepared by Dr. Abhijit Kar Gupta, Physics Department, Panskura Banamali College (e-mail: [email protected])

km. This massive loosely-bound cloud of dust, ice particles, and gases (primarily hydrogen and helium) had some small rate of rotation due to the method in which it was formed. Over time, this nebular cloud began to collapse inward. The collapse may have itself been triggered by a supernova that sent shockwaves through the cloud causing it to compress. As the cloud compressed on itself, the gravitational attraction of the matter within increased and pulled the material in even further. The nebula continued to contract under the influence of gravity causing it to spin faster. The more the cloud contracted, the faster it rotated due to the conservation of angular momentum. The rate of contraction was the greatest near the center of the cloud where a dense central core began to form. As the rate of rotation of the nebula continued to increase, centrifugal effects caused the spinning cloud to flatten into a disk with a bulge at its center. The middle of this spinning disk further condensed to eventually form the Sun at the center of the solar system. The material spinning around this new star also condensed into several large chunks of material called planetoids. As these planetoids collided, they coalesced into larger bodies to form the planets that exist today. Because the Sun and planets all formed from the same nebular cloud, they all rotate in the same direction that was induced on the disk of material as it coalesced. Not only do the planets all rotate counter-clockwise around the Sun, but the Sun and nearly all the planets rotate counter-clockwise about their axes. The solar equator and the plane containing the orbits of the planets are also nearly identical, further supporting the formation theory described above. Only a few exceptions to this explanation can be found in the solar system. The planet Venus actually rotates clockwise around its axis in what is called a retrograde motion. Uranus is also in an unusual orientation since the world is tilted on its side with its north and south poles in about the same plane as the planet's orbit around the Sun. Tiny Pluto is also tilted on its side and is the only planet with an orbit considerably outside of the ecliptic plane. While the reasons for these eccentricities are unknown, it is believed that large collisions with other large bodies during the formation of the solar system may account for the unusual characteristics of these planets. In Short:

   

A massive cloud, called solar nebula, was disturbed by the shock wave coming from a nearby supernova. The cloud then collapsed under its own gravitational attraction and the sun was formed in the central region. The planets were formed from the outer regions of the accretion disk. The initial collapse of solar nebula is supposed to take less than 100,000 years. The age of formation of solar system is estimated to be little more than 4.5 billion years.

Some Old theories of origin of solar system: The meteorite theory of Immanuel Kant :

6

Prepared by Dr. Abhijit Kar Gupta, Physics Department, Panskura Banamali College (e-mail: [email protected])

The German philosopher Immanuel Kant (1724 - 1804) put up a theory which is believed to be the oldest meteorite theory. Kant believed that a huge cloud of meteorites existed at the beginning of our solar system. This cloud turned slowly around itself. Through this circling the cloud got a disc shape. Through their attraction under each other the meteorites started to form a mass-centre. Then it became so hot through contractions that it started to glow. That could have been the birth of the Sun. In some distance from the sun there were some other, smaller mass-centres which started to rotate around the sun – they formed the planets. The Nebular theory of Laplace:

The French mathematician Pierre Simon Marquis de Laplace (1749 - 1872) developed the ideas of Kant further. Because of this his theory is also known as the Kant-Laplace Nebular theory. Laplace believed that in the beginning there was a huge cloud of hot gas which rotated slowly. It started to condense while it cooled off. Through this the gravitation of the particles got big enough and formed a discusshaped ‘pre-sun’. Through further condensing, the centre of the pre-sun became hot again and the rotation speed became faster. So the more the gas cloud condenses, the hotter the centre became and the faster became the rotation speeds. Because of this the centrifugal force became stronger than the gravitational pull, so a ring would detach itself. Because of the different attractions of the particles in the ring it would tear apart and from the different pieces, the planets would form that are still circling the sun. What is a meteorite? A meteorite is a natural object originating somewhere in outer space. We often see an impact of such objects with the Earth’s atmosphere and earth’s surface. When it enters the atmosphere, impact pressure causes the body to heat up and emit light, thus forming a fireball – we call it meteor or shooting star.

An old Hypothesis and some facts about our Planet Tetrahedron Hypothesis: The Tetrahedral hypothesis by geologist Arthur Holmes is an obsolete scientific theory attempting to explain the arrangement of the Earth's continents and oceans by referring to the geometry of a tetrahedron. Although it was a historically interesting theory in the late 19th and early 20th century, it was superseded by the concepts of continental drift and modern plate tectonics. (Note: A tetrahedron is a solid formed by 4 equilateral triangles, like Pyramid.) Key considerations of the Hypothesis:  More than 75% of the Earth's land area is in the northern hemisphere.  Continents are roughly triangular. 7

Prepared by Dr. Abhijit Kar Gupta, Physics Department, Panskura Banamali College (e-mail: [email protected])

   

Oceans are roughly triangular. The North Pole is surrounded by water, the South Pole by land. Exactly opposite the Earth from land is almost always water. The Pacific Ocean occupies about one third of the Earth's surface.

What is the age of Earth?

Throughout this century the race has been on to discover the oldest rocks in the world. The oldest volcanic rock found so far has been dated at 3.75 billion years old, but this is not the whole story. Meteorites created at the same time as the Earth, hit us all the time, radioactive dating shows that they are about 4.55 billion years old. Earth’s atmosphere and early life:

The Earth’s atmosphere was not like this as it is now. The compositions were different. During the first billion years, the Earth's surface was originally molten, as it cooled the volcanoes ejected huge amounts of Carbon Dioxide, steam, ammonia and methane. There was no Oxygen! The steam condensed to form water which then produced oceans. Evidence points to bacteria flourishing 3.8 billion years ago so this means that life came into existence about 700 million years after the Earth was created. Such early forms of life existed in the shallow oceans close to thermal sources, which were reservoirs of heat and minerals.

8