What is Big Bang Theory

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‘ What is Big Bang Theory? ‘ is challanging question for physist. It was remained unsolved till the expansion of universe was detected by the Hubble Space Telescope. Today we know the origine but not the cause.

Evidence of Big Bang

Our modern picture of the universe dates back to only 1924, when the American astronomer Hubble demonstrated that ours wasn’t the sole galaxy. There were actually many others, with vast tracts of empty space between them. In order to prove this, he needed to work out the distances to those other galaxies, which are thus far away that, unlike nearby stars, they really do appear fixed. Hubble was forced, therefore, to use indirect methods to live the distances.

Now, the apparent brightness of a star depends on two factors: what proportion light it radiates (its luminosity), and the way far it’s from us. For nearby stars, we will measure their apparent brightness and their distance, then we will compute their luminosity. Conversely, if we knew the luminosity of stars in other galaxies, we could compute their distance by measuring their apparent brightness. Hubble noted that certain sorts of stars always have an equivalent luminosity once they are near enough for us to measure; therefore, he argued, if we found such stars in another galaxy, we could assume that they had the same luminosity – and so calculated the distance to that galaxy.

If we could do that for variety of stars within the same galaxy, and our calculations always gave an equivalent distance, we might be fairly confident of our estimate. In this way, Hubble figured out the distances to nine different galaxies. We now know that our galaxy is merely one among some hundred thousand million which will be seen using modern telescopes, each galaxy itself containing some hundred thousand million stars.

We sleep in a galaxy that’s about 100 thousand light-years across and is slowly rotating; the celebs in its spiral arms orbit around its center about once every several hundred million years. Our sun is just an ordinary, average-sized, yellow star, near the inner edge of one of the spiral arms.

We have certainly come a long way since Aristotle and Ptolemy, when thought that the earth was the center of the universe! Stars are thus far away that they seem to us to be just pinpoints of sunshine . We cannot see their size or shape. So how can we tell differing types of stars apart?

For the overwhelming majority of stars, there’s just one characteristic feature that we will observe – the colour of their light. Newton discovered that if light from the sun passes through a triangular-shaped piece of glass, called a prism, it breaks up into its component colors (its spectrum) as during a rainbow. By focusing a telescope on a private star or galaxy, one can similarly observe the spectrum of the sunshine from that star or galaxy.

Different stars have different spectra, but the relative brightness of the different colors is always exactly what one would expect to find in the light emitted by an object that is glowing red hot. (In fact, the sunshine emitted by any opaque object that’s glowing hotdog features a characteristic spectrum that depends only on its temperature – a thermal spectrum. This means that we will tell a star’s temperature from the spectrum of its light.)

Moreover, we discover that certain very specific colors are missing from stars’ spectra, and these missing colors may vary from star to star. Since we know that each chemical element absorbs a characteristic set of very specific colors, by matching these to those that are missing from a star’s spectrum, we can determine exactly which elements are present within the star’s atmosphere. In the 1920s, when astronomers began to look at the spectra of stars in other galaxies, they found something most peculiar: there were the same characteristic sets of missing colors as for stars in our own galaxy, but they were all shifted by the same relative amount toward the red end of the spectrum. To understand the implications of this, we must first understand the Doppler Effect.

As we’ve seen, light consists of fluctuations, or waves, within the electromagnetic field. The wavelength (or distance from one wave crest to the next) of sunshine is extremely small, starting from four to seven ten-millionths of a meter. The different wavelengths of sunshine are what the human eye sees as different colors, with the longest wavelengths appearing at the red end of the spectrum and therefore the shortest wavelengths at the blue end. Now imagine a source of sunshine at a continuing distance from us, like a star, emitting waves of sunshine at a continuing wavelength. Obviously the wavelength of the waves we receive are going to be an equivalent because the wavelength at which they’re emitted (the field of the galaxy won’t be large enough to have a significant effect).

Suppose now that the source starts moving toward us. When the source emits subsequent wave crest it’ll be nearer to us, therefore the distance between wave crests are going to be smaller than when the star was stationary. This means that the wavelength of the waves we receive is shorter than when the star was stationary. Correspondingly, if the source is moving faraway from us, the wavelength of the waves we receive are going to be longer.

In the case of sunshine , therefore, means stars moving faraway from us will have their spectra shifted toward the red end of the spectrum (red-shifted) and people moving toward us will have their spectra blue-shifted. This relationship between wavelength and speed, which is named the Doppler effect , is an everyday experience. Listen to a car passing on the road: because the car is approaching, its engine sounds at a better pitch (corresponding to a shorter wavelength and better frequency of sound waves), and when it passes and goes away, it sounds at a lower pitch. The behavior of sunshine or radio waves is analogous .

Indeed, the police make use of the Doppler effect to live the speed of cars by measuring the wavelength of pulses of radio waves reflected off them. ln the years following his proof of the existence of other galaxies, Rubble spent his time cataloging their distances and observing their spectra. At that point most of the people expected the galaxies to be traveling quite randomly, then expected to seek out as many blue-shifted spectra as red-shifted ones.

It was quite surprise, therefore, to seek out that the majority galaxies appeared red-shifted: nearly all were moving faraway from us! More surprising still was the finding that Hubble published in 1929: even the dimensions of a galaxy’s Doppler effect isn’t random, but is directly proportional to the galaxy’s distance from us. Or, in other words, the farther a galaxy is, the faster it’s moving away! And that meant that the universe couldn’t be static, as everyone previously had thought, is actually expanding; the space between the various galaxies is changing all the time. The discovery that the universe is expanding was one among the good intellectual revolutions of the 20 th century.

The Big Bang Theory

The Big Bang is really not a “theory” in the least , but rather a scenario or model about the first moments of our universe, that the evidence is overwhelming.

It is a standard misconception that the large Bang was the origin of the universe. In reality, the large Bang scenario is totally silent about how the universe came into existence within the first place. In fact, the closer we glance to time “zero,” the less certain we are about what actually happened, because our current description of physical laws don’t yet apply to such extremes of nature. The Big Bang scenario simply assumes that space, time, and energy already existed.

But it tells us nothing about where they came from or why the universe was born hot and dense to start with. But if space and everything with it’s expanding now, then the universe must are much denser within the past. That is, all the matter and energy (such as light) that we observe within the universe would are compressed into a way smaller space within the past. Einstein’s theory of gravity enables us to run the “movie” of the universe backwards—i.e., to calculate the density that the universe must have had within the past. The result: any chunk of the universe we will observe—no matter how large—must have expanded from an infinitesimally small volume of space.

By determining how briskly the universe is expanding now, then “running the movie of the universe” backwards in time, we will determine the age of the universe. The result is that space started expanding 13.7 billion years ago. This number has now been experimentally determined to within 1% accuracy.

It’s a common misconception that the whole universe began from some extent . If the entire universe is infinitely large today (and we do not know yet), then it might are infinitely large within the past, including during the large Bang. But any finite chunk of the universe—such because the a part of the universe we will observe today—is predicted to possess started from a particularly small volume.
Part of the confusion is that scientists sometimes use the term “universe” when they’re pertaining to just the part we will see (“the observable universe”). And sometimes they use the term universe to ask everything, including the a part of the universe beyond what we will see.
It’s also a common misconception that the Big Bang was an “explosion” that took place somewhere in space. But the large Bang was an expansion of space itself. Every part of space participated in it. For example, the a part of space occupied by the world , the Sun, and our Milky Way galaxy was once, during the large Bang, incredibly hot and dense. The same holds true of each other a part of the universe we will see.

We observe that galaxies are rushing apart in just the way predicted by the Big Bang model. But there are other important observations that support the large Bang.
Astronomers have detected, throughout the universe, two chemical elements that would only are created during the large Bang: hydrogen and helium. Furthermore, these elements are observed in just the proportions (roughly 75% hydrogen, 25% helium) predicted to have been produced during the Big Bang.

This is the nucleosynthesis of the light elements. This prediction is based on our well-established understanding of nuclear reactions—independent of Einstein’s theory of gravity. Second, we will actually detect the sunshine left over from the age of the large Bang. This is the origin of the cosmic microwave background . The blinding light that was present in our region of space has long ago traveled off to the far reaches of the universe. But light from distant parts of the universe is simply now arriving here at Earth, billions of years after the large Bang.

This light is observed to possess all the characteristics expected from the large Bang scenario and from our understanding of warmth and lightweight . The standard Hot explosion model also provides a framework during which to know the collapse of interest form galaxies and other large-scale structures observed within the Universe today. At about 10,000 years after the large Bang, the temperature had fallen to such an extent that the energy density of the Universe began to be dominated by massive particles, instead of the sunshine and other radiation which had predominated earlier.

This change within the sort of matter density meant that the gravitational forces between the huge particles could begin to require effect, in order that any small perturbations in their density would grow. Thirteen point seven billion years later we see the results of this collapse within the structure and distribution of the galaxies.

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The First Fractions of a Second

The standard model of big bang cosmology states that the universe began 13.8 billion years ago as a very small, dense region at a temperature of 1032 degrees K. The universe underwent a period of rapid expansion after 10-43 seconds, a period referred to as Planck time.

After 10-33 seconds, just a short time after it began, the inflation ended and the universe had grown by a factor greater than 1035. Quantum fluctuations that occurred during inflation left very small density fluctuations or inconsistencies in the composition of the infant universe.

At first the universe was pure energy, but after the large bang, that energy converted into matter. Due to some reason, conversion resulted in slightly more matter than matter’s counterpart, antimatter. (Antimatter is almost just like matter except antimatter particles have the other charge and spin of their corresponding matter particles. The two annihilate when they meet.)

The antimatter annihilated with the matter, but for some reason, for every billion antiparticles there were a billion and one particles. As a result, the antimatter was destroyed and some matter remained. The asymmetry between matter and antimatter at the beginning of the universe is a mystery to scientists and a topic of current research.

By the first second after the big bang, the universe consisted of a 10-billion-degree K soup of neutrons, protons, electrons, anti-electrons, photons, and neutrinos. At this point, the region of the known universe was a minimum of 1019.5 cm across.

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Big Bang and Big Crunch

In Friedmann’s model, all the galaxies are moving directly faraway from one another . The situation is quite sort of a balloon with variety of spots painted thereon being steadily blown up. As the balloon expands, the space between any two spots increases, but there’s no spot which will be said to be the middle of the expansion. Moreover, the farther apart the spots are, the faster they’re going to be moving apart.

Similarly, in Friedmann’s model the speed at which any two galaxies are moving apart is proportional to the space between them. So it predicted that the Doppler effect of a galaxy should be directly proportional to its distance from us, exactly as Hubble found. Despite the success of his model and his prediction of Hubble’s observations, Friedmann’s work remained largely unknown within the West until similar models were discovered in 1935 by the American physicist Howard Robertson and therefore the British mathematician Arthur Walker, in response to Hubble’s discovery of the uniform expansion of the universe.

Although Friedmann found only one , there are literally three differing types of models that obey Friedmann’s two fundamental assumptions. In the first kind (which Friedmann found) the universe is expanding sufficiently slowly that the gravity between the various galaxies causes the expansion to hamper and eventually to prevent . The galaxies then start to maneuver toward one another and therefore the universe contracts.

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Figure 1 shows how the space between two neighboring galaxies changes as time increases. It starts at zero, increases to a maximum, then decreases to zero again. In the second quite solution, the universe is expanding so rapidly that the gravity can never stop it, though it does slow it down a touch .

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Figure 2 Shows the Separation between neighboring galaxies during this model. It starts at zero and eventually the galaxies are moving apart at a gentle speed. Finally, there’s a 3rd quite solution, during which the universe is expanding barely fast enough to avoid recollapse. In this case the separation, shown in

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Figure 3, also starts at zero and increases forever. However, the speed at which the galaxies are moving apart gets smaller and smaller, although it never quite reaches zero. A remarkable feature of the primary quite Friedmann model is that in it the universe isn’t infinite in space, but neither does space have any boundary.

Gravity is so strong that space is bent round onto itself, making it rather a bit like the surface of the planet . If one keeps traveling during a certain direction on the surface of the world , one never comes up against an impassable barrier or falls over the sting , but eventually comes back to where one started. In the first quite Friedmann model, space is simply like this, but with three dimensions rather than two for the earth’s surface. The time , time, is additionally finite in extent, but it’s sort of a line with two ends or boundaries, a beginning and an end.

We shall see later that when one combines general theory of relativity with the indeterminacy principle of quantum physics , it’s possible for both space and time to be finite with none edges or boundaries. The idea that one could go right around the universe and find yourself where one started makes good fantasy , but it doesn’t have much practical significance, because it can be shown that the universe would recollapse to zero size before one could get round.

You would got to travel faster than light so as to finish up where you started before the universe came to an end – which isn’t allowed! In the first quite Friedmann model, which expands and recollapses, space is bent in on itself, just like the surface of the world . It is therefore finite in extent. In the second quite model, which expands forever, space is bent the opposite way, just like the surface of a saddle. So in this case space is infinite.

Finally, within the third quite Friedmann model, with just the critical rate of expansion, space is flat (and therefore is additionally infinite). But which Friedmann model describes our universe? Will the universe eventually stop expanding and begin contracting, or will it expand forever? To answer this question we’d like to understand this rate of expansion of the universe and its present average density. If the density is a smaller amount than a particular critical value, determined by the speed of expansion, the gravity are going to be too weak to halt the expansion.

If the density is bigger than the critical value, gravity will stop the expansion at a while within the future and cause the universe to recollapse. We can determine this rate of expansion by measuring the velocities at which other galaxies are moving faraway from us, using the Doppler effect . This can be done very accurately. However, the distances to the galaxies aren’t alright known because we will only measure them indirectly. So all we all know is that the universe is expanding by between 5 percent and 10 percent every thousand million years. However, our uncertainty about this average density of the universe is even greater.

If we add up the masses of all the celebs that we will see in our galaxy and other galaxies, the entire is a smaller amount than one hundredth of the quantity required to halt the expansion of the universe, even for rock bottom estimate of the speed of expansion.

Our galaxy and other galaxies, however, must contain an outsized amount of “dark matter” that we cannot see directly, but which we all know must be there due to the influence of its gravity on the orbits of stars within the galaxies. Moreover, most galaxies are found in clusters, and that we can similarly infer the presence of yet more substance in between the galaxies in these clusters by its effect on the motion of the galaxies. When we add up all this substance , we still get only about one tenth of the quantity required to halt the expansion.

However, we cannot exclude the likelihood that there could be another sort of matter, distributed almost uniformly throughout the universe that we’ve not yet detected which might still raise the typical density of the universe up to the critical value needed to halt the expansion.

The present evidence therefore suggests that the universe will probably expand forever, but all we will really make certain of is that albeit the universe goes to recollapse, it won’t do so for a minimum of another ten thousand million years, since it’s already been expanding for a minimum of that long. This should not unduly worry us: by that point , unless we’ve colonized beyond the system , mankind will long ago have died out, extinguished along side our sun! All of the Friedmann solutions have the feature that at a while within the past (between ten and twenty thousand million years ago) the space between neighboring galaxies must have been zero. At that point , which we call the large bang, the density of the universe and therefore the curvature of space-time would are infinite.

Because mathematics cannot really handle infinite numbers, this means that the general theory of relativity (on which Friedmann’s solutions are based) predicts that there is a point in the universe where the theory itself breaks down. Such some extent is an example of what mathematicians call a singularity. In fact, all our theories of science are formulated on the idea that space-time is smooth and nearly fiat, in order that they break down at the large bang singularity, where the curvature of space-time is infinite.

This means that albeit there have been events before the large bang, one couldn’t use them to work out what would happen afterward, because predictability would break down at the large bang. Correspondingly, if, as is that the case, we all know only what went on since the large bang, we couldn’t determine what happened beforehand.

As far as we are concerned, events before the large bang can haven’t any consequences, in order that they shouldn’t form a part of a scientific model of the universe. We should therefore cut them out of the model and say that point had a beginning at the large bang.


  • (Advanced Series in Astrophysics and Cosmology, Vol 8) Stephen W. Hawking – Papers on the Big Bang and Black Holes-World Scientific Publishing Company (1993)
  • (Great Discoveries in Science) Rachel Keranen – The Big Bang Theory-Cavendish Square Publishing (2017)
  • David L. Alles – The Evolution of the Universe (2005)
  • Stephen W Hawking – The theory of everything-Phoenix Books (2006)
  • Stephen W. Hawking, Ron Miller, Carl Sagan – A Brief History of Time_ From the Big Bang to Black Holes -Bantam (1988)
  • https://www.hawking.org.uk/news/black-holes-the-edge-of-all-we-know

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