Before
the creation of time, a mysterious event, or more precisely, an explosion,
started the path of this universe, which people all over the world know as the
Big Bang. (The link is given for details about the Big Bang https://spacejagat.blogspot.com/2022/02/the-big-bang-first-most-mysterious.html)
Since then, matter, time, energy, and radiation have arrived, and the universe
has been expanding ever since. Today, in very simple words, we will try to
understand this cosmic expansion from all angles.
So
now the question is what exactly do we mean by the expansion of the universe?
In answer to this question, we can simply state that the gradual increase in
the distance between any two gravitationally bound objects in the observable
universe over time is characterized by the expansion of the universe. This
phenomenon was first known a hundred years ago from today
when the American scientist Vesto Slipher first observed in 1912 that distant
galaxies were gradually moving away from us. However, he could not understand
the reason. Ten years later, in 1922, Alexander Friedman, a Russian scientist,
explained for the first time with his theory that the universe was expanding.
The first observational evidence in this regard was presented to us in 1924 by
the Swedish scientist Kurt Lundmark. Three years later, in 1927, the Belgian
Catholic priest Georges Lemaître made a theoretical explanation similar to
Friedmann's, trying to establish a relationship between galaxies' distances and
their velocities, and presented the whole thing as the Big Bang. Finally, the
American scientist Edwin Hubble, through his observations, brought Hubble's
theory to further establish the phenomenon of the expansion of the universe.
One thing to note here, however, is that the distance between the distant
galaxies and the rate of expansion of universe found in the Kurt Lundmark study
were quite accurate. Today, 100 years after his research work, only 1 percent
error has been found, using highly sophisticated equipments.
After
extensive research and observations over a long period of time, scientists
around the world, have unanimously agreed that the universe was created by the
Big Bang approximately 1,375 crore years ago and began to expand in less than 1
second of the explosion that continues today and as time goes on, the velocity
of this expansion is getting faster and faster. The exact answer to why the
universe is expanding is not yet known. However, it is said that when the
expansion of the universe began 1032 seconds after the Big Bang,
there was an expansion equal to 100 million light years in each part of the
universe smaller than an atom. This event accelerated the rapid expansion of
the entire universe. The phenomenon of expansion of this universe is called
'Metric Expansion' in the language of science. This metric term implies that
the concept of distance in the universe is always variable or in other words relative.
The
modern explanation for this metric expansion was given in 1979 by the American
scientist Allen Guth. He showed in his mathematical explanation that if there
were any vacancies with positive energy in this space, then according to
general relativity there could be expansion. (General Theory of Relativity:
Its basic idea is that it is an invisible force for which objects attract
each other which we call gravitational force, the curvature of time and space
is observed for this gravitational force) This cosmic inflation has solved
many problems of the universe but failed to solve all the problems. However, we
do not yet know the exact answer to why this cosmic inflation occurred.
To
understand the metric expansion of the universe, we need to know what metric
expansion is and how it happens. Metrics is a method of expressing the concept
of distance by referring mathematically to how the distance between two points
in space is measured in terms of coordinate system. The coordinates here refer
to the arrangement of straight lines and curves to determine the position of a
particular point relative to a place. The position in this system is denoted by
X and Y. In other words, a metric is a path that describes how the distance
between two points can be measured.
One
thing is very important here. Distance is not always measured in straight
lines, but sometimes it is also measured in curves, as in the case of earth,
the distance to the airway is determined. In this case the curvature of the
smallest distance between any two points on the surface of a curve is called
geodesic. However, using the conventional method of determining the distance
between two places in space, various errors and deviations can occur. So we use
a special coordinate system called Commoving coordinate system to determine the
distance between galaxies. This method uses time, speed of light, etc. to
measure distance. Here first the brightness of the light, coming from a distant
object, is measured then the value of redshift of that light is determined. All
this information is then calculated using the Lambda-CDM Model to determine the
distance to an object farther from earth (Redshift: The farther an
object goes, the longer the wavelength of light coming from that object
increases and the frequency decreases so that the object turns red. This
phenomenon is called redshift) It is not possible to measure distances
directly in the universe, so this indirect method is used.
One
thing that has become clear from what has been discussed so far is that the
constant expansion of the universe is a very common feature. Albert Einstein's
Theory of General Relativity mentioned this subject long before the 'Expansion
of Space' into the realm of scientists. Friedmann's mathematical equations,
Hubble's law, and Einstein's theory all point to the expansion of the universe.
At first, Hubble initially had doubts about the initial size and density of
universe, and even about the velocity of expansion of universe. Recent
measurements of galaxies' distance from earth and velocity have shown a value
of 9 percent difference from Hubble's constant (Hubble constant: the
proportional relationship between the velocity of distant galaxies and their
distance from earth called the Hubble constant). In 2001, Wendy Friedman
has calculated the velocity of expansion of universe which is 72 km / second /
parsec. In 2013 this velocity increased slightly and finally in 2016 its value
becomes 73 km / second / parsec. At present the universally accepted velocity
of space expansion is 73.24 km/ sec/ mega parsec. (1 mega parsec = 32,61,563.78
light years or 3.086 X 1019 km) In other words, for an observer, for every 1
million or 10 lakh parsecs or 32,61,563.78 light-years or 3.086 X 1019 km
distance, the light, received from that distance, moving away or redshift is
73.24 km per second. (Light Year: The distance that light travels in
a year) Redshift can be more or less 1.74 km per second. Another measurement
found for space expansion to be 67.4 km/ sec/ mega parsec. Here, too, this
redshift of light can be more or less 0.5 km per second. In this case, the two
values of space expansion prove that the conventional space-related model
must have left out some important formula of physics which we have not yet
fully understood. However, it is true that there is a working force on every
object in the universe, the existence of which is understood, but we have not
yet formed a good idea about it. For the mystery of this form of energy we have
identified this energy as Dark energy. (Details link to know more about Dark
energy: https://spacejagat.blogspot.com/2021/10/dark-energy-master-of-universe.html)
Cosmologists
have compared the universe to Euclidean space on cosmic scale. (Euclidean
space: This is a two- or three-dimensional space where Euclidean
geometry is self-evident and the hypotheses apply.) A four-dimensional
space-time mathematical model has been developed across one-dimensional time
with three-dimensional space. According to Einstein's theory, mass and energy
can give curvature to this four-dimensional space-time, which is similar to the
postulate of Euclidean geometry.
To understand the
expansion of the universe, we need to understand a few more things. Although
'Space' or 'Universe' is one in English, it is different from the point of view
of science. The first creates a mathematical concept where cosmic objects are
embedded and the second term is used in a broader sense where there are cosmic
objects, cosmic rays and various known, unknown forces. So our discussion
should always be Universe-centric, when we talk about the space expansion,
science only discusses about three-dimensional structures, but there are many
other dimensions beyond that are existed in universe.
The
whole universe can be either infinite or finite. What we call universe, or more
precisely, observable universe, is actually just the smallest part of the
largest universe or super universe. This is stated by all the combined models
of mathematics and physics. This vast universe may have no edges. It is
pointless to think that universe is expanding into something. Again, many are
comparing universe to bubbles, which is also quite confusing. As the distance
between two fixed points in universe is increasing, so our universe seems to be
expanding. Since the expansion of this universe is infinite, the greatest
universe around our observable universe can never exist.
The
black hole is a very dense cosmic object. But in the initial state after the
Big Bang, the universe was many times denser than the black hole. So the question
is why the state of the universe is not like a black hole? This is because even
though there is motion of matter inside or around the black hole, the black
hole itself is fixed to its place relative to other objects. But this universe
has been and still is very dynamic since its inception. The gravitational
collapse of a place depends on the stability of the object in that particular
place. So at the beginning of the universe its condition was not like the black
hole.
If
one tries to understand the expansion of universe in a very small scale or
area, then it can be understood that this law is not very effective there. For
example, matter can come together and create a variety of cosmic objects such
as comets, satellites, planets, stars, galaxies, etc. It goes without saying
that, there is almost no cosmic expansion. This is because where the
concentration of matter is higher, the force responsible for cosmic expansion
cannot in any way become stronger than the gravitational force. According to
the Principle of Equality of General Relativity, the laws of Special Relativity
are applicable only in such small areas (Principle of Equality: In
the theory of General Relativity, the Principle of Equality is the equilibrium
state of gravitational force and inert mass) For example, the moon, the
only satellite of the earth, is moving away from us by 3.78 cm every year.
Again Andromeda galaxy is gradually coming towards us under the influence of
the gravitational force of our Milky Way galaxy and will merge with our galaxy
at some point of time in future. This has happened many times in the past which
is a very common incident. So it can be said that for this gravitational force
the cosmic expansion in that vast sense within the local cluster of galaxies is
not going to be noticed. However, this expansion can be understood and measured
outside of this local group.
The
rapid expansion of universe is due to the decrease in the density of matter in
per unit area, as the density of matter decreases, the gravitational force also
decreases. As a result, redshift is observed. Galaxies that are 4.5 Giga parsec
or 14.7 billion or 14,700 crore light years away from earth are outside the
Hubble Radius (Hubble Radius: The specific distance from an observer
after which the object will move faster than light from him) Although the
redshift of cosmic objects has been occurred, the visibility of those distant
cosmic objects depends on the history of the expansion of the universe. If the
cosmic object were more than 16 billion or 16,000 light-years away, light would
never reach the earth from there. The last light that was radiated before that
object crossed that boundary will reach the earth.
The
phenomenon of increasing distance between two cosmic objects over time is also
called Scale Factor. At one time the value of this scale factor was zero but in
the future this value will reach infinity. But some models suggest that the
value of this scale factor may decrease, meaning that the universe may shrink.
According to some models, this value may be fixed in the future, meaning that
the universe will no longer expand. In a word, scientists are talking about
three possibilities. Some of the most popular models that scientists are
speculating about cosmic expansion are 'Ant on Rubber Model', 'Rubber Seat
Model', 'Balloon Model’, 'Raisin Bread Model', and so on.
Scientists
have presented some evidence that the universe is expanding, such as:
1.
The universe looks the same from all sides (Isotropic) and consists of a
mixture of homogeneous matter.
2.
According to Copernican theory, no special place in universe was given
priority.
3.
Distant galaxies are gradually moving away from us over time (Hubble's Law)
4.
Explosion and radiation of gamma rays evenly distributed throughout space along
with the existence of supernovae (Supernova: Every star explodes when
it runs out of fuel)
5.
Uniform temperature of Cosmic Microwave Background radiation that has been
scattered throughout the universe for billions of years (Cosmic Microwave
Background Radiation or CMB Radiation: Dim red light , due to redshift
of matters, scattered throughout the universe after the Big Bang, it is the
last ray of light to prove the Big Bang)
6.
By measuring the temperature of interstellar clouds, made up of dust and
hydrogen, through CMB Radiation. It has been observed that this temperature
decreases uniformly everywhere in Universe with time.
7.
The Redshift Survey has shown that all cosmic objects are evenly distributed
around the universe.
Not all of the evidence cited here can be seen with the naked eye, but it can be said with certainty that all of these features must be fully present in the universe. Not only that, but there may be more authentic facts or sources scattered around the universe that we have not yet fully understood. It is hoped that over the next few decades, through more advanced scientific models, we will be able to identify these unknown features or evidence to understand and explain this mysterious cosmic expansion with more logical approach.
DECLARATION: All The Images Have Been Sourced From Google.
