sun as center of universe ( Heliocentric theory or model)

Heliocentrism, or heliocentricism is the astronomical model in which the Earth and planets revolve around the Sun at the center of the Solar System. The word comes from the Greek (ἥλιος helios "sun" and κέντρον kentron "center"). Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. The notion that the Earth revolves around the Sun had been proposed as early as the 3rd century BC by Aristarchus of Samos but at least in the post-ancient world Aristarchus's heliocentrism attracted little attention—possibly because of the loss of scientific works of the Hellenistic Era.
It was not until the 16th century that a fully predictive mathematical model of a heliocentric system was presented, by the Renaissance mathematician, astronomer, and Catholic cleric Nicolaus Copernicus, leading to the Copernican Revolution. In the following century, Johannes Kepler elaborated upon and expanded this model to include elliptical orbits, and Galileo Galilei presented supporting observations made using a telescope
With the observations of William Herschel, Friedrich Bessel, and others, astronomers realized that the sun was not the center of the universe as heliocentrists at the time of Copernicus had supposed. Modern thinking is that there is no specific location that is the center of the universe, per Albert Einstein's principle of relativity.
Galileo Galilei 15 February 1564 – 8 January 1642),

[U][COLOR="rgb(255, 0, 255)"]was an Italian astronomer, physicist, engineer, philosopher, and mathematician who played a major role in the scientific revolution during the Renaissance. He is widely heralded as one of the greatest scientists of all time. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism. Galileo has been called the "father of modern observational astronomy"the "father of modern physics", the "father of science" and "the father of modern science".
His contributions to observational astronomy include the telescopic confirmation of the phases of Venus, the discovery of the four largest satellites of Jupiter (named the Galilean moons in his honour), and the observation and analysis of sunspots.

Galileo proposed that a falling body would fall with a uniform acceleration, as long as the resistance of the medium through which it was falling remained negligible, or in the limiting case of its falling through a vacuum. in 1593 Galelio invented thermometer, in 1605 he was first person who use a telescopeto observe the sky.
origin of universe
"the problem of origin of universeis a bit like old question " which came first the chicken or egg". in the other words what egency created the universe and who created this agency?? or perhap the universe and agency that created it,existed forever and did not need to be created (Stephen Hawking)

In 1959, a survey was taken of leading American scientists. Among the many questions asked was, "What is your estimate of the age of the universe?" Two-thirds of the scientists gave the same answer. The answer that two-thirds - an overwhelming majority - of the scientists gave was, "Beginning? There was no beginning. Aristotle and Plato taught us 2400 years ago that the universe is eternal. Oh, we know the Bible says 'In the beginning.' That's a nice story; it helps kids go to bed at night. But we sophisticates know better. There was no beginning."
. In 1965, Arno Penzias and Robert Wilson discovered the echo of the Big Bang in the black of the sky at night, and the world paradigm changed from a universe that was eternal to a universe that had a beginning.
9they discover microwave signal burried in data. they attempted to filter out signal assiming that it was merely unwanted noise , however thy soon realised , what signal actully was, they had unknowingly the cosmic microwave background (CMB).
the CMB is after glow of big bang .( the event that set our universe in motion) . today CMB represent a temprature of about 2.7 kelven which menifists(clear) itself as a microwave radiation . CMB is uniform across the entire obserable universe . this discovry was the first evidence that universe had i beginning.
Big bang After the answer of this question that universe had beginning a new question arise whis is how did it come into existence and what existed before it ?
The Big Bang theory is an effort to explain what happened at the very beginning of our universe.
It states that the universe was in a very high density state and then expanded. universe sprang into existence from a singularity ( it is a term that physicists used to describe region of space that defy(appear to be challenging (someone) to do or prove something.) the law of physics.we know very little about singularities, but we believe that other probably exist in black holes.
second part of quetion, what existed before the big bang has bewildered scientists ?

By definition, nothing existed prior to the beginning, but that fact creates more questions than answers. For instance, if nothing existed prior to the Big Bang, what caused the singularity to be created in the first place? Once the singularity was created (however it happened), it began to expand through a process called inflation. The Universe went from very small, very dense, and very hot to the cool expanse that we see today. This theory is now referred to as the Big Bang, a term first coined by Sir Fred Hoyle during a British Broadcasting Corporation (BBC) radio broadcast in 1950.
Interestingly, there really wasn’t any sort of explosion (or bang) as the name suggests, but rather the rapid expansion of space and time. It is like blowing up a balloon, as you blow air in, the exterior of the balloon expands outward.

The Moments after the Big Bang

The early Universe was not bound by the laws of physics as we know them today. Consequently, we cannot predict with great accuracy what the Universe looked like during the first minutes of creation. In spite of this, scientists have been able to construct an approximate representation of how the Universe evolved.

Scientists believe that the Universe was initially so hot and dense, that even elementary particles like protons and neutrons could not exist. Instead, different types of matter (called matter and anti-matter) collided together, creating pure energy. But as the Universe began to cool during the first few minutes, protons and neutrons began to form. Then slowly over time these protons, neutrons and electrons came together to form Hydrogen and small amounts of Helium. During the billions of years that followed, stars, planets and galaxies formed to create the Universe as we see it today.

Big Bang Theory - Evidence for the Theory.

What are the major evidences which support the Big Bang theory?

• First of all, we are reasonably certain that the universe had a beginning.
• Second, galaxies appear to be moving away from us at speeds proportional to their distance. This is called "Hubble's Law," named after Edwin Hubble (1889-1953) who discovered this phenomenon in 1929. This observation supports the expansion of the universe and suggests that the universe was once compacted.
• Third, if the universe was initially very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat. In 1965, Radioastronomers Arno Penzias and Robert Wilson discovered a 2.725 degree Kelvin (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave Background radiation (CMB) which pervades the observable universe. This is thought to be the remnant which scientists were looking for. Penzias and Wilson shared in the 1978 Nobel Prize for Physics for their discovery.
• The CMB signal detected by Penzias and Wilson, a discovery for which they later won a Nobel Prize, is often described as the “echo” of the Big Bang. Because if the Universe had an origin, it would leave behind a signature of the event, just like an echo heard in a canyon represents a “signature” of the original sound. The difference is that instead of an audible echo, the Big Bang left behind a heat signature throughout all of space.
• 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.
  Alternatives to the Big Bang Theory

While the Big Bang theory is the most widely accepted theory about the origins of the Universe, and is supported by all the observational evidence, there are other models that explain the evidence equally well.

Some of the other theories argue that the Big Bang theory is based on a false premise -- that the Universe is built on an ever expanding space-time. Some, instead, prefer to build theories on a static Universe, which is what was originally predicted by Einstein’s theory of general relativity. Einstein’s theory was only later modified to accommodate the way the Universe appears to be expanding.
important note

GEORGES LEMAITRE consider father of big bang theory because he first proposed this theory in 1927