Who gave the first evidence of the Big-Bang theory?

1. The Origin of the Universe: The Big Bang Theory (basic)

Concept: The Origin of the Universe: The Big Bang Theory

2. Galaxies and Large Scale Structures (basic)

To understand the universe, we must look at its building blocks: galaxies. A galaxy is a massive system comprising stars, stellar remnants, interstellar gas, dust, and dark matter, all bound together by gravity. The scale is truly mind-boggling—the universe is estimated to contain about 100 billion galaxies, and each of these holds an average of 100 billion stars Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.1. These galaxies are not scattered randomly but are organized into clusters and super-clusters, forming the "large-scale structure" of the cosmos.

Galaxies come in different shapes and life stages. The two most prominent types are Spiral and Elliptical galaxies. Their differences tell us a story about how stars are born and age:

Feature	Spiral Galaxies	Elliptical Galaxies
Shape	Disc-shaped with a central bulge and "arms."	Ovoid or spherical; star distribution is non-uniform.
Star Age	Contains a mix of young, bright stars and old stars.	Consists almost entirely of very old stars.
Star Formation	Active; well-supplied with gas and dust.	Little to no new star formation; lacks gas.
Brightness	Often smaller and less bright than the largest ellipticals.	Can be the brightest galaxies in the universe.

Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.7-8

Our home, the Milky Way, is a spiral galaxy. It isn't just a static collection of stars; it is a dynamic, rotating system. Our Solar System is located in the Orion Arm, about 26,000 light-years from the galactic center Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.8. At the very heart of the Milky Way lies a supermassive black hole known as Sagittarius A*. Interestingly, while we often focus on stars like our Sun, the most common stars in our galaxy are actually red dwarfs—stars that are much cooler and dimmer than the Sun.

Finally, it is helpful to know our neighbors. The Andromeda Galaxy is the closest large galaxy to the Milky Way, situated about 2 million light-years away Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.9. Even though galaxies are moving away from each other due to the expansion of the universe, gravity still pulls local neighbors like the Milky Way and Andromeda toward one another.

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.1; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.7; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.8; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.9

3. Life Cycle of Stars and the Chandrasekhar Limit (intermediate)

To understand the life of a star, think of it as a constant battle between two opposing forces: the outward pressure generated by nuclear fusion and the inward pull of gravity. A star begins its journey in a Nebula, a massive cloud of hydrogen gas and dust. As gravity pulls this gas together, it forms a Protostar, which eventually heats up enough to ignite nuclear fusion. Once fusion begins, the star enters the Main Sequence phase—the stable 'adulthood' of a star where our own Sun currently resides Physical Geography by PMF IAS, Chapter 1, p.9.

As the star exhausts its hydrogen fuel, its fate is decided entirely by its mass. Low-to-medium mass stars (like the Sun) expand into Red Giants before shedding their outer layers. What remains is a White Dwarf—a small, incredibly dense core of 'degenerate matter' where atoms are packed so tightly that a single spoonful would weigh several tonnes Physical Geography by PMF IAS, Chapter 1, p.11. Over trillions of years, these will eventually cool into invisible Black Dwarfs, though the universe is currently too young for any to exist yet Physical Geography by PMF IAS, Chapter 1, p.12.

The transition point between a 'peaceful' death as a White Dwarf and a violent collapse is known as the Chandrasekhar Limit. Formulated by the Indian-American astrophysicist Subrahmanyan Chandrasekhar, this limit is approximately 1.44 times the mass of the Sun (1.44 M☉). If the mass of the stellar remnant exceeds this limit, electron degeneracy pressure can no longer resist gravity. The star will then collapse further, resulting in a Supernova and leaving behind either a Neutron Star or, if the mass is great enough, a Black Hole Physical Geography by PMF IAS, Chapter 1, p.14.

Core Mass After Fuel Exhaustion	Final Evolutionary Stage
Below 1.44 Solar Masses (Chandrasekhar Limit)	White Dwarf (Stable core)
Above 1.44 Solar Masses	Neutron Star or Black Hole

Sources: Physical Geography by PMF IAS, Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.9; Physical Geography by PMF IAS, Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.11; Physical Geography by PMF IAS, Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.12; Physical Geography by PMF IAS, Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.14

4. Forces Shaping the Universe: Dark Matter and Dark Energy (exam-level)

To understand our universe, we must first accept a humbling truth: everything we can see—stars, planets, and even ourselves—makes up less than 5% of the cosmos. The rest is governed by two mysterious components: Dark Matter and Dark Energy. While they sound similar, they perform opposite roles in the cosmic drama. Think of Dark Matter as the 'invisible glue' that holds galaxies together, and Dark Energy as the 'invisible expander' that pushes the universe apart.

Dark Matter was first hypothesized when astronomers noticed something strange about spiral galaxies like our Milky Way. Based on the amount of visible stars and gas, the outer arms of these galaxies should be rotating slowly. However, observations show they rotate much faster than expected. As noted in Physical Geography by PMF IAS, Chapter 1, p.8, this high rotation velocity is only possible if there is a massive amount of invisible 'extra mass' providing the necessary gravitational pull. This Dark Matter accounts for roughly 85% of all matter in the universe and is believed to consist of undiscovered subatomic particles that do not emit or reflect light.

On the other hand, Dark Energy is the force responsible for the accelerated expansion of the universe. For decades, scientists thought gravity would eventually slow down the expansion caused by the Big Bang. Instead, they discovered that about 5 billion years ago, the universe began expanding even faster Physical Geography by PMF IAS, Chapter 1, p.3. Dark Energy acts as a repulsive force that permeates all of space. Combined, Dark Matter and Dark Energy constitute a staggering 95.1% of the total content of the universe.

Feature	Dark Matter	Dark Energy
Primary Role	Acts as 'Cosmic Glue' (Attractive)	Acts as 'Cosmic Expander' (Repulsive)
Effect	Holds galaxies together; explains rotation speeds	Accelerates the expansion of the universe
Evidence	Galactic rotation curves; gravitational lensing	Redshift of distant supernovae; CMB observations

We know the universe is expanding because of Hubble’s Law, which states that galaxies are moving away from us at speeds proportional to their distance. This is supported by the Redshift phenomenon (light stretching as galaxies move away) and the Cosmic Microwave Background (CMB) radiation. The CMB is the 'relic radiation' or the oldest light in the universe, serving as a landmark proof for both the Big Bang and the accelerating expansion Physical Geography by PMF IAS, Chapter 1, p.4.

Sources: Physical Geography by PMF IAS, Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.3; Physical Geography by PMF IAS, Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4; Physical Geography by PMF IAS, Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.8

5. Space Observation and Gravitational Waves (intermediate)

In our journey through the cosmos, we have moved beyond simply "looking" at the stars with our eyes. Modern astronomy relies on observing the universe through different mediums. Traditionally, we used the Electromagnetic Spectrum (visible light, radio waves, X-rays). However, Albert Einstein’s General Theory of Relativity (1916) predicted a revolutionary new way to "hear" the universe: Gravitational Waves. These are not light waves; they are literally "ripples" in the fabric of spacetime itself, caused by the most violent and energetic processes in the cosmos Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4.

Imagine spacetime as a giant trampoline. If you drop a heavy bowling ball on it, it curves the fabric. If you then swirl two heavy balls around each other rapidly, they create ripples that travel outward. In space, these "heavy balls" are massive accelerating objects like neutron stars or black holes orbiting and eventually merging with one other. These waves travel at the speed of light, carrying information about their cataclysmic origins. While predicted a century ago, they were only physically detected in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which sensed the tiny distortions from two black holes colliding 1.3 billion light-years away Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.5.

Aside from waves, we observe the universe's history through relic radiation. While the space between galaxies looks dark to an optical telescope, a sensitive radio telescope detects a faint, uniform glow called the Cosmic Microwave Background (CMB). This is the "afterglow" of the Big Bang—the oldest light in the universe. Along with Redshift (the stretching of light as galaxies move away), the CMB provides the foundational proof for the Big Bang Theory and the accelerating expansion of the universe Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.5-6.

Feature	Electromagnetic Waves (Light)	Gravitational Waves
Nature	Vibrations of electric and magnetic fields.	Distortions or "ripples" in the fabric of spacetime.
Origin	Atomic transitions, heated matter, stars.	Accelerating massive objects (Black hole mergers, Supernovae).
Interaction	Can be blocked or absorbed by dust and gas.	Pass through matter almost entirely unimpeded.

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.5; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.6

6. The Doppler Effect and Redshift (intermediate)

To understand why we believe the universe is expanding, we must first understand the Doppler Effect. In our daily lives, we experience this with sound: as an ambulance speeds toward you, its siren sounds higher in pitch (higher frequency) because the sound waves are compressed. As it moves away, the pitch drops (lower frequency) because the waves are stretched out. In astronomy, we apply this same principle to light waves.

When an object in space, like a galaxy, moves relative to us, the wavelength of the light it emits changes. This is described through two key terms: Redshift and Blueshift. If a galaxy is moving away from Earth, its light waves are stretched toward the longer, red end of the visible spectrum (Redshift). Conversely, if it were moving toward us, the light would compress toward the shorter, blue end (Blueshift). In the 1920s, astronomer Edwin Hubble observed that almost all distant galaxies exhibit a 'galactic redshift,' meaning they are moving away from us Physical Geography by PMF IAS, Chapter 1, p.3.

Hubble’s findings led to the formulation of Hubble’s Law, which states that the velocity at which a galaxy recedes is proportional to its distance from us. Simply put: the farther away a galaxy is, the faster it appears to be moving away. To measure these vast distances and the rate of expansion precisely, scientists use 'standard candles' like Type Ia supernovae. Because these explosions have a remarkably consistent maximum brightness, their observed dimness (cosmological redshift) allows us to calculate exactly how much the space between us and the star has expanded Physical Geography by PMF IAS, Chapter 1, p.13.

Scenario	Wave Behavior	Resulting Shift
Object moving toward observer	Waves are compressed (shorter)	Blueshift
Object moving away from observer	Waves are stretched (longer)	Redshift

Sources: Physical Geography by PMF IAS, Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.3; Physical Geography by PMF IAS, Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.13

7. Hubble's Law and Observational Evidence (exam-level)

For centuries, the universe was thought to be static and eternal. However, in the 1920s, American astronomer Edwin Hubble revolutionized our understanding by providing the first observational evidence for the Expanding Universe Hypothesis, commonly known as the Big Bang Theory FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 1, p.13. Hubble’s breakthrough relied on the observation of light from distant galaxies, leading to two critical discoveries: the phenomenon of Redshift and the mathematical relationship we now call Hubble’s Law.

Redshift occurs when an object moving away from an observer emits light that appears shifted toward the longer (red) wavelengths of the spectrum Physical Geography by PMF IAS, Chapter 1, p.3. By analyzing this "galactic redshift," Hubble demonstrated that galaxies are not stationary; they are drifting apart. To visualize this, consider the Balloon Analogy: if you mark dots on a balloon (representing galaxies) and inflate it, the dots move away from each other. Crucially, it is the space between the dots that is expanding, not just the dots moving through space FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 1, p.13.

Hubble’s Law formalizes this observation by stating that the recession velocity of a galaxy is directly proportional to its distance from Earth. In simpler terms: the farther away a galaxy is, the faster it appears to be moving away from us. This relationship is expressed through the Hubble Constant (H₀), a value that describes the current rate of the universe's expansion Physical Geography by PMF IAS, Chapter 1, p.5. While traditional methods used light (starlight), modern scientists are now looking toward gravitational waves—emitted by colliding black holes or neutron stars—to act as "sirens" to calculate a more precise value for the Hubble Constant Physical Geography by PMF IAS, Chapter 1, p.6.

Concept	Description	Significance
Redshift	Light stretching toward longer wavelengths as objects recede.	Evidence that galaxies are moving away.
Hubble's Law	Velocity = H₀ × Distance	Proves the expansion is uniform and predictable.
Hubble Constant	The unit measuring the expansion rate.	Helps estimate the age and future of the universe.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Geography as a Discipline, p.13; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.3, 5, 6

8. Solving the Original PYQ (exam-level)

Now that you have mastered the concepts of cosmic evolution and the expanding universe hypothesis, this question serves as the perfect application of how theoretical physics meets observational reality. While the Big Bang Theory describes the origin of the universe from a singular point, its scientific acceptance relied heavily on the observational evidence of galaxies moving apart. As discussed in FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), this breakthrough bridged the gap between abstract mathematical models and the physical reality of our cosmos, transforming a hypothesis into a foundational pillar of modern science.

To arrive at the correct answer, (A) Edwin Hubble, you must focus on the keyword "evidence." In 1929, Hubble utilized the redshift phenomenon to demonstrate that galaxies are receding from Earth at speeds proportional to their distance—a principle we now call Hubble’s Law. This was the first empirical proof that the universe is not static but is continuously expanding. When you see "evidence" in a UPSC question, always look for the figure who observed the phenomenon rather than the one who simply theorized it. This distinction is crucial for navigating the concepts found in Physical Geography by PMF IAS.

UPSC often includes high-profile names as "distractor" traps to test the depth of your conceptual clarity. Albert Einstein provided the General Theory of Relativity, which is the mathematical framework for the Big Bang, but he initially (and famously) favored a static universe. Similarly, S. Chandrasekhar is noted for his work on stellar evolution and the Chandrasekhar limit, while Stephen Hawking is celebrated for his later 20th-century work on black holes and singularity theorems. By eliminating these specialists who focused on different cosmic phenomena, you can confidently identify Hubble as the pioneer of the expanding universe evidence.