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What is the Universe? What do we know, and don't? | Class Notes | Meethi Sharma, Study notes of Physics

At its core, the universe is governed by the four fundamental forces: gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. These forces dictate the interactions between particles and the behavior of matter and energy on both the cosmic and quantum scales.

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2023/2024

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Sharma | Spring 24 1
BYRNE SEMINAR SPRING 2024 | WHAT IS THE UNIVERSE?
The universe, as understood by physics, is the entirety of space, time, matter, and energy,
encompassing all known forms of existence, from the smallest particles to the largest galaxies
and everything in between. Physics seeks to understand the fundamental laws and principles
that govern the behaviour and evolution of the universe.
At its core, the universe is governed by the four fundamental forces: gravity, electromagnetism,
the weak nuclear force, and the strong nuclear force. These forces dictate the interactions
between particles and the behaviour of matter and energy on both the cosmic and quantum
scales.
One of the most fundamental concepts in physics is the theory of general relativity, proposed by
Albert Einstein in the early 20th century. General relativity describes how matter and energy
curve the fabric of spacetime, giving rise to the force of gravity. According to this theory, massive
objects like stars and galaxies warp spacetime, causing smaller objects to move along curved
paths in their vicinity.
On the other hand, quantum mechanics, developed in the early 20th century, describes the
behaviour of matter and energy at the smallest scales, such as atoms and subatomic particles.
Quantum mechanics introduces concepts such as wave-particle duality, uncertainty principle,
and quantum entanglement, challenging our intuitive understanding of reality.
The universe began around 13.8 billion years ago in an event known as the Big Bang. At this
moment, the universe was incredibly hot and dense, and it has been expanding and cooling
ever since. The Big Bang theory is supported by a wealth of observational evidence, including
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BYRNE SEMINAR SPRING 2024 | WHAT IS THE UNIVERSE?

The universe, as understood by physics, is the entirety of space, time, matter, and energy, encompassing all known forms of existence, from the smallest particles to the largest galaxies and everything in between. Physics seeks to understand the fundamental laws and principles that govern the behaviour and evolution of the universe. At its core, the universe is governed by the four fundamental forces: gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. These forces dictate the interactions between particles and the behaviour of matter and energy on both the cosmic and quantum scales. One of the most fundamental concepts in physics is the theory of general relativity, proposed by Albert Einstein in the early 20th century. General relativity describes how matter and energy curve the fabric of spacetime, giving rise to the force of gravity. According to this theory, massive objects like stars and galaxies warp spacetime, causing smaller objects to move along curved paths in their vicinity. On the other hand, quantum mechanics, developed in the early 20th century, describes the behaviour of matter and energy at the smallest scales, such as atoms and subatomic particles. Quantum mechanics introduces concepts such as wave-particle duality, uncertainty principle, and quantum entanglement, challenging our intuitive understanding of reality. The universe began around 13.8 billion years ago in an event known as the Big Bang. At this moment, the universe was incredibly hot and dense, and it has been expanding and cooling ever since. The Big Bang theory is supported by a wealth of observational evidence, including

the cosmic microwave background radiation, the abundance of light elements, and the large-scale structure of the universe. As the universe expanded and cooled, matter began to form, eventually giving rise to stars, galaxies, and other cosmic structures. Gravity played a crucial role in this process, pulling matter together to form the first stars and galaxies. Over billions of years, galaxies collided and merged, giving rise to the diverse array of structures observed in the universe today. Within galaxies, stars are born from clouds of gas and dust, fueled by nuclear fusion reactions in their cores. Stars undergo various stages of evolution, eventually exhausting their nuclear fuel and either collapsing into dense remnants like neutron stars and black holes or exploding in violent supernova explosions. These cosmic events play a critical role in shaping the universe, dispersing heavy elements forged in stellar interiors into space, which later contribute to the formation of new stars and planetary systems. Planets, moons, and other celestial bodies form from the remnants of stellar debris, orbiting stars in diverse configurations. The evolution of the universe is also influenced by dark matter and dark energy, two mysterious components that dominate its composition. Dark matter, although invisible and detectable only through its gravitational effects, provides the gravitational scaffolding for the formation of cosmic structures. Dark energy, on the other hand, is thought to be responsible for the observed accelerated expansion of the universe, pushing galaxies away from each other at an ever-increasing rate.