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• Introduction

Earliest conceptions of the universe

Astronomical theories of the ancient greeks.

• The system of Aristotle and its impact on medieval thought
• The Copernican revolution
• Kapteyn’s statistical studies
• Shapley’s contributions
• Hubble’s research on extragalactic systems

• What are the planets in the solar system?
• How did the solar system form?
• Why do stars tend to form in groups?
• Why do stars evolve?

Our editors will review what you’ve submitted and determine whether to revise the article.

• LiveScience - How Did the Universe Begin?
• Chemistry LibreTexts - The Origins of the Universe
• NeoK12 - Educational Videos and Games for School Kids - Universe
• Salt Lake Community College Pressbooks - Physical Geography and Natural Disasters - The Universe
• universe - Children's Encyclopedia (Ages 8-11)
• universe - Student Encyclopedia (Ages 11 and up)
• Table Of Contents

Recent News

universe , the whole cosmic system of matter and energy of which Earth , and therefore the human race, is a part. Humanity has traveled a long road since societies imagined Earth, the Sun , and the Moon as the main objects of creation, with the rest of the universe being formed almost as an afterthought. Today it is known that Earth is only a small ball of rock in a space of unimaginable vastness and that the birth of the solar system was probably only one event among many that occurred against the backdrop of an already mature universe. This humbling lesson has unveiled a remarkable fact, one that endows the minutest particle in the universe with a rich and noble heritage: events that occurred in the first few minutes of the creation of the universe 13.7 billion years ago turn out to have had a profound influence on the birth, life, and death of galaxies , stars , and planets . Indeed, a line can be drawn from the forging of the matter of the universe in a primal “ big bang ” to the gathering on Earth of atoms versatile enough to serve as the basis of life . The intrinsic harmony of such a worldview has great philosophical and aesthetic appeal, and it may explain why public interest in the universe has always endured.

The “ observable universe ” is the region of space that humans can actually or theoretically observe with the aid of technology. It can be thought of as a bubble with Earth at its centre. It is differentiated from the entirety of the universe , which is the whole cosmic system of matter and energy, including the human race. Unlike the observable universe, the universe is possibly infinite and without spatial edges.

This article traces the development over time of humanity’s perception of the universe, from prehistoric observations of the night sky to modern calculations on the recessional velocity of galaxies. For articles on component parts of the universe, see solar system , star , galaxy , and nebula . For an explanation of the scientific study of the universe as a unified whole, see cosmology . For an article about the possible existence of other universes, see multiverse .

All scientific thinking on the nature of the universe can be traced to the distinctive geometric patterns formed by the stars in the night sky. Even prehistoric people must have noticed that, apart from a daily rotation (which is now understood to arise from the spin of Earth ), the stars did not seem to move with respect to one another: the stars appear “fixed.” Early nomads found that knowledge of the constellations could guide their travels, and they developed stories to help them remember the relative positions of the stars in the night sky. These stories became the mythical tales that are part of most cultures .

When nomads turned to farming, an intimate knowledge of the constellations served a new function—an aid in timekeeping, in particular for keeping track of the seasons . People had noticed very early that certain celestial objects did not remain stationary relative to the “fixed” stars; instead, during the course of a year, they moved forward and backward in a narrow strip of the sky that contained 12 constellations constituting the signs of the zodiac . Seven such wanderers were known to the ancients: the Sun , the Moon , Mercury , Venus , Mars , Jupiter , and Saturn . Foremost among the wanderers was the Sun: day and night came with its rising and setting, and its motion through the zodiac signaled the season to plant and the season to reap. Next in importance was the Moon: its position correlated with the tides , and its shape changed intriguingly over the course of a month. The Sun and Moon had the power of gods; why not then the other wanderers? Thus probably arose the astrological belief that the positions of the planets (from the Greek word planetes , “wanderers”) in the zodiac could influence worldly events and even cause the rise and fall of kings. In homage to this belief, Babylonian priests devised the week of seven days, whose names even in various modern languages (for example, English, French, or Norwegian) can still easily be traced to their origins in the seven planet-gods.

The apex in the description of planetary motions during classical antiquity was reached with the Greeks , who were of course superb geometers . Like their predecessors, Greek astronomers adopted the natural picture, from the point of view of an observer on Earth , that Earth lay motionless at the centre of a rigidly rotating celestial sphere (to which the stars were “fixed”), and that the complex to-and-fro wanderings of the planets in the zodiac were to be described against this unchanging backdrop. They developed an epicyclic model that would reproduce the observed planetary motions with quite astonishing accuracy. The model invoked small circles on top of large circles, all rotating at individual uniform speeds, and it culminated about 140 ce with the work of Ptolemy , who introduced the ingenious artifact of displaced centres for the circles to improve the empirical fit. Although the model was purely kinematic and did not attempt to address the dynamical reasons for why the motions were as they were, it laid the groundwork for the paradigm that nature is not capricious but possesses a regularity and precision that can be discovered from experience and used to predict future events.

The application of the methods of Euclidean geometry to planetary astronomy by the Greeks led to other schools of thought as well. Pythagoras ( c. 570– c. 490 bce ), for example, argued that the world could be understood on rational principles (“all things are numbers”); that it was made of four elements—earth, water , air , and fire; that Earth was a sphere; and that the Moon shone by reflected light . In the 4th century bce Heracleides Ponticus , a follower of Pythagoras, taught that the spherical Earth rotated freely in space and that Mercury and Venus revolved about the Sun . From the different lengths of shadows cast in Syene and Alexandria at noon on the first day of summer, Eratosthenes ( c. 276–194 bce ) computed the radius of Earth to an accuracy within 20 percent of the modern value . Starting with the size of Earth’s shadow cast on the Moon during a lunar eclipse , Aristarchus of Samos ( c. 310–230 bce ) calculated the linear size of the Moon relative to Earth. From its measured angular size, he then obtained the distance to the Moon. He also proposed a clever scheme to measure the size and distance of the Sun. Although flawed, the method did enable him to deduce that the Sun is much larger than Earth. This deduction led Aristarchus to speculate that Earth revolves about the Sun rather than the other way around.

Unfortunately, except for the conception that Earth is a sphere (inferred from Earth’s shadow on the Moon always being circular during a lunar eclipse), these ideas failed to gain general acceptance. The precise reasons remain unclear, but the growing separation between the empirical and aesthetic branches of learning must have played a major role. The unparalleled numerical accuracy achieved by the theory of epicyclic motions for planetary motions lent great empirical validity to the Ptolemaic system . Henceforth, such computational matters could be left to practical astronomers without the necessity of having to ascertain the physical reality of the model. Instead, absolute truth was to be sought through the Platonic ideal of pure thought. Even the Pythagoreans fell into this trap; the depths to which they eventually sank may be judged from the story that they discovered and then tried to conceal the fact that the square root of 2 is an irrational number (i.e., cannot be expressed as a ratio of two integers ).

The history of the universe: Big Bang to now in 10 easy steps

Take a trip through time to discover the history of the universe.

Step 1: How it all started

Step 2: the universe's first growth spurt, step 3: too hot to shine, step 4: let there be light, step 5: emerging from the cosmic dark ages, step 6: more stars and more galaxies, step 7: birth of our solar system, step 8: the invisible stuff in the universe, step 9: the expanding and accelerating universe, step 10: we still need to know more, additional resources, bibliography.

The history of the universe and how it evolved is broadly accepted as the Big Bang model, which states that the universe began as an incredibly hot, dense point roughly 13.7 billion years ago. So, how did the universe go from being fractions of an inch (a few millimeters) across to what it is today? Here is a breakdown of the Big Bang to now in 10 easy-to-understand steps.

The Big Bang was not an explosion in space, as the theory's name might suggest. Instead, it was the appearance of space everywhere in the universe, researchers have said. According to the Big Bang theory, the universe was born as a very hot, very dense, single point in space. Cosmologists are unsure what happened before this moment, but with sophisticated space missions, ground-based telescopes and complicated calculations, scientists have been working to paint a clearer picture of the early universe and its formation. A key part of this comes from observations of the cosmic microwave background , which contains the afterglow of light and radiation left over from the Big Bang. This relic of the Big Bang pervades the universe and is visible to microwave detectors, which allows scientists to piece together clues of the early universe. In 2001, NASA launched the Wilkinson Microwave Anisotropy Probe (WMAP) mission to study the conditions as they existed in the early universe by measuring radiation from the cosmic microwave background. Among other discoveries, WMAP was able to determine the age of the universe — about 13.7 billion years old.

When the universe was very young — something like a hundredth of a billionth of a trillionth of a trillionth of a second (whew!) — it underwent an incredible growth spurt. During this burst of expansion, which is known as inflation, the universe grew exponentially and doubled in size at least 90 times.

"The universe was expanding, and as it expanded, it got cooler and less dense," David Spergel, a theoretical astrophysicist at Princeton University in Princeton, N.J., told SPACE.com. After inflation, the universe continued to grow, but at a slower rate. 

As space expanded, the universe cooled and matter formed.

Light chemical elements were created within the first three minutes of the universe's formation. As the universe expanded, temperatures cooled and protons and neutrons collided to make deuterium, which is an isotope of hydrogen. Much of this deuterium combined to make helium.

For the first 380,000 years after the Big Bang, however, the intense heat from the universe's creation made it essentially too hot for light to shine. Atoms crashed together with enough force to break up into a dense, opaque plasma of protons, neutrons and electrons that scattered light like fog.

About 380,000 years after the Big Bang, matter cooled enough for electrons to combine with nuclei to form neutral atoms. This phase is known as "recombination," and the absorption of free electrons caused the universe to become transparent. The light that was unleashed at this time is detectable today in the form of radiation from the cosmic microwave background. Yet, the era of recombination was followed by a period of darkness before stars and other bright objects were formed.

Roughly 400 million years after the Big Bang, the universe began to come out of its dark ages. This period in the universe's evolution is called the age of re-ionization. This dynamic phase was thought to have lasted more than a half-billion years, but based on new observations, scientists think re-ionization may have occurred more rapidly than previously thought. During this time, clumps of gas collapsed enough to form the very first stars and galaxies. The emitted ultraviolet light from these energetic events cleared out and destroyed most of the surrounding neutral hydrogen gas. The process of re-ionization, plus the clearing of foggy hydrogen gas, caused the universe to become transparent to ultraviolet light for the first time.

Astronomers comb the universe looking for the most far-flung and oldest galaxies to help them understand the properties of the early universe. Similarly, by studying the cosmic microwave background, astronomers can work backwards to piece together the events that came before. Data from older missions like WMAP and the Cosmic Background Explorer (COBE), which launched in 1989, and missions still in operation, like the Hubble Space Telescope, which launched in 1990, all help scientists try to solve the most enduring mysteries and answer the most debated questions in cosmology.

Our solar system is estimated to have been born a little after 9 billion years after the Big Bang, making it about 4.6 billion years old. According to current estimates, the sun is one of more than 100 billion stars in our Milky Way galaxy alone, and orbits roughly 25,000 light-years from the galactic core.

Many scientists think the sun and the rest of our solar system was formed from a giant, rotating cloud of gas and dust known as the solar nebula. As gravity caused the nebula to collapse, it spun faster and flattened into a disk. During this phase, most of the material was pulled toward the center to form the sun.

In the 1960s and 1970s, astronomers began thinking that there might be more mass in the universe than what is visible. Vera Rubin , an astronomer at the Carnegie Institution of Washington, observed the speeds of stars at various locations in galaxies. Basic Newtonian physics implies that stars on the outskirts of a galaxy would orbit more slowly than stars at the center, but Rubin found no difference in the velocities of stars farther out. In fact, she found that all stars in a galaxy seem to circle the center at more or less the same speed. This mysterious and invisible mass became known as dark matter . Dark matter is inferred because of the gravitational pull it exerts on regular matter. One hypothesis states the mysterious stuff could be formed by exotic particles that don't interact with light or regular matter, which is why it has been so difficult to detect. 

In the 1920s, astronomer Edwin Hubble made a revolutionary discovery about the universe. Using a newly constructed telescope at the Mount Wilson Observatory in Los Angeles, Hubble observed that the universe is not static, but rather is expanding. Decades later, in 1998, the prolific space telescope named after the famous astronomer, the Hubble Space Telescope , studied very distant supernovas and found that, a long time ago, the universe was expanding more slowly than it is today. This discovery was surprising because it was long thought that the gravity of matter in the universe would slow its expansion, or even cause it to contract.

– How big is the universe?

– What is the coldest place in the universe?

– How many black holes are there in the universe?

– What color is the universe?

Dark energy is thought to be the strange force that is pulling the cosmos apart at ever-increasing speeds, but it remains undetected and shrouded in mystery. The existence of this elusive energy, which is thought to make up 80% of the universe, is one of the most hotly debated topics in cosmology.

While much has been discovered about the creation and evolution of the universe, there are enduring questions that remain unanswered. Dark matter and dark energy remain two of the biggest mysteries, but cosmologists continue to probe the universe in hopes of better understanding how it all began.

The James Webb Space Telescope (JWST), launched in 2021, will continue the hunt for the elusive dark matter, as well as peering back to the beginning of time and the evolution of the universe using its infrared instruments.

For more information about the evolution of the universe check out, " The History of the Universe " by David H. Lyth or " A Brief History of Time " by Stephen Hawking. You can also keep up to date with the discoveries of JWST, visit NASA's dedicated webpage or the European Space Agency's dedicated webpage . 

Scientific American, " The Evolution of the Universe ", October 1994. 

Walter Perry, " Origin and Evolution of the Universe ", Journal of Modern Physics, Volume 12, November 2021.

Bharat Ratra and Michael S. Vogeley, " The Beginning and Evolution of the Universe ", Publications of the Astronomical Society of the Pacific, Volume 120, March 2008, 

NASA, " Brief History of the Universe ", December 2006. 

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Denise Chow is a former Space.com staff writer who then worked as assistant managing editor at Live Science before moving to NBC News as a science reporter, where she focuses on general science and climate change. She spent two years with Space.com, writing about rocket launches and covering NASA's final three space shuttle missions, before joining the Live Science team in 2013. A Canadian transplant, Denise has a bachelor's degree from the University of Toronto, and a master's degree in journalism from New York University. At NBC News, Denise covers general science and climate change.

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Origins of the Universe 101

How old is the universe, and how did it begin? Throughout history, countless myths and scientific theories have tried to explain the universe's origins. The most widely accepted explanation is the big bang theory. Learn about the explosion that started it all and how the universe grew from the size of an atom to encompass everything in existence today.

Earth Science, Astronomy

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• The Universe

The Universe is everything we can touch, feel, sense, measure or detect. It includes living things, planets, stars, galaxies, dust clouds, light, and even time. Before the birth of the Universe, time, space and matter did not exist.

The Universe contains billions of galaxies, each containing millions or billions of stars. The space between the stars and galaxies is largely empty. However, even places far from stars and planets contain scattered particles of dust or a few hydrogen atoms per cubic centimeter. Space is also filled with radiation (e.g. light and heat), magnetic fields and high energy particles (e.g. cosmic rays).

The Universe is incredibly huge. It would take a modern jet fighter more than a million years to reach the nearest star to the Sun. Travelling at the speed of light (300,000 km per second), it would take 100,000 years to cross our Milky Way galaxy alone.

No one knows the exact size of the Universe, because we cannot see the edge – if there is one. All we do know is that the visible Universe is at least 93 billion light years across. (A light year is the distance light travels in one year – about 9 trillion km.)

The Universe has not always been the same size. Scientists believe it began in a Big Bang, which took place nearly 14 billion years ago. Since then, the Universe has been expanding outward at very high speed. So the area of space we now see is billions of times bigger than it was when the Universe was very young. The galaxies are also moving further apart as the space between them expands.

Story of the Universe

• Extreme life
• In the beginning
• The Big Bang
• The birth of galaxies
• What is space?
• Black Holes
• The mystery of the dark Universe
• Cosmic distances

Welcome to the Universe

Discover the universe: Learn about the history of the cosmos, what it's made of, and so much more.

Worlds beyond our solar system.

Giant balls of hot gas that burn for millions to billions of years. 

Black Holes

Concentrations of matter with gravity so powerful not even light can escape.

Collections of stars, planets, and vast clouds of gas and dust bound together by gravity.

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Scale of the Universe

Scale of Universe is an interactive experience to inspire people to learn about the vast ranges of the visible and invisible world.

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February 10, 2022

Scientists unveil most accurate virtual representation of the universe

by Royal Astronomical Society

Scientists have produced the largest and most accurate virtual representation of the universe to date. An international team of researchers, led by the University of Helsinki, and including members from Durham University, in the U.K., used supercomputer simulations to recreate the entire evolution of the cosmos, from the Big Bang to the present. The findings are published in the Monthly Notices of the Royal Astronomical Society .

The simulation, named SIBELIUS-DARK, is part of the Simulations Beyond the Local universe (SIBELIUS) project, and is the largest and most comprehensive constrained realization simulation to date. The team meticulously compared the virtual universe to a series of observational surveys to find the correct locations and properties for the virtual analogies of the familiar structures.

It was found that our local patch of the universe may be somewhat unusual as the simulation predicted a lower number of galaxies on average due to a local large-scale 'underdensity' of matter . While the level of this underdensity is not considered to be a challenge to the standard model of cosmology, it could have consequences for how we interpret information from observed galaxy surveys.

The simulation covers a volume up to a distance of 600 million lightyears from Earth and is represented by over 130 billion simulated particles, requiring many thousands of computers working in tandem over several weeks and producing large amounts of data. The simulation was performed on the DiRAC COSmology MAchine (COSMA) operated by the Institute for Computational Cosmology at Durham University.

These cosmological simulations developed by the team used relevant physics equations to describe how dark matter and cosmic gas evolve throughout the universe's lifetime. Dark matter is a hypothetical form of matter thought to account for a large amount of all matter in the universe.

First, the dark matter coalesces into small clumps, called haloes, and the surrounding gas is gravitationally attracted towards these clumps, eventually fragmenting into stars to form galaxies. Over time, haloes grow large enough to host galaxies like our own Milky Way.

Over the past 20 years, cosmologists have developed a standard model of cosmology—the cold dark matter model—which can explain much observed astronomical data, including the properties of heat left over from the big bang and the number and spatial distribution of galaxies observed today.

When simulating a virtual cold dark matter universe, most cosmologists follow a typical, or random, patch, one that is similar to our own observed universe, yet only in a statistical sense.

The simulations carried out in this study are different. By using advanced generative algorithms (models how the data was generated in order to categorize a signal), the simulations are conditioned to reproduce our specific patch of universe, thus containing the present day structures in the vicinity of our own galaxy that astronomers have observed over decades.

This means that the familiar structures within our local universe , such as the Virgo, Coma and Perseus clusters of galaxies, the Great Wall and the Local Void—our cosmic habitat—are reproduced in the simulation. At the center of the simulation is perhaps the most important structure, a pair of galaxies, the virtual counterparts of our own Milky Way and our nearby massive neighbor, the Andromeda galaxy.

Professor Carlos Frenk, Ogden Professor of Fundamental Physics at the Institute for Computational Cosmology, at Durham University, said: "It is immensely exciting to see the familiar structures that we know exist around us emerge from a computer calculation. The simulations simply reveal the consequences of the laws of physics acting on the dark matter and cosmic gas throughout the 13.7 billion years that our universe has been around. The fact that we have been able to reproduce these familiar structures provides impressive support for the standard Cold Dark Matter model and tells us that we are on the right track to understand the evolution of the entire universe."

Dr. Matthieu Schaller from Leiden University added: "This project is truly ground-breaking. These simulations demonstrate that the standard Cold Dark Matter Model can produce all the galaxies we see in our neighborhood. This is a very important test for the model to pass."

Former Durham PHD student Dr. Stuart McAlpine, postdoctoral researcher at the University of Helsinki, said: "By simulating our universe , as we see it, we are one step closer to understanding the nature of our cosmos. This project provides an important bridge between decades of theory and astronomical observations."

The international research team will further analyze the simulation created with the hope of providing further stringent tests of the standard model of cosmology.

Journal information: Monthly Notices of the Royal Astronomical Society

Provided by Royal Astronomical Society

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This mind-blowing map shows Earth’s position within the vast universe

See the circle of galaxy clusters and voids that surround us in this map of the nearby cosmos, extending 200 million light years in each direction

By Leah Crane

26 June 2024

Richard Powell/NASA

This story is part of our Cosmic Perspective series, in which we confront the staggering vastness of the cosmos and our place in it. Read the rest of the series here . Subscribers can see a larger version of this map at the bottom of the page.

This map shows the circle of the cosmos that surrounds us, extending to a distance of 200 million light years. At this scale, space is comprised of clusters of galaxies and voids, the latter being areas with relatively few galaxies. The Milky Way, at the centre, is part of the Local Group of galaxies, with the Virgo cluster our nearest neighbour.

Majestic spiral

The Milky Way’s spiral structure is dominated by two main arms called Scutum-Centaurus and Perseus. It also features a dense region known as the central bar. Our solar system lies on a more modest structure called the Orion spur.

However tangled the question of our metaphorical place in the universe, we can use astronomy to grasp Earth’s physical location.

Earth orbits the sun at a distance of 150 million kilometres and the sun orbits the centre of the Milky Way . Specifically, we are in the Orion arm, around 26,500 light years from the centre.

The Milky Way resides in the Local Group of galaxies. About 2.5 million light years away is our closest neighbouring galaxy, Andromeda, the largest galaxy in the Local Group. Right now, we are hurtling towards Andromeda at more than 100 kilometres per second; in about 4 billion years, the two galaxies will collide.

The Local Group

That will shake up the Local Group,…

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Expansion of the Universe. Natural consequence of the basic field equations of the General Theory of Relativity (GTR) When GTR was first developed in the 1920s, everyone thought the Universe was static

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• universal expansion
• expansion rate
• perfect cosmological principle
• critical density matter crit

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Expansion of the Universe • Natural consequence of the basic field equations of the General Theory of Relativity (GTR) • When GTR was first developed in the 1920s, everyone thought the Universe was static • Albert Einstein’s static solution: He arbitrarily introduced a cosmological constant Λ (later called it his “greatest blunder”) • Edwin Hubble’s observations of receding galaxies led to the discovery that the Universe is expanding • Hubble’s law: v = H0 d — where H0 ~ 70 km/s/Mpc (Hubble constant) 1

Hubble’s Law 3

Nature of the Universal Expansion • Hubble’s law implies the Universe is expanding uniformly • Expansion of Universe can be thought of as the expansion of space itself • Observed redshift of distant galaxies — wavelength of photons “stretched” by the expansion of space • Not everything is expanding — if it were, we couldn’t detect the expansion since our rulers would be expanding in proportion to everything else in the Universe 4

The Big Bang • Reverse extrapolation of Universal expansion → There must have been an instant of infinite density and temperature → The BIG BANG! • Phrase “Big Bang” was derisively coined by Fred Hoyle, but the name stuck! • It was not like an explosion: it happened everywhere! • Universe has a finite age: t ~ 1 / H0 • We are not allowed to ask the question: What happened before the Big Bang? 5

The Future of the Universe • Is the expansion slowing down (i.e., decelerating) because of the mutual gravitational attraction of all the matter in the Universe? …….or……. • Is the expansion speeding up (i.e., accelerating) because of a repulsive anti-gravity force (analogous to Einstein’s cosmological constant Λ? • One of the most exciting developments of the last decade is the discovery that the Universal expansion rate is actually ACCELERATING! 7

Dark Matter and Dark Energy • Critical density of matter needed to barely halt the Universe’s expansion: ρcrit ~ 10-30 g/cm3 • Most of the matter in the Universe is DARK — total density of matter in the Universe ρ may or may not be equal to this critical density: Ωmatter ≡ ρ / ρcrit • How much DARK ENERGY (anti-gravity) is present in the Universe? • Determination of Ωmatterand ΩΛ are major goals / achievements of modern cosmology • Methods: • Density/dynamics of galaxies in the Universe • Geometrical properties of the Universe • Expansion rate in the Universe’s past • Search for dark matter particles / physical origin of dark energy 8

Olbers’ Paradox: Why is the Night Sky Dark? • Assume uniform and infinite distribution of stars — night sky should be infinitely bright(!), but is observed to be dark • Paradox phrased by Olbers in 1823, though already well known for about a hundred years at the time • Stars are distributed over a finite volume (our Galaxy, for example) but the argument can be extended to the distribution of individual galaxies • Can absorption by dust in galaxies solve the paradox? NO! Dust would heat up and glow as a black body radiator • Universe has finite aget → observable Universe has a horizon at a finite distance ct — this is the most important factor in solving Olbers’ Paradox! 10

Alternative to Big Bang theory:Steady State Cosmology • Cosmological Principle: Universe is homogenous and isotropic — the same everywhere in space • Perfect Cosmological Principle: Universe is the same at all times also — our Universe probably does not obey the Perfect Cosmological Principle • Steady State theory proposed as alternative to Big Bang by Bondi, Gold, & Hoyle in 1940s — a theory is based on Perfect Cosmological Principle • Requires that galaxies constantly be created at the expense of energy out of the so-called C-field • Not widely believed — discovery of the Cosmic Microwave Background Radiation is considered definitive evidence against Steady State cosmology 12

Cosmic Microwave Background Radiation (CMBR) • Relic of the Big Bang (“afterglow” of initial fireball) predicted in late 1940s • Discovered by Penzias & Wilson in 1965 — they won the Nobel Prize for this discovery • CMBR studied in detail by satellites (COBE, WMAP) • Radiation comes from era of decoupling of matterand radiation in the early Universe (~300,000 years old) when neutral H atoms first formed 13

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