THE 60-MINUTE UNIVERSE
The earliest cosmological models of the universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center. Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of universal gravitation, Isaac Newton built upon Copernicus's work as well as Johannes Kepler's laws of planetary motion and observations by Tycho Brahe.
Further observational improvements led to the realization that the Sun is one of a few hundred billion stars in the Milky Way, which is one of a few hundred billion galaxies in the universe. Many of the stars in a galaxy have planets. At the largest scale, galaxies are distributed uniformly and the same in all directions, meaning that the universe has neither an edge nor a center. At smaller scales, galaxies are distributed in clusters and superclusters which form immense filaments and voids in space, creating a vast foam-like structure. Discoveries in the early 20th century have suggested that the universe had a beginning and that space has been expanding since then at an increasing rate.
According to the Big Bang theory, the energy and matter initially present have become less dense as the universe expanded. After an initial accelerated expansion called the inflationary epoch at around 10−32 seconds, and the separation of the four known fundamental forces, the universe gradually cooled and continued to expand, allowing the first subatomic particles and simple atoms to form. Dark matter gradually gathered, forming a foam-like structure of filaments and voids under the influence of gravity. Giant clouds of hydrogen and helium were gradually drawn to the places where dark matter was most dense, forming the first galaxies, stars, and everything else seen today.
From studying the movement of galaxies, it has been discovered that the universe contains much more matter than is accounted for by visible objects; stars, galaxies, nebulas and interstellar gas. This unseen matter is known as dark matter (dark means that there is a wide range of strong indirect evidence that it exists, but we have not yet detected it directly). The ΛCDM model is the most widely accepted model of the universe. It suggests that about 69.2% of the mass and energy in the universe is a cosmological constant in the form of dark energy, which is responsible for the current expansion of space, and about 25.8% is dark matter. Ordinary ('baryonic') matter is therefore only 4.84% of the physical universe. Stars, planets, and visible gas clouds only form about 6% of ordinary matter.
There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible. Some physicists have suggested various multiverse hypotheses, in which our universe might be one among many universes that likewise exist.
The Multiverse/Parallel Universes
Parallel universes are no longer just a feature of a good sci-fi story. There are now some scientific theories that support the idea of parallel universes beyond our own. However, the multiverse theory remains one of the most controversial theories in science.
Our universe is unimaginably big. Hundreds of billions, if not trillions, of galaxies (opens in new tab) spin through space, each containing billions or trillions of stars (opens in new tab). Some researchers studying models of the universe speculate that the universe's diameter could be 7 billion light-years (opens in new tab) across. Others think it could be infinite.
But is it all that's out there? Science fiction loves the idea of a parallel universe, and the thought that we might be living just one of an infinite number of possible lives. Multiverses aren't reserved for "Star Trek," "Spiderman" and "Doctor Who," though. Real scientific theory explores, and in some cases supports, the case for universes outside, parallel to, or distant from but mirroring our own.
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many are thought to have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant galaxy observed. [WT: Note that the James Webb Space Telescope--which went into operation in 2022--will likely discover older examples.] GN-z11 has a distance of 32 billion light-years from Earth and is seen as it existed just 400 million years after the Big Bang.
In 2021, data from NASA's New Horizons space probe was used to revise the previous estimate to roughly 200 billion galaxies, which followed a 2016 estimate that there were two trillion or more galaxies in the observable universe, overall, and as many as an estimated 1×1024 stars (more stars than all the grains of sand on all beaches of the planet Earth). Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 light years) and is separated from the Andromeda Galaxy (with diameter of about 220,000 light years), its nearest large neighbor, by 780,000 parsecs (2.5 million light years).
The space between galaxies is filled with a tenuous gas (the intergalactic medium) with an average density of less than one atom per cubic meter. Most galaxies are gravitationally organized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which it dominates along with Andromeda Galaxy. The group is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea.
A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Its total mass is the main factor determining its evolution and eventual fate. A star shines for most of its active life due to the thermonuclear fusion of hydrogen into helium in its core. This process releases energy that traverses the star's interior and radiates into outer space. At the end of a star's lifetime, its core becomes a stellar remnant: a white dwarf, a neutron star, or—if it is sufficiently massive—a black hole.
Stellar nucleosynthesis in stars or their remnants creates almost all naturally occurring chemical elements heavier than lithium. Stellar mass loss or supernova explosions return chemically enriched material to the interstellar medium. They are then recycled into new stars. Astronomers can determine stellar properties—including mass, age, metallicity (chemical composition), variability, distance, and motion through space—by carrying out observations of a star's apparent brightness, spectrum, and changes in its position in the sky over time.
Stars can form orbital systems with other astronomical objects, as in the case of planetary systems and star systems with two or more stars. When two such stars have a relatively close orbit, their gravitational interaction can significantly impact their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
The word planet probably comes from the Greek planḗtai, meaning "wanderers", which in antiquity referred to the Sun, Moon, and five bodies visible as points of light that moved across the background of the stars. These five planets were Mercury, Venus, Mars, Jupiter and Saturn. Historically, planets have had religious associations. Multiple cultures identified celestial bodies visible to the naked eye with gods, and these connections with mythology and folklore persist in the schemes for naming newly-discovered Solar System bodies. Earth was recognized to be a planet when heliocentrism supplanted geocentrism during the sixteenth and seventeenth centuries.
With the development of the telescope, the meaning of planet broadened to include objects not visible to the naked eye: the ice giants Uranus and Neptune; Ceres and other bodies later recognized to be part of the asteroid belt; and Pluto, later found to be the largest member of the collection of icy bodies known as the Kuiper belt. The discovery of other large objects in the Kuiper belt, particularly Eris, spurred debate about how exactly to define a planet. The International Astronomical Union (IAU) adopted a standard by which the four terrestrials and four giants qualify, placing Ceres, Pluto and Eris in the category of dwarf planet, though many planetary scientists have continued to apply the term planet more broadly.
Further advances in astronomy led to the discovery of over five thousand planets outside the Solar System, termed exoplanets. These include hot Jupiters—giant planets that orbit close to their parent stars—like 51 Pegasi b, super-Earths like Gliese 581c that have masses in between that of Earth and Neptune, and planets smaller than Earth like Kepler-20e. Multiple exoplanets have been found to orbit in the habitable zones of their respective stars, but Earth remains the only planet known to support life.
- HISTORY OF UNIVERSE: https://en.wikipedia.org/wiki/Chronology_of_the_universe
- ORIGINS OF THE UNIVERSE (NatGeo): https://www.nationalgeographic.com/science/article/origins-of-the-universe
- HISTORY OF UNIVERSE: https://www.space.com/13320-big-bang-universe-10-steps-explainer.html
- MULTIVERSE OVERVIEW: https://www.nationalgeographic.com/science/article/what-is-the-multiverse
- MULTIVERSE OVERVIEW (with videos): https://www.livescience.com/multiverse
- GALAXY OVERVIEW: https://en.wikipedia.org/wiki/Galaxy
- GALAXY OVERVIEW (Swinburne University): https://astronomy.swin.edu.au/cosmos/g/galaxy
- MILKY WAY GALAXY OVERVIEW: https://en.wikipedia.org/wiki/Milky_Way
- MILKY WAY GALAXY OVERVIEW: https://www.space.com/19915-milky-way-galaxy.html
- OLDEST GALAXY (James Webb): https://earthsky.org/space/oldest-galaxy-yet-seen-by-webb-telescope/
- BLACK HOLE OVERVIEW: https://science.nasa.gov/astrophysics/focus-areas/black-holes
- SUPERMASSIVE BLACK HOLE: https://en.wikipedia.org/wiki/Supermassive_black_hole
- MILKY WAY BLACK HOLE: https://www.smithsonianmag.com/science-nature/heres-what-the-black-hole-in-the-center-of-the-milky-way-looks-like-180980078/
- SAGITTARIUS A: https://en.wikipedia.org/wiki/Sagittarius_A
- STAR FORMATION: https://en.wikipedia.org/wiki/Star_formation
- TYPES OF STARS: https://www.universetoday.com/24299/types-of-stars/
- EXOPLANETS: https://exoplanets.nasa.gov/
- EXOPLANETS OVERVIEW: https://en.wikipedia.org/wiki/Exoplanet
- EXOPLANETS OVERVIEW: https://www.planetary.org/worlds/exoplanets
- OUR SOLAR SYSTEM (Overview): https://solarsystem.nasa.gov/solar-system/our-solar-system/overview/
- OUR SOLAR SYSTEM (Detailed Overview): https://en.wikipedia.org/wiki/Solar_System
- OUR SOLAR SYSTEM (Overview with Videos): https://www.space.com/16080-solar-system-planets.html
- HUBBLE SPACE TELESCOPE: https://www.nasa.gov/mission_pages/hubble/main/index.html
- JAMES WEBB SPACE TELESCOPE: https://www.nasa.gov/mission_pages/webb/observatory/index.html
- JAMES WEBB TELESCOPE WEBSITE: https://webbtelescope.org/
- THE UNIVERSE IN 8 MINUTES: https://www.youtube.com/watch?v=C2Ebp7IsPW4
- HISTORY OF UNIVERSE (from 2009; some material dated, but overall excellent): https://www.youtube.com/watch?v=wY5C_MmOk8I&list=PL69A4C488E2FD55D1
- UNIVERSE 101 (NatGeo): https://www.youtube.com/watch?v=HdPzOWlLrbE&t=1s
- MULTIVERSE OVERVIEW (Neil deGrasse Tyson): https://www.youtube.com/watch?v=h6OoaNPSZeM
- MULTIVERSE OVERVIEW: https://www.youtube.com/watch?v=TLGcmDbleWQ
- GALAXIES OVERVIEW (2016; pre James Webb): https://www.youtube.com/watch?v=O57DyNMRGY8
- ANDROMEDA GALAXY: https://www.youtube.com/watch?v=-ybiXR2WCFQ
- MILKY WAY GALAXY: https://www.youtube.com/watch?v=SPzqXRFgV_g
- JAMES WEBB TELESCOPE: https://www.youtube.com/watch?v=shPwW11MEHg
- TYPES OF STARS: https://www.youtube.com/watch?v=ab_5MNP2OfQ
- TYPES OF STARS (PBS): https://www.youtube.com/watch?v=ld75W1dz-h0&t=10s
- EXOPLANETS 101: https://www.youtube.com/watch?v=EUU0-ZpFoK4
- ROGUE PLANETS 101: https://www.youtube.com/watch?v=wgfvL4xhK-w
- OUR SOLAR SYSTEM OVERVIEW (2020): https://www.youtube.com/watch?v=6SlDNHSOwdc
- OUR SOLAR SYSTEM 101 (NatGeo 2017): https://www.youtube.com/watch?v=libKVRa01L8&t=99s