“How can you look at the galaxy and not feel insignificant?” ~ English movie maker Ridley Scott
Since its inception, the cosmos has undergone enormous evolution, which is reflected in galactic dynamics.
A galaxy is a cluster of star systems and stellar remnants, swirling in an interstellar mixture of gas and dust. The 1st galaxies coalesced 13.5 billion years ago. There were already mature galaxies 1 billion years later.
The ballet of galaxies glides along invisible corridors. Gravitational filaments thread the universe in an invisible web which ensnares galaxies and spurs their formation.
There are now some 4 trillion galaxies, spread out spherically in a diameter over 93 billion light-years wide. Roughly half of the galaxies have light, and half are dark: detectable only by their gravitational wake. Dark galaxies have scant visible stars.
Each galaxy may contain many millions or even billions of stars. Almost all visible star systems have planets.
A few billion years after galaxies started to form, there were 10 times as many galaxies as there are today. Cosmic evolution reduced the number of galaxies through extensive merging. At every scale, existence gyrates in an intricate dance.
Cosmic expansion raises an obvious question: what is the universe expanding into? The answer is: nothing.
Spacetime itself is delimited by the universe. The cosmos has no edge, no wrapper. All that we know is that distant objects in space appear to be moving away from us in every direction, indicating an expanding universe.
The Milky Way
According to Greek mythology, the randy god Zeus had a son from a mortal woman. Zeus placed the infant, Heracles, on the breast of his goddess consort, Hera, while she was asleep, so the baby could suckle divine milk, and thus become immortal.
Hera woke up while breastfeeding Heracles. Realizing the child was not hers, she pushed him away. A jet of her milk sprayed the night sky, producing the Milky Way.
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Earth is tucked into an infinitesimal spot on an arm of the Milky Way galaxy, which formed 13.2 billion years ago. The Milky Way is now over 150,000 light-years in diameter, with over 400 billion stars and at least 640 billion planets, cumulatively weighing in at 3 trillion Suns. It all spins at 250 kilometers per second. One revolution takes 240 million years.
At the center of the Milky Way is a massive black hole that barely spins. This black hole served as the gravitational pivot around which the galaxy formed. Its girth is equivalent to 4 million solar masses.
Despite the black hole’s current sloth, that ponderous nothingness produces a fearsome whirl upon the galaxy that orbits it. The star systems at the ends of the Milky Way’s galactic spirals seem to be orbiting so fast that they should fly off, but they do not. We don’t know why.
What has been gleaned about the birth, evolution, and state of the cosmos cultivates more mysteries than answers. For one, the universe is nearly flat as a sheet. For that to be true, there must be a critical level of mass/energy density. It is a practically miraculous balance, as what is has been defined by what is not. All that exists has evolved because of what is not there. Everything is entangled with nothing.
“What is not makes what is useful.” ~ Lao Tzu