What is the universe like? – click to expand or collapse

The universe as seen from Earth seems filled with dense hot stars separated by vast stretches of empty space within the Milky Way galaxy and by even greater stretches of emptiness between galaxies. These stars and the gases and dust observed from Earth appear to be made mostly of ordinary matter similar to that in our solar system - protons, neutrons, electrons - and radiant energy composed of photons. Some stars and galaxies seem to be moving toward us and others away from us in random ways. Close examination of the motion of stars moving within a galaxy reveal that the gravitational attraction of a galaxy far exceeds the amount of matter that appears to be present, leading to the realization that galaxies contain massive amounts of dark matter composed of unseen and unknown particles. Near a star or galaxy, massive gravitational forces curve space locally as observed in the motion of photons whose paths deviate from straight lines.

Despite the fact that stars and galaxies seem to curve space locally and despite the fact that stars and galaxies clearly are not isotropic or homogeneous, scientists have concluded that the universe is flat, isotropic, and homogeneous on the grandest scale when averaged over enormous spaces far greater than individual galaxies or even clusters or superclusters of galaxies. In other words, on the grandest scale on average, the universe does not curve the path of photons, appears the same in all directions, and has a uniform density of particles everywhere.

Observations do show that the universe is expanding and has been expanding for 13.8 billion years and that the rate of expansion slowed for nearly ten billion years but began to accelerate about four billion years ago. This acceleration is evidence of the presence of an unseen energy source called dark energy, whose composition is unknown so far.

From its earliest days, the universe has been expanding. The early universe contained far more radiation in the form of photons and neutrinos than matter when compared using Einstein's famous equivalency formula E = mc². Over time, the expansion of the universe produced a redshift of radiation to longer wavelengths with lower energy. Eventually after about 50,000 years, the energy density of radiation equaled the lower energy density of matter. For billions of years, matter dominated the universe, but as the universe continued to expand, the density of radiation became insignificant and the density of matter decreased to that of the unseen dark energy. For the last four billion years the density of dark energy which never changes has become ever more dominate relative to radiation and matter. These days dark energy is twice as dense as matter.

As remarkable as the composition, properties, and evolutionary processes of the universe have been for billions of years since the earliest seconds, far more astronishing and unfamiliar processes occurred in the very early history, particularly the first tiny fraction of a second following the Big Bang. Before delving into the nature of the very early universe, it is helpful to take the time to understand the universe as it exists today and its evolutionary history beginning after the first few seconds, days or years.

Future updates will address more remarkable aspects of the universe and some equations and graphs for those seeking advanced and precise explanations.

Stars vary in temperature and luminosity – click to expand or collapse

Hertzsprung-Russell diagram animation gif based on

https://youtu.be/NvEaLlVd5iw - "In this video, the stars in globular cluster Omega Centauri are rearranged according to their intrinsic brightness (vertical axis) and their temperature (horizontal axis). The temperature of a star dictates its apparent colour, with cooler stars being red and hotter ones being blue. The majority of stars at any given time fall into a wide band known as the main sequence, which passes from the top left (hot, bright stars) to the bottom right (cool, dim stars). However this is just a snapshot in time — as stars evolve they do not stay fixed to one point on the diagram for the whole of their lives. Credit: NASA, ESA, J. Anderson and R. van der Marel (STScI)" also see https://www.spacetelescope.org/videos/heic1017b/

Universe timeline illustrated – click to expand or collapse

How Galaxies Came From Nothing

All the large-scale structure in the universe may owe its existence to nothing. Let's see how clearly I can explain this. We think of empty space as, well... empty, the epitome of nothingness. But as our understanding of physics has evolved we have realized that it's not truly empty. Space is filled with fields. There is a field for every subatomic particle. One for electrons, up quarks, down quarks, neutrinos and so on. In empty space these fields are basically zero, flat, nil. But it's impossible to make them perfectly zero so there are always some quantum fluctuations in the fields, even in a perfect vacuum. These are sometimes called virtual particles but they should really just be thought of as little disturbances in the field. Vacuum fluctuation play a role mediating the interactions of subatomic particles but they don't really have an impact on the large-scale structure of the universe, EXCEPT during inflation, right after the big bang when the universe increased in size 10^26 times. Due to this rapid expansion, those tiny fluctuations were blown up to the scale of the observable universe. And we know this by looking at the cosmic microwave background radiation where we can see slightly hotter and cooler parts of the early universe that correspond to density fluctuations. And it is these density fluctuations that allowed matter to clump together into large structures like the gigantic gas clouds that would go on to contain stars and planets. In case the video isn't clear, this is what I've been trying to say.    Animations by Gustavo Rosa

Universe radius vs. time – click to expand or collapse

about the Helix Nebula image – click to expand or collapse

image from https://en.wikipedia.org/wiki/Helix_Nebula.

This infrared image from NASA's Spitzer Space Telescope shows the Helix Nebula, a cosmic starlet often photographed by amateur astronomers for its vivid colors and eerie resemblance to a giant eye. The nebula, located about 700 light-years away in the constellation Aquarius, belongs to a class of objects called planetary nebulae.Discovered in the 18th century, these colorful beauties were named for their resemblance to gas-giant planets like Jupiter.

Planetary nebulae are the remains of stars that once looked a lot like our sun. When sun-like stars die, they puff out their outer gaseous layers. These layers are heated by the hot core of the dead star, called a white dwarf, and shine with infrared and visible colors. Our own sun will blossom into a planetary nebula when it dies in about five billion years.

In Spitzer's infrared view of the Helix nebula, the eye looks more like that of a green monster's. Infrared light from the outer gaseous layers is represented in blues and greens. The white dwarf is visible as a tiny white dot in the center of the picture. The red color in the middle of the eye denotes the final layers of gas blown out when the star died. The brighter red circle in the very center is the glow of a dusty disk circling the white dwarf (the disk itself is too small to be resolved).

This dust, discovered by Spitzer's infrared heat-seeking vision, was most likely kicked up by comets that survived the death of their star. Before the star died, its comets and possibly planets would have orbited the star in an orderly fashion. But when the star blew off its outer layers, the icy bodies and outer planets would have been tossed about and into each other, resulting in an ongoing cosmic dust storm. Any inner planets in the system would have burned up or been swallowed as their dying star expanded.

So far, the Helix nebula is one of only a few dead-star systems in which evidence for comet survivors has been found. This image is made up of data from Spitzer's infrared array camera and multiband imaging photometer. Blue shows infrared light of 3.6 to 4.5 microns; green shows infrared light of 5.8 to 8 microns; and red shows infrared light of 24 microns.