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# 17.1 Cosmology

The story of modern cosmology began with Albert Einstein and Edwin Hubble. From Einstein's theories, the shape of space is described as being warped by gravity. From the observations of Hubble, we learned the universe is expanding. Together these two men described the evolving shape of space and they opened up a new field of science called cosmology. In addition to building on their insights, cosmology also is rooted in three principles: (1) the universe is isotropic, the same in all directions, (2) the universe on the largest scales is smooth or homogeneous, and (3) we do not live in a special time or a special place in space. On scales larger than about 300 Mpc, the universe appears smooth or homogeneous, and we do not have a special or privileged place in the vast cosmos.

Our modern cosmological theory describes an expanding universe that was generated through a "big bang" origin event. Observationally, every galaxy is rushing away from every other galaxy, carried by the expansion of space itself. Since galaxy red shifts are caused by the expansion of space, red shift can be used as a distance indicator. The Milky Way is not at the center of this expansion; observers on any other galaxy would measure the same Hubble relation. Moreover, the enormous size of the universe, coupled with the finite speed of light, means that we do not see the distant universe as it is now. When astronomers look far out in space, they observe ancient light from distant galaxies. The slope of the relationship between red shift and distance yields the Hubble constant, which is a measure of the rate of expansion of the universe. The universe is 13.8 billion years old and 92 billions light years across — and the Hubble constant, in conjunction with the mass density and energy density of the universe can allow us to calculate the age of the universe.

As the universe expands over time, small voids in space grow larger and larger, while galaxies gravitationally pull one another together into larger and larger structures. Galaxy surveys have mapped out the large-scale structure of the universe in rich detail. Galaxies are gregarious, and surveys reveal that most live in small groups or large clusters, and the clusters are often bound by gravity into enormous super clusters. These sheet-like structures are separated by voids up to 100 Mpc across. Yet, these impressive structures describe only 5% of the matter in the universe. The remainder of the universe is made up of dark matter (23%) and dark energy (72%).

The scientific story of creation is the big bang model. The earliest parts of the story are speculative and uncertain. The beginning was a time of such high energy and density that mass and energy were freely interchangeable. As a result of inflation, the universe became flat, smooth, and virtually empty. The expanding universe was filled primarily with radiation, along with a residue of matter. (This residue was nevertheless sufficient to eventually form all the billions of galaxies in the universe.) Over the next 3 minutes, while the entire universe was still hotter than the center of a star, a quarter of the mass of the universe was fused from hydrogen into helium. As the universe expanded and cooled, radiation thinned out more rapidly than matter, until at 10,000 years, the universe became matter-dominated. After 380,000 years, the universe became transparent as electrons were able to join atomic nuclei to form stable atoms. Today, billions of years later, the radiation from this era has been diluted and red shifted to microwaves. The big bang model rests on a solid base of three pillars of evidence: the expansion traced by galaxies, the cosmic background radiation, and the abundance of helium created by cosmic nucleo synthesis.

Gradually, gravity sculpted the ripples in the early universe into structures on all scales. This process was influenced by the large amount of unseen dark matter. About half a billion years after the big bang, enormous gas clouds collapsed into the first galaxies. Within them, legions of stars switched on for the first time. Stars followed a pattern of birth and death for 6 or 7 billion years. Most stars died a quiet death, as slowly fading embers. The massive ones exploded, seeding the space between stars with heavy elements. Many stars ended up surrounded by the debris of their own creation. On one such rocky fragment, life evolved out of a watery, organic broth. Four and a half billion years later, warmed by a yellow star and bathed in the microwave afterglow of the creation event, humans point their telescopes at the skies and ponder the meaning of it all.

As we look across the universe, observing different cosmological times, we can observe galaxies undergoing behaviors we see rarely in the local and recent universe. For instance, a small fraction of galaxies called active galaxies have violent events occurring in their nuclei. Termed active galaxies, they exist in increasing numbers at increasing distances. The activity in their cores can generate strong radio emission from high-velocity gas near the nucleus and sometimes from associated jets. Many galaxies with nuclear activity also have peculiar morphologies and strong bursts of star formation. The most powerful active galaxies are called quasars, and their nucleus outshines the light from the entire rest of the galaxy. Luminous quasars may cram 1000 times the luminosity of the entire rest of the galaxy into a region not much larger than the Solar System. Quasars are distant beacons in the universe — light from the most distant examples was emitted when the universe was only 5% of its current age. The best model for the power source involves a super massive black hole feeding on gas and stars from the surrounding galaxy. The central regions of quasars and other active galaxies can act as enormous particle accelerators, spitting out relativistic jets of glowing material. Some quasars emit non-thermal radiation that spans the entire electromagnetic spectrum. We have learned that all galaxies harbor super massive black holes, but they are inactive for most of the time.

The universe is vast and very old. The finite speed of light and the finite age of the universe limit our view to an observable fraction of a larger and possibly infinite physical universe. Three numbers govern the past, present, and future of a universe described by the big bang model. One is the present rate of expansion or the Hubble constant, H0. The Hubble constant is determined to an accuracy of about 4%. The other are the mean density of matter and dark energy, which act respectively as a "brake" and an "accelerator" on the expansion. Based on the values given above and measurements by recent microwave satellites, we know that our universe is geometrically flat and will expand forever at an increasing expansion rate.