Chapter 17: Cosmology

The detailed, all-sky picture of the infant universe created from nine years of WMAP data. The image reveals 13.77 billion year old temperature fluctuations (shown as color differences) that correspond to the seeds that grew to become the galaxies. The signal from our galaxy was subtracted using the multi-frequency data. This image shows a temperature range of ± 200 microKelvin.

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.

According to the Big Bang model, theuniverse expanded from an extremely dense and hot state and continues to expand today.

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 redshifted 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 nucleosynthesis.

The Core of Galaxy M87

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 has 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 supermassive 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 supermassive black holes, but they are inactive most of the time.