In the early days, people thought our galaxy was all there was to the universe, and that the tiny smears of light were nebulas, or clouds of dust and gas. When Edwin Hubble noticed that these objects had extremely high redshifts, it was learned that they were galaxies, like our own galaxy, and that they were very far away. It was also observed that the bigger the distance to a galaxy, the faster it was moving away from us. That prompted Hubble, Einstein and Joseph LeMaitre to realize that the universe was expanding.
To understand how they made that discovery, we need to consider how they learned about the speeds of the galaxies from analyzing the light signatures of the galaxies. Since everything we know about the universe comes to us from the light emitted from stars and galaxies, we will need to begin by learning about the nature of light and how it interacts with matter.
When a particle of light, called a photon, interacts with an atom, the energy of the photon can be absorbed by an electron and raise it to an excited state. The electron quickly drops to a lower state, and emits a photon. The energies of the emitted photons correspond to a wavelength of light. Sometimes these wavelengths of light are visible light, and we can see the various colors emitted by the electrons of a gas. The color spectrum is like a fingerprint, identifying the chemical nature of the gas. The image above shows the colors of visible light emitted for four common elements.
When a light source moves with respect to an observer, the signal gets compressed in the direction of the motion and stretched in the opposite direction. This phenomenon is called the Doppler shift. If we know what the frequency of the emitted light is, we can tell how fast the object is moving.
Please check out this video illustrating the Doppler shift:
The smears on this photograph are not mistakes with the image, they are evidence that space itself acts like a lens. Gravity distorts spacetime and causes light to take a curved path. We can use the smears caused by gravity lensing to help us see farther, a sort of cosmic magnifying glass. Looking farther away is equivalent to looking backward in time, since light takes more time to get to us, the farther away it was emitted. Looking farther back in time is helpful in understanding the evolution of the universe.
We will be studying galaxies, beginning with our own galaxy, the Milky Way since it is our nearest source of information for studying galaxies. Even from looking from within, we can tell that the Milky way is a spiral galaxy, and that our place in the galaxy is out on one of the arms.
We will extend our study of galaxies by looking at various kinds of nearby galaxies, including other types of spiral galaxies, elliptical galaxies, and merging galaxies. The above simulation shows what might happen as two spiral galaxies collide and merge into a larger galaxy.
As galaxies merge and evolve, they develop supermassive black holes at their centers. We believe that the supermassive black hole at the center of the Milky Way contains as much mass as four million suns. It is quiet for now. When galaxy mergers happen, the orbits of stars in the galaxies become disrupted and some stars can get flung right at the central black hole. We will study events like this, in active galactic nuclei.
After beginning with the study of light and galaxies, we will turn our focus to cosmology, or the science of the universe itself. We will develop an understanding of the chronology of the universe and pay particular attention to the large scale structure of the universe and its makeup, including dark matter and dark energy. We will finish by considering the possibility of finding life in the universe outside of our own planet and solar system.
The journey we will take in this course begins with the Big Bang, through the beginning of matter and energy, and the formation of stars and galaxies, to the present day universe. We will consider what the current scientific evidence indicates about the eventual fate of the universe.
The study of the universe is rich and rewarding, raising as many questions as it answers.
© Kathryn Hadley PhD 2020
Hubble Deep field
In 2014, astronomers pointed the Hubble telescope at a small region of the sky, a tiny space between stars, to look beyond our galaxy. What they saw in this was that space was teeming with galaxies. In this image, even the tiniest dots shown in this image are distant galaxies, five to ten billion light years away.
A light year is the distance that light travels in a year. Since light travels at 300 million meters per second, a light year is about six trillion miles, or 63,000 times as far away as the sun is from earth. In astronomy, the typical distances that we talk about are so vast that we need really big units, like light years, to discuss them.