The Expanding Universe

The ancient Greeks believed that the universe contained Earth at the center and that the sun, moon, and stars were connected around the planet. This idea held for many centuries until Galileo’s telescope helped allow people to realize that Earth is not the center of the universe. They also found out that there are many more stars than were visible to the naked eye. All of those stars were in the Milky Way Galaxy.

In the early 20th century, an astronomer named Edwin Hubble discovered that what scientists called the Andromeda Nebula was over 2 million light-years away, many times farther than the farthest distances that had ever been measured. Hubble realized that many of the objects that astronomers called nebulas were not clouds of gas, but were collections of millions or billions of stars that we now call galaxies.

Hubble showed that the universe was much larger than our galaxy. Today, we know that the universe contains about a hundred billion galaxies, about the same number of galaxies as there are stars in the Milky Way Galaxy. Edwin Hubble went on to measure the distance to hundreds of other galaxies after discovering that there are galaxies beyond the Milky Way. His data would eventually show how the universe is changing, and would even yield clues as to how the universe formed. Today we now know that the universe in nearly 14 billion years old.

Redshift

If we look at a star through a prism, we will see a spectrum or a range of colors through the rainbow. The spectrum will have specific dark bands where elements in the star absorb light of particular energies. By examining the arrangement of these dark absorption lines, astronomers can determine the composition of elements that make up a distant star. The element helium was first discovered in our Sun, not on Earth, by analyzing the absorption lines in the spectrum of the Sun.

While studying the spectrum of light from distant galaxies, astronomers noticed something strange. The dark lines in the spectrum were in the patterns they expected, but they were shifted toward the red end of the spectrum. This shift of absorption bands toward the red end of the spectrum is known as redshift.

Redshift occurs when the light source is moving away from the observer or when the space between the observer and the source is stretched. What does it mean that stars and galaxies are redshifted? When astronomers see redshift in the light from a galaxy, they know that the galaxy is moving away from Earth. What astronomers are noticing is that all the galaxies have a redshift, strongly indicating that all galaxies are moving away from each other, causing the Universe to expand.

Redshift can occur with other types of waves, too, called the Doppler Effect. An analogy to redshift is the noise a siren makes as it passes. As it passes by, the ambulance will seem to lower the pitch of its siren. This is because the sound waves shift towards a lower pitch when the ambulance speeds away. Though redshift involves light instead of sound, a similar principle operates in both situations.

The Expanding Universe

Edwin Hubble combined his measurements of the distances to galaxies with other astronomers’ measurements of redshift. From this data, he noticed a relationship, called Hubble’s Law, that states that the farther away a galaxy is, the faster it is moving away from us. What this leads to is the hypothesis that the universe is expanding.

The figure above by NASA shows a simplified diagram of the expansion of the universe. If we look closely at the diagram, the formation of the universe and the energy is relatively high. Over the 13.7 billion years, the energy begins to cool enough to create trillions of stars and, over time, develop into galaxies. Over time, the galaxies continue to cool and expand farther apart from each other.

Formation of the Universe

Before Hubble, most astronomers thought that the universe did not change. However, if the universe is expanding, what does that say about where it was in the past? If the universe is expanding, the next logical thought is that it had to have been smaller in the past.

The Big Bang Theory

The Big Bang Theory is the most widely accepted cosmological explanation of how the universe formed. According to the Big Bang theory, the universe began about 13.7 billion years ago. Everything that is now in the universe was squeezed into a tiny volume of hot, chaotic mass. An enormous explosion, a big bang – caused the universe to start expanding rapidly. All the matter and energy in the universe and even space itself came out of this explosion. Currently, there is not a way for scientists to know since there is no remaining evidence.

After the Big Bang

In the first few moments after the Big Bang, the universe was unimaginably hot and dense. As the universe expanded, it became less dense and began to cool. After only a few seconds, protons, neutrons, and electrons formed. After a few minutes, those subatomic particles came together to create hydrogen. The energy in the universe was significant enough to initiate nuclear fusion, and hydrogen nuclei were fused into helium nuclei. The first neutral atoms that included electrons did not form until about 380,000 years later.

The matter in the early universe was not smoothly distributed across space. Dense clumps of matter held close together by gravity were spread around. Eventually, these clumps formed countless trillions of stars, billions of galaxies, and other structures that now form most of the universe’s visible mass. If we look at an image of galaxies at the far edge of what we can see, we are looking at great distances. However, we are also looking across a different type of distance. Because it takes so long for light from so far away to reach us, we are also looking back in time.

Dark Matter and Dark Energy

The Big Bang Theory is still the best scientific model we have for explaining the formation of the universe, and many lines of evidence support it. However, recent discoveries continue to shake up our understanding of the universe. Astronomers and other scientists are now wrestling with some unanswered questions about what the universe is made of and why it is expanding. Many cosmologists create mathematical models and computer simulations to account for these unknown phenomena, such as dark energy and dark matter.

The things we observe in space are objects that emit electromagnetic radiation. However, scientists think that matter that emits light makes up only a small part of the matter in the universe. The rest of the matter, about 80 percent, is dark matter. Dark matter emits no electromagnetic radiation, so we cannot observe it directly. However, astronomers know that dark matter exists because its gravity affects the motion of objects around it. When astronomers measure how spiral galaxies rotate, they find that the outside edges of a galaxy rotate at the same speed as parts closer to the center. This can only be explained if there is a lot more matter in the galaxy than they can see.

Gravitational lensing occurs when light is bent from a very distant bright source around a supermassive object. To explain strong gravitational lensing, more matter than is observed must be present. With so little to go on, astronomers do not know much about the nature of dark matter. One possibility is that it could just be ordinary matter that does not emit radiation in objects such as black holes, neutron stars, and brown dwarfs, objects more massive than Jupiter but smaller than the smallest stars. However, astronomers cannot find enough of these types of objects, which they have named MACHOS (massive astrophysical compact halo object), to account for all the dark matter, so they are thought to be only a small part of the total.

Another possibility is that the dark matter is thought to be much different from the ordinary matter we see. Some appear to be particles that have gravity, but do not otherwise appear to interact with other particles. Scientists call these theoretical particles WIMPs, which stands for Weakly Interactive Massive Particles. Most scientists who study dark matter think that the dark matter in the universe is a combination of massive astrophysical compact halo objects (MACHOS) and some exotic matter such as weakly-interacting massive particles (WIMPs). Researching dark matter is an active area of scientific research, and astronomers’ knowledge about dark matter is changing rapidly.

Astronomers who study the expansion of the universe are interested in knowing the rate of that expansion. Is the rate fast enough to overcome the attractive pull of gravity? If yes, then the universe will expand forever, although the expansion will slow down over time.  If no, then the universe would someday start to contract, and eventually get squeezed together in a big crunch, the opposite of the Big Bang. Observations of the universe show that it is expanding faster now than ever before, and in the future, it will expand even faster. So now, astronomers think that the universe will keep expanding forever. However, it also proposes a problematic new question: What is causing the expansion of the universe to accelerate? One possible hypothesis involves a new, hypothetical form of energy called dark energy. Some scientists think that dark energy makes up as much as 72 percent of the total energy content of the universe.

Star Systems

“Stars are the most widely recognized astronomical objects, and represent the most fundamental building blocks of galaxies. The age, distribution, and composition of the stars in a galaxy trace the history, dynamics, and evolution of that galaxy. Moreover, stars are responsible for the manufacture and distribution of heavy elements such as carbon, nitrogen, and oxygen, and their characteristics are intimately tied to the characteristics of the planetary systems that may coalesce about them. Consequently, the study of the birth, life, and death of stars is central to the field of astronomy.” (Stars | Science Mission Directorate, n.d.)

Although constellations have stars that usually only appear to be close together, stars may be found in the same portion of space. Stars that are grouped tightly together are called star systems. Larger groups of hundreds or thousands of stars are called star clusters. The image shown here is a famous star cluster known as Pleiades, which can be seen with the naked autumn sky.

Although the star humans know best is a single star, many stars – in fact, more than half of the bright stars in our galaxy – are star systems. A system of two stars orbiting each other is a binary star. A system with more than two stars orbiting each other is a multiple star system. The stars in a binary or multiple star system are often so close together that they appear as only through a telescope can the pair be distinguished.

Star clusters are divided into two main types, open clusters and globular clusters. Open clusters are groups of up to a few thousand stars that are loosely held together by gravity. Pleiades is an open cluster that is also called the Seven Sisters. Open clusters tend to be blue and often contain glowing gas and dust and are made of young stars formed from the same nebula. The stars may eventually be pulled apart by gravitational attraction to other objects.

Globular clusters are groups of tens to hundreds of thousands of stars held tightly together by gravity. Globular clusters have a definite, spherical shape and contain mostly reddish stars. The stars are closer together, closer to the center of the cluster. Globular clusters do not have much dust in them — the dust has already formed into stars.