Big Bang Theory: Edwin Hubble

Key Concepts

• Galaxy

• Universe

• Hubble’s postulates

• Big bang theory

Introduction

Cosmology is the study of the origin, structure, and future of the universe. People who study cosmology are called cosmologists.  

An important discovery regarding the nature of the universe was made by the astronomer Edwin Hubble in 1929. Hubble noticed that galaxies tend to move away from the Earth. Hubble’s observations also revealed that the speed at which they moved depends on their distance from Earth. Galaxies that were far away from Earth moved more rapidly.  

Like other theories are tested, theories about the origin and evolution of the universe must be tested constantly against new observations and experiments. Many recent theories of the universe started with observations made long time back, may be less than 100 years ago. 

Most of the area between stars is a great vacuum, plenty of matter has assembled in some places to form visible clouds of gas, ice, and dust (small solid particles), known as nebulae. Nebulae come in many shapes and sizes. The density of matter in a nebula may be 1,000 to 10,000 times that of matter in the vacuum of space, but it is even much less than that of the air we breathe. 

The Expanding Universe 

In the 1920s, astronomers such as Edwin Hubble observed many a frosty night under the open dome of a mountain top observatory in order to record distant galaxies too pale to be seen with the bare eye, but observable through large telescopes. At first, they only wanted to document the locations and shapes of the galaxies. But these astronomers studied how to decide whether the galaxies were moving towards the Earth or moving away from the Earth, and to determine how fast they were moving by calculating it. Hubble studied the results of these measurements and understood that all faraway galaxies—irrespective of their direction from the Earth—are moving away from us with a great velocity. He assumed that this could occur only if the universe was expanding. Till Hubble’s miracle moment, astronomers thought that the size of the universe remained unaffected. To visualize the expansion of the universe, think of a ball of dough just mixed to make raisin bread. The ball is compact, and the raisins in it are close together. When the dough is kept in a warm oven, the dough will start to expand. During this baking process, each raisin moves away from other resins in the dough. 

Hubble’s postulate passed so many tests and it achieved the status of a theory, now called the Expanding Universe Theory. It guided astronomers to understand that the Earth is simply one planet in one of over 300-billion-star systems in our galaxy, and that more than a trillion galaxies are zooming away from one another through the vast distances of space. 

Hubble’s Observations 

The two points to  Hubble’s discovery were created by others in the 1910s. 

The first point, the discovery of period luminosity scale by Henrietta Leavitt, permitted astronomers to calculate the distance to variable stars from Earth. Hubble had already used this knowledge in his 1924 invention that the Andromeda nebula, comprising a variable star, was more than 900,000 light years from Earth, i.e., way away from our own galaxy. With this measurement and other devices, Hubble had found and measured 23 other galaxies out to a distance of about 20 million light years. 

The second point was the effort of Vesto Slipher. He had studied the spiral nebulae, before Hubble’s discovery of Andromeda . These bodies produce light which can be split up into its component colors on a spectrum. Lines then emerge in this spectrum in specific patterns depending on the elements in the light source. However if the light source is moving away, the lines are moved into the red part of the spectrum. Examining the light from the nebulae, Slipher noticed that nearly all of them seemed to be moving away from Earth. Slipher knew that a shift toward red indicated the body was moving quickly away from the viewer. But he did not had a way to calculate the distances to these reddish bodies. 

Hubble’s observation showed that the red shift of galaxies was directly proportional to the distance of the galaxy from earth. This means that things that are farther away from Earth were moving away more rapidly. In other words, it can be said that the universe must be expanding. He declared his conclusion in 1929. The ratio of distance to redshift was 170 kilometers/second per light year of distance, now it is known as Hubble’s constant. The numbers were not precisely correct, and improvements in measuring techniques and technology have changed all of Hubble’s early numbers. But the basic principle did not changed.  

Determining Movement of Galaxies  

Blueshift and Redshift 

When light passes through the drop of water, it separates into various color bands because white light is a combination of rainbow colors. This band of various colors is called the spectrum. The heated gases of stars produce light, and when this light passes through an outer atmosphere of a star, some light is absorbed by the outer atmosphere of the star. When scientists noticed a spectrum of this starlight, they saw the absorbed light had fallen from the spectrum and formed dark lines called absorption lines. When you look at a spectrum from a galaxy, you can see that the absorption line pattern does not exist at the same point in the spectrum. The position of the pattern looks shifted because the galaxies are moving away from each other and the space between them has increased. 

When the absorption lines of the spectrum shift towards the end of the blue color of the spectrum, it is called a blueshift. This means the galaxy is moving in the direction of us. If the absorption lines are shifted towards the red zone of the spectrum, then it is called a redshift; this means that the galaxy is moving away from us. The absorption lines of almost all galaxies are red shifted. As there is an expansion in space, the absorption lines show an expansion redshift. There is no center point to this enlargement. Suppose if we consider ourselves to be in the center of the galaxy, then we can see that the other galaxies are moving away from each other. 

Astronomers think that in the past galaxies must have been close to each other. In the beginning, the universe was very small, hot, and dense, but after that, the universe expanded very quickly. This expansion of the universe is called the big bang, and it sent matter in all directions. 

What is Big Bang? 

According to Big Bang theory, all the energy and matter of the universe was in a strangely hot and dense state. Around 13.7 billion years ago, our universe started as a catastrophic explosion, which continued to expand, cool, and evolve to its present state. In the initial moments of this expansion, only energy and quarks (subatomic particles that are the building blocks of protons and neutrons) present. Not until 380,000 years after the initial expansion did the universe cool adequately for electrons and protons to come together to form hydrogen and helium atoms. These are the lightest elements in the universe. For the first time, light traveled all through space. Temperatures decreased adequately to allow clusters of matter to collect. This material produced huge clouds of dust and gases (nebulae), which rapidly developed into the first stars and galaxies. Our Sun and planetary system formed around 5 billion years ago (almost 9 billion years after the Big Bang).  

As the universe expanded, due to gravity, matter accumulate into large “clusters” and “strands” of interstellar matter called as nebulae (meaning “clouds;” singular – nebula, Plural – nebulae). In addition, substantial amounts of interstellar matter once present in the interiors of stars and were later returned to space. Some stars discharged matter as part of their normal life cycle, while some burst when they died, and some created black holes that emitted streams of matter through structures known as jets. Interstellar matter lives between stars in the galaxies and comprises of nearly 90% hydrogen and 9% helium. The rest, interstellar dust, is comprised of atoms, molecules, and larger dust grains of the heavier elements. These enormous concentrations of interstellar dust and gases are very diffuse, similar to fog, without distinctive edges or boundaries. Though, because nebulae are huge, their total mass is most of the times that of the Sun. 

If a nebula is sufficiently dense, then it will contract because of gravity, which leads to processes that create stars and planets. When nebulae are in close vicinity to very hot (blue) stars, they shine and are known as bright nebulae. By contrast, when clouds of interstellar material are far away from bright stars to be illuminated, they are called as dark nebulae. 

The Cosmic Microwave Background Radiation  

In 1965 scientists discovered that microwaves seem to be coming from all directions in space. This radiation was predicted by the big bang theory and is termed cosmic background radiation. Lately, thorough measurements of the cosmic background radiation have been done by an orbiting observatory known as the Wilkinson Microwave Anisotropy Probe (WMAP). A map of these measurements is shown in Figure 5. The bright areas on the image show regions where the density of matter was high, and galaxies first formed. The WMAP data and other data indicate that the big bang occurred about 13.7 billion years ago. 

What is the Universe Made of? 

Stars and galaxies are almost made of hydrogen and helium gas, with little amounts of heavier elements. Hydrogen, helium, and the other elements form the stars, galaxies, and other objects that can be identified with various types of telescopes. But, over the past many decades, astronomers have found proof for another type of matter that can not be identified with telescopes. This type of matter is known as dark matter and can be identified by only its gravitational effects on ordinary matter. Observations have shown that there is around five to six times as much dark matter in the universe as ordinary matter. 

 

All matter present in the universe exerts a gravitational force on all other matter, and also on dark matter. These gravitational forces could be strong enough to slow down the expansion of the universe. But, in the late 1990s, astronomers have found that the rate of expansion was increasing.  Astronomers postulated that an unidentified repulsive force was causing the expansion to speed up and it is due to the presence of a type of energy known as dark energy. According to astronomers that of all the matter and energy in the universe, we see around 75% is dark energy, around 20% is dark matter, and only around 5% is ordinary matter.

Summary

• Cosmology is the study of the origin, structure and future of the universe. • People who study cosmology are called cosmologists.

• Clusters of stars held together by gravity is called a galaxy.

• Hubble noticed that galaxies tend to move away from Earth.

• If a nebula is sufficiently dense, then it will contract because of gravity, which leads to processes that create stars and planets.

• In 196S scientists discovered that microwaves seem like coining from all directions in space.

• Astronomers use redshift and bluesh ift to determine how near or far the object is from Earth

• As per Big Bang theory, the universe is enlarging, and its density and temperature are decreasing.

• Galaxies and stars are formed by the collection of matter into clumps due to gravity.

• Dark matter does not give off light.

• An unidentified repulsive force causing the expansion to speed up and it is due to the presence of a type of energy known as dark enemy.