Wait, Cosmology? Isn't that the study of beauty?

Not quite!

"Cosmology is concerned with the makeup of the universe. Cosmetology is concerned with the universe of makeup."

- Rocky Kolb, Physicist

 

The Ancient Universe

Early philosophers believed the universe was made up of five "elements": Earth, Air, Fire, Water, and the mysterious Quintessence (also known as aether). Our understanding of physics has come a long way since then as we now know that the Universe consists of a multiplicity of subatomic particles held together by four fundamental forces; which are the Strong force, the Weak force, the Electromagnetic force, and the force of Gravitation.

Models of the Universe

One of the first cosmological models was the geocentric model developed by the Greek astronomer Ptolemy. Ptolemy's model of the universe placed the Earth at the center with the sun and planets located in concentric crystal spheres surrounding Earth. These spheres rotated, causing the sun and planets to appear to rise and set. The stars were fixed in a stationary outer sphere. During the Middle Ages, this model became widely accepted in Europe, because the central location of Earth reaffirmed the importance of man.

 

Nicolaus Copernicus (1473-1543)

By the 1400s, scientists were beginning to question Ptolemy's model. In his book On the Revolutions of the Heavenly Spheres, the church canon and astronomer Nicolaus Copernicus proposed a heliocentric model which placed the sun, instead of the earth, at the center of the solar system. Copernicus's model would later be championed by the famed scientist Galileo Galilei.

Today, cosmologists have a much grander view of the universe, extending far beyond our solar system. Far from being the center, our solar system is but a single system swirling around in the arm of our galaxy, the Milky Way, a spiral galaxy containing billions of stars. In fact one of the cornerstones of modern cosmology, the Cosmological Principle, asserts that the universe has no center at all!

So what else is out there? Move forward to see what is in our Cosmic Backyard...

 

The Modern Universe

Like astronomers throughout history, modern cosmologists are interested in making an accurate model of the universe. Starting with the laws of physics which explain how fundamental particles and forces interact, physicists derive general equations describing the evolution of the universe's structure. Cosmologists use experimental evidence to select a set of initial conditions enabling them to solve the general equations, and calculate the state of the universe at times in the past, present, or future. This generates a possible model, which can be tested by comparing the phenomena it predicts with observational data. In this manner, following the rigorous scientific method, cosmologists work to build a successful universal model.

In the next section we will examine evidence for the current Big Bang model, one which arises from a set of initial conditions describing a hot dense universe. But first, in order to understand what cosmologists do, we'll have to look at the fundamentals of light, matter, and time.

Looking Back in Time

The first concept we must discuss involves how cosmologists observe the universe. Scientists use a very special tool; one that enables them to see the universe both in the present and in the past...

Apparent Magnitude

Some astronomical objects and their apparent magnitudes from Earth.

Before telescopes, people looked at the sky and classified the objects they saw by their brightness. Hipparchus, a Greek mathematician, classified over 850 cosmic objects into six categories of brightness. Scientists later adopted the word magnitude, keeping and extending the scale developed by Hipparchus. The brightest stars were called first magnitude stars, the next brightest being second magnitude stars, etc. Today, we measure the brightness of an object using this same scale, but with much more precision and using a much larger scale. The scale is formatted so that the lower the magnitude the brighter the object, which means a star with a magnitude of -1 is brighter than a star with magnitude 2.

Luminosity, Distance, and Brightness are interrelated. Observed Brightness is what we see here on Earth, while Luminosity is the actual light energy produced by a star.

You probably know from direct experience that a light source seen from far away appears much dimmer than the same source viewed from up close. This is the basic idea that enables scientists to use light to measure astronomical distances. A measure of the energy emitted from a star (in the form of light) per unit time is called the star's luminosity. This value is independent of distance, hence astronomers treat it as a measure of the star's intrinsic brightness. Observed brightness, what we see, varies with the distance to the observer.

Sidequest: Absolute and Apparent Magnitude

The Relation Between Brightness and Luminosity

The brightness of a star is proportional to its luminosity divided by its distance from the observer squared.For this example at one unit of distance from the star the brightness is one, but at three units of distance from the star the brightness is nine times smaller.

As light from a star spreads out, its energy covers larger and larger areas causing the energy per unit area to decrease in an inverse square relationship. This means that doubling the distance actually cuts the energy by four. With more area to cover, the light from the star appears dimmer. We call "energy per unit area" brightness.

From our vantage point on Earth, it is relatively simple to measure the apparent magnitude of a distant star. Measuring luminosity on the other hand is incredibly difficult (one cannot simply send a probe out to a star many lightyears away). In fact measuring stellar distances is one of the most difficult ongoing problems in cosmology. Often scientists must utilize several different methods in combination to arrive at a reasonable estimate for the distance to a stellar object. Many of these methods make use of unique astronomical objects called standard candles.

Down the Rabbit Hole: Luminosity versus Observed Brightness

Standard Candles

Standard Candles are used to calculate astronomical distances.Each of these candles have the same intrinsic luminosity, the only difference is the distance from the observer.

A standard candle is a general term for any class of objects that have the same intrinsic luminosity (produce the same amount of light energy per unit of time). Objects that astronomers have traditionally used as standard candles include the largest galaxies in clusters, type Ia supernovae, and particularly bright stars called cepheid variables. Since standard candles of the same type possess the same luminosity astronomers can calculate distance ratios simply by measuring the apparent magnitude of two candles, because distant candles will appear dimmer than closer candles according to the inverse square law.

These distant objects that we see give off light and consist of visible matter.

 

Blackbody Radiation

Click the button to see the stove in action.

All matter emits light, which is a type of electromagnetic radiation. The wavelength of this radiation depends on temperature: higher temperature corresponds to a short wavelength while lower temperature means a longer wavelength. For example, think of the coils on a metal stove; When the stove is cold (off) the coils look black, but When you turn the stove on you begin to feel the coils giving off heat. If you turn the stove to its highest setting, the coils will begin to glow red. If you could set it any higher the coils would glow yellow, then white.

Temperature

The atoms in ice are more densely packed and move less than atoms in lava.

From our stove example, we can see that temperature and electromagnetic radiation are related, but what exactly is temperature? If one considers a sample of matter on the molecular scale temperature is a measurement of the sample's internal energy: the random motion or excitement of particles comprising the sample. The less internal energy there is in a system the lower the temperature; the greater the internal energy the higher the temperature. A sample of water, for example, exists in a solid state below 0° C. The constituent H₂O molecules are densely packed and move very little. At higher temperatures the water sample transitions into liquid or gaseous forms in which the molecules are widely separated and rapidly moving. These molecules (as with all forms of visible matter) consist of atoms.

Matter and Atoms

Matter is made of atoms, and atoms are comprised of protons, neutrons, and electrons.

Everything in the Universe is made of matter. Though matter exists in many different forms, each form is made out of the same basic constituents: small particles called atoms. Atoms themselves are made of smaller particles: protons, neutrons, and electrons. Protons and neutrons are composed of even smaller particles called quarks.

Scientists postulate that there are even smaller particles, but for the purpose of studying cosmology, we will focus on the atom.

Down the Rabbit Hole: Electric Charge

Anatomy of an Atom

The classical orbit representation of the ground state of a Helium atom on the left and the Quantum Mechanical Model of the Helium ground state showing the 1s orbital on the right.

In the center of the atom, the strong force binds protons and neutrons tightly together forming the atom's nucleus. Electrons are found in the surrounding region, rapidly orbiting the nucleus. Classical mechanics describes the electrons as being held in distinct orbits by the electromagnetic force, very similar to the way gravity holds the moon in orbit around Earth. A more accurate model is given by quantum mechanics which describes the electrons as occupying regions of greater probability density called orbitals. On much larger scales, matter is attracted to other matter through the gravitational force.

Gravity: The Main Attraction

Gravity's effect is apparent even at the largest scales: just as gravity keeps the Earth orbiting the sun, it holds these two irregular galaxies M32 and M110 in orbit around the larger Andromeda galaxy.

In the late 1600s, the English mathematician Sir Isaac Newton gave the first scientific description of gravitation. Gravity is an attractive force existing between any two objects that have mass, causing them to accelerate towards each other. It is the weakest of the four fundamental forces but can act over great distances and is responsible for the formation of planets, stars, galaxies, and even larger scale structures such as groups and superclusters. Gravity is also the force that governs the motion of celestial bodies.

Gravitation

Gravity and Mass

From the graph we can see that as the mass of the alien increases, the gravitational force (the alien's weight) also increases: mass varies directly with gravitational force. The more massive two objects are, the greater the gravitational attraction between them.

Force_g ∝ Mass

Click the "Increase Mass" button to see the effect of increased mass on gravitational attraction.

Gravity and Distance

The force of gravity is inversely related to the distance the alien is from Earth; this means that force of gravity decreases as the distance between the alien and Earth increases.

As the distance between the alien and the surface of the planet increases, the force of gravity decreases. This relationship is true in all cases: as the distance increases between any two objects, the gravitational force gets much smaller very fast in an inverse square relationship.

Force_g ∝ 1/d²

As distance increases, gravitational attraction decreases.

Putting it Together: The Universal Law of Gravitation

Newton combined the inverse square relation between distance and gravitational attraction with the direct relation between mass and gravitational attraction as well as an additional constant of proportionality. His end result would be one of the most powerful laws in classical physics: the Universal Law of Gravitation.

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