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Cosmology: A brief history

• Ancient peoples thought the universe was much much smaller than it really is.

• Many ancient cultures considered mountain tops to be sacred ground. This was because the top of the mountain is closer to the heavens.

• Ancient Egyptians considered the sky to be like a giant tent suspended over head and held up by mountains at the four corners of the Earth.

• The ancient Greeks considered the celestial sphere on which the stars resided, to be anywhere from tens of miles to thousands of miles overhead.

Water in ancient cosmologies

• The Mesopotamian civilizations of Sumer, Babylon, Cannan, and Judea all had a common concept of the cosmos. First, it was based on water as the fundamental primordial substance.

• The water surrounded the Earth and was separated from the Earth by the firmament.

Water

Earth

Firmament

• In this world view, the Earth was a disk and the firmament rested at the edges of the Earth. It held the waters of the universe out.

• The universe, of the Sun, Moon, planets and stars was tiny and inside the firmament.

• This is the same firmament that God opened in order to flood the Earth in the Noah story.

Educated guesses for the distance to the stars

• Ptolemy (90 – 168 AD) developed a mathematical model to predict the locations of the planets in the sky. He assumed the Earth was at the center of the universe. He estimated from the speeds that the stars need to move to orbit the Earth that the stars were about 50 million miles away.

• Archimedes (287 – 212 BC) uses the measurements of Aristarchus to estimate the distance to the stars. He arrived at 6 trillion miles. (1 light year)

Galileo (1564-1642 AD)• There was little progress on cosmology until the

invention of the telescope. Up until this point in time it was assumed that the stars were all equidistant and resided on the celestial sphere. When Galileo turned his telescope to the Milky Way he realized this was wrong.

• “I have observed the nature and the material of the Milky Way… The galaxy is, in fact, nothing but congeries of innumerable stars grouped together in clusters. Upon whatever part of it the telescope is directed, a vast crowd of stars is immediately presented to view. Many of them are rather large and quite bright, while the number of smaller ones is quite beyond calculation.”

New technologies• Friedrich Bessel in 1838 made the first parallax

measurement of a star, 61 Cygni. This was the first measurement showing the distance to the stars is enormous.

• William Herschel (1738-1822) observed, by eye through his telescope, many star clusters, star forming regions, spiral nebula (galaxies) and did star counts through his telescope to map the Milky Way.

Herschel’s 40 foot

telescope

Herschel’s sketch of the Milky Way

Lord Rosse spiral nebula sketch in 1845

The spiral nebula debate

• Through the 1800s and into the 1900s, a debate raged as to whether the spiral nebula where star forming regions in the Milky Way or if they were individual galaxies beyond the Milky Way.

• The question was finally settled with data from the new 100” Mt Wilson telescope (Completed in 1919) and observations by Edwin Hubble in 1925.

The Mount Wilson 100” Telescope

• Hubble’s observation of cepheid variable stars (which have a known luminosity) in M31 and several other spiral nebula, allowed him to calculate the distance to these spiral nebula. He found them to be millions of light years away, well beyond the established size of the Milky Way.

• This landmark discovery meant that all the spiral nebula were individual galaxies and that some must be hundreds of millions to billions of light years away.

Relatively close by NGC 3370 and other more distant galaxies.

NGC 7331 (50 million light years away) and other background galaxies that are farther away.

Estimate the distance of the other galaxies in the image

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25% 25%25%25%

301. About 100 million

light years2. About 500 million

light years3. About 5000 million

light years4. There is no way to

tell

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What assumption did you make to estimate the distance?

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33% 33%33%1. I didn’t assume

anything, it is impossible to tell.

2. I assumed that the smaller galaxies were dwarf galaxies

3. I assumed all the galaxies were about the same size.

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Starting in the 1980s large surveys of galaxies in the universe began to reveal that the universe has structure.

Filaments

Voids

Model of filaments and voids.

• Filaments are long strands of galaxy clusters which are loosely bound by gravity and are being stretched out by the expansion of the universe.

• Voids are volumes of space where there are very few galaxies. These regions have opened up about 5 billion years ago as the universe expanded. They continue to grow in size with the expansion

Scaling the universe as we know it today.

• When we scaled the Milky Way galaxy, it was necessary to shrink the entire solar system to the width of a hair. Then the distance to the next nearest star system, Alpha Centauri, was one meter. The size of the Milky Way on this scale is the size of Lexington, with a thickness of about 700 feet. M31, the Andromeda galaxy on this scale is at about, St. Louis.

• Now we scale the observable universe. To do this we need to shrink the distance from the Earth to Alpha Centauri to 1 millimeter.

• On this scale the Milky Way is about the size of Yankee Stadium in New York.

• On this scale, the most distant galaxies ever observed are at the distance of the Rose Bowl in Pasadena, California.

• Remember, the distance from the Sun to Alpha Centauri is the size of the head of a pin.

• The solar system is 10,000 times smaller than the head of a bin, and the Earth is 30 billion times smaller than the head of a pin.

• In about 2500 years human understanding of the size of the universe went from a little bigger than the Earth to 120 quintillion times the size of the Earth. (1.2 x 1020) that’s 120 billion billion times size of the Earth.

Back to Einstein and his biggest blunder• Einstein published his theory of general relativity in 1915.

This theory not only described how gravity worked it made prediction about phenomena, such as gravitational lensing, which had never been observed. The theory also allowed the calculation of the space-time curvature of the entire universe.

• When Einstein attempted to do this, he found that his theory predicted that the universe could not be static. It had to be either expanding or contracting.

• In 1915 it was accepted by the scientific community that the universe was infinite in extent and infinitely old. It was also accepted that the universe was static, and unchanging.

• Einstein had the ability to predict, just from calculations using his general theory of relativity, that the universe was either expanding or contracting.

• For the only time in Einstein’s career he was unable to trust his own theory to this extent.

• He postulated an additional force, which he called the cosmological constant, which he added to his equations in order to make his theory predict a static, unchanging universe.

• Einstein was wrong.

Edwin Hubble’s next project.

• Having established in 1925 that galaxies were not in the Milky Way, Hubble used the Mt Wilson telescope to obtain spectra of galaxies.

• When he analyzed the spectra he found that, with the exception of the galaxies in our local group, all other galaxies where moving away from us. He found this using the Doppler shift of absorption lines in the galaxy spectra.

• Not only were all the galaxies found to be moving away, the farther away a galaxy was, the faster it was moving away.

• In order to establish the relation between distance and recessional velocity, Hubble needed to determine the distance to the galaxies. He did this by assuming all the galaxies he observed were of similar size, and figured out their relative distance from their apparent size.

• He had already calculated the actual distance to several nearby galaxies, so this allowed him to find the true distance to the galaxies that were farther away.

• Here is a plot from his 1929 publication.

Recessional velocity verses Distance.

Modern Day Hubble plots

What is the equation for a straight line?

1 2 3

33% 33%33%1. y = ax2

2. y = 1/x + b3. y = mx + b

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• So a straight line has the equation of • y = mx + b• In the Hubble plot the y-axis is velocity (v) and

the x-axis is distance (d)• So we can write the equation as • v = md + b

• What are m and b?

What are m and b?

1 2 3

33% 33%33%1. The slope and y-intercept

2. The slope and the rise over run

3. The x-intercept and the y-intercept

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Modern Day Hubble plots

The Hubble Constant – The slope of the line

• The Hubble constant is a very fundamental quantity, which tells us the age of the universe.

• Today we see that the universe is expanding, and it is growing larger every day. This means that the distance between galaxies are growing in size.

• What would happen if we could run the clock in reverse? Move backward in time.

• The galaxies would get closer together, with the most distant galaxies moving toward us at the greatest speed. The result is that all galaxies would arrive at the same point, at the same time.

• The equation of a straight line for the Hubble plot looks like this, with b = 0

• v = Hod velocity = Ho times distance

• What does our normal velocity equation say?

• v = d/t velocity = distance divided by time• Or• v = (1/t)d compared to v = (Ho)d

• So Ho tells us the time it takes for all the galaxies in the universe to collapse to the same point. This is the age of the universe.

• Ho = 1/tage Notice: Ho has units of km/s/Mpc

• Or • tage = 1/Ho Today the best value is Ho = 71 km/s/Mpc

• We can use this value to compute the age of the universe. We first need consistent distance units.

• 1 Mpc = 3.1 x 1019 km

• So Ho = (71km/s/Mpc)(1 Mpc/3.1 x 1019 km)• Ho = 2.3 x 10-18 (1/seconds)

• tage = 1/Ho = 1/2.3 x 10-18 (1/seconds)• tage = 4.37 x 1017 seconds

• There are 3.14 x 107 seconds in a year. So dividing we get:

• tage = 13.7 x 109 years • tage = 13.7 billion years

• The age of 13.7 billion years is consistent with the age of the oldest known stars, which are found to be between 12 and 14 billion years old.

• Note that star age is based off of stellar physics and has nothing to do, whatsoever with the calculation of the age of the universe using the Hubble constant.

Quiz 11

• Explain why we see many high luminosity quasars at extremely large distances but none in our local area of the universe.