How Do You Make a Universe? Geary Albright Saturday Morning Physics March 3, 2012
The history of the Universe can be summed up like this: Hydrogen is a light, odorless, colorless gas, that if given enough time, turns into people. • In this lecture, I am going to try to fill in some of the gaps in this statement….
Whence the Universe? Ripple in still water When there is no pebble tossed Nor wind to blow lyric/haiku, R. Hunter
The Great Nebula Debate • Starting with Charles Messier in 1781, astronomers began cataloging faint nebulae (nebula is Latin for cloud) in the sky. • Initially, they were assumed to be part of the Milky Way (sipral nebulae). However, as better telescopes came along, it was clear that they were composed of stars. Therefore, they might be other distant systems of stars.
The Shapely-Curtis Debate • In 1920, a debate was held at the National Academy of Sciences in Washington, D.C. to settle the question. • Harlow Shapley argued that they must be part of our Galaxy. • Heber Curtis, who was a graduate of the University of Virginia, argued that they are separate “island universes.” • Shapley won the debate, Curtis was correct.
The Shapely-Curtis Debate Harlow Shapely Heber Curtis
Distances to Galaxies • In 1924, Edwin Hubble used the 100-inch Hooker Telescope on Mt. Wilson to resolve individual stars in the Andromeda Galaxy. • Among these stars were some Cepheid variables. • Using the method pioneered by Henrietta Leavitt, Hubble was able to calculate the distance to these nebulae. • This proved once and for all that the spiral nebulae were outside the Milky Way and were galaxies just like the Milky Way.
The Local Group • The Milky Way is a member of the Local Group of galaxies. • The local group is spread over 3 million LY and contains about 40 members. • There are three large spirals (The Milky Way, M-31 the Andromeda Galaxy, M-33).
Hubble Ultra Deep Field Area is about 1/100 the size of the full Moon and 10,000 Galaxies!
The Expanding Universe • In 1912, the American astronomer VestoSlipher measured the spectra of about 40 faint spiral nebulae. • He found that almost all the galaxies showed a redshift. That is, the absorption lines in the spectrum were shifted to longer (redder) wavelengths. • This indicated that all the galaxies were moving away from our Galaxy.
0 of 140 If almost all the galaxies that we see are moving away from the Milky Way, what does that tell us about our location in the Universe? • We are at the exact center. • We are near the center, but not necessarily at the exact center. • We are near the edge of the universe. • It tells us nothing about our location in the universe.
The Expanding Universe • In the late 1920’s, Edwin Hubble and Milton Humason, began a systematic survey of the distances to nearby spiral galaxies and their recession velocity. • In 1929, they published a paper showing that the recession velocities of the galaxies are directly proportional to their distances.
The Expanding Universe • The relationship between velocity and distance is: • Where v is the recession velocity, H0 is the Hubble constant (equals the slope of the line), and D is the distance to the object. • This equation is called the Hubble Law.
The Hubble Law • The most widely accepted value for the Hubble constant is: • An accurate value of the Hubble constant is difficult to obtain. • It requires accurately measuring the recession velocity (easy) and distance (difficult) to a number of galaxies. • Mpc = Mega-parsec ~ 3 million light years
The Hubble Law • The Hubble Law simply says that a galaxy moves away from us at a speed of 71 km/s for every million pc of its distance. • A galaxy at 1 Mpc is receding at 71 km/s. • A galaxy at 2 Mpc is receding at 142 km/s. • A galaxy at 5 Mpc is receding at 355 km/s
The Expansion of the Universe • The Hubble Law implies that the universe is expanding. • At first, the idea that all galaxies are moving away from us seems to indicate that we are at the center of the universe. However, in a uniformly expanding space, every galaxy is moving away from every other galaxy.
The Expansion of the Universe • Imagine a rubber ruler with four ants on it. • Assume that this ruler is expanding so that it uniformly doubles it length every minute. • Consider the distances from the ant at 2 cm to the other ants after 1 minute (when the ruler has doubled in length). Note that ant 2 is not at the center of the ruler.
0 of 140 As the ruler stretches, do all of the ants see all of the other ants moving away from them? • Yes • No
The Expansion of the Universe • Ants that are farther apart move away from each other with a greater velocity. • The speed at which the distant ants recede is proportional to their distance, just like the Hubble Law! • On this ruler, all the ants will see all the other ants moving away from them. Just like galaxies in our universe!
The Expansion of the Universe • For a three dimensional analogy, consider raisins in a loaf of rising bread. • Regardless of which raisin you pick, all the other raisins are moving away from it. • The reason they are receding from each other is the bread between them is expanding. The same is true for the universe, the space between the galaxies is expanding.
The Birth of the Universe • The Hubble Law expansion of the universe increases the distance between any two points as time moves forward. • This implies that at some time in the distant past the distance between any two points was zero and that the density of the universe was infinite. • The expansion of the universe from this beginning is called the Big Bang.
The Birth of the Universe • Where, then, is the center of the expansion? How far are we from the Big Bang that created the universe? • The universe has no center! All points in space are expanding away from all other points in space. They are NOT expanding away from a single central point. • To picture this, imagine living on the surface of a balloon as it was being inflated.
The Birth of the Universe • It is incorrect to think of the Big Bang as an explosion from a single point. • Since the entire universe was created in the Big Bang, including all of space and time, the explosion occurred everywhere at once. • The universe is not expanding into some “empty space” beyond the galaxies, since the space in which the galaxies exist was created in the Big Bang.
The Birth of the Universe • What came before the Big Bang? • Since all of our laws of physics were created in the Big Bang, we cannot use them to probe back to a time before the Big Bang occurred. • Are there parallel universes where, for example, my parallel identity is getting an A+ in all my high school classes? • Again, our laws of physics apply only to our universe and are contained within it. We may not be able to use them to probe the existence of other universes outside our own.
The Birth of the Universe • In fact, the expansion of the universe is the expansion of space itself. • A better way to think about the expansion of the universe is not that the distant galaxies are moving away from us, but that they are stationary and the space between the galaxies is expanding. • As the space between the galaxies expands, distant galaxies appear farther away. • In addition, this expansion of space stretches the waves of light as they travel. This expansion of the wavelength creates the redshift.
Some say the world will end in fire, Some say in ice. From what I’ve tasted of desire I hold with those who favor fire. But if I had to perish twice, I think I know enough of hate To say that for destruction ice Is also great And would suffice. Robert Frost
Olber’s Paradox • Consider this question “Why is the night sky dark?” • The answer has profound cosmological implications. • It was believed that the universe was infinite in size and infinitely old. Everywhere you look your line of sight will eventually end on the surface of a star. Therefore, the night sky would be as bright as the surface of a star!
Olber’s Paradox • Edmund Halley thought the darkness was due to the distance of stars, making them appear dim. However, as feeble as the light may be, the combined light from an infinite number of stars would still light the sky. • Dust in the universe cannot solve the problem either, since the dust would eventually heat up and be as bright as a star.
Olber’s Paradox • The modern answer to Olber’s paradox is that the universe has a finite age (about 13.7 billion years old). • We can only see the light from objects that are less than 13.7 billion LY away, only those objects are close enough so their light has had the time to get here since the origin of the universe
Olber’s Paradox • This solution was first proposed by Edgar Allen Poe in 1848.
The Age of the Universe • Since all galaxies are receding from us, and their velocity is proportional to their distance, indicates that the Universe is expanding. • If we could somehow make time run backward, we would see the Universe contracting, that is, the galaxies would be getting closer together. • We can use this fact to calculate the age of the Universe by determining when the distances between the galaxies goes to zero.
The Age of the Universe • To calculate the age of the Universe use: Hubble’s Law
The Age of the Univserse • That is, the age of the Universe is simply 1/H0 if the universe expanded at a constant rate since the beginning. • Note: you must convert the Mpc to kilometers and the seconds to years to get an age expressed in years.
The Age of the Universe • For H0=71 km/s per megaparsec we have