We now look at the origin of the Universe itself.

Key points: Cosmological Principle; Olber's Paradox; Universal expansion; Cosmic background radiation

Cosmology is the study of the origin and evolution of the Universe.

Newton was one of the first to approach cosmology on a scientific basis. He said

"the hypothesis of matter, being at first evenly spread through the heavens, is in my opinion inconsistent with the hypothesis of innate gravity without a supernatural force to reconcile them" -- that is, he felt that matter would unavoidably be attracted into concentrations, unless God kept the matter spread through space.

He did think that an infinite distribution of static matter could be stable.

Now, we summarize a thought similar to Newton's statement as the Cosmological Principle: On a sufficiently large scale, the Universe is homogeneous and isotropic (meaning that it is uniform and is the same in all directions)

Note that this is an assumption that is subject to test and experimental verification. As far as we can tell by our most powerful observations, the universe is homogenous and isotropic provided we look at a large enough scale (e.g., clusters of galaxies are obviously not uniform: we have to look over distances of ~100-1000 million parsecs for things to be uniform)

The cosmological principle is very similar to stating that we do not have a preferred viewpoint

(for example, we are not at the center of the Universe)

Two simple but key cosmological observations:

1) We exist.

Proves that the Universe does not consist of equal amounts of matter and anti-matter that are well-mixed. If there are equal amounts, they must be kept very well separated.

2) The sky is dark at night.

(Also know as Olber's Paradox after Wilhelm Olber who first posed this apparent problem)

Olber reasoned that if the Universe contains an infinite distribution of stars, then every line-of-sight would intersect a star, and the sky would be as bright as the surface of a star . (from Univ. of Oregon, http://zebu.uoregon.edu/~js/ast123/lectures/lec01.html) So why is the sky dark? ---Maybe dust absorbs the light?         no, because the dust would be         heated up by the bath of starlight. --- Maybe the Universe is finite?         if so, where's the edge?

Hubble's Law

Hubble made a fundamental discovery about the Universe by studying distant galaxies. A galaxy is a system of stars, containing hundreds of billions or even trillions of them. Galaxies are good probes of the distant Universe because they are relatively easy to recognize and bright enough that we can see them to great distances. We will discuss them in depth later, but now just consider them as probes of the Universe.

By measuring the distance to a galaxy (using a variety of indicators we will discuss when we cover galaxies in depth) and comparing with the Doppler shift of the galaxy, Hubble found:

1) Virtually all galaxies are moving away from us, e.g. they are redshifted. 2) The more distant the galaxy, the larger its redshift, that is the faster it is moving away. This graph summarizes these statements: the greater the distance the larger the velocity away from us (From Ned Wright). This modern version is far better than Hubble's original. His distance scale was hopelessly wrong and he derived an expansion that is six to seven times greater than the correct value. Nonetheless, his basic result started off the inquiry into theories of cosmology that could explain this fundamental property of our Universe. (from Ned Wright's cosmology tutorial http://www.astro.ucla.edu/~wright/cosmolog.htm)

This expansion is summarized in Hubble's Law:

V_R = H₀ x d

V_R = radial velocity, d=distance

H₀ = Hubble's constant, about 70 Km/sec/Mpc

Hubble's Law, once it is calibrated, provides an easy way to measure the distance to a galaxy. Just take a spectrum and measure the redshift, use it to calculate the velocity, and plug the result into Hubble's Law rearranged to solve for distance: d = V_R/H₀.

Implications of Hubble's Law

1) Hubble's Law with its proportionality between a galaxy's distance and its velocity is most easily explained as the result of the Universe expanding

==> Space itself is getting bigger, galaxies are moving further apart, but there is no "center" to the expansion.

Here is an analogy using an expanding balloon (from Ned Wright, http://www.astro.ucla.edu/~wright/balloon0.html.  This animation shows an expanding universe model with yellow blobs for galaxies and moving, redshifting photons. The animation starts at a redshift of 3 when the Universe was 4 times smaller than it is now, and finishes at the present. Notice that the galaxies do not expand, they just get farther apart.

2) H₀ is closely related to the age of the Universe:

Distance = Velocity x time

Hubble's Law: V_R = d x H₀ ==> d = V_R/H₀

so 1/H₀ is the age of the Universe (or more precisely, the length of time that the Universe has been expanding).

1/H₀ ~ 13 billion years.

3) the expansion also explains Olbers' Paradox - why the sky is dark. The light from distant objects is shifted to longer and longer wavelengths by the Doppler shift from their recessional velocity. When the recessional velocity approaches the speed of light, the light is shifted so far we can no longer see the object, thus solving Olber's Paradox.

The concept of an expanding Universe opened the question: What caused the expansion? The Belgian scientist and priest Georges Lemaitre envisioned allowing time to run backwards and watching what must have happened. He postulated the existence of a "primeval atom" that contained all of the matter in the Universe. An explosion split this primeval atom apart thus starting the expansion of the Universe. This explosion could have been responsible for the formation of the elements.

We now know that Lemaitre was right, and we call the explosion he predicted the "Big Bang." However, the concept of the expansion of the Universe and an explosion as its cause fell into disrepute almost immediately:

1) The value of H_o , the expansion rate of the Universe, as measured by Hubble, implied that the Universe is only 2 billion years old whereas much older fossils and rocks had been found on Earth. [We now know that Hubble's distance scale was inaccurate. The small value for the age of the Universe was due to his value for the expansion rate of 500 km/sec/Mpc, while the modern value is 70 km/sec/Mpc]

2) Many scientists simply rejected the notion of the Universe beginning in an explosion for philosophical reasons -- they didn't like questions such as what came before the explosion!

Read about Einstein's biggest mistake, or biggest insight - your choice

An alternative to the explosion theory was expounded by Fred Hoyle and others. This theory was called the Steady State Theory

Resolution of the Big Bang versus Steady State "Dispute"

In 1948 George Gamow predicted that if the Big Bang had occurred, the radiation from it should be observable in the microwave portion of the spectrum. However, the technology for observing at these wavelengths did not yet exist. In the mid 1960s, microwave techniques had advanced to where they could be applied to astronomy. A group at Princeton decided to build detectors to look for the radiation from the Big Bang. A few miles down the road from Princeton, Arlo Penzias and Robert Wilson were working for Bell Labs trying to improve the use of microwaves for carrying telephone calls. To their annoyance, their microwave receiver picked up some "static" regardless of where it was pointed. They learned of the Princeton group's research, and realized that their "static" was in fact the remnant radiation from the Big Bang. Penzias and Wilson received the Nobel Prize for their work. (from Bell Labs, http://www.bell-labs.com/user/apenzias/nobel.html)

The COBE (Cosmic Background Explorer) satellite made very careful measurements of the shape of the spectrum of this emission. It is a perfect blackbody at a   temperature of 2.728K; it is often termed the "3K background". (From R. McCray, http://cosmos.colorado.edu/astr1120/lesson12.html)

The 3K radiation is remarkably uniform in all directions. The temperature in one direction is the same as in 180 degrees the  opposite direction to an accuracy of 1 part in 100,000! Here is a map of the whole sky from COBE, scaled so blue would be 0K and red 4K -- the fact that it is all the same color shows how uniform the 3K radiation is. The presence of this highly uniform cosmic remnant radiation field has confirmed the Big Bang theory and it is the foundation of our understanding of cosmology.

A summary of modern cosmology can be found at: http://background.uchicago.edu/~whu/beginners/introduction.html

An advanced discussion with a broad historical perspective is at http://www.seas.columbia.edu/~ah297/un-esa/universe/index.html

J. Hevelius at the telescope		In Hindu mythology, the dance of Shiva Nataraja expresses the birth and death, and rebirth of the Universe. from http://webonautics.com/mythology/shiva5.html
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