A1 24 Cosmology

The Birth of the UniverseLACC: §28.2, 28.4, 28.5

• Olber’s Paradox

• Hubble’s Law

• The Big Bang Theory

An attempt to answer the “big questions”: How did we get here?

1Thursday, May 20, 2010

Olber’s Paradox

http://www.williams.edu/astronomy/Course-Pages/330/images/olbers_paradox.gif






Olber’s Paradox

http://hyperphysics.phy-astr.gsu.edu/HBASE/Astro/olbers.html

Why is the Sky Dark at Night?

If the universe were infinite and filled with stars in a uniform distribution, then every line of sight would terminate on the surface of a star and should be bright. To be sure, those further away would be fainter, but there would be more of them. Careful analysis suggests that the sky should be as bright as the surface of an average star.




Olber’s Paradox

http://math.ucr.edu/home/baez/physics/Relativity/GR/olbers.html

There are many possible explanations which have been considered. Here are a few:

1. There's too much dust to see the distant stars.2. The Universe has only a finite number of stars.3. The distribution of stars is not uniform. So, for example, there could

be an infinity of stars, but they hide behind one another so that only a finite angular area is subtended by them.

4. The Universe is expanding, so distant stars are red-shifted into obscurity.

5. The Universe is young. Distant light hasn't even reached us yet.




The Expanding Universe& Hubble’s Law

http://www.youtube.com/watch?v=IwMFBqzpxDU

3:52

The further away a galaxy is away from us, the greater the red-shift of its spectrum. The accepted explanation is that the further away a galaxy is, the faster it is moving away from us. This means galaxies are moving away from each other, i.e. the universe itself is expanding.











The Expanding Universe

http://www.pas.rochester.edu/~afrank/A105/LectureXVI/LectureXVI.html

But if its space that is expanding why do we see redshifts?

The light itself is stretched as space expands.

The more distant an object is the more space has expanded while it was traveling





Smoking Gun of the Big Bang

The cosmic microwave background is a thermal relic of a hot, dense phase in the early universe. The subsequent expansion of the universe shifts the radiation to colder temperatures but does not otherwise change the spectrum: in the absence of later non-equilibrium interactions, the cosmic microwave background will follow a blackbody spectrum.





This map shows a range of 0.0005 K from the coldest (blue) to the hottest (red) parts of the sky. Note that there is no part of the Earth at right that is not included in the oval, and thus there is nothing "outside" the WMAP map.

The Cosmic Microwave Background [CMB]




Evidence of the Big Bang:Cosmic Microwave

Background

http://www.haydenplanetarium.org/universe/duguide/exgg_wmap.php

When this recombination event took place, the light from the Big Bang peaked at about 1 micrometer in the infrared. At that time the gas would have been about 3,000 Kelvin and would have glowed orange-red in the visible spectrum. However, the Universe has expanded 1,000 times since, and the light within space has been redshifted to longer and longer wavelengths. Today the peak wavelength is close to 1 mm (1 micrometer x 1,000 = 1 mm) and corresponds to a gas temperature around 3 Kelvin (3, 000K ÷ 1, 000 = 3K).




Olber’s Paradox


There are many possible explanations which have been considered. Here are a few:

1. There's too much dust to see the distant stars.2. The Universe has only a finite number of stars.3. The distribution of stars is not uniform. So, for example, there could

be an infinity of stars, but they hide behind one another so that only a finite angular area is subtended by them.

4. The Universe is expanding, so distant stars are red-shifted into obscurity.

5. The Universe is young. Distant light hasn't even reached us yet.




Olber’s Paradox


1. The first explanation is just plain wrong. In a black body, the dust will heat up too. It does act like a radiation shield, exponentially damping the distant starlight. But you can't put enough dust into the universe to get rid of enough starlight without also obscuring our own Sun. So this idea is bad.

2. The premise of the second explanation may technically be correct. But the number of stars, finite as it might be, is still large enough to light up the entire sky, i.e., the total amount of luminous matter in the Universe is too large to allow this escape. The number of stars is close enough to infinite for the purpose of lighting up the sky.

3. The third explanation might be partially correct. We just don't know. If the stars are distributed fractally, then there could be large patches of empty space, and the sky could appear dark except in small areas.

4. But the final two possibilities are surely each correct and partly responsible. There are numerical arguments that suggest that the effect of the finite age of the Universe is the larger effect. We live inside a spherical shell of "Observable Universe" which has radius equal to the lifetime of the Universe. Objects more than about 13.7 [billion] years old (the latest figure) are too far away for their light ever to reach us.

5. Historically, after Hubble discovered that the Universe was expanding, but before the Big Bang was firmly established by the discovery of the cosmic background radiation, Olbers' paradox was presented as proof of special relativity. You needed the red shift to get rid of the starlight. This effect certainly contributes, but the finite age of the Universe is the most important effect.

References: Ap. J. 367, 399 (1991). The author, Paul Wesson, is said to be on a personal crusade to end the confusion surrounding Olbers' paradox.Darkness at Night: A Riddle of the Universe, Edward Harrison, Harvard University Press, 1987




Cosmological Principles

http://www.astronomynotes.com/cosmolgy/s3.htm

Recall that there are two aspects of the [weak] cosmological principle:

• The universe is homogeneous. This means there is no preferred observing position in the universe.• The universe is also isotropic. This means you see no difference in the structure of the universe as you look in different directions.

The strong cosmological principle adds• The universe looks the same at all times.

So, which describes our universe, the weak or the strong cosmological principle?




The Birth of the UniverseLACC: §28.2, 28.4, 28.5

• Olber’s Paradox: Why is the night sky dark? The universe is not of infinite age (and some objects are so red-shifted we can’t see them).

• Hubble’s Law: Distance = H0 x Velocity implies the universe is expanding and that at some point in the past, the universe was a hot, dense, singularity.

• The Big Bang Theory: cosmological redshifts, cosmic microwave background

An attempt to answer the “big questions”: How did we get here?


LACC HW: Franknoi, Morrison, and Wolff, Voyages Through the Universe,

3rd ed.

• Ch. 28, pp. 669: 5.

Due beginning of next class period.

Test covering chapters 24-28 next class period.


The Fate of the UniverseLACC: §28.2, 28.4, 28.5

• The Big Crunch

• The Big Rip

• Heat Death

An attempt to answer the “big questions”:

What is going to happen to us?


The Expanding Universe

http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question28.html




Big Bang Expansion

http://www.jwst.nasa.gov/firstlight.html




Dark Energy

http://www.nasa.gov/missions/deepspace/f_dark-energy.html

The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart at a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.Image courtesy of NASA/STScI/Ann Feild.

The diagram [left] shows the changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion.




Dark Energy


Probing dark energy, the energy in empty space causing the expanding universe to accelerate, calls for accurately measuring how that expansion rate is increasing with time. Dark energy is thought to drive space apart.

Astronomers used NASA's Hubble Space Telescope to hunt for supernovae (an energetic explosive event that occurs at the end of a star's lifetime), using their brightness, astronomers could measure if the universe was expanding faster or slower in the distant past.

In its search, Hubble discovered 42 new supernovae, including six that are among the most distant ever found. The farthest supernovae show that the universe was decelerating long ago, but then "changed gears" and began to accelerate.

Cosmologists believe about 70 percent of the universe consists of dark energy, 25 percent is dark matter, and only four percent normal matter (the stuff that stars, planets and people are made of). Hubble observations suggest the dark energy may be ... an energy percolating out of the vacuum of the space between galaxies.




The Fate of the Universe: The Big Crunch

http://science.howstuffworks.com/big-crunch3.htm

Open Universe. In this scenario, the universe will expand foreverFlat Universe. It will consume all of the energy from the big bang and, reaching equilibrium, coast to a halt far into the future....it will take, literally, forever for the universe to reach the equilibrium point.Closed Universe. Its expansion will slow down until it reaches a maximum size. Then it will [collapse] back on itself.




The Fate of the Universe: The Big Rip

http://science.nationalgeographic.com/science/enlarge/universe-death.html

reveal that the stretching of space is actually speeding up. If it picks up even more, the universe could be headed for a "big rip." An artist's conception of this scenario—one of many possible fates—shows how, some 20 billion years from now, unchecked expansion could tear matter apart, from galaxies all the way down to atoms.

The death of the universe could rival its birth in explosive drama if a puzzling form of energy continues to accelerate the expansion of space-time. Since the 1920s astronomers have thought the expansion was slowing down, but recent observations of distant stars




The Fate of the Universe: Heat Death

http://www.astroengine.com/?p=98

Basically, as the universe expands, it cools. Eventually, everything is cold and dead (in 1

followed by a thousand 0’s years).




The Fate of the UniverseLACC: §28.2, 28.4, 28.5

• The Big Crunch: if the universe exceeds some critical density, gravity wins, the universe stops expanding and collapses back on itself; a closed universe; accelerated expansion implies this is not the case

• The Big Rip: dark energy goes crazy and starts expanding everything--atoms eventually rip apart

• Heat Death: the universe expands forever; an open universe; stars die out, black holes evaporate, universe becomes cold, dead, and void of objects

An attempt to answer the “big questions”: What is going to happen to us?


LACC HW: Franknoi, Morrison, and Wolff, Voyages Through the Universe,

3rd ed.

• Ch. 28, pp. 669: 13.

Due beginning of next class period.

Test covering chapters 24-28 next class period.


Review for the Test (5th of 5):The Universe

[10 pts] Our Milky Way Galaxy• morphology: central bulge (barred, Sag A), disk

(spiral arms, Orion arm), halo (globular clusters)• formation and evolution: collapse of protogalactic

clouds, globular clusters, population I & II stars, collisions

• observing our own galaxy: radio 21 cm (neutral hydrogen), infrared (dust), visible (globular clusters); distance ladder (stellar parallax, main-sequence fitting, Cepheid variables)

[10 pts] Normal Galaxies • galaxy types: Hubble Tuning Fork Diagram

(ellipticals, spirals, barred spirals, irregular, dwarf galaxies, giant ellipticals); typical masses, sizes, stellar populations, supermassive black holes, mass-luminosity ratios, collisions, etc.;

• distance ladder (Cepheid variables, Tully-Fisher Relation, type 1 supernovae, brightest cluster galaxy, Hubble’s Law)

• groupings of galaxies: clusters, superclusters; Local Group (LMC, SMC, Andromeda, Sagittarius Dwarf Galaxy, Canis Major Dwarf Galaxy, etc.); Local Supercluster (Local Cluster, Virgo Cluster, etc.)

[10 pts] Active Galaxies • types: Seyfert Galaxies vs. Radio Galaxies, vs.

Quasars• feeding a supermassive black hole: accretion disk,

normal part of early galaxy development and/or galaxy mergers

• observing active galaxies: radio (radio lobes, radio jets), visible (bright cores, parent galaxies), X-rays (hot jets)

[10 pts] The Universe • dark matter: galactic rotation curves, missing

luminous matter in galaxy clusters (only ~10% of matter is visible!), dark matter candidates (MACHOS, neutrinos, WIMPS etc.)

• Structure: walls, filaments, voids; Olber’s Paradox, cosmological principles (homogeneous, isotropic)

• Evolution: The Big Bang (cosmological redshift, cosmic microwave background, dark energy), The Big Crunch, Heat Death, The Big Rip

[10 pts] Identify Objects from a picture• Milky Way galaxy: bulge, disc, spiral arms, halo• galaxy types: E0 elliptical, E7 elliptical, spiral,

barre spiral, irregular; galaxy collision; quasar• Active Galactic Nuclei: supermassive black hole,

accretion disk, dust torus, X-ray jet(s), radio lobes


Education

A1 24 Cosmology