Upload File

olbers paradox why is the sky dark at night? of course, the sun’s gone down! but more careful...

  • Home
  • Documents
  • Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to
33
Olbers paradox • Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to get the first constraints on cosmological models

Upload: cory-goodwin

Post on 23-Dec-2015

215 views

Category:

Documents


0 download

Report

Tags:

TRANSCRIPT

Page 1: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

Olbers paradox

• Why is the sky dark at night?

Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to get the first constraints on cosmological models

Page 2: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

Olbers paradox

• Why…

If the universe were infinite in size & contained an infinite number of stars that live forever, then every line-of-sight would eventually lead to a star

Stars light dims as 1/r2 but the volume of space sampled increases by the same factor

So, the night sky should be as bright everywhere as the average surface of a star

Page 3: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

Olbers paradox

• How can it be dark then?

• Pondered as early as Kepler and Newton. • Newton wanted the universe to be infinite to avoid

collapse under his theory of gravity• One of the scientists associated with discussing the puzzle

was Heinrich Olbers, and his name remained associated with “Olbers paradox”

• Some people suggested the distant stars light to be absorbed and thus diminished before reaching us

• No good - why?

Page 4: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

Olbers paradox

• Any material which absorbed the starlight should heat up and re-emit it, we would see this gas glowing !

• Flaw in arguments was assumption stellar lifetimes are infinite

• In fact, if we look far enough we look back to a time when no stars existed

• Further, an universe with finite age or which is expanding has a limit to how far we can see, light has to have had time to reach us

Page 5: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

Olbers paradox

• Anyway, number of stars is too small, and stellar lifetimes to short to fill space with light

• The darkness of the night sky rules out the simplest idea that the universe is infinite and filled with unchanging stars

Page 6: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

The Cosmic Microwave Background (CMB)

• However, there is more radiation filling the universe than that from stars

• Something called the Cosmic Background Radiation (CBR) fills the sky in all directions at wavelengths too long for our eyes to see

Page 7: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

The Cosmic Microwave Background (CMB)

• The expanding universe does tell us something about this new version of Olbers paradox

• The expansion has caused CBR to redshift to longer wavelengths from its original energy!

• The cosmos must have once been ablaze with this radiation-this is the radiation predicted to have been produced in the Big Bang!

Page 8: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

The Cosmic Microwave Background (CMB)

• Observational discovery of the CMB

• The Big Bang model

• What can we learn from the CMB?

Page 9: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

1964 Penzias & Wilson (Bell-Labs) and antenna

THE OBSERVATIONAL DISCOVERY OF THE COSMIC MICROWAVE

BACKGROUND

Page 10: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

CBR

• Arno Penzias & Robert Wilson (1964)– Attempted to study radio emission from our Galaxy

using sensitive antenna built at Bell-Labs– Needed to characterize and eliminate all sources of

noise– They never could get rid of a certain noise source…

noise had a characteristic temperature of about 3 K– They figured out that the noise was coming from the

sky, and was approximately the same in all directions…

Page 11: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

THE HOT BIG BANG MODEL• Penzias & Wilson had discovered radiation left

over from the early universe…• The big bang model…

– Independently developed by James Peebles and George Gamov

– They suggested that the universe started off in an extremely hot state

– As the universe expands, the energy within the universe is spread over in increasing volume of space…

– Thus the Universe cools down as it expands

Page 12: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to
Page 13: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to
Page 14: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

CBR

• Why did they suggest this model?– If the early universe was hot (full of energy), a lot of

features of the current universe could be explained…– Could explain where the matter that we see around us

came from (H, He, Li created by the big bang)– Could explain the observed ratio of elements

(nucleosynthesis occurred within first few minutes)– Predicted that there should be left over radiation in the

present universe…

Page 15: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

A brief look at the stages of the Universe’s life…

• We will discuss this diagram in detail in future classes.

• Most crude description…– t=0: The Big Bang– For first 380,000ys, universe is an

expanding “soup” of tightly coupled radiation & matter

– After 380,000yrs, radiation & matter “decouple”. Radiation field reduced enough (due to expansion) that protons can now capture electrons & hang on to them

– Left over radiation that we see known as the CMB…

Page 16: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

THE COSMIC BACKGROUND EXPLORER (COBE)

Page 17: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

COBE• The COBE mission

– Built by NASA-Goddard Space Flight Center– Launched Nov. 1989– Purpose was to survey infra-red & microwave emission

across the whole sky (BB predicts 3 K black body) – Primary purpose – to characterize the CMB

• Had a number of instruments on it:– FIRAS (Fair infra-red absolute spectrophotometer)– DMR (Differential Microwave Radiometer)– DIRBE (Diffuse Infrared background Experiment)

Page 18: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

The CMB: DMR map of the microwave sky…

Page 19: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

COBE Results

• Map of the microwave sky (frequency of 50GHz)…– We’re looking at the CMB– The map is very uniform.– Means that the CMB is extremely isotropic (i.e. the same in every

direction we look)– Supports the idea that the universe is isotropic (one of the basic

cosmological principles).– In fact, if we measure the universe to be isotropic, and we’re not

located at a special place in the Universe, we can also deduce that the Universe is homogeneous!

Page 20: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

The spectrum of the CMB (FIRAS)

Page 21: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

CBR-Spectrum

• Spectrum has precisely the shape predicted by the theory…– So-called “Blackbody” spectrum– Characteristic temperature of 2.728K

Page 22: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

Subtract off average level…

Page 23: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

CBR

• What causes this pattern of redshift and blueshift?

Page 24: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

CBR

• What causes this pattern of redshift and blueshift?

• Earths motion through space causes this dipole (anisotropy with 2 well-defined/opposite points)

• (Earth orbits Sun at 30 km/s, Sun orbits MW at 220 km/s, MW has motion around center of local group etc)

Page 25: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

Subtract the “dipole”…

Page 26: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

CBR

• Subtract off the dipole resulting from the Earth’s motion…

• Are left with…– Bright ridge corresponding to microwave

emission from our Galaxy– Pattern of random fluctuations in the CMB

Page 27: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

Subtract off the emission from our Galaxy…

Page 28: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

CBR fluctuations

• Can use the different spectrum of the Galaxy’s emission and the CMB to distinguish them.

• So, can subtract off the emission from our Galaxy…

• Left with a random pattern of fluctuations in the CMB… correspond to temperature differences of 30 millionths of a Kelvin

Page 29: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

CBR fluctuations

• What are these fluctuations…– The early universe was very close to being perfectly homogeneous

– But, there were small deviations from homogeneity… some regions were a tiny bit colder and some were a tiny bit hotter.

– When matter and radiation decoupled, this pattern of fluctuations was frozen into the radiation field.

– We see this nowadays as fluctuations in the CMB.

Page 30: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

CBR Fluctuations

• Why are the fluctuations important?– Before decoupling, fluctuations in the radiation field

also meant fluctuations in the mass density

– After decoupling, these small fluctuations in density can get amplified (slightly dense regions get denser and denser due to gravity).

– These growing fluctuations eventually collapse to give galaxies and galaxy clusters.

– So, by studying these fluctuations, we are looking at the “seeds” that grow to become galaxies, stars, planets…

Page 31: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

Microwave Anisotropy Probe (MAP)

Page 32: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

Microwave Anisotropy Probe (MAP)

• NASA mission to map out the fluctuations in the CMB in fine detail…– Will characterize these seeds for structure formation– Will determine fine detail of the CMB fluctuations that

depend upon the curvature of space (k) and .

• Launched last year…

Page 33: Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to

New WMAP results - Tues Feb 11

NASA's Wilkinson Microwave Anisotropy Probe (WMAP)

Patterns in the big bang afterglow were frozen in place only 380,000 years after the big bang, a number nailed down by this latest observation. These patterns are tiny temperature differences within this extraordinarily evenly dispersed microwave light bathing the universe, which now averages a frigid 2.73 degrees above absolute zero temperature. WMAP resolves slight temperature fluctuations, which vary by only millionths of a degree. Theories about the evolution of the universe make specific predictions about the extent of these temperature patterns

A NASA satellite has captured the sharpest-ever picture of the afterglow of the big bangDates universe to 13.7 billion years.


Indian Astronomers

Level R/40 Astronomers - Amazon S3 · Astronomers Astronomers Skills & Strategies Level R/40 Anchor Comprehension Strategies ... • Leedy, Loreen. Postcards from Pluto. Holiday House,

Calculation of Sun’s Pos

1.4: The Sun’s Energy. Science Starter Mr. Houtz asked his students how astronomers figure out what stars are made of: Ka’Ron : “I think it’s impossible

The Sun’s Dynamic Atmosphere

The Sun’s Surface Features

Ancient Astronomers 100 pts

Sun’s Project JXTA

Sun’s path equator

The profession of Astronomers

Astronomers Demonstrate a Global Internet Telescopeastro/aovlbi/press_release/eVLBI_AR.pdf · Astronomers Demonstrate a Global Internet Telescope European and US radio astronomers

Astronomy 218 › class › a218_s20 › lecture23.1.pdfto modern cosmology is Olbers’ Paradox. As pointed out by numerous astronomers over time (Thomas Digges in 1576, Johannes

Astronomers and Data

Ancient Astronomers -Example Version

Balle R.S. Great Astronomers

Music reflects astronomers' contributions

Women Astronomers of IIA

Ch. 22: Astronomers

î - WordPress.com010 Kepler Thomas Digges Edward Halley Heinrich Olbers Olbers Paradox

EXPANDING UNIVERSE. OLBERS PARADOX Olbers’ paradox “The night sky is dark.” This statement is called Olbers’ paradox, after astronomer who discussed

Famous Astronomers And Astronauts

Early Astronomers

San Francisco Sidewalk Astronomers

The origin of lunar crater rays - UCLAjewitt/class/Surfaces_Papers/Hawke04.pdf3 Olbers A ray intersection 1 1.02 0.66 10.00.34 4 Olbers A ray intersection 2 1.00 0.66 7.90.31 5 Olbers

Meet the Astronomers!

Famous Astronauts And Astronomers

Great Astronomers: Isaac Newton

Cosmology The Origin and Future of the Universe Part I Olbers’ Paradox

Famous Astronomers And Astronauts1

The Sun’s Path at Night - Zoo Torahzootorah.com/RationalistJudaism/Sixteenth Century Attitudes to Talmudic... · The gentile astronomers, on the other hand, knew that the world

The Sun’s Rays and Climate Demonstration - … ·  · 2011-04-14Latitudes near the poles always receive the Sun’s rays at lower angles, ... The Sun’s Rays and Climate Demonstration

The Astronomers’ Data Manifesto

The Astronomers: Claudius Ptolemy

The Sun’s enduring mysteries

î · 2020. 11. 27. · 010 Kepler Thomas Digges Edward Halley Heinrich Olbers Olbers Paradox